Bricks, cement and asbestos have major role in building and road construction. Construction industry is the largest consumer of material resources, of both the natural ones (like stone, bricks, cement, lime) and the processed and synthetic ones. Each material which is used in the construction, in one form or the other is known as construction material (engineering material). No material, existing in the universe is useless; every material has its own field of application. A brick is a block of ceramic material used in masonry construction, usually laid using various kinds of mortar. It has been regarded as one of the longest lasting and strongest building materials used throughout history. Brick is the most commonly used building material which is light, easily available, and uniform in shape and size and relatively cheaper except in hilly areas. Bricks are easily moulded from plastic clays, also known as brick clays or brick earth. Bricks can be moulded by any of the three methods; soft mud process, stiff mud process and semi dry process. There are various kinds of bricks; silica bricks, carbon bricks, magnesite bricks, dolomite bricks, alumino silicate bricks, refractory bricks, etc. Cement is a binder, a substance that sets and hardens independently, and can bind other materials together. The most important use of cement is the production of mortar and concrete the bonding of natural or artificial aggregates to form a strong building material that is durable in the face of normal environmental effects. Cement is made by heating limestone (calcium carbonate) with small quantities of other materials (such as clay) to 1450 °C in a kiln, in a process known as calcination, whereby a molecule of carbon dioxide is liberated from the calcium carbonate to form calcium oxide, or quicklime, which is then blended with the other materials that have been included in the mix. The resulting hard substance, called clinker, is then ground with a small amount of gypsum into a powder to make Ordinary Portland Cement, the most commonly used type of cement (often referred to as OPC). Asbestos is a set of six naturally occurring silicate minerals used commercially for their desirable physical properties. Asbestos mineral have an almost unique combination of physical and chemical properties. The most widespread modern uses of asbestos are in fireproof textiles, papers and boards and in brake and clutch linings for many kinds of vehicle and machinery. The three main kinds of asbestos which have had wide commercial exploitation are chrysolite, amosite and crocidolite.
Some of the major contents of the book are moulded and ornamental bricks and blocks, including copings and lintels, cutters and rubbers, fireplace bricks, fire bricks and other refractory bricks
mixing, tempering mills or wet pans, the addition of water, souring, de airing, shaping the bricks, bricks made of calcined clay or grog, silica bricks, transition temperatures of silica on cooling, alumino silicate bricks, magnesium silicate bricks (forsterite bricks), high alumina bricks, spinel bricks, developments in refractory brick, production of cement clinker, introduction, preparation of kiln feed, wet and semi wet processes, dry and semi dry processes, pyroprocessing: principal manufacturing processes, wet and semi wet processes, dry processes, semi dry (lepol) process, clinker cooling, refractories, electric power consumption , plastic moulding by machinery the machine moulding process, moulding machines, the wire cut or extrusion process, selection of machinery, power, individual machines, shredding machines , grids, feeders, proportioning, proportioning feeders, crushing rolls, high speed rolls, dressing the rolls, edge runner mills, tempering mills etc.
The present book contains processes of different types of bricks making, cement manufacturing and production of asbestos. The book is very resourceful for new entrepreneur, existing units, professionals, institutions related to building construction, research scholars etc.
Moulded and Ornamental Bricks and
Blocks, Including Copings and
Lintels, Cutters and Rubbers,
Fireplace Bricks, Etc.
Ornamental
bricks and
bricks of special shape are generally made by hand moulding, but where
the
nature of the ornamentation permits them to be made by the wire cut
process or
the stiff plastic process these are cheaper and applicable to most
shapes such
as mullions and squints, of which the profile can be cut in a
mouthpiece or die
(sec Cutter and Rubber Bricks and Wire cut Ornamental Bricks).
Where
a very small number of special shapes are required, ordinary bricks may
be made
by the wire cut process or by the stiff plastic process and then cut by
a taut
wire preferably in a frame or guillotine.
Where
a sufficient number of bricks of the same pattern and size are
required, a
metal die may be used and where only a small number of such bricks are
desired
they should be moulded by hand in wooden or metal lined moulds, but for
more
ornate work plaster moulds sometimes made in several pieces must be
used. A
brick of the required design is first curved in plastic clay a little
larger
than the size of the finished brick, so as to allow for contraction in
diying
and firing. This model must he very carefully and accurately made, as
any defects
in it will be reproduced in future bricks. As soon as the modeller has
completed his work, the mould maker places it on a board and brushes it
over
with a solution of soft soap in water to which a little sallow has been
added,
all the boards being similarly treated. He next places several boards
or a
piece of linoleum around the model, wins them tightly tngcthet to form
a strong
casing, and carefully stopping up any holes with clay paste, so that a
case is
formed into which the liquid piaster can be poured without any leaking
awiy.
Plenty of clay paste should be used, as a leak is very troublesome.
The
model and the inside of the case are brushed over with the soap
solution, and
the mould maker next mixes a quantity of superfine plaster of Paris
with water
in a bucket, so as to obtain a thick slip, and stirs this well with his
hands,
so as to mix it thoroughly. The amount of plaster needed must be judged
by
experience the beginner will not go far wrong if he half fills a bucket
with
water and sprinkles the plaster rapidly into it until it no longer
sinks into
the water, but the proper proportions can only be ascertained by trial.
They
usually lie between 3 and 5! b. of plaster to each quart of water.
The
plaster slurry must be worked with the hands until it is free from
lumps and is
of a smooth, creamy consistent it is then poured slowly and steadily
into the
case by an assistant, whilst the mould maker uses one or both hands to
stir it
slightly and prevent air bubbles forming between the model and the
plaster.
Sufficient plaster must be poured in to cover the model to a depth of
about 2
or 3 in. The whole is now left until the plaster has set, after which
the
casing is removed, the plaster mould turned upside down, and the clay
cut out
with a knife or torn out with the fingers, great care being taken not
to damage
the mould. Sometimes the mode! will drop out whilst the mould is being
turned,
but if it does not do so it must be cut out. The mould is then set
aside to dry
and harden before it is used.
If
sufficient care is taken not to spoil the moulds by overheating them,
they may,
with advantage, be dried by heating them in a warm stove, or even by
placing
them on the boiler.
When
complex designs are required, it may be necessary to make the mould in
several
pieces, especially if some part of the work is undercut, i.e. with part
of the
surface projecting beyond an adjacent (lower) part, and so gripping the
mould
that the article cannot be withdrawn. By making the mould in several
pieces
this difficulty may be overcome, but it is often cheaper to make a
single piece
mould for a modified model and afterwards to undercut portions of the
brick
where required.
To
reproduce bricks in such a mould, it is laid on a bench and a piece of
clay
paste thrown into it with considerable force and pressed well into the
crevices
of the mould. More paste is thrown in and pressed in until the mould is
full.
Any excess of clay is removed by drawing a strike or a stretched wire
across
the face of the mould, the clay being then smoothed (if necessary) with
a
large, flexible bladed pallet knife. The mould with its contents is
then set
aside until the clay is sufficiently dry for it to be turned out of the
mould.
If the mould is properly made and filled, the bricks should not require
any
further finishing, but it will often be found necessary to touch them
up
slightly with a modelling tool before drying them. The burning may be
carried
out in any ordinary kiln, but as the colour of ornamental bricks is
usually
important, they should be so placed in the kiln as to be discoloured by
dust or
not flame.
Ornamental
moulded bricks, especially those which are not symmetrical, require
special
care in drying, as the ornamental portion tends to dry more rapidly
than the
remainder of the bricks and to crack or flake. Such bricks are best
dried on
hot floors or on shelves in intermittent steam heated chambers, and not
in
tunnel or other continuous dryers. The drying should be slow and
without
draughts.
There
is no end to the shapes of bricks that can be made by hand moulding,
although
the cost of some of the moulds used in producing the beautiful bricks
used in
some Tudor architecture would be prohibitive today. Some of the old
Tudor
chimneys required fourteen separate moulds for a total of 144 pieces.
There
is still a large demand for hand made bricks, and although it is very
difficult
to copy exactly the beautiful colours of the old bricks which were
arrived at
more by accident than design, owing to the manner in which the bricks
were
burned in those days. Innumerable different colours and mouldings can
still be
produced using the various sands and stains available, which give
different
effects, both in the texture of the faces of the bricks and also in the
colour.
Carved
Brickwork may be produced by carving and finished bricks in situ in the
wall,
but as this removes the skin and renders the bricks less resistant to
weather,
it should be avoided. Another method consists in making a large slab of
plastic
clay, modeling the desired design on it, and then cutting it with wires
into
bricks, which are afterwards burned in the usual way. In doing the
carving, the
artist must remember to allow for the effect of the joints when the
bricks are
laid in mortar or a ludicrous effect may be produced in the finished
brickwork.
The
design may also be modeled on separate plastic bricks, which are laid
on thin
boards instead of mortar, the modeling being afterwards cut so that the
bricks
can be separated and burned. When this method is used, the burned
bricks should
be assembled again before they leave the works, so as to ensure the
design
being accurately produced.
When
the burned bricks are to be cut or carved, those made of very sandy
loams and
known as cutter bricks are generally used.
The
carving of brickwork is still practiced, but not to so large an extent
as
formerly.
Cutter
and Rubber Bricks are made of very sandy loams, and are so soft that,
when
burned, the former can easily be cut with a hammer and chisel, whilst
the
latter can be rubbed to the required shape on other bricks or on a
stone. They
are made by hand moulding or in a box mould.
Both
Sir Christopher Wren and Inigo Jones did much to popularize rubbed and
gauged
work, which was very fashionable about the middle of the seventeenth
century,
and large surfaces of wall were built with rubbed and gauged bricks,
and in
addition to fiat surfaces, bricks were cut and rubbed to form mouldings
and
quite elaborate cornices and projecting string courses with several
courses of axed
or axed and rubbed bricks. Some of these cornices had projections of
more than
19 in.
For
some purposes, ordinary bricks can be cut to a special shape or size
(e.g. key
bricks, wedges, tapers, semi tapers, and closers) by means of a masonry
saw
such as those supplied by the Clipper Manufacturing Co., Ltd.,
Leicester.
For
cutting over size or distorted bricks to the correct size an abrasive
(carborundum) wheel is usually employed. It is dangerous to press the
brick
against the circular face of the wheel either the edge must be used or
a cup
wheel suitable for surface grinding must be employed.
Fire
bricks and Other Refractory Bricks
The
subject of refractory bricks is now so large that it cannot be dealt
with fully
in the present volume. Readers requiring more information than is
contained in
this one should see
A. B.
Searle, Refractory
Materials their Manufacture and Uses (London Charles Griffin &
Co., Ltd.).
J. H.
Chesters, Steel Plant Refractories (Sheffield the United Steel
Companies,
Ltd.).
J. R. Rait,
Basic
Refractories (London Iliffe & Sons, Ltd.) and the current
Literature.
A. T. Green
and G. H. Stewart, Ceramics a Symposium (Stoke on Trent British Ceramic
Society).
The
manufacture of fire bricks and blocks was carried on for many years in
a
somewhat rudimentary manner, and it is only during the last forty years
or so
that the more important firms attempted to improve their product and
bring it
up to date. In earlier times fire bricks and blocks were only required
to
withstand relatively low temperatures, but, with the increasingly
stringent
requirements of modern metallurgists and other users of furnaces, it is
necessary at the present time to make use of every available assistance
which
science can render to the fire brick maker.
With
this development has come an increasing use of the term refractory
bricks
primarily to distinguish those with a greater resistance to heat
(refractoriness) from bricks unsuitable for use at temperatures above
about
1500ºC. The term refractory is used rather loosely and many ungraded
fire
bricks are sold as refractories.
Investigations
have shown that various users require widely different characteristics
in fire
bricks and blocks, and a material which suits one customer well may be
entirely
unsuitable for another. It is, therefore, necessary to know what
characteristics are required before the value of a fire clay can be
stated.
The
materials from which fire bricks and blocks are made are of four main
classes
(1) fire clay (2) rocks consisting of almost pure silica (3) rocks
composed
chiefly of silica, but containing about 10 per cent of clay and known
as ganister
(artificial imitations of ganister are also used) (4) neutral and basic
materials such as chromite, alumina, and magnesia.
The
treatment of the materials depends on their nature, and the three chief
processes used must therefore be described
Fire
clay Bricks are made from seams of fire clay found in several parts of
the
country, the most noted deposits being in West Scotland,
Northumberland, Yorkshire,
the Midlands (including Burton on Tren! and Ashby de la Zouche),
Cheshire,
Stourbridge, Shropshire, Devonshire, and Wales. The materials from
these
various sources differ widely in composition and character.
The
West Scotland fire clays (including those of Glenboig) are noted for
their
unusual heat resisting power. They require to be fired at a very high
temperature, as otherwise they are soft and weak.
The
North Umbrian fire days are chiefly found near the Tyne, and are richer
in
alumina than most of those of Scotland, though this advantage is more
than
neutralised in several cases by the presence of an excessive proportion
of
fluxing material (alkalies and lime), which greatly reduces the heat
resisting
power of the bricks. Several seams in Northumberland and Durham are,
however,
of excellent quality.
The
Yorkshire fire clays are found chiefly near Leeds and Halifax, but the
material
crops up unexpectedly in several other parts of the county. In South
Yorkshire
it is associated with ganister (silica). The fire clays in Yorkshire
are
peculiarly variable in composition, the alumina varying from 15 to 39
per cent.
The clays richest in alumina are found nearer the surface, but are much
more
tender than the stronger ones found at greater depths. Taken as a
whole, the
Yorkshire fire clays are amongst the most refractory, but they have not
hitherto been worked so as to develop this property to the fullest
extent, as
they are almost invariably under fired, and so shrink abnormally when
in use at
high temperatures.
The
Midland fire clays are more readily vitrified than must others of equal
quality, and are, therefore, in great demand for the manufacture of
close
grained bricks and sanitary pipes. They are not usually so resistant to
heat as
some others, but where other factors (such as the cutting or corrosive
action
of dust and fire gases) have to be considered, they are very valuable,
and
under some conditions prove more durable than more infusible bricks
from other
districts.
The
Stourbridge fire clays have a world wide reputation for refractoriness.
The
composition is remarkably constant, though unexpected variations occur
at
times. The average proportion of alumina is about 22 per cent thus
corresponding to the Scotch and some Leeds clays but portions of clay
with over
36 per cent of alumina have been found.
The
Devonshire fire clays, like those of the Ashby district, are relatively
easily
vitrified, but considerable variations in quality exist. The most noted
fire
clays in this country are found in the Teign valley, and often contain
considerable proportions of undecomposed granite. They are, therefore,
used for
the manufacture of vitrified bricks where the greatest resistance to
heat is
not required, but where a brick which will stand what is ordinarily
considered
to be a high temperature is needed.
The
Welsh fire clays in some ways resemble those of Stourbridge, but are
seldom so
pure, and must, therefore, be worked with caution. Even the best
deposits in
this district are not of first class quality for refractory work, yet
are
excellent with respect to resistance to abrasion.
The
fire clays are chiefly found associated with the Coal Measures and the
Millstone Grit, and are usually obtained by mining. Some brickmakers
work up
the rubbish heaps of collieries, but the best fire clays are obtained
by direct
mining.
The
seams vary in thickness, just as do those of coal, but are less uniform
than
the latter, and it has generally been considered that the only seams
which can
be worked at a profit are thick ones near the surface or those mined
along with
coal. Curiously enough, the best fire clay is often raised from pits
containing
little or no coal.
The
chief constituent of fire clays is a mineral resembling kaolinite and
also
halloysite but not identical with cither, as shown by Roberts in 1947
and named
livesite. In some fire clays the other two minerals are also present in
small
proportions. This mineral, when heated, behaves like kaolinite and is
decomposed into free alumina, free silica and water vapour. On further
heating
a liquid glass formed by the reaction ofalkalis and other fluxes on
some of the
silica this glass contains most of the impurities in the fire clay and
gradually dissolves the alumina and silica. At and above 1200ºC mullite
is formed
by the catalytic action of the alkalis liron oxide and other fluxes. At
a still
higher temperature, mullite crystallizes from the molten glass, the
size of the
crystals depending on the temperature and duration of the healing. The
amount
of mullite formed can be increased by the use of more flux, but this is
commercially unsatisfactory and a much better way is to increase the
temperature of the kiln and to prolong the heating at that temperature.
The
essential constituents of fire clay bricks as well as of other
aluminosilicate
refractory bricks consist of crystals of mullite and silica (tridymite
or
cristobalite) and a glassy matrix. Minor constituents may include
unaltered
quartz, free alumina (corundum) calcium and other silicates.
The
proportion
of mullite crystals is always small in proportion to the whole mass,
but for
the best, bricks it should be as large as possible as it forms the
skeleton or core
of the bricks.
There
appear to be several modifications of mullite with slight differences
in properties
and composition. Natural and fusion cast mullite appears to contain
more
alumina in solution than the mullite obtained in the firing of fire
clay
bricks.
Formerly,
the manufacturer of fire bricks had chiefly to see that his material
was right
and that the men worked well. A few degrees more or less in the kiln
made but
little difference, and so long as his goods were saleable little else
mattered.
Within the last forty years, however, a great change has come over the
fire
clay industry. This is due to a variety of causes, the chief of which
is the
demand for better bricks and blocks from various users. This demand is
increasing as progress with high temperature work continues, and the
fire clay
worker of the future must use his best endeavours to meet the demand.
Fortunately, the cost of building and rebuilding is so high compared
with the
cost of fire bricks that a good price can be obtained for a really
satisfactory
article.
The
Stiff
plastic Process of Brickmaking
The
stiff plastic process owes its name to the fact that the bricks appear
to have
been made of plastic material, though they are stiffer and stronger
than most
bricks made by a plastic process. The stiff plastic process is
specially
suitable for certain shales, which are becoming increasingly popular
for the
manufacture of hard burned, slightly vitrified building bricks.
The
chief advantages lie in the saving of (i) the capital cost of a dryer,
(ii) the
cost of placing the bricks in the dryer, (iii) the cost of labour in
working
the dryer, and (iv) part of the cost of the fuel. The total of these
amounts is
sometimes quite large. These advantages can best be gained with bricks
which
can be safely and rapidly dried in a continuous kiln, but the stiff
plastic
process can also be made suitable for (i) materials whose plasticity or
excess
of water can be reduced sufficiently by drying by artificial heat or by
adding
a suitable non plastic material and (ii) materials which only require
the
addition of water to enable them to be pressed satisfactorily.
Hence,
the stiff plastic process can be used for almost all clays and shales
if they
are first subjected to a suitable preliminary treatment. If no such
treatment
(other than grinding and screening) is desired, the process is confined
to
clays and shales which are dry enough to be ground and screened.
THE SIMPLE STIFF PLASTIC
PROCESS
When
no such preliminary treatment is required, the clay or shale is taken
from the
pit in wagons and fed into a grinding mill, generally of the edge
runner type,
with a revolving perforated pan, though a preliminary breakage of the
large
lumps is desirable. The clay is ground dry or in a slightly moist
state, and is
then taken by an elevator to the screens, of which there is generally
one to
each mill. The clay which passes through the screens goes down a chute
into a
mixer, where a little water is usually added and the whole is then
thoroughly
mixed. It next goes into the making machines and is pressed into rough
blocks
or clots about the size of a brick. These are then re pressed, this
latter
operation giving the brick its proper shape, making the well or frog
and
printing the name of the firm. The bricks are then dried, if necessary,
and
taken to the kilns. Drying is avoided when possible, this being the
great
advantage claimed by the stiff plastic process, though even where it
cannot be
entirely avoided its cost is greatly reduced. The kilns are the same as
those
used for bricks made by the plastic process but it may be noted here
that as
the stiff plastic process is generally used for large outputs some form
of
continuous kiln is usually employed.
The
material must be sufficiently ground, and for the best bricks must be
able to
pass through a sieve with twenty holes per linear inch without leaving
any residue
though for common bricks a coarser sieve may be used, one with eight
holes per
linear inch being popular.
The
ground material may require the addition of a little water, hut in any
case it
should be mixed so as to form a granular material of uniform
composition and of
constant stiffness, and the machinery used must be kept in first class
order.
Economical
grinding and pressing by this system requires the provision of a
comparatively
dry clay, or one in which a wet clay can be mixed with a large amount
of dry
material so as to make a relatively dry mixture. This is necessary,
because in
this process the clay is ground and sifted, and this cannot be done if
the clay
is very moist. If these matters are attended to and the material is
suitable,
no serious difficulties should occur in the manufacture of stiff
plastic
bricks.
Fine
grinding and accurate screening are essential, and avoid many
difficulties
which otherwise arise. Saleable brick can be made with imperfectly
ground
material, but the process is costly and the results are always
uncertain.
A convenient
arrangement of the plant for the stiff plastic process in its simplest
form is
shown in Fig. 1, in which I represents the grinding pan, 2 the
elevators, and 3
the brickmaking machine in this instance a Fawcett brick making machine
being
included.
Various
modifications of this simple process are usually desirable.
CRUSHING
Although
many firms using the Stiff plastic process send the clay or shale
direct to the
grinding mills, it is usually more economical to subject it to a
preliminary
crushing.
Stone
breakers or Jaw Crushers can only be used for dry hard shales. If damp
clay is
passed through such a machine it soon clogs it and may cause serious
damage!
Many attempts made to use them for damp clays have failed.
Jaw crushers
are very satisfactory for the preliminary crushing of burned clay
(waste bricks
used for grog) and for soft sandstone used for reducing the shrinkage
of clay
during drying, but not for wet (slightly sticky) shales.
No
attempt should be made to crush the material very small and there is no
need
for the jaws to be set closely, and consequently they can be arranged
to give a
large output. The jaws should be examined occasionally and any wear and
tear
made good, as the machine will waste power if it is unduly worn.
Crushing
Rolls are often very satisfactory for a preliminary crushing of lumps
of
material prior to their entering an Edge runner Mill.
Such
a machine with two rolls, each 18 in. in diameter and 16 in. long,
using 25 to
30 h.p. will crush sufficient shale or hard clay in an hour to make
6000 bricks
and will greally reduce the wear and tear on the Edge runner Mill.
Prior
to the material entering the crusher it is often advantageous to pass
it over a
live grizzly or other form of screen to separate pieces which are too
large for
the crusher and to break these by hand. A second screen in take all
pieces
smaller than the outlet of the crusher will also save power, though
when the
proportion of such small pieces is insignificant the whole of the
material can
be passed through the Fig. 3. Section of Light type of Breaker crusher.
A jaw
crusher is generally the best machine for reducing large lumps of dry
clay or
shale, though a gyratory crusher has a greater range of reduction.
Hammer
Mills (Disintegrators) In the United States the term disintegrator is
applied
to crushing rolls, but in Great Britain it is used for an entirely
different
type of machine, consisting of a series of hammers hunt loosely on a
shaft
which rotates at the rate of about 1000 revolutions per minute, and so
rapidly
reduces any moderately dry clay or shale to a coarse powder. Another
type of
disintegrator consists of two cylindrical cages, one inside the other,
which
revolve in opposite directions, and so break up lumps of clay, shale,
etc., and
reduce them to a coarse powder.
The
Lightning Crusher shown in Fig. 4 consists essentially of a casing
enclosing a
rotating shaft, bearing two discs or flanges which carry two or more
shaped
hinged hammer bars, which are carried round by the revolving shaft. As
these
bars strike a lump of material they deliver a violent hammer like blow,
which
splits the lump rather than crushes it, so that the product is more
cubic than
that from edge runner mills and crushing rolls. A grid is provided when
a liner
product is desired.
In
the disintegrator made by British Jeffrey Diamond Ltd., the rotor
consists of a
series of discs mounted on a strong shaft, with a number of hammers
loosely
mounted on pins on each disc. The end discs are flanged to form a seal.
In
all hammer mills, the number of hammers and their position on the rotor
should
be made to depend on the nature of the material to be ground and on the
desired
fineness of the product.
Most
machines of this type are not suitable for plastic clay, but are
excellent for
shale, stone, or grog which is to be reduced lo pieces 1/16 in. or less
in
diameter.
These
machines are not generally suitable for fine grinding, but a
disintegrator,
which can both dry and grind shales and hard clays (if they are not too
sticky)
is the Atritor, made by Alfred Herbert, Ltd., Coventry. It requires the
material to be reduced to pieces not more than ¼ in. by a preliminary
crusher an edge
runner mill being
usually the most convenient and
effects
the drying by a current of hot air which also conveys the ground
material to a
cyclone separator from which it is delivered to a storage bin or
conveyor. The
fineness of the product of this machine often improves the appearance
and
texture of the bricks or hollow blocks.
Whilst
disintegrators are not usually regarded as fine grinders, an Atritor
will grind
hard shale and clay sufficiently fine for more than half of it to pass
through
a 100 mesh sieve. One pattern of the Atritor has been specially
designed for
grinding shale or clay to specified degrees of fineness from 5 to 200
mesh. In
this machine, the air is passed through it by means of an independently
driven
fan, so that the particle size of the product can be controlled by
varying the
speed of the mill without interfering with the air supply. By pre
heating the
air, damp clays and shales can be dried sufficiently to be ground
satisfactorily.
Disintegrators
usually require the material supplied to them to be in pieces not more
than 3
in. diameter, and they do not work economically when the articles of
the
product are less than 1/8 in. moreover, they are chiefly useful for
clays
employed for common bricks in which the minute particles of metal from
the
hammers do not spoil the colour. With a suitable material they require
less
power than an edge runner mill, but the wear and tear is greater. To
obviate
damage by stray pieces of metal entering the machine, the beaters
should be
hinged so as to stand straight out by centrifugal action in the
ordinary course
of grinding, but to fall back when a mass of metal is encountered. One
section
of the casing is hinged and held in place by an easily opened catch,
and upon
the attendant hearing the noise caused by a stray article he at once
opens the
catch with a long pole, standing well aside and out of the way of the
material
which is ejected from the machine.
When
using a hammer mill or disintegrator it is important to adjust the
machine so
as to give a product of the desired particle size with one passage
through the
machine, as such mills depend on hammeraction (shattering) and not on
direct
crushing pressure and the larger the pieces of material fed into the
mill the
greater is the shatter effect. For the same reason it is seldom
advisable to
pass any material a second or third time through the mill unless it has
previously been mixed with a large proportion of fresh material. If the
product
is too coarse, increasing the speed of the mill will grind it finer but
care
must be taken not to exceed the safety limit. A considerable increase
of speed
may also require the substitution of superior bearings and may involve
the use
of smaller hammers arranged in a staggered position instead of a few
larger
hammers.
It
is unwise to use bars with too small a space between them, as the
function of
these bars is not that of a screen and insufficient space between them
merely
results in clogging the mill, i.e. the material is carried round and
round
inside the mill, without being ground. It is usually possible to adjust
the
particle size by moving the striking plate closer to or further away
from the
hammers.
Gyratory
or Conical Crushers are seldom used for clays and shales in this
country, but
they are extensively used in some of the much larger works in the
United States.
Hand
Moulding Processes
Hand
Making is chiefly practiced in an area to the south and east of a line
drawn
from Kings Lyrin to Portland Bill for ordinary building bricks and in
the
Midlands and North for the manufacture of fire bricks, specially
moulded
bricks, and terca cotta. As almost any clay with sufficient plasticity
can be
moulded into bricks by hand, the number of clays of widely differing
characteristics described as brick earth is very large, and the
prospective
brick maker must be careful in his choice of material, for some clays
are
impossible to use commercially, even when, apart from the cost of
manufacture,
it is quite possible to make good bricks from them.
Only
about 5 per cent of the total output of building bricks made in the
British
Isles is made by hand moulding, as such a process is much more costly
than
others which are available.
Bricks
were made by hand moulding during more than 4000 years and in fairly
large
numbers in the British Isles during the Roman occupation but after the
Romans
left in the fifth century it was a long time before bricks were again
made in
appreciable quantities, viz., in the thirteenth century.
They
were very largely used in the fifteenth and sixteenth centuries when
some of
the most beautiful Tudor buildings were erected and ever since they
took an
increasingly important part as a constructional material suitable for
almost
every kind of building and for many other purposes until the use of
machinery
greatly reduced their production relative to the total output of bricks.
Notwithstanding
its great age, hand moulding is by no means an obsolete method, for by
it
bricks of a beautiful appearance can be made better than by any other
method.
It is costly and slow, and skilled moulders are scarce, so that the use
of
machinery has many advantages but where a beautiful appearance is an
important
consideration, hand made bricks still occupy the first place.
Materials.
Hand moulded building bricks are usually made from surface clays of a
mild
character highly plastic or tough clays must be mixed with a suitable
non
plastic material before they can be used satisfactorily. The materials
are
selected according to the kind of bricks to be made thus, for an
artistic
facing brick a suitable loam or sand may be added to the clay, and the
moulding
process may also be arranged so as to produce bricks coated with sand.
The well
known stock bricks of Kent and Essex, on the contrary, are made of clay
rich in
calcium carbonate, or this is added purposely to the clay in the form
of chalk.
Clays
used for hand made bricks should usually be of such a nature as not to
require
very powerful machinery to convert them into a suitable paste. Hence,
they
should be free from gravel, nodules, and stones, or these materials
must be
removed by washing. Some clays which contain an excess of sand can be
used
satisfactorily after being washed.
The
most popular clays for hand brickmaking are the Kent, Reading, Bagshot,
and
Gault beds in the South and East, and some Midland beds, but many
surface clays
in different parts of the country are locally considered to be of great
value
for this purpose.
Many
clays which are too strong or tough can be made suitable by the
addition of 20
to 30 percent of sand, which must be thoroughly mixed with the day,
either by
repeatedly turning over with a spade or by using an open trough mixer
and a pug
mill or brick machine.
HAND MADE FACING BRICKS
Hand
made bricks vary greatly in appearance Kent Stocks are usually smoother
than
Sand faced Facing bricks and some antique Hand made Facing bricks have
(purposely) a very coarse texture. Many imitation handmade bricks now
made are
described as antiques, rustics and by other names.
For
red facing bricks, the clay must be as free as possible from lime, as
this
would affect the colour. If necessary, the clay may be washed to free
it from
objectionable impurities, but this is not usually necessary. When the
bricks
are sand faced their colour is largely due to the sand used in
moulding, and by
selecting this carefully, using different sands for different coloured
bricks,
many distinct and many beautiful shades of colour ranging from red to
purple
may be produced. Whatever sand be used, the colour of the clay body
will show
wherever the brick is chipped, and predominate eventually on the large
face of
the brickwork. The inherent colour of the burned clay produces the main
effect
of colour, but it can be varied to a limited extent by the use of sand.
The
stock bricks made in Essex, Kent, and Middlesex from natural or
artificial
marls are made by washing the brick earth, with or without the addition
of
chalk, and then running the liquid into wash backs where it remains
until
sufficiently solid to walk on. The material is then covered with a
layer of
fine ashes (Soil) and when required the mixture is dug vertically and
sent 10 a
pus mill to be tempered. The resultant paste is moulded by hand, the
bricks are
then dried on hacks or in dryers and afterwards burned in clamps or
kilns.
Many
of the resultant bricks are by no means pleasing when viewed singly
they may
have a bad arris, be much chipped, and are often irregularly burned and
sorted,
and when they arrive on the job they do not appear to be in such good
condition
as machine made bricks. On the other hand, they form a very strong mass
when
built up, on account of their adhesion to the mortar they are excellent
for
plastering, and are the most durable of any bricks when exposed to the
London
atmosphere. If well made and properly burned, they are as strong as
many other
building bricks, though samples picked at random vary greatly in this
respect.
Rubbers
are relatively soft bricks which must be fine grained and of uniform
colour
throughout. Some of the best of these are box moulded and so uniformly
burned
that even when the outer skin is removed by carving or rubbing, the new
surface
exposed will weather perfectly.
HAND MADE FIRE BRICKS
The
materials used in the manufacture of fire bricks are too hard to be
sent direct
from the mine to the pug mill. They are crushed or ground before being
made
into a paste. It is possible to use crushing rolls, but fire clays are
usually
best crushed in an edge runner mill and, after sifting, are mixed with
water in
a pug mill until a uniform paste is obtained of a consistency suitable
for hand
moulding. It is not advisable to mould it immediately, but to keep it
for
several days or even for a month in a moist state, and then to pass it
a second
time through the pug mill. The reason for this is the souring, or
putrefaction,
which most clays undergo when kept in a moist state, ensures the water
being
move uniformly distributed, and a more homogeneous paste is the result.
The soured
or matured and re pugged material is then slop moulded and then the
bricks are
dried on a hot floor or in another dryer, and are afterwards burned in
single
or continuous kilns. As fire bricks are required to be highly resistant
to high
temperatures, they should be burned under such conditions that they
will not
shrink seriously when in use. For this reason, the temperature attained
in
burning fire bricks should not be less than the bending point of Seger
cone 5a
(1180ºC.), and may be as high as that of Seger cone 18 (1500ºC.).
Glazed
Bricks
Glazed
bricks are used for three distinct purposes (a) to provide a smooth and
readily
washable surface which is impervious to dirt, (b) to increase the
resistance of
the body of the bricks to acids, e.g. by salt glazing acid proof
bricks, and
(c) to provide a pleasing and ornamental facing to the building.
For
the last named, the surface may be glossy or matt and in the United
States very
attractive effects are produced by spraying one colour irregularly over
another, so that walls built of a dark shade of brick at their base
tone
gradually to a light shade in the upper courses of the buildings, or
contrasting shades of light and dark are used in columns running to the
height
of the building, thus emphasizing the vertical construction a striking
example
is the American Radiator Building with its manganese and gold tower.
In
conjunction with larger glazed blocks (known as glazed terra cotta and
by
various trade names) very effective facings are provided.
There
is a general impression amongst brick makers that any kind of brick can
be
glazed, provided that the composition of the glaze is known. This half
truth
has been the cause of much trouble and loss of money, because few
people have
realized that unless the brick to which the glaze is to be applied is
practically perfect the glazed brick will be a failure. Trifling
defects in a
facing brick are often overlooked, but even smaller defects in a brick
which is
afterwards glazed will render attempts to sell it entirely abortive.
Thus, a
few tiny specks of lime in a facing brick may be passed unnoticed by
the
purchaser, but, if such a brick be glazed, the glaze will shell off
above each
lime speck and the brick will be of no value. Again, small defects in
the arris
of an unglazed brick are not obvious, but in a glazed brick they are at
once
noticeable.
Speaking
generally, red burning clays are very liable to defects which are
trifling in
themselves, but which render successful glazing impossible, and, whilst
a few
firms have succeeded in building up a good trade in glazed bricks made
of red
burning clay, the majority of those who have attempted to use this
material on
a large scale have failed to show any profit. Glazed bricks are
therefore,
chiefly made of fire clay, the second grade clays with a refractoriness
corresponding to cone 26 to 30 being used.
A
brick to be suitable for glazing must be regular in shape, exact in
size, with
clean arrises, and a fine face free from small irregularities or
discoloured
spots. It must be sufficiently porous to absorb the water in the glaze
slip,
and must be refractory enough to keep its shape whilst heated at a
temperature
which will suit the glaze.
Such
bricks are usually made by the plastic process and are re pressed
before being
fired, so as to obtain a good shape and face and to make them accurate
in size.
Any of the re presses may be used that by Pullan and Mann has a special
measuring mechanism which automatically makes all pressed bricks of the
same
thickness, as any excess of clay is absorbed by making a somewhat
shallower
frog than usual.
When
made of fire clay, bricks to be glazed are often hand moulded, as are
fire
bricks and are re pressed. Stiff plastic and semi dry prcssed bricks
are slowly
coming into use for glaring purposes, but they have not proved popular
so far,
owing to their liability to develop tiny surface cracks, which are of
little or
no importance in unglazed bricks, but prevent the glaze from adhering
properly.
Much
difference of opinion has been expressed from time to time on the
desirability
or otherwise of burning bricks before glazing them. It is considered
that the
cost of burning the bricks is so much wasted money, as they have to be
reburned
when glazed. Experience shows, however, that if the glaze is applied to
unfired
(green) bricks, the damage suffered in handling makes a large
proportion of the
bricks useless when they come from the kiln. These spoiled glazed
bricks cannot
be sold except as rubbish, as it is obvious that they are damaged. If
on the
contrary, the bricks are first burned without glaze, any defective ones
sorted
out may be sold as building bricks of good quality, or even as fire
bricks at a
higher price. The bricks selected to be glazed are stronger and less
liable to
damage, the amount of glaze wasted is reduced, and the number of
unsaleable
glazed bricks is brought to a minimum. These various savings often
combine to
make it cheaper to burn bricks twice instead of once.
At
the same time, it is often possible with extraordinarily careful
handling to
glaze the unfired bricks and put them into the kilns in a remarkably
perfect
condition, and if work people will give sufficient care to the matter,
it is
quite possible (though seldom realized) to obtain a large proportion of
excellent glazed bricks with a single tiring.
A
mistake made by several purchasers of glaze recipes is to consider that
they
can buy all the bricks they require from a neighbouring yard. Such
people
forget that bricks intended for glazing need most careful handling, and
when
chipped at the edges they are useless. As few bricks which have been
carted
from one yard to another are not slightly chipped, it is practically
impossible
to buy bricks suitable for glazing unless the glazer is allowed to work
on the
same premises as the brick maker.
The
glazed brick manufacturer cannot be too stringent or careful in the
selection
of his bricks.
The
clay being suitable for the purpose of making a clean, well shaped
brick, free
from any impurities which could affect the body or the glaze, the most
important part of the manufacture is the pressing. The presses should
be placed
conveniently near to the drying floor, or to the dipping sheds,
according as
the bricks are glazed in the burned or green state, as a little
roughness in
handling the impressed bricks will do no damage, but the pressed bricks
must be
handled as little as possible and carried to the dryer and after wards
to the
kiln with all necessary care to prevent them from being damaged
especially at
the arrises and corners.
Two
serious errors, which sometimes arise in pressing, must be prevented at
all
costs. The first is due to the use of worn moulds or dies, whereby the
bricks
are formed with an arris or rough edge on them, which does not leave a
clean
edge unless the arris is very skillfully removed. The second is where
the
pressman fails to clean out the die completely, with the result that
succeeding
bricks have small pieces of clay forced into their faces, and these
rise during
the dipping and later cause the glaze to peel.
Pressing
bricks for glazing is necessarily a slow operation (about four bricks
per
minute being the maximum), and any attempt to hurry the pressman may
result in
the loss of several hundred bricks, because these are spoiled by loose
arris
getting on to the faces of the bricks, or in other ways.
Glazed
bricks must be laid with the thinnest possible joints, and, for this
reason,
must be pressed accurately. Any good power press may be used for this
purpose,
but it is sometimes a convenience lo use one in which the die can be
drawn out
on slides to the front of the press in order to discharge the brick,
and enable
the die to be cleaned before pressing another brick. When the die is
movable in
this way, it is much easier for the workman to see that it is properly
cleaned
and oiled than when a die fixed permanently beneath the plunger is
used. It is,
however, essential that the slides on which the die moves are kept
perfectly
clean, or the male part of the die will not fit accurately into the
female
portion and the die will be damaged.
Bricks
which are glazed previous to burning require to be set in the kilns
with the
greatest care to prevent chipping, and the temperature throughout the
kiln must
be as uniform as possible, or the bricks will be unevenly glazed.
Bricks to be
glazed in the green state are often first beaten with a flat wooden
blade to close
up the face, but with a good press in charge of a careful man this
operation is
not necessary.
The
burned bricks to be dipped are conveniently sorted at the kiln, then
placed on
a large off bearing barrow fitted with ample springs to prevent undue
vibration,
and are taken to the dipper, who has a small wagon carrying his tub of
slip.
If
the bricks are to be dipped before firing, they are placed directly
they come
from the press on to the barrow already mentioned, a sufficient number
of these
barrows being provided to allow the bricks to dry somewhat after they
have been
pressed. This is better than placing the bricks on the floor, as the
double
handling thus necessary is certain to damage them, and ihecost of a few
additional barrows is not usually prohibitive.
The
barrows with the bricks on them may be run into a warm shed so as to
allow the
bricks to stiffen and dry sufficiently within two or three hours, or
they may
be left overnight, bricks pressed one day being dipped on the next. The
bricks
must not be so dry as to show a lighter colour at the edges. Some firms
dip the
bricks after they have been dried white hard, but this is seldom
satisfactory,
as the sudden soaking of the dried face often cracks it.
GLAZES AND BODIES
Glazes
are seldom applied directly to the bricks as the result would not be
pleasing.
Anengobe, body or dip, consisting of white burning clays, such as
chinaand ball
clays together with a flux such as Cornwall stone or felspar and some
free
silica (usually flint) is therefore made into a slip or dip and applied
to the
surface of the bricks. This engobe is later covered with a coating of
glaze.
Engobes
are intermediate in composition between the clays of which the bricks
are made
and the glazes used and so forms a suitable buffer between them it
covers small
defects in the surface of the brick, prevents any staining impurities
in the
brick from coming into contact with the glaze and enables the glaze to
adhere
more tightly to the engobe than it wouid to the brick.
The
compositions of some engobes and a glaze which are largely used for
glazed
bricks are shown below they are merely guides as they will probably
have to be
adjusted to any bricks which the reader desires to giaze.
The
glazes used for bricks must be sufficiently durable to withstand
ordinary
climatic changes without crazing1 or forming hair like cracks. They
must be
sufficiently hard to withstand accidental blows, and must adhere to the
bricks
so completely that they will not chip or peel off. Glazes which melt at
low
temperatures (below 1000ºC.) do not usually possess these necessary
characteristics when fired on a porous body, but tend to craze or peel.
Glazes
fired at a higher temperature are, therefore, employed for glazed
bricks, as
the higher temperature enables a mixture of material to be used which
produces
a mass more nearly resembling the brick itself. Low temperature glazes
are
frequently termed soft fired or soft, and high temperature ones are
spoken of
as hard fired or hard the terms hard and soft when applied to glazes
have no
necessary connection with the softness or hardness of the glaze.
The
materials used in the preparation of glazed bricks are very numerous,
and would
require a large volume to describe them fully. For temperatures near
1000°C.
they are similar to those used by potters, but for the higher
temperatures less
fusible glazes are employed, and these are usually composed of felspar,
Cornwall stone, flint and whiting, the corresponding bodies being
composed of
china clay, ball clay, Cornwall stone, and flint, a little of the
finely ground
or finely screened brick clay often being used in the first dip. Other
materials, such a barytcs, zinc oxide, soda, and plaster of Paris, may
be added
at the discretion of the glaze maker, and the materials must, in some
cases, he
fritted into a kind of glass and ground before use.
Lead
compounds are seldom necessary in hard fired glazes, and their use
should be
avoided whenever possible, for several reasons.
Enamelled
Bricks are, strictly, those with a coating of enamel (i.e. opaque
glaze), but
the term is applied to all glazed bricks.
Production
of cement clinker
Introduction
The
objective in making cement clinker, namely the combination of the four
principal oxides to make a material high in di and tri calcium
silicates but
low in free lime, was discussed in terms of the composition and
reactivity of
the raw materials. In this chapter, commercial methods used will be
outlined
and some of the chemical and physical processes that occur at high
temperature
considered. The manufacturing process is primarily concerned with the
selection
of the most efficient engineering methods for crushing, grinding,
blending and
conveying of solids on a large scale as well as with their heat
treatment (pyroprocessing).
Energy usage is considerable and is constantly monitored so that
improvements
can be made.
Two
distinct processes are employed in the production of clinker. In the
wet
process a slurry of the finely divided raw materials is made and pumped
into a
long rotary kiln. In the dry process the raw materials are prepared for
pyroprocessing as a blend of finely ground powders and initial heating
is
usually carried out in a preheater using the hot gases from a
relatively short
kiln. However, in those parts of the world where the raw materials are
relatively dry, and fuel costs are not prohibitive, drying, calcining
and
clinkering may be carried out in a long dry kiln. The wet process was
at one
time predominant but a rapid increase in fuel costs in the 1970s
accelerated
the final stages of its replacement, since it involves the evaporation
of a
substantial quantity of water, typically 30 35% of the mass of the kiln
feed.
However, in areas where the primary raw material is a porous, high
moisture
content chalk containing flint, the wet process has survived in the
preparation
of raw materials.
Preparation of kiln feed
After
extraction from the quarry, raw materials must be crushed, ground and
blended
to make the raw feed (meal) for the kiln. The choice of equipment is
dependent
on the physical properties of the materials. The importance of adequate
grinding and blending of raw materials cannot be overemphasised, as
there is a
limit to the size of the regions of different chemical composition
which can be
eliminated by the rate determining processes of dissolution and ionic
diffusion
in clinkering.
Wet and semi wet processes
Soft
materials are converted to a slurry with water in a wash mill. This
involves
vigorous agitation with harrows and tynes hanging into a cylindrical
tank from
a centrally pivoted rotating arm. Fine material in suspension passes
through a
vertical screen at the side of the tank, against which it is thrown by
the
harrows. The slurry produced, controlled by measurement of its density
and the
addition rates of the materials should contain the highest
concentration of
solids at which it is pumpable. Reduction of the water content
necessary for
pumping can often be achieved by adding a deflocculant, such as sodium
carbonate or silicate, at a cost which is less than that of the kiln
fuel
saved. However, if significant amounts of a smectite (swelling clay)
are
present in the clay, the addition required will be excessive.
Either
clay or chalk may be slurried first and the second component blended
with it in
a second wash mill. When chalk contains flints it may be added to the
clay
slurry in a form of tube mill, called a wash drum, in which the flints
act as
grinding media and are then scalped off. Remaining coarse material is
removed
from the slurry by fine screens or by hydrocyclones. The suspension
enters a
cyclone tangentially in an upper cylindrical section that produces a
downward
spiral motion in it (a vortex), which carries it into the conical
section. The
quantity of material leaving the bottom is limited so that an upward
vortex is
produced in the centre. Heavy material is thrown outwards and downwards
and
light material is carried up by the fluid flow to the second exit.
Further
adjustment, using minor components such as ground sand, pulverised fuel
ash or
iron oxide, may be necessary to optimise chemical composition. The
refined
slurry is monitored by determination of residues when a sample is
washed
through standard sieves. Acceptable residues depend on the reactivity
of the
coarse material but 0.5% greater than 300 mm and 12% greater than 90 mm
(in
would be typical. Chemical composition is checked by sampling at
several
stages. An automated X ray fluoresence spectrometer is used to
determine
silicon, aluminium, iron and calcium, and an on line instrument can
control
equipment feeding the blending mill. The slurry is then held in tanks
in which
it is agitated both mechanically and by compressed air to prevent
segregation.
Water
has a high enthalpy of evaporation, and so the curve related kiln fuel
consumption to the water content of a slurry is a steep one [(Fig. 1
(a)].
Consequently, a number of the remaining wet process plants have been
converted
to the semi wet process by installing filter presses (Fig. 2) which
produce a
cake with a water content of about 18 20% as feed for a kiln or
preheater. In
some variants of this process part of the kiln feed is introduced to
the
preheater in a dry state.
Dry and semi dry processes
The
sequences of operations used in preparing the raw meal for the dry and
semi dry
processes are shown schematically in Fig. 3. Raw materials are crushed
and put
into stockpiles, usually under cover. A considerable degree of
homogenisation
is obtained by laying them down in strips or layers and systematically
reclaiming
from the stockpiles produced. They are then continuously proportioned
into the
milling and drying system by weight.
Gases
from the kiln or the cooler are used for drying, although supplementary
firing
may be necessary. Grinding is carried out either in a ball mill or a
vertical
spindle mill in which rollers grind the material in a pan (Fig. 4). The
latter
is now usually selected for grinding raw materials in a new plant
(unless they
are exceptionally abras ive) as it uses significantly less energy for a
given
fineness than a Fig. 4. Vertical roller (spindle) mill used in drying
and
grinding raw materials A feed B roller on grinding table hot air flow
(arrows)
carries material to classifier–C–from which the finer particles are
carried to
exit–D. A peripheral dam ring determines the depth of the bed on the
table
which rotates. Hydrostatic pressure is applied, to the rollers ball
mill. Early
problems of wear and maintenance have been reduced by using replaceable
wear
resistant alloy surfaces, and steadier running is achieved by recycling
some of
the ground product to establish a denser bed in the pan.
Separation
of coarse from fine material leaving the mill may be effected by
entraining the
powder in exhaust gas from the kiln, followed by separation of coarse
material
in a separator and/or cyclone system. Alternatively, the mill product
may be
transferred to the separator by a bucket elevator. The separator or
classifier
makes use of the balance between centrifugal and air drag forces,
produced by a
rotating plate which strikes the particles and by a fan. The milled raw
meal is
transferred to blending silos, which may be more than 30m high, and
then lo
final stage silos. To reduce the capital investment required in the
blending
and storage silo system, some new plants now employ limited storage
capacity
with direct on line chemical analysis and continuous adjustment of the
kiln
feed composition, using corrective materials such as ground limestone,
sand and
iron oxide.
Pyroprocessing principal
manufacturing processes
The
processes that occur when a raw meal is heated to clinkering
temperatures, and
where they take place in the principal types of plant employed, are
summarised
in Table 1. In wet process and some semi wet plants they all take place
in the
rotary kiln, which can be divided into the zones in which they occur
(Fig. 5).
In the most modern dry process plants almost all but the clinkering is
carried
out by suspending the powder feed in hot combustion gases before it
enters a
short kiln.
Material
moves down a rotary kiln by sliding and rolling induced by the rotation
and
inclination of the kiln. The powder begins to form nodules as melting
begins in
the approach to the clinkering zone, the driving force for this being
the high
surface tension and low viscosity of the melts formed. Timashev
reported a
linear dependence of nodule size on melt surface tension. Gas
velocities in a
kiln are high. In the wet kiln, where a high fuel input is needed for
drying
the slurry, velocities may exceed 5 m/s so that dust entrainment
occurs. Any
dust not captured by the slurry in the drying zone and leaving the kiln
constitutes a significant energy loss since its sensible heat (heat
content) is
lost. However, in the dry process most the dust/heat is retained in the
preheater.
Dust leaving the preheater may be captured where exhaust gases are used
in a
raw mill/drying system. Before going to atmosphere, gases are passed
through a
cooler and bag filter or a multi chamber electrostatic dust
precipitator after
conditioning to a suitable humidity and temperature. Collected dust
which is
not too rich in alkali may be returned to the raw feed blending system.
The
choice of process for a plant is determined by balancing fuel economy
against
the capital investment required and its depreciation, as well as the
amount and
quality of the cement demanded by the (mainly) local market. Where
there is
limited demand and a relatively unsophisticated product suffices, a
simple
vertical shaft kiln may be used. A new dry process plant is unlikely to
be
constructed to produce less than about 800 000 t per annum. Kilns are
fired by
pulverised coal, heavy fuel oil or natural gas, according to local
availability
and cost. Coal has usually been employed in the UK but with a
substantial
supplement of petroleum coke. Other low cost materials employed as fuel
supplements include old tyres and solvent wastes, use of the latter
being
subject to the constraints imposed by any deleterious effect on cement
quality,
refractory life or potential emissions to atmosphere from the system.
Wet and semi wet processes
A
long kiln (length/diameter (L/D)~30) is necessary to carry out the
endothermic
processes of drying the slurry, decomposing the clay minerals and the
calcite,
and then to raise the temperature of the feed to a level at which
clinkering
takes place. Some 30 minutes residence in the burning zone is usually
required.
The strongly endothermic nature of both water evaporation and
calcination is
reflected in the plateaux in the material temperature profile in Fig. 5
Their
length is an indication of the time taken for completion of these
processes,
involving minutes rather than the seconds needed in a fluidised powder
suspension. In the drying zone, heat transfer from gas to slurry is
optimised
by heavy chains fixed to the shell of the kiln. These transfer heat
from the
combustion gases to the slurry and lift it to increase the surface at
which
evaporation can occur. In the semi wet process, filter cake is either
introduced 10 a shorter kiln chain section or it enters a kiln after
being
dried on a Lepol grate or in a crusher dryer.
Grinding
and fineness of cement
After
it leaves the cooler, clinker is conveyed to a covered store in which
some
blending may be possible. Cement is produced by grinding clinker and
gypsum,
usually in a tube mill. This is divided into two or three chambers by
means of
slotted partition walls (diaphragms) which permit the forward movement
of
cement but retain the size graded grinding media. Milling is continuous
and the
residence time of the material in the mill, and therefore the fineness
of the
cement, depends on the rate at which clinker and gypsum are introduced.
A large
mill drawing 4500 kW, 4.6m in diameter and 14m long, would contain
about 2801
of steel balls with diameters from 90 mm in the first chamber down to
15 mm in
ihc last chamber. A mill may operate either on open circuit that is
with the
product going direct to a storage silo, or on closed circuit with the
product
being conveyed by air or a mechanical elevator to a separator
(classifier) from
which coarse material is returned for further grinding.
The
wall of a ball mill lifts the media as it rotates and at a certain
height they
fall to grind the cement the mill must not rotate above the critical
speed producing
a centrifuging action. The shell of the mil! is protected by liner
plates which
may have a rippled profile to optimise lifting, since slippage of the
media
results in energy loss. Efficiency is rated in terms of the surface
area
produced per unit of electrical energy consumed and a standard energy
requirement of 1.15 x 104m2/kWh may be used as a basis of comparison.
Energy
consumption is approximately linear up to about 300m2/kg, above which
it
increases progressively per unit increase in surface area as cushioning
becomes
more serious. In everyday running, the residues in the mill product on
90 mm
and 45 mm sieves are used to monitor mill performance, which may
decline as a
result of media wear, for example. Increase in such residues at a given
surface
area will result in a change in the compressive strength/curing time
relationship of the cement. A comparison of the effects of open and
closed
circuit milling on these residues can be seen in Fig. 1.
Fig.
l. Effect of classifier efficiency on the particle size grading of
ordinary
(42.5N) Portland cement Power consumption in ball milling Portland
cement is of
the order of 45kWh/t for a surface area of 360m2/kg. This may be
reduced by
employing a closed circuit system, the saving of 2 5 kWh/t depending on
the
efficiency (fan power requirement) of the separator. The principal
variables to
be considered in optimising energy consumption in a ball mill include
the speed
of rotation of the mill, its ball size grading and loading, and the
design of
its lining. The use of a grinding aid reduces energy consumption,
especially
with higher surface area products. A mill is designed on the basis of
the
throughput required, using data for the grindability of the clinker
determined
in the laboratory.
The
ideal way to grind a material would be to break each crystal or
aggregate of
crystals separately by simple cleavage. The energy consumed is then the
surface
energy created plus that lost to the fragments and media as heat.
However,
those milling systems which most efficiently keep the particles
separated for
grinding, such as the roller (vertical spindle) mill or the roll press
are the
most susceptible to wear with a hard, abrasive material like clinker.
This
resulted in serious maintenance problems in early versions of these
mills
before replaceable, wear resistant alloy surfaces were developed.
Consequently,
although a ball mill wastes energy in multiple impacts (cushioning), it
has
remained predominant in cement grinding. Capital costs usually preclude
the
complete replacement of a grinding plant in an established works.
Initial
grinding of large clinker nodules in a ball mill is particularly
inefficient.
In existing plants the introduction of a roll press for preliminary or
semi
finish grinding, with finish grinding in a ball mill, has proved a cost
effective way of significantly increasing both energy utilisation and
throughput, the latter making it possible to maximise use of low tariff
(off
peak) electricity. Improvements in both are even greater in raw
materials grinding.
This combination of grinding techniques has the advantage of avoiding
two
adverse effects when a roller mill or roll press is used for finish
grinding,
namely an increase in the water demand of the cement and the
possibility of an
unacceptable reduction in initial setting time. The former is ascribed
to the
narrowing of the particle size distribution resulting in an increased
voidage
in the cement (decreased bulk density) and the latter to the production
of
coarser gypsum particles and a lower degree of dehydration resulting
from a
lower grinding temperature.
The
relatively recently introduced Horomill (horizontal roller mill), which
is
suitable for the finish grinding of cement clinker and raw materials,
is
essentially a tube mill in which a cylindrical roller constitutes the
grinding
component. Cordonnier describes the performance of the first industrial
(25t/h)
installation of this mill in Italy. For a similar capacity to a ball
mill, it
has a slightly smaller diameter and is only one third of the length.
Energy
saving was quoted as 30 50% with wearing surfaces having a satisfactory
life.
For a cement with a surface area of 360m2/kg, an energy consumption
below
30kWh/t can be expected. Cement produced by the Horomill had a similar
particle
size distribution and similar physical properties to one produced in a
ball
mill.
Factors
influencing the grindability of clinker The grindability of clinker
depends on
its chemistry and on the conditions it experiences in burning and
cooling. Hard
burning and high melt content resulting from a low silica ratio
increase
initial grindability since they result in a clinker with a low
porosity.
(Grindability increases with increasing difficulty of grinding. Maki et
al.
observed that grinding was impaired in clinker containing clusters of
belite
crystals. After most of the larger aggregates of crystals have been
broken, the
fracture properties of the individual phases assume greater importance,
although it must be remembered that a majority of the final cement is
made up
of multiphase particles. Hardness of a crystal is less important than
its
brittleness in comminution and since alite cracks much more readily
than belite
in a microhardness measurement, clinkers with a high lime saturation
(and
substantially complete chemical combination) can be ground more readily
than
those with a low lime saturation.
Hornain
and Regour found that the grindability of a clinker sample sintered to
a
density of 3000 kg/m3 was determined by its fracture energy and the
size of the
microcracks present. The number and size of the latter could be related
to the
cooling regime experienced by the clinker. They measured fracture
energies in
the range 12 20J/m2, using notched, sintered prisms of clinker. From
measurements of the impression made in each clinker phase by a Vickers
micro
indenter and the size of the cracks radiating from the indentation,
they
calculated values for a brittleness index C3S 4.7 C3A 2.9 C2S and C4AF
2.0.
Scrivener
examined cement particles by BSE imaging and X ray microanalysis and
found
that fracture of alite crystals predominated in polymineralic clinker
particles
rather than fracture along phase boundaries. Many smaller particles had
surfaces rich in interstitial phases. Bonen and Diamond used the same
techniques to examine two chemically very similar cements, one produced
in a
ball mill the other in a roller mill. The latter contained more nearly
isodimensional particles and a significantly smaller proportion of the
finest
particles present in the sample from the ball mill. The particles in
the ball
milled sample also exhibited much greater surface
roughness. X
ray microanalysis revealed that the surfaces of the particles in the
two
cements differed in composition, that from the roller mill having a
higher
content of belite and interstitial material, apparently reflected in a
significant decrease in heat release in the first day of hydration.
Minor additional
constituents
The
current British Standard follows the European prestandard to be ground
with
clinker as part of the nucleus of a Portland cement that is excluding
the
gypsum. Unlike the European specification, it limits the materials
which can be
used to one or more of a natural pozzolana, blastfurnace slag, or
pulverised
fuel ash but not if a blended cement is being manufactured with one of
these as
a main constituent or if a filler such as ground limestone is being
added. The
term filler covers any inorganic natural or artificial material which,
owing to
its particle size grading when ground, enhances the physical properties
of
cement without any detrimental effect on concrete durability. Moir
concluded
that ground limestone is likely to be a preferred mac for practical and
economic reasons. It aids the control of cement workability and
strength
development and inhibits bleeding.
Tests
of
cement quality
Introduction
The
assessment of cement quality relies primarily on direct performance
tests
because of the complexity of the factors influencing its rate of
hydration and
its hydraulicity. It was seen in the last chapter that the value
obtained for
the specific surface area of cement is particularly method dependent,
so that a
prescribed procedure must be followed and named when referring to the
result
obtained. A similar constraint applies to the determination of the
hydraulicity
of cement by incorporating it in a concrete or mortar.
Committees
of specialists, representing all interested parties (manufacturers,
consumers,
government and academic institutions), have in some countries produced
national
specifications and test methods for the assurance of cement quality. In
other
countries, British Standards or those published by the American Society
for
Testing and Materials are used. Standard test procedures have also been
published by the International Standards Organisation and by the Comite
Europeen de Normalisation (CEN). Those developed by the latter, Methods
of
Testing Cement EN 196, have been published with useful National Annexes
by the
British Standards Institution as pans of BS EN 196. CEN is currently
finalising
a specification for cements commonly available in Europe. A pre
standard is
voluntary (ENV 197) but drafts have formed the basis of the present
British
Standard, BS 12 1996. It should be noted that standards and specified
test
procedures are regularly reviewed and revised when deemed necessary. BS
12 was
first published in 1904. ASTM standards are published annually in book
form
cement and concrete specifications and testing are included in Volume
04.01.
A
standard specification lays down the chemical, physical and performance
characteristics required of a cement for it to be sold as conforming to
the
standard. In the current European approach, a programme of product
sampling
(defining minimum frequency and method) is indicated for the
manufacturer who
must sample at the point the product leaves a works and Employ the test
methods
laid down in EN 196. Two statistical procedures, and values for the
necessary
statistical parameters, are provided for the assessment of test results
obtained against the specification (manufacturers auto control).
Criteria for
conformity take the form of specified characteristic values for
properties,
which can only be breached by a defined number of test results in a
given set
(test period). They are derived from probability theory using a
defined, low
risk of acceptance of a batch not meeting the required characteristic
value.
These procedures are described in detail by Brook banks with helpful
worked
examples. In addition, for some properties, limit values are also
specified and
no individual auto control test result must fall outside these.
The
British Standard BS 12 1996 specifies chemical physical and performance
requirements for Portland cement following ENV 197. In addition to
characteristic values, the British Standard specifies acceptance limit
values
for certain properties which are somewhat more stringent than the limit
values
in ENV 197. They can be used with results obtained for single samples
by a
customer or independent test laboratory. Maximum permitted deviations
above or
below the stated acceptance limits for individual results are also
specified.
Chemical composition
The
compositional requirements specified for Portland cement cover both
clinker and
cement. The test methods to be employed are those described in BS EN
196. A
detailed discussion of these is given by Taylor. The compositional
requirements
for clinker are C3S + C2S > 66.7% C/S > 2.0 MgO <
5.0%. They are
comfortably met in the UK. The requirements and acceptance limit values
specified
for cement in BS 12 1996 are given in Table 1. The limit for chloride
ion
content is necessary to reduce the risk of corrosion of steel in
reinforced and
prestressed concrete. Limits for loss on ignition and insoluble residue
protect
the consumer from a product which has suffered either excessive
exposure to the
atmosphere during storage or contamination.
Setting times
These
are the times after completion of mixing at which a neat cement paste
presents
specified resistances to the penetration of a needle. The principle
variables
influencing penetration are the water content of the paste, the
temperature,
the load on and dimension of the needle and, of course, the reactivity
of the
cement. The needle employed (diameter 1.13 mm, total load 300 g) is
named after
Vicat. It is released at the surface of the hydrating paste at
intervals until
it penetrates only to a point 4 ± 1 mm from the bottom of the standard
mould.
When the paste has attained this degree of stiffness it is said to have
reached
initial set, for which a minimum value is specified in BS 12. A second
similar
needle with a concentric ring attached can then be used to determine
final
setting time, although a maximum value for this is no longer specified
in the
British Standard. Final set is reached when the needle makes an
impression on
the surface of the paste but does not penetrate the 0.5 mm necessary
for the
ring to mark the surface.
The
higher the water content of the paste, the longer it will take for the
cement
hydration products to form a structure with the chosen resistance to
penetration. BS EN 196 does not, however, specify a fixed water/ cement
ratio.
Instead, pastes are examined at a range of ratios toestablish that
needed to
produce a paste into which a 10mm dia. plunger, which is held in the
apparatus
used for the Vicat needle, penetrates to 6 ± 1 mm from the bottom of
the same
mould. This paste is described as having standard consistence and since
the
result is sensitive to shear history, the mixing procedure is specified.
Compressive strength
The
most important test of cement quality involves the determination of the
compressive strength it produces in a mortar or concrete. In the past,
a
specified concrete mix was usually tested in the UK using British
Standard 4550
although this also gave a procedure for a mortar. The USA and many
European
countries favoured mortar testing and widespread use of the ISO RILEM
R679
mortar strength test is encountered in the literature. In the spirit of
membership of the European Union, the mortar prism test method of EN
196 was
adopted in BS 12.
The
mortar mix specified is 3 1 0.5 by weight of sand, cement and water,
respectively. It is cast into 40 x 40 x 160mm moulds. Flatness of the
resulting
surfaces of the mortar prisms is important because surface
irregularities would
concentrate stresses during compressive strength measurement and affect
the
result. Compressive strength may be related to the volumes of cement
(C), water
(W) and air (A) in a mix by Ferets empirical law
Where
k is a constant for the aggregates, cement and curing employed.
The volume
of
air present depends on the degree of compaction achieved and the object
is to
achieve full compaction using, in the EN 196 procedure, a jolting table
or a vibrating
table giving equivalent results. Each prism then contains approximately
600 g
of mortar. Excessive compaction must be avoided, however, as it causes
particle
segregation. Some air may be entrained during mixing of the mortar and
checks
against a reference sand are important because some sands cause more
entrapment
than others, possibly because of an abnormal, although small, amount of
clay
and/or organic matter adhering to the grains.
The
effect of the amount of water used is marked but easily controlled.
Compressive
strength is usually measured after 2 and 28 days. The latter gives what
is
referred to as the standard strength and BS 12 classifies cements on
the basis
of the level attained for low strength classes the 2 day test is
replaced by
one at 7 days. After curing, a prism is superficially dried and tested
immediately. It is first broken in flexure in a specified manner and
then the
separate halves broken in compression across the 40mm thickness. Prisms
are
cured in batches of three yielding six results for compressive strength
at each
age. If any one result deviates by more than 10% from the mean it is
rejected
and if any one of the remainder deviates by more than 10% from the new
mean,
then all the results must be rejected and the test repeated.
Where
p is the mortar prism compressive strength (N/mm2), c the concrete cube
compressive strength (N/mm2) and d the curing period in days at test.
At 28
days this is equivalent to a strength ratio p/c of 1.30.
Some
results for concretes prepared using the sand, granite and the
procedure
specified in BS 4550 Part 3 1978 are given in Fig. 2. They illustrate
the
effect of cement surface area and that of water/cement ratio. In
construction
contracts, test cubes are prepared from samples of production concrete
taken as
it is placed. Since 28 day strength is regarded as an important
indication of
concrete quality for structural engineering purposes, various
accelerated
curing procedures involving elevated temperatures have been proposed to
reduce the
time needed to get an indication of its potential value. Unfortunately,
correlations with strength developed under normal curing conditions are
poor,
presumably because at elevated temperatures there is a coarsening of
the pore
size distribution in the on the development of strength in BS 4550
concrete
mixes (the w/c specified in the standard is 0.60) hydration products of
the
cement. A prediction of 28 day strength from the 7 day strength, using
a
knowledge of the form of the growth curve, is preferred. This assumes
that
curves such as those in Fig. 2 may be displaced parallel to the
strength axis
but retain their shape, an assumption which is acceptable as long as
the
chemistry of the cement employed does not change significantly.
Admixtures
and special cements
The
development of new cements has been a major part of research activity
for most
of the twentieth century, driven by the need to obtain cement
compositions
which improve on Portland cement by providing particular properties
such as
rapid setting and hardening, improved workability, or increased
durability in
severe environments. In the past 25 years, the need to reduce energy
consumption and, where possible, emissions of the greenhouse gas CO2,
have
added to the incentives to introduce new cementitious compositions as
well as
improved production processes. Since long term satisfactory performance
in use
is a major requirement of any new cement, testing to a point where it
is widely
accepted can involve a prolonged examination of its durability in the
environments in which it is to be used.
In
this chapter, some special cements are described briefly and, since the
properties of Portland cement mortars or concretes can be given special
properties by the use of admixtures, a short account of their nature
and
applications is included.
Admixtures
The
properties of a concrete or mortar containing Portland cement can often
be
beneficially modified for a particular use by the addition of small
amounts of
certain chemicals. When the addition is made as the concrete mix is
being
prepared, the material is described as an admixture. A classification
of the
commonest admixtures groups them as accelerators or retarders of set
and
hardening and water reducers, although a particular substance may
combine one
of the first two characteristics while also reducing the water needed
to
produce a mix with a given workability. The addition is usually made
with the
substance in solution to maximise the uniformity of its dispersion,
although
its introduction may be slightly after the addition of the main bulk of
the
water where experience has shown that this increases effectiveness.
Sensitivity
to dose of the admixture is first examined with a sample of the
particular
cement to be used since the addition required may be influenced by
cement
fineness and chemistry, in particular the contents of C3A, free lime,
and
soluble alkali sulfates. Dose is usually expressed as mass percentage
on cement
of the active ingredient of an admixture.
Accelerators
These
may be employed in precast concrete production or cold weather
concreting. They
act by increasing the rate of hydration in the acceleratory period,
leaving the
dormant period largely unaffected. Calcium chloride at 2% is
particularly
effective in increasing early strength development but the corrosive
effect of
the chloride ion means that it cannot be used in reinforced or
prestressed
concrete. Calcium formate, nitrate and nitrite are less effective
alternatives
but no single accelerator is widely accepted. It is often more
practicable to
employ a water reducing admixture to enhance early strength development.
Retarders
These
are valuable in extending the working time of a concrete or mortar in
warm
conditions since their effect is primarily confined to the dormant
period.
Hydroxycarboxylic acids (citric acid and those, such as gluconic acid,
derived
from sugars) and sugars themselves are examples, the latter having
drastic effects
if an overdose are used. Additions of about 0.25% are usual, but
sucrose can
extend setting time by as much as 10 h at levels as low as 0.05%. The
low
dosage of retarders needed is considered to indicate that they function
by
adsorption on the surfaces of cement grains or, more probably, the
hydrates
formed initially on them. The poisoning of portlandite (CH) nuclei has
also
been suggested as a mechanism of retarding set. With cements having a
high C3A
content, a greater retardation with a given dose may be obtained by a
delay of
just two minutes in adding the retarder after the bulk of the water.
The
initial interaction of C3A with gypsum is believed to reduce its
interaction
with the admixture.
Fluorides,
phosphates, zinc and lead salts are all retarders which are
precipitated from
solution (the first two as calcium salts, the second two as hydroxides)
as
coatings on the surface of cement grains. Phosphate based admixtures
show
superior retention of their effectiveness at elevated temperatures.
Proprietary
blends of retarders and plasticisers are employed in ready mixed
mortars with a
life of 36 48 h.
Water reducing
(plasticising) admixtures
These
enable a reduction of up to 15% in water content to be made while
retaining a
chosen workability. Usually, a sodium or calcium ligno sulfonate (by
products
of wood pulp manufacture) or a hydroxycarboxylic acid is employed.
Sugars in
unrefined samples of the former are said to give it an additional
function as a
retarder. These materials inhibit segregation and can be used in pumped
concrete.
Sodium
salts of sulfonated naphthalene formaldehyde co polymers (PNS) or
sulfonated
melamine formaldehyde co polymers (PMS) are described as
superplasticisers
because they can be used at levels of addition (ca. 0.5%) which make
possible
water reductions of around 30% without introducing either air
entrainment or retardation.
The production of very high strength concrete becomes practicable and
Neville
cites 28 day compressive strengths of
150N/mm2 for
concrete made at a water/ cement ratio of 0.2. Alternatively, the water
content
may be maintained with no loss of cohesion even with slumps as high as
200mm.
These materials are therefore important in pumpable concrete and self
levelling
screeds for flooring. A mix will progressively stiffen (exhibit slump
loss) but
an additional dose of superplasticiser may be used to prolong working
time.
The
enhancement of flow by superplasticisers is attributed to their
adsorption on
initial cement hydration products resulting in an increase in the zeta
(z)
potential at the shear plane of the electrical double layer at the
interface of
particle and aqueous phase.Bonen and Sarkar found that the adsorption
capacity
of a cement depended on the molecular weight of the PNS, the fineness
of the
cement and its C3A content, while slump loss was strongly dependent on
the
ionic strength of the aqueous phase. Cement grains in a paste usually
possess a
low z potential, presumably because of the high ionic strength, but the
strongly acidic (dissociated) sulfonate groups (SO3H) maintain a
negative
surface charge and C potentials in the region of 30 mV have been
quoted. This
would significantly reduce the extent to which particles form
agglomerates
trapping water which cannot contribute to flow, a factor which is
particularly
important in concentrated suspensions.
Uchikawa
et al. found lower z potentials (ca. 10 mV) in a cement paste to which
a PNS co
polymer had been added, although it exhibited satisfactory flow
characteristics. They used the relatively recently introduced
electrokinetic
sonic method, which does not require dilution of a cement paste, and
the lower
potential could be ascribed to the higher ionic strength of the
undiluted
suspension. However, the most interesting observation was that a co
polymer of
polyacrylic acid and polyacrylic ester was especially effective in
increasing paste
fluidity, although it did not produce a significant change in the z
potential
of the paste (ca. –1 mV). They concluded that this was evidence of
steric
stabilization. Everett has provided a useful introduction to the
electrical
double layer, as well as the possible roles that polymers can play in
either
coagulating or stabilizing colloidal dispersions, depending on their
molecular
size and structure.
Calcium
aluminate cement (CAC)
Formerly
referred to as high alumina cement (HAC), calcium aluminate cement was
developed in France in the early years of the twentieth century to meet
several
needs rapid hardening, concreting in cold conditions, sulfate (sea
water)
resistance and refractoriness. The major product, Ciment Fondu, is
manufactured
by total melting of a mix of a ferruginous bauxite and limestone at
about 1700°
in a reverbatory, open hearth furnace. A typical major oxide analysis
of the
product is SiO2 4.5% Al2O3 38% CaO 38% Fe2O3 10% FeO 6%. The principal
phases
present are CA (40 50%) and a ferrite phase (20 40%), the A/F ratio of
which
varies from grain to grain but is generally less than 1. Solid solution
effects
are extensive and minor phases include C2S and C2AS. Some C12A7 and a
glass may
also be present. The minor iron containing phases present, which
include FeO
(wusite), depend on the ratio of Fe2+ to Fe3+ and they give the clinker
its
black colour. The cooled, solidified product is ground without addition
to a
fineness of 300 400 m2/kg.
Concrete
made with Ciment Fondu, unless very vigorously mixed, usually sets more
slowly
than that made with Portland cement but it can develop the same
strength in 1
day as that found in Portland cement concrete in 28 days and the heat
generated
aids hydration in cold conditions. Strength is mainly derived from the
rapid
hydration of CA which occurs by a process of dissolution followed by
nucleation
and precipitation of a crystalline hydrate. The hydrate precipitated
depends on
the temperature of hydration. If C12A7 is present, C2AH8 is formed even
at the
lower temperatures. The hydration of calcium aluminate cements involves
the
conversion of Al from the 4 coordination in the anhydrous phases to the
6
coordination present in the crystalline hydrates and AH3, so that it
can be
followed by 27Al NMR.
Where
AH3 is an aluminium hydroxide gel which
subsequently crystallises to
form gibbsite. However, following the conversionreaction by synchrotron
radiation energy dispersive diffraction, Rashid el al. found that in
the
temperature range 70 90°, C2AH8 was an initial, transitory
intermediate. In a
later investigation of the reaction at 50°, they found that both
crystal forms
of this phase were formed sequentially. The direct conversion to C3AH6
was only
observed after its nuclcation by the indirect route.
The
reaction in Equation (1) is accompanied by a significant increase in
the
density of the solid phases and, consequently, a marked increase in the
porosity of the concrete and reduction in its strength result, although
some of
the water formed may react with remaining anhydrous cement to give a
modest
subsequent increase. The design of a structure can be based on the
converted
strength and the value for this can be optimised by using a
water/cement ratio
of less than 0.4. This produces a concrete in which the reduction in
strength
is not too severe but ensuring adequate strength by use of such low
water
contents has been considered too difficult for normal working
conditions. In a
humid environment serious problems have also arisen where CAC concrete
structural members have been in contact with Portland cement concrete.
Loss of
strength in the former is the result of alumina leaching from the CAC
by
soluble alkali from the Portland cement (alkaline hydrolysis) and loss
of
integrity in the concrete is accelerated by simultaneous carbonation.
Calcium
aluminate cements are now primarily used in the UK for non load bearing
and
refractory applications. The more refractory cream and white grades are
produced by a clinkering process using low iron content bauxites, or
gibbsite
from the Bayer process, and either a pure limestone or quick lime. They
may
contain CA2, CA6, and alumina itself, as the proportion of the latter
is
increased in the raw mix to raise the refractoriness of the product.
Characterisation
of Portland Cement Clinker
Introduction
Provided
that the kiln feed has been properly prepared with a regularly checked
composition and fineness, then it is only necessary to check the
adequacy of
the burning process and this is done by frequent determination of the
free lime
in the clinker. However, as an additional check, elemental chemical
composition
is determined regularly by X ray, fluorescence analysis. The sulfate
content of
the clinker is also frequently determined so that the level of gypsum
to be
added in cement grinding can be calculated. In this chapter the
application of
a number of methods of phase characterization will be briefly described.
In
routine checks on the kiln product, the nodule size grading is watched
for
variation from the norm. Well nodulised clinker with a minimum of dust
is
needed for efficient air flow in the cooler. Nodule formation and
densification
also indicate a high degree of chemical combination, although if the
burning
temperature is too high the relationship breaks down and dust is
formed. For a
given burning time and temperature the clinker size range and density
also
depend on the silica and alumina ratios of the kiln feed. Portland
cement
clinker is normally black and the appearance of a paler than usual
product has
in the past been taken to suggest excessive chemical reduction of Fe3+
in the
kiln. However, Scrivener and Taylor found significant reduction in some
black
samples of clinker and, perhaps more surprisingly, they also found that
a
lighter colour did not necessarily mean that reduction had occurred.
Very pale
clinker indicates serious under burning.
When
problems arise, indicated either by the checks mentioned above or by
test
results obtained with cement produced from a clinker then a more
fundamental
investigation is necessary and clinker phase composition and
microstructure are
examined, usually by optical microscopy. Scrivener has provided a
useful
introductory account of both optical and electron microscopy and their
application to cement and concrete.
Chemical
analysis by selective dissolution
Free
lime can be extracted from ground clinker (or cement) by treatment with
a hot
glycerol ethanol mixture or by hot ethanediol (ethylene glycol). The
latter is
more usual and the extraction produces a solution of calcium glycollate
which,
as the salt of a very weak acid, can be titrated after filtration with
a
standard solution of HCl using a methyl red bromocresol green
indicator.
Calcium hydroxide present is also extracted.
The
total silicates plus free lime in a clinker can be determined by
dissolving
them from a ground sample using either 20% salicylic acid or 20% maleic
acid in
dry methanol and weighing the washed and dried residue which consists
of C3A,
ferrite, MgO and sulfates. From this assemblage it has been suggested
that C3A
can be determined by dissolving it in 3% aqueous saccharose solution,
although
the ferrite phase is also slowly attacked and the soluble sulfates
would be
extracted. Selective dissolution is a considerable aid to qualitative
phase
analysis and characterisation by X ray diffraction. Gutteridge used a
solution
of sucrose in aqueous potassium hydroxide to dissolve the aluminate and
ferrite
phases in order to identify the forms of alite present in cement
samples.
Optical microscopy
Optical
microscopy has proved very effective in characterising cement clinker
because
much of the microstructural detail occurs from 1 mm upwards. Sampling
of a
clinker requires great care since only a very small proportion of any
batch can
actually be examined under the microscope and clinker from each of the
range of
sieved size fractions should be included. Pieces of lightly crushed
clinker and
fines are stabilised by vacuum impregnation with an epoxy or polyester
resin,
which is hardened in situ.
Although
a thin section of a specimen is sometimes examined in transmitted
light, which
makes the determination of optical properties easier, polished surfaces
are
more frequently examined. Nitric acid in ethanol and hydrofluoric acid
vapour
are common eichants. Both attack the silicates at rates dependent on
their
silica content, forming surface films which in reflected light produce
interference colours. These provide good contrast between the di and
tri
calcium silicates, the colours depending on the thickness of the
reaction layer
and therefore varying somewhat with expo sure to the etchant and the
reactivity
of the individual crystal. The use of HF also allows the (more
difficult)
distinction of free lime, periclase, C3A and the sulfatc phases with
care.
Differences in reflectivity and morphology are also useful aids to
identification. Reflectivity is greatest for the ferrite phase.
Microscopy
is used to examine clinkers qualitatively and, less frequently,
quantitatively.
Even without point counting of individual phases, an experienced
microscopist
can make a very useful assessment of the proportions of the phases
present.
However, the principal value of microscopy is in detecting non
equilibrium
effects such as the heterogeneity of phase distribution porosity,
crystal form
and size. The size of the crystals of the interstitial phases, for
example, may
indicate the cooling rate experienced by the clinker. Maki and his co
workers
related the crystal size and fine structure of alite to the conditions
it
experienced in the burning zone of a kiln and the microstructure of
belite to
the cooling regime.
Microscopy
is also valuable in a number of other ways in the cement industry, in
particular in the identification of the mineralogy of build ups and
deposits
which form in kiln and preheater systems and in the examination of
clinker
refractory interactions in kiln coatings.
Characteristics of the
principal clinker phases
Alite
(C3S density 3150 kg/m3). Crystals of alite are prismatic, sometimes
pseudo
hexagonal, frequently having clearly defined faces. However, less
regular
shapes with rounded corners and faces with re entrant angles are found.
Internal structure observed includes lamellae and other manifestations
of
twinning and inclusions of belite are common. Thin sections can be used
to
distinguish trigonal, monoclinic and triclinic forms but to distinguish
between
variants with the same symmetry may require X ray diffraction.
Sizes
of crystal sections within a specimen usually range up lo 100/mm with
an
average around 30 mm. In general, the size distri bution in a clinker
is not
random. Smaller alite crystals form in the lime rich regions while
larger
crystals form in the silica rich regions of a nodule by conversion of
belite.
Crystal size is also influenced by burning time and temperature, melt
quantity
and composition, and, if present, mineralisers such as fluoride ions.
Twin
planes or fault lines may be marked by material exsolved from solid
solution
during cooling. For example, Fe3+ may be formed on cooling as striae of
C2F by
oxidation of Fe2+ present in solid solution at clinkering temperatures.
If
clinker cooling is too slow, dissolved iron can significantly
accelerate the
decomposition of alile below 1250°. Scrivener and Taylor described the
complex
sequence of reactions which can occur between 1200° and 1100º. When
severely
reducing conditions exist in the burning zone (ca. 1400°) alite absorbs
Fe2+
and S2– from the melt, the latter being derived from the sulfur
present. In the
absence of oxidation during cooling, some of the alite decomposes with
the
formation of belite, free lime and CaO FeO solid solution.
Subsequently, an
aluminium rich ferrite and metallic iron are formed. Reduction can
result in a
serious fall in cement quality but is minimised by proper flame
control. This
problem is usually suspected if yellow brown clinker nodules, or nodule
cores,
are produced and free lime rises significantly.
Belite
(C2S density 3280 kg/m3). Belite usually occurs as rounded crystals,
frequently
made up of lamellae or marked by striations indicating twinning and/or
exsolution from solid solution, both effects resulting from polymorphic
transformations. Belite crystals may occur in clusters which, if large,
indicate an unacceptable level of heterogeneity in the kiln raw feed.
If a
belite cluster is centred on a pore, a site formerly occupied by a
silica
particle is indicated. In a clinker from a coal fired kiln, inefficient
dispersion and incorporation of the ash can be detected as a localised
concentration of belite, occasionally in the outer part of a nodule.
In
a slowly cooled clinker, small belite crystals separate from the melt.
When the
A/F ratio is high they are also formed by interaction between alite and
the
melt, typically fringing alite crystals. Together with the crystal size
of the
aluminate and ferrite phases, these features of the clinker
microstructure are
indicators of cooling rate.
Interstitial
phases. Tricalcium aluminate (density 3030 kg/m3) and the ferrite phase
crystallise from the melt with a degree of separation which increases
with
decreasing rate of cooling. The crucial cooling is thai occurring in
the kiln
itself, that is above about 1300°. If clinker is air quenched from the
burning
temperature then these individual phases may not be resolved by optical
microscopy. Normally C3A is cubic but, in the presence of alkali not
combined
with sulfate, alkali containing solid solutions may form. The alkali is
retained by the C3A during cooling and part or all of the resulting
solid
solution will be orthorhombic. This phase grows as elongated, lath like
crystals and exhibits birefringence.
The
ferrite phase (density in the region of 3700 3900kg/m3 variable as a
result of
variation in its composition) is distinguished by its colour in thin
section
and its high reflectivity in polished section. The black colour of
clinker is
believed to result from elements such as magnesium, titanium and
silicon in
solid solution in the ferrite phase and oxidation as it cools in the
kiln
cooler system.
Minor
phases. Magnesia When
the magnesia
content of a clinker is greater than that which can be taken into solid
solution (ca. 1.5%) periclase crystals may be observed, sometimes well
formed
and in clusters. More often it is observed as relicts of a dolomitic
material
in the kiln feed or present in the coal. Alkali sulfates and double
sulfates
calcium langbeinite (K2SO4.2CaSO4) the sulfates are the last phases to
crystallise during cooling of the clinker.
They
may also condense directly from the vapour phase. A solid state phase
inversion
of potassium sulfate leads to characteristic cracking. If an aqueous
etchant is
used, these phases are removed leaving pores. Free lime free lime is
often
globular as a result of attack by the melt but where interaction is
limited it
exists as relicts of the original calcite particles, usually surrounded
by
alite crystals.
Quantitative determination
of phase composition
Using
a suitably prepared polished section, point counting with a crosswire
and steps
of a fixed size in a grid pattern is employed with each field examined
in the
optical microscope. The phase observed at each step is noted and some
2000 4000
counts are made per clinker specimen. Automatic, instrumental counting
may be
used as in metallurgical analysis, but a number of different etches may
be
needed to obtain the range of contrast required for the instrument to
operate
satisfactorily. Counting gives volume percentages of the phases present
which
are converted to mass percentages using individual phase densities. The
principal
sources of error in quantitative microscopy derive from the difficulty
in
obtaining a representative sample and incorrect phase identification,
especially where the crystals of the interstitial phases are very small.
The
Mineralogy of Asbestos
INTRODUCTION
The
asbestos minerals have an almost unique combination of physical and
chemical
properties. They take the form of extremely thin and flexible fibres,
which at
the same time have great thermal stability and tensile strength. In
appearance
the fibres are often silky, and their flexibility is such as to allow
them to
be spun into yarn and subsequently made into woven fabric. The most
widespread
modern uses of asbestos are in fireproof textiles, papers and boards
and in
brake and clutch linings for many kinds of vehicle and machinery. The
low
thermal conductivity of asbestos products can be as important as their
incombustibility for fire prevention. Composites of asbestos with
cement,
resins, and plastics are commonly manufactured.
The
term asbestos derives from the Greek meaning inextinguishable and was
perhaps
used by the ancient Greek writers in the sense of indestructible to
describe
material which was unscathed by flames. The first use of the term in
the
context of mineralogy came in the middle of the 19th Century as applied
to a
fibrous amphibole mineral discovered in the Italian Alps.
The
three main kinds of asbestos which have had wide commercial
exploitation are
chrysolite, amosite, and crocidolite. Of these, chrysolite is by far
the most abundant
and most used. Three others, in decreasing order of importance, are
anthophyllite, tremolite and actinolite. The last material, although
not
uncommon as a mineral, is rare in asbestiform habit.
Of
the above six varieties of asbestos, chrysotile alone is a member of
the
serpentine group of minerals, all of the others belonging to the
amphibole
group. Chrysotile has yellowish or greenish white fibres which are
usually
silky in nature, crocidolite is blue and less silky, and amosite has
white,
grey, pale yellow, or pale brown fibres which are more brittle than
those of
the former two varieties. Typical specimens are shown in the
frontispiece.
Amosite
is not strictly a mineral name but is a commercial name derived from
the
initial letters of Asbestos Mines of South Africa. Amosite usually
consists
mainly of the amphibole mineral grunerite, but appreciable amounts of
other
amphibole fibres are present in some specimens.
Silicates
other than serpentines and amphiboles, and also some non silicates, can
occur
in asbestiform habit, but they are not exploited because of their
limited
occurrence or inappropriate physicochemical properties.
DEFINITIONS
Some
would confine the use of the term asbestos to material which, if
present in
sufficient amount, would be commercially exploitable because of the
special
properties mentioned above, but it is probably difficult to exclude
specimens
which fall slightly short of this because of greater coarseness,
brittleness,
or weakness of fibre.
The
most abundant minerals which could be termed asbestos are chemically
hydrous
silicates. Those hydrous silicates that are asbestos are composed of
particles
which are extremely thin and so generally have a very high length to
breadth
ratio. We are concerned, therefore, with particle morphology and a
property
known in mineralogy as crystal habit. Figure 1 shows a range of crystal
habits
and their descriptive terms, namely tabular, equant, prismatic,
acicular, and
asbestiform.
The
limiting lengths to breadth ratios that differentiate between these
terms have
not been precisely defined, and it has not in general been thought
necessary to
do so. Thus, it is a matter of subjective judgment as to whether a
crystal is
described as acicular (meaning needle like) or prismatic, but it is
doubtful if
anyone would use the term acicular for material with a length to
breadth ratio
much less than 10 1.
Since
the fundamental fibrils of asbestos have typical widths in the range 20
200 nm,
a fibril of any appreciable length will have a very high length to
breadth
ratio. Thus, for example, assuming an average width of 100 nm, a fibril
of
length 1 mm will have a length to breadth ratio of 10 000 1. Even a
fibril as
short as 10 mm has a length to breadth ratio of 100 1, and only a 1 mm
length will
give, a ratio as low as 10 1. The characteristic feature of high length
to
breadth ratio is not, however, applicable on a single fibre basis, but
it is
more useful if applied statistically. In natural untreated asbestos,
although
some very short fibres will be present, a high proportion of fibrils
with a
length to breadth ratio of the order of 100 1 or greater can be
expected. In
samples of milled asbestos most particles were found to have a length
to
breadth ratio between 5 1 and 20 1, whereas for a non asbestos
amphibole the
majority had a ratio of less than 3 1. For milled chrysotile asbestos
most
particles had a ratio of greater than 50 1.
Related
to, but not entirely dependent upon, the length to breadth ratio is the
property of flexibility. Needle shaped crystals of some substances will
be very
brittle, others may flex slightly without breaking and spring back
elastically,
while others can deform inelastically. The term acicular, meaning
needle shaped
would not normally be applied to crystals that are curved.
A
bundle of asbestos fibrils will tend to bend and recover elastically.
but when
the diameter of the fibre bundle is small so that only few fibrils are
involved, then inelastic deformation occurs, giving the curved fibrils
common
for natural asbestos.
Variations
in minerals can be produced not only by the different habits of their
component
crystals, as described above, but also by the way in which these
crystals are
aggregated. An important feature of asbestos is that it is an aggregate
of hair
like fibrils. These have their lengths approximately parallel to one
another
but lie in more then one azimuthal orientation. More extensive
discussion of
definitions pertaining to asbestos is given by Campbell et al.
Serpentine
minerals
The
serpentine
minerals, of which chrysotile is one variety, have a layered silicate
structure. The layers can be regarded as made up of Si atoms, each
surrounded
by four oxygen atoms at the corners of an almost regular tetrahedron,
and Mg
atoms each surrounded by six oxygen atoms at the corners of an
approximately
regular octahedron. Si O tetrahedra lying on their triangular bases and
with
apices all pointing in the same direction are linked by sharing all
basal
oxygen atoms to form a continuous layer. This has approximately 3 fold
symmetry
but is more conveniently described in terms of a rectangular unit cell
which
has repeat distances a 5.3
Å, b 9.2 Å. Gaps in
the plane formed by apical
oxygen atoms are filled with (OH) ions to form a regular and
approximately close
packed array of (O, OH) at this level. Lying above this plane is an
array of Mg
atoms and above these a plane of (OH) ions such that the Mg atoms are
surrounded by octahedra of (O, OH) ions. Each Mg has a triangle of 3
(OH) above
it and a triangle below of 2(O) and one (OH), rotated by 60° with
respect to
the first. Plan and elevation views of this structure are presented in
Figure
2. The chemical content of a unit cell can be seen to be a multiple of
Mg3Si2O5(OH)4.
In
building up the three
dimensional serpentine structure, composite layers of the type
described are
superimposed, and the inter layer distance is about 7.3 Å. However, one
layer
may be placed directly above another, or there may be some displacement
or
rotation between layers. This leads to the possibility of various
stacking
sequences, different symmetries and multi layer the symmetry may be
trigonal,
hexagonal, or monoclinic. Disordered stacking can also occur.
The
comparatively simple structure described above is that of the
serpentine mineral
called lizardite. Lizardites are mostly fine grained and have platy
morphology,
but some have lath like particles with elongation parallel to a.
The
asbestiform morphology of chrysotile is not obviously reconcilable with
a
layered crystal structure, and this paradox has been the subject of
much
painstaking research. The ultimate and complete solution came with the
direct
evidence from high resolution electron microscopy that in chrysotile
the
structural layers are curved about the direction to form either scrolls
or
concentric cylindrical tubes. The diameters of such tubes are of the
order of
200 Å thus; a sliver of chrysotile asbestos with cross section 0.1 mm
square
contains about 20 x 106 tubular fibrils all in approximately parallel
orientation. It is possible, therefore, to strip from an asbestos fibre
bundle
very fine threads each of which still contains many thousands of
fibrils.
A
theoretical reason for the curvature of serpentine layers can be found
in the
composite and polar character of the fundamental sheet. The Si
component tends
to have a smaller repeat dimension than the Mg component, and the mis
match
between the two can be overcome by bending, with the Mg octahedra on
the
outside of the curve. In another variety of serpentine, antigorite, the
layers
invert at regular intervals thus producing a regularly repeating
corrugation.
Well formed platy crystals, not tubes, are the result. In lizardites
the platy
crystals tend to be buckled and disordered, and they are usually
limited to
very small (sub microscopic) dimensions.
The
growth of very thin fibrils is more readily understood for chrysotile
than for
amphibole asbestos (discussed later). When chrysotile tubes are formed,
a
particular radius of curvature may be the most stable, and this may
impose an
upper limit on the diameters of the tubes.
Amphibole
minerals
The
fundamental unit of the amphibole structure is a chain of SiO4
etrahedra linked
by sharing corner oxygen atoms in the manner shown in Figure 4. The
characteristic chain formula is Si4O11 and the repeat distance along
its length
is approximately 5.3Å. The chains are four tetrahedra wide and of very
great
length fibres of asbestiform amphiboles run parallel to the chain
length.
In
the amphibole structure the Si4O11 chains are linked laterally by
cations as
shown in Figure 5. In tremolite the cations are Mg and Ca, the Mg ions
linking
chains by means of strips of Mg (O, OH) octahedra. The oxygen atoms in
these
strips are the apices of tetrahedra, and the (OH) ions occur as in
Figure 4.
The Ca ions link neighbouring chains across the bases of the tetrahedra
and
they occur in distorted polyhedra of oxygen atoms. The alternative
occupation
of Mg (Y) and Ca (X) sites in different am phiboles has been discussed
above.
Most
amphiboles are monoclinic in crystal symmetry, as a result of the way
in which
successive chain units are stacked with respect to each other.
Anthophyllite,
however, is orthorhombic, and the relation of its unit cell to that of
a
monoclinic amphibole is shown in Figure 6. The cell parameters of some
amphiboles are given in Table 2.
The
chain like structure in amphiboles leads to their possession of good
cleavages
on (110) planes. If the chains themselves and their linkage across Y
sites are
regarded as the strongest elements of the structure, paths of weakness
can be
traced as in Figure 7 and an average path can be taken as defining a
likely
cleavage plane.
It
is tempting to associate the physical nature of asbestos with the chain
like
structure and with cleavages, but while these features may be
indirectly
pertinent they are not fully responsible. Thus, many amphibole
specimens are
not asbestiform although they possess the chain structure and good
cleavage. A
distinction should be recognized between the process of cleaving
fragments from
a single crystal of an amphibole, and that of stripping a fibril or
bunch of
fibrils from the aggregate that constitutes a specimen of asbestos.
The
processes that lead to growth in asbestiform habit are not clearly
understood.
To form asbestos there presumably has to be multiple nucleation
followed by
relatively rapid growth along the fibre direction and very limited
growth at
right angles to it. Such a process might be influenced by chemical
factors, and
it is noteworthy that the class of amphibole called hornblendes, in
which Al is
an important substituent for Si in the structural chains, are not often
found
with fibrous and still less with asbestiform morphology. Major element
chemistry cannot, however, be the only factor, since we find among
tremolites
some which are and some which are not fibrous or asbestiform. One can
only
conjecture that other factors such as the temperature regime during
crystallization, trace element chemistry, speed of growth, or a
combination of
these, exert an influence on crystal habit.
The
question of variations in nature and properties from one amphibole to
another
is worthy of further discussion. Although the crystal structures of all
amphiboles are broadly the same, these structures are derived (by X ray
diffraction methods) from specimens which, although small, contain
about 1015
unit cells. They are therefore average structures. Real crystals do not
have
the ideal exact repetition of unit cells, but contain defects of
various kinds,
and the abundance and distribution of defects are known to influence
mechanical
properties, and may also influence morphology and surface properties.
Structural detail at this level is not easily detected by X ray
diffraction,
but may be investigated more readily by electron microscopy.
An
important kind of structural imperfection in amphiboles is the stacking
fault.
In a perfect crystal of a monoclinic amphibole, slabs of structure
parallel to
(100) are stacked alongside one another with regular and identical
displacements. In a faulted structure, errors in the direction and
magnitude of
these displacements occur and the frequency of such faults can vary
from one
specimen to another. When the faults are relatively infrequent, the
result can
sometimes be described as a twinned crystal. When the faults are
frequent and
regularly repeating, they are no longer really faults but are the
regular
displacements of a structure with larger unit cell and perhaps
different
symmetry. This latter condition describes approximately the
relationship
between monoclinic and ortho rhombic amphiboles.
Monitoring
and identification of airborne asbestos
INTRODUCTION
Airborne
asbestos dust is usually monitored for one of three reasons. Firstly,
large
numbers of samples are taken to check compliance with legislation. As
part of
the standards issued by most controlling authorities recommended
methods are
described by which this monitoring should be done. Secondly within the
asbestos
industry regular sampling is carried out to determine the efficiency of
dust
suppression equipment. Here it is frequently necessary to know only the
relative amount of dust present, and direct reading dust monitoring
instruments
play a key role. Finally, an increasing number of samples are taken for
epidemiological purposes. For this it is essential that standard
methods be
used which can be related to one another, and which remain constant
over many
years. This work includes monitoring the exposure of people outside the
asbestos industry and may involve measuring extremely small amounts of
asbestos.
In
practice, it is not possible to obtain an absolute measure of the dust
inhaled
and retained in the lung. With any sampling method that is adopted,
therefore,
there will be inherent errors that must be understood and allowed for
in the
interpretation of the sample results. Care must be taken to ensure that
the
samples obtained are representative of the airborne dust at the
sampling point,
and are sufficient in number so that variations of concentration with
time and
space can be allowed for. Initially, dust sampling instruments were
either too
heavy to be easily portable (e.g. the thermal precipitator) or only
took very
short duration samples (e.g. konimeter, Draeger pump). The long period
sampling
instruments are normally used to monitor the environmental air, and
take what
are commonly called static samples. The short period or snap samplers
operate
most satisfactorily in high dust concentrations, where a measurable
amount of
asbestos can be captured. Neither static nor snap samples may be
representative
of the airborne dust breathed by exposed personnel, however. Hygiene
standards
normally refer to Threshold Limit Values (TLVs). These are the levels
at which
it is believed that nearly all workers may be repeatedly exposed
throughout a
40 h week without adverse effect. Many asbestos workers do a series of
different jobs throughout a shift and may be exposed to dust for only
relatively short periods. In such instances, and indeed in the majority
of
situations, a sample taken with the instrument attached to the person
being
monitored has distinct advantages. This provides what is known as a
personal
sample. In this case the instrument should be completely portable and
the air
sampled in the breathing zone of the person concerned. Normally this is
taken
to be anywhere within 30cm of the ori nasal region.
In order for the results of
the monitoring to
have any meaning, it is necessary that they provide a consistent
measure of the
dust of pathogenic significance. When evaluating a potentially
hazardous dust
either its mass or number concentration is normally measured. The mass
can be
either that of all the airborne particles (total dust), or that of the
proportion thought to be capable of reaching the lung alveoli, commonly
referred to as respirable dust. For coal dust the work of Jacobsen et
al. has
shown that the mass of respirable dust present in the air corresponds
much more
closely with radiological change than does particle number. This work
was based
on a long term study in British coalmines, but unfortunately similar
epidemiological data are not available for asbestos exposure, and both
fibre
mass and number concentration measurements are therefore used. The
total mass
of airborne dust is monitored in the U.S.S.R. provided that the
asbestos
content is greater than 10%. This, however, creates a difficulty when
interpreting the results, because whereas in an asbestos textile
factory most
of the dust will be asbestos, in mining or the asbestos cement industry
the
majority of the dust may be other material. A more satisfactory method
of
evaluation is to determine the weight of asbestos in this total dust
sample
using chemical or physical methods of analysis. This, however, will
monitor all
asbestos fibres, including those too large to penetrate into the
alveoli. One
of these fibres could have a mass of many times that of the potentially
harmful
dust in the sample, and so this method in fact only provides an upper
limit to
the amount of asbestos present. Alternatively, instruments containing
particle
size selectors, known as elutriators can be used for sampling the dust.
These
were designed, however, using the aerodynamic properties of spherical
particles
and rely on the larger particles separating out by virtue of their
higher
falling speed. At present, however, the sampling characteristics of
these
instruments with fibrous dusts are not fully understood and an
additional
degree of uncertainty is introduced when they are used. One of the
alternative
German Federal Republic standards for asbestos, however, requires the
use of
one of these instruments. Once again physical or chemical analysis can
be used
to determine the amount of asbestos present.
The
measurement of the airborne fibre number concentration is normally
preferred in
most countries. This avoids some of the difficulties and uncertainties
of mass
monotoring by using a microscopical method to separate out only those
particles
thought to be potentially harmful from a sample of all the dust from
the air.
The results obtained, however, depend upon the microscopical
magnification and
technique used and also upon any restriction placed upon the type of
particle
counted. Some early attempts to monitor the exposure of asbestos
workers
recorded all the particles thought to be respirable, but most present
methods
involve counting only those fibres within a limited size and shape
range. One
advantage of this method is that, unlike the mass standard, the 2
fibres/cm3
British Standard for chrysotile asbestos (which has been adopted by
several
other countries), is based upon an epidemiological study. This type of
measurement is also likely to provide the basis of any revised standard
within
the foreseeable future.
Outline
of
technique
Measurements
are taken by drawing a known volume of air through a membrane filter.
This
filter is then made transparent, and the number of fibres fitting a
standard
definition of size and shape which are in the deposit are counted using
a phase
contrast microscope. The mean fibre concentration during the sampling
period
can then be calculated. Where fibre identification is needed, different
types
of sampling filters and analytical techniques may be required.
It
is also possible to use an entirely different technique, with a green
or black
membrane, which is fixed with Perspex but not cleared. This is then
mounted
without a coverglass and examined by incident light using a 4 mm
objective and
a suitable vertical illuminator (such as the Cooke Universal
Illuminator).
Tests seemed to indicate that this method gives similar results to the
usual
phase contrast method, but in general it has not found wide scale usage
for
asbestos monitoring, probably because of the increased difficulty in
setting up
and operating the microscope.
Definition
of the Fibres which are Evaluated
The membrane
filter method was developed by the British Asbestosis Research Council
in order
to try to monitor only those fibres thought to be capable of causing
lung
damage. At the time of its inception, asbestos or ferruginous bodies
were
thought to play a major role in the development of asbestosis. These
bodies are
fibres surrounded by protein and iron frequently found in the sputum of
asbestos workers. These fibres are normally longer than 10 mm and it
was
therefore concluded that it was necessary to evaluate only the longer
fibres.
Initially it was decided to monotor those fibres between 5 and 100 mm
in
length, although this upper limit is no longer applied in most
countries,
except South Africa. As only a relatively small proportion of airborne
fibres
are longer than 100 M.m, however, the difference is insignificant. In
addition,
at the time of inception of the membrane filter method, it was thought
that the
number of longer fibres was a constant proportion of the total number,
but it
has since been found that some processes, notably carding,
preferentially
produce longer fibres. More recent experimental evidence, however, has
shown
that the longer fibres (>10 mm) are in fact the most potentially
dangerous,
in particular in the development of cancer, snowing that the limitation
of
monitoring to the larger fibres was probably justified.
An
added advantage of monitoring the longer fibres was that this can be
done by
optical microscopy, thereby avoiding the sophisticated electron optical
equipment that is required in order to detect the smaller fibres. It
was also
found that by restricting the type of microscope and range of
magnification
used, good agreement could be obtained between different laboratories
evaluating the same samples. A magnification of approximately 500x was
chosen
in the U.K. With increasing magnification more fibres are seen, but the
level
of inter laboratory agreement depends more and more upon the quality of
the
microscope and the skill of the observer. The lack of ability to see
all fibres
does not invalidate the method, however, as it is only required to
produce an
index of the hazard, which may in fact not be proportional to all the
fibres
present. In order to distinguish a fibre from the other dust in the
sample, the
definition of a fibre having a length to diameter ratio (i.e. aspect
ratio) of
at least 3 1 was chosen. This is now used internationally by
occupational
hygienists when monitoring fibrous dusts, whereas 10 1 is more commonly
used by
engineers and fibre technologists.
In
order to cause disease, the airborne particles must be capable of
penetrating
into the lung. Many, however, are caught in the nose and larger airways
by
sedimentation or impaction. For spherical particles a generalization of
experimental measurements of the proportion of airborne particles of
different
sizes able to reach the alveoli produced a plot known as the
Johannesburg
curve. Here the size of a particle is expressed in terms of its
aerodynamic
diameter, that is the diameter of a sphere of density 1 g/cm3 with the
same
falling speed in air as the particle. According to this convention, all
particles larger than 7.1 mm aerodynamic diameter and about 50% of the
5 mm
aerodynamic diameter particles are removed before reaching the lungs.
Fibres,
however, behave very differently aerodynamically, and it has been shown
that
those with high aspect ratios fall through the air at a rate which is
proportional to their diameters, but independent of their lengths, and
that
asbestos fibres have aerodynamic diameters approximately three times
their
actual diameters. Asbestos fibres with high aspect ratios and with
diameters
greater than approximately 2.5 mm are therefore unable to reach the
alveoli.
When monitoring asbestos, the diameter limit is made greater than this
so as to
ensure that all of the respirable fibres are evaluated. In Australia,
Belgium,
France, Sweden and the U.K., only fibres with diameters less than 3 mm
are
evaluated, whereas a 5 mm limit is used in South Africa and Finland.
Canada,
Denmark and the U.S.A., however, do not place any restriction on the
diameter
of the fibres counted, provided that it is less than one third of its
length. A
summary of the definitions of a countable fibre used in different
countries,
together with the standards to which they apply, is given in Table l.
A
major advantage of using the microscope to select a definite size range
of
fibres is that this permits the operator to sort by size and exclude
fibres not
considered to be respirable. All fibres can therefore be collected on
the
filter and there is no necessity for a pre selector such as a cyclone
or
horizontal elutriator, as used for spherical dust. This is important as
a
satisfactory air elutriation method for asbestos fibres has not yet
been
reported.
A
further constraint placed by some codes of practice for evaluating
asbestos is
that the fibres fitting the above classification shall be asbestos.
This can
lead to difficulties as some non asbestos fibres have the same
morphology as
asbestos fibres, for example organic fibres and chrysotile, and gypsum
fibres
and amphibole asbestos. In a comparison of the counting procedures in
nine
countries, it was noted that many laboratories counted all fibres
fitting the
size definition, whereas others limited them selves to those fibres
whose morphology
appeared to be that of asbestos.
The
membrane filter
Cellulose
ester filters
are normally used for asbestos fibre monitoring. The Asbestosis
Research
Council in the U.K. recommends the use of 0.8 5.0 mm pore size filters,
whereas
the Australian code of practice suggests that only a pore size of 0.8
mm should
be used. Although many asbestos fibres have diameters much less than
this, they
are in fact captured by the filter and the optical fibre count is not
affected
by penetration. Smaller pore sizes (e.g. 0.2 mm) may be used when it is
essential for all fibres to be retained on the filter surface, for
example when
coating for transmission electron microscopy. These samples are
normally
restricted to special situations, as a large pump is required to
overcome the
pressure drop across the filter. When it is necessary to evaluate a
sample by
scanning electron microscopy, Nuclepore filters are frequently used.
These are
manufactured by etching the tracks of high energy particles through
polycarbonates, and have a very smooth surface, giving a uniform
background on
which the fibres can be easily identified. These filters, however, have
different
filtration characteristics from the cellulose ester membranes, and very
significant fibre penetration occurs when sampling at pore sizes of 5
mm and
above.
Filters
of diameter 25 and 37 mm are most commonly used, and sampling filter
holders
are available commercially. Smaller 13mm filters, however, are being
developed
for some uses in particular where there is very little dust or where
the
sampling volume is limited. These, however, present difficulties at
high
sampling rates where larger pore size filters may be required to
overcome the
increased pressure drop. Some laboratories prefer cellulose ester
membrane
filters with a gridded pattern printed on the surface. This enables the
microscopist to find more easily the plane of the dust deposit, which
may be
difficult for sparse samples.
Sampling
The
filter is placed in a
holder, where it is supported by a gauze or thick pad, which helps in
controlling the distribution of air through the filter. The 25 mm
Gelman holder
is normally used with the filter surface completely exposed. The 37 mm
Millipore holder can also be used in this way, or alternatively the
face cap
may be left in place and the small plug removed. The latter reduces the
risk of
damage to the filter by large high velocity particles, but has the
disadvantage
that if any large particles are present, a small portion of the centre
of the
filter may be obscured by them. This should not alter the count, but
the effect
can be overcome by drilling six additional 4 mm holes in some face caps
and fitting
these to the holders during sampling. The addition of a 1¾ in. plenum
between
the air inlet and the filter has been reported to give a more uniform
deposit
with this holder. Breslin and Stein measured the collection efficiency
of the
Millipore holder in still air and found that it was good for respirable
sizes
of spherical particles. In wind velocities of 2 m/s, however, its
sampling
characterstics were less satisfactory.
When
monitoring asbestos in moving air streams, for example in air ducts of
filtration
or exhaust systems, isokinetic sampling is required. This involves
matching the
velocity of the air entering the sampling equipment to that flowing in
the
duct. A sampling head must therefore be designed so that when it is
inserted in
the air stream it does not alter the flow pattern.
Personal
samples require the filter to be placed in a holder within the
operators
breathing zone. Various methods of wearing the holder have been
evaluated, e.g.
using a head harness, plastic jacket, shoulder harness, or lapel filter
holder,
but no significant difference was found. Normally a shoulder harness or
a lapel
filter holder is used with the sampling surface facing downwards or
vertically.
Upwards facing filters are avoided because of the high risk of
contamination.
In addition, the filter may be protected by a cap. or by leaving the
end on the
Millipore holder, in order to prevent the contamination from extraneous
sources
such as dusty overalls. The volume of air sampled is determined by
measuring
the flow rate of the air through the filter. This can be carried out
connecting
a variable gap flow merer or bubble meter on the sampling side of the
filter
holder. Flow meters are usually calibrated at atmospheric pressure and,
where
they are incorporated between the pump and filter head, allowance must
be made
for the reduced pressure in which they are operating. Flow meters
incorporated
in the sampling pumps are liable to error in that they record all of
the air
drawn through the pump, including any from leaks or bleed systems. The
flow
rate should be checked at regular intervals and at the end of sampling,
so that
any changes due to filter blockage or pump malfunctions can be noted.
Some
personal sampling pumps are, in fact, able to compensate for small
increases in
pressure drop across the filter. A comparative study of the pumps at
present
available on the U.K. market has recently been completed.
Alternatives
to Asbestos in Industrial Application
Introduction
The
numerous applications of asbestos are a consequence of its desirable
physical
and chemical properties, combined with a low material cost. It is this
unique
combination that makes the replacement of asbestos very difficult in
many
applications.
Some
of the properties of asbestos are summarized in Table 2, together with
the
comparable properties of some of the synthetic fibre materials that
have been
suggested as replacements for asbestos in some applications. Some of
these
properties require further comment.
Thermal
properties
The
most widely known
property of asbestos is its heat and fire resistance, although this
resistance
is not as great as is popularly believed. Asbestos cannot be classed as
refractory, although normally its properties are sufficient to
withstand super
heated steam and other high temperature industrial environments.
Degradation of
the crystal structure of asbestos and major loss of strength occur at
temperatures in the range 300 500 °C. However, a useful performance can
be
obtained at higher temperatures than this specified working
temperatures for
some asbestos products may be as high as 600ºC. The reasons for this
are
unclear, but some points of significance are apparent.
Chrysotile
contains 14% by weight of hydroxyl groups, which are lost from its
structure as
water vapour (2OH– > H2O + O2–) at temperatures greater than
450°C. The
latent heat of vaporization of this water content is thought to be a
potent
heat sink, protecting the remaining undegraded fibre. Further, the
solid
decomposition products are inert and of low thermal conductivity,
providing
additional protection to the remaining fibres, and maintaining
structural
integrity. It has been shown that, in some cases, asbestos can maintain
its
integrity at temperatures up to 1700°C.
Mechanical
properties
It
can be seen from Table 1 that the values quoted for the strength of
asbestos
fibres are very high. However, even the average values quoted in the
table may
not tell the whole story. The measurements of strength are inevitably
derived
from testing in controlled laboratory conditions, and the values
obtained may
not be representative. Discussion with suppliers of asbestos has
suggested that
the reliable strength of chrysotile fibres, as produced and used
commercially,
is no higher than approxi mately 700MN/m2.
Other
properties
Various
other properties make asbestos a valuable material. For instance, its
resistance to chemical and biological attack is valuable in
applications
involving hostile environments, and in achieving a useful service life.
The
friction and wear characteristics of chrysotile and its thermal
decomposition
product forsterite, a non fibrous silicate, make chrysotile a widely
used
material in such applications as friction clutches, brake linings, and
bearings.
The
high aspect ratio of asbestos fibres makes them useful as a mechanical
reinforcement in both polymer and cement based products.
Price
and
availability
This
favourable
combination of properties in one material, which is obtainable at a
price
significantly lower than that of its competitors in specific
applications,
makes asbestos an extremely attractive material. However, as noted in
Table l,
prices of asbestos are likely to rise. Further, since asbestos is a
limited
natural resource, with an estimated resource life of about 25 years,
prices are
likely to continue to increase, and alternative materials will have to
be found
on availability and cost as well as on health grounds.
Applications
The
breakdown of U.K. usage of asbestos is given in Table 1. The function
of
asbestos in the different indust rial applications and the alternatives
that
are available are discussed.
INDUSTRIAL APPLICATIONS OF
ASBESTOS PRODUCTS
Asbestos
textiles
Chrysotile
fibre forms the basic raw material for almost all of the activities of
the
asbestos textile industry. The length and flexibility of the longer
grades of
chrysotile are such that spinning into yarn and cloth weaving are
possible. Two
basic types of yarn are produced plain, possibly braced with an organic
fibre
and reinforced, which incorporate either wire or another yarn such as
nylon,
cotton, or polyester. The wire reinforced yarns and textiles can retain
their
mechanical properties at temperatures up to 600 °C. Recently developed
textiles
combined with resins and ceramic binders have successfully withstood
short term
exposure to temperatures up to 2200ºC. The main applications of
asbestos
textiles are represented in Figure 1. Some of the applications and
their
alternatives are considered in other sections as indicated.
Fire
and
heat protection clothing
These
garments are
manufactured from asbestos cloth which is aluminized to give a heat
reflecting
surface. The metallic layer is bonded to the cloth by a thermosetting
resin.
As
an alternative, clothing made from temperature resistant nylon fibre
has found
application in fire fighting and foundry work and as protective
underclothing
for racing car drivers. The materials in suitable form can provide
short term
protection from exposure to temperatures up to 1370°C and also for
protection
against molten metal impingement. Gloves made from this material are
suitable
for use with contact temperatures up to 300°C. The product is marketed
and
manufactured by Du Pont under the name Nomex.
Nomex
is suitable for protection against most chemical hazards, with the
exception of
some strong acids, and can be laundered with little deterioration in
properties. Nomex is about 3 times more expensive than asbestos.
However, the
ability to launder Nomex clothing means that its useful life is much
longer
than asbestos clothing.
Clothing
for heat protection is also made from special wool blends, such as
Multitect,
manufactured by Multifabs Ltd., Derby. These materials are suitable for
direct
heat and metal splash protection and have been tested for protection
against
splashes of molten steel at 1500°C. Aluminized grades are available for
greater
protection from radiant heat. The materials are resistant to chemical
attack
and can be laundered with no deterioration in properties. The wool
fibres are
surprisingly resistant to ignition and flame spread and the clothing is
competitive in price with asbestos based products.
Bleached
Teflon fluorocarbon fibres, also manufactured by Du Pont, are
advertized as
being the most fire resistant organic fibre in oxygen rich and high
pressure
atmospheres. Garments woven from this material have been worn by Apollo
astronauts and for missile fuel handling. These suits protect the
wearer from
extremely high flash temperatures and corrosive missile fuels.
Fire
blankets, curtains, and aprons
In
general, the materials
detailed above are also suitable for these applications. Additionally,
a
recently developed product, a sandwich of a layer of ceramic fibres
between two
woven glass cloths has become available from Marglass Ltd. This has
been shown
to be effective against flame and molten metal hazards. In the latter
case, the
surface glass layer melts and the ceramic fibres provide the
protection. In
certain circumstances blankets or rolls of mineral wool or ceramic
fibres may
be used, although these may tend to disintegrate more readily than the
woven
products.
Electrical
insulation
Most
cases in which asbestos textiles are employed for electrical insulation
also
demand a degree of thermal and/or chemical protection.
Filters
Asbestos
cloths are widely used for filtration of bulk liquids such as beer,
which has
latterly been the cause of some concern in relation to possible cancers
of the
stomach and gut. Expanded Perlite has been used successfully as a
substitute
for asbestos in some applications. Vermiculite products are similarly
employed.
Filter
bags of woven fabrics and needled felts of 100% Teflon fluorocarbon
fibres are
used in many filtration applications where temperatures up to 300 °C
coupled
with corrosive chemical environments are encountered.
Ropes,
yarns, tapes, etc.
In
general, satisfactory
substitution of asbestos may be made with glass for many of the
applications of
these materials, provided that the softening of the glass at about 300
°C is
not significant. For higher temperature applications, textile forms of
the
continuous ceramic and silica fibres may be suitable replacements if
price
permits.
Other
applications
Other
textile
applications for which Nomex is a suitable substitute include press
covers and
pads for laundry dry cleaning and the textile industry, racing car
seats and
iron rests on ironing boards. Teflon is employed for wicking felts and
fuel
cell membranes. The small number of minor textile applications for
which no
satisfactory alternative exists at present include lamp and stove
wicks, wipes
for molten metal, diaphragms for some of the electrolytic cells
currently
employing asbestos, and some filter cloths.
Getting,
Cleaning, and Delivering the Clay
Clays
and shales may be obtained by (a) open working a pit, hole or quarry,
if the
material is sufficiently near the surface, or by (b) mining, if the
material is
deep seated. The mining of clay is usually subsidiary to that of coal,
and may
be regarded as outside the scope of the brick manufacturers work, who
usually
purchases the mined clay when it has been delivered to his works or
erects the
latter near a heap of mined clay, shale, or blaes. For this reason, the
mining
of clay is not described in this volume the methods used are
practically the
same as those for coal and described in text books on coal mining.
In
open working, the clay or shale is obtained from the quarry by digging
by hand,
or by a mechanical excavator, with or without the aid of explosives to
loosen
the harder portions of the material. As in most clay deposits the
composition
of the bed varies at different parts, it is necessary to exercise much
care in
choosing portions of the bed from which the clay or shale has to be
taken. It
is, therefore, usual to work horizontally in a series of terraces or
steps,
each step being the height of the particular strata worked, but
conditions vary
so in different deposits that each brick manufacturer must, to a large
extent,
be left to use his own judgment in the matter. Care and attention are
required
if the clay hole is to be worked economically, as otherwise a large
amount of
useless material may be shifted. Water in the clay hole is often a
source of
trouble, as its removal entails considerable expense. This can be
minimized if
the clay or shale is excavated in such a way that the bottom of the
working
slopes to a well or sump, into which all the water can drain rapidly
and be
pumped out as fast as it accumulates. A little foresight in arranging
this sump
and the necessary drains leading to it will save much trouble, expense,
and
annoyance at a later stage. As the water will contain clay in
suspension, a
form of pump suitable for dirty water should be used.
When
steam can be carried to the clay hole, a Pulsometer pump is the most
suitable,
as it can deal with very dirty water and has no wearing parts otherwise
some
form of diaphragm or centrifugal pump should be substituted. The
ordinary types
of pump with valves, whilst excellent for clean water, are not
desirable for
use in clay holes,
REMOVAL OF OVERBURDEN
The
first step in opening a new quarry or extending an existing one is to
remove
the overburden, fen, or callow, i.e. the soil and other material lying
above
the brick earth. Some manufacturers of common bricks do not take this
precaution, and so reduce the quality of the bricks.
The
overburden need not consist merely of the soil in some localities it is
desirable to remove material 3 ft. or more in thickness in order to
expose the
brick earth. This is particularly the case where the overburden is of a
different nature from the brick earth. For instance, if a clayey
Glacial Drift,
4 ft. thick, overlies a compact clay rock 50 ft. or more in thickness,
it will
be profitable to treat the whole of the Drift as overburden and discard
it
otherwise it will spoil the bricks made of the underlying material, as
the
Drift requires a different type of machinery from that most suitable to
the
compact clay rock beneath. Failure to recognize this has ruined several
brickworks.
The
overburden should be removed so far back from the face that there is no
risk of
any overburden falling down the face, should a slip or landslide occur.
The
minimum distance from the face will depend on the nature of the brick
earth it
is usually wise to have a strip not less than 30 ft. in width cleared
of
overburden, and to keep this width continuously clear.
Whilst
ample surface should be cleared of obstruction it is not satisfactory
to clear
too large a space, because coarse vegetation may grow on the cleared
surface
and may cause more trouble and spoil more bricks than if the overburden
had not
been removed.
The
overburden was formerly removed by digging by hand and loading into
small
wagons, which run on rails and tip the material where it can do no
harm. This
method is too costly nowadays and mechanical means for removing the
overburden
are more common.
Any
trees of sufficiently large size should be felled, sorted, and sold, or
stored
for timber. When this has been done any of the following machines may
be used
according to local conditions
A
Plough may be used to loosen the soil if there is no serious quantity
of
brushwood. It is followed by a bulldozer or scraper to form the
loosened
material into heaps.
A
Tractor rooter may be used when the surface is exceptionally hard or
contains
trees, roots or scrub vegetation or high boulders, or if the brick
earth is
hard and dry. This machine will break up the overburden and loosen it,
as well
as uproot trees and bushes. A gyro tiller can also be used for this
purpose.
Both these machines require considerable skill and it is usually
advisable to
hire a man with them for this purpose.
A
Scoop or Scraper drawn by a tractor and provided with teeth or blades
to loosen
the ground and enable a suitable quantity to enter the scoop. A popular
type of
scraper will hold 9 cub. yd. of clay the tractor hauling it must
develop 80 90
h.p. This is too large a machine for constant use in many brickworks
and it is
therefore, preferable to hire a machine (with driver) when required. On
distances of more than 300 ft. rubber tyred machines are often cheaper
to use
and maintain than caterpillar tractors.
A
scraper may be towed by a tractor or mounted on the tractor chassis or
it may
be connected to the boom of a single bucket excavator (the bucket
having been
removed) by ropes.
A
Bullgrader or Bulldozer consists of a tractor with a powerful blade in
front
which pushes the loosened material forward so as to form a series of
shallow
heaps which can then be loaded mechanically into wagons and removed. It
is not
economical to use a bullgrader for distances of more than 200 ft. Where
possible the machine should work down hill.
A
Skimmer, which may be regarded as a single bucket excavator arranged to
dig
horizontally. A Skimmer does not remove the whole of the
overburdencleanly and
where one is used it is generally advisable to regard the top 3 in. of
the
clay, or shale as part of the overburden and to remove it. This
procedure will
often avoid the use of a second machine to follow the skimmer.
A Dragline
Scraper,
consisting of a scoop, with teeth in fioni. This is attached to two
ropes one
to pull it to where the material is to lie dut and to cause the scoop
to travel
forward and fill itself the other rope returns the scoop to where it
can be
discharged.
A
Single bucket Excavator or Navvy may be used where the overburden is
sufficiently thick to justify its use.
A
small Multi bucket Excavator may be used for a friable, fairly thick
overburden
not less than 3 ft. or more than 8 ft. thick. Under suitable conditions
the
overburden may be removed the same time as the clay, yet each be kept
separate,
by attaching a separating device to a multi bucket excavator.
When a
Ladder
Excavator is used it can sometimes be provided with a fitment which
enables the
overburden to be removed separately without a second machine being
needed.
Where
the overburden is very thick and the output of bricks is very large, as
in some
parts of the Oxford Clay Areas, it is advantageous to use a conveyor
bell,
tipping frame or other suitable device to carry the excavated
overburden over
the face of the pit and to deposit it mechanically some distance away.
In
smaller works the overburden is usually loaded into wagons and tipped
in part
of the worked out portion of the pit.
Much
greater care than is usual should be exercised in deciding where the
overburden
is to be tipped. In some works it has been deposited on some of the
best clay!
As
roots and stones in a brick earth are very objectionable, it is more
profitable
to remove a small excess of material as overburden than, by attempting
an undue
saving, to permit roots and stones to be mixed with the brick earth.
Special
oversight is needed to prevent the wrong strata becoming mixed with
those
containing suitable material, particularly at certain stages in the
quarrying,
but with capable men no special difficulty in this direction need be
experienced.
It
is very important to see that the overburden is completely removed.
Carelessness in this respect may result in many defective bricks being
made.
The cost of
removing overburden varies very greatly because of the differences in
its
thickness in various localities. It bears no necessary relation to the
number
of bricks made, though it tends to be less (in proportion to the
output) in the
larger than in the smaller works.
When
machines of the types described are used, continuous operation by men
of
sufficient skill is of great importance. With unskilled men the cost
can easily
be trebled.
As
the removal of overburden is an intermittent process
sufficient being removed at once to liberate
a year or mores supply of clay or shale
it is often cheaper to hire an independent firm to
undertake this work.
In this case, great care should be taken to insure its being properly
done or
the damage may be serious.
DIGGING AND EXCAVATING
The
main bulk of usable clay or shale is obtained by digging or excavating,
but
fireclays and some colliery shales are mined.
Hand
digging. In works with an output of less than three million bricks a
year, hand
digging is usual, and notwithstanding the fact that hand labour is more
expensive than mechanical excavation, a very large proportion of the
bricks in
this country is made of hand dug clay. This is due to several reasons
of which
the most important are (i) the output is so small that it is thought it
would
not justify a mechanical excavator this may be incorrect and is worth
further
investigation (ii) several kinds of clay have to be dug separately and
afterwards mixed in suitable proportions (some may have to be rejected
as
harmful) (iii) even when the bulk of the clay is excavated mechanically
some
portions must be dug by hand and separated or they would damage the
bricks and
(iv) hand digging is often necessary in clearing up part of the pit or
in
preventing clay lying in a dangerous position from doing harm to
workmen or to
useful portions of the clay.
Mechanical
Excavating. Where practicable, mechanical excavation is much cheaper
and is
used in most of the larger brickworks.
Excavation
in Benches. When the bed of clay or shale is too thick for an excavator
to dig
the whole thickness at a single cut, the machine may be made to form a
series
of terraces, or benches, each of suitable height. In a few very large
works two
or more excavators may work simultaneously at different levels and the
products
of their total working mixed on arrival at the machine house.
Thin,
Shallow
Beds. Shallow beds near the surface are best excavated in the same
manner as
the overburden (p. 41), and thin beds of harmful material lying between
thicker
beds of usable clay or shale may be similarly treated.
Plastic
Moulding by Machinery
Plastic
bricks may be made by machinery by two entirely different groups of
processes,
known respectively as the Machine moulding and the Wire cut Processes,
they
produce quite different kinds of bricks.
The
machines used for shaping bricks in both these groups of processes
require to
be supplied with a rather soft plastic paste, though in some cases
several
machines are united in one structure so as to prepare the paste as well
as
shape it. It is in the use of a somewhat soft plastic paste that the
processes
described in this chapter differ from those in the three succeeding
ones.
THE MACHINE MOULDING PROCESS
The
chief purpose of the machine moulding process is to produce bricks as
similar
as possible to those made by hand, but using machinery instead of
skilled
moulders. In so far as they are successful they overcome the difficulty
of
obtaining skilled moulders a difficulty which has been serious for
about forty
years.
Materials
The loams and other mild clays generally used for hand moulded bricks
are the
most suitable for machine moulding, but many other clays, shales, etc.,
which
can be made into a soft plastic paste can be used satisfactorily.
In
the large American works on the River Hudson, the sandy alluvial clay
is so wet
and soft that it can best be shaped in box moulds. It receives no
preliminary
treatment prior to entering the large square pug mills which force it
into the
moulds. Preparation of the Plastic Mass Some clays merely require to be
passed
through a pug mill with sufficient water, but others require to be
weathered or
soaked before pugging as these processes add considerably to the cost
of
manufacture, they are much less used than in former years when labour
was
cheaper and brickyards were usually small and apparently primitive.
Some tougher
materials require to be passed through a pan mill, crushing rolls, and
a trough
mixer, and then pugged.
Impure
clays may need to be washed and if machine moulded London Stocks are
required,
the clay must be washed and mixed with chalk and soil. Stones may be
removed by
means of a clay cleaner or by eliminating rolls. Rock clays and shales
may be
ground to powder, and then passed through a trough mixer and pug mill,
in order
to produce a suitable paste, though such elaborate plant is unusual in
this
country. When much mechanical treatment is required in the preparation
of the
clay, it is usual to shape the bricks by the wire cut, stiff plastic,
or semi
dry process, and not in mechanically operated moulds.
Whichever
method is used, the plastic paste must be as homogeneous as possible,
or it
will shrink irregularly, and in other ways produce defective bricks.
To
Increase the Plasticity of a Clay or Brick Earth, one or more of the
following
methods may be used
The
clay may be wetted and exposed to the weather (see Weathering) Hot
water,
followed by immediate tempering, is often more effective than cold
water, but
is seldom used in brickmaking.
When
it is difficult to get sufficient water into the clay and the addition
of soda
or mineral acid is undesirable, the use of a wetting agent is sometimes
beneficial. The most useful ones are various alkyl aryl sulphonates,
alkyl
naphthalene, sodium sulphonate. dioctyl sodium sulphosuccinate and the
sodium
salts of sulphonated hydrocarbons. These wetting agents are usually
sold under
registered names. They act by reducing the surface tension between the
clay or
shale and the water. Very small proportions suffice, sometimes as
little as 0.3
per cent. Cationic wetting agents are less useful.
The
prepared clay paste (pug) may be kept in a cool place for a suitable
time which
may be several days or several months according to the nature of the
material.
By
the addition of a very small proportion of soda or sodium silicate (or
both)
and mixing very thoroughly with the clay. It is generally better to
have the
clay in the form of a slip but some clays can he effectively treated in
the
paste form. An excess of the alkali will do no good and may do harm. A
few
clays are made more plastic by adding a little acid it depends on the
nature of
the Base Exchange portion of the clay.
Some
clays respond best to the addition of quicklime (only about 10 lb. per
ton)
followed (after thorough mixing) by a solution of aluminium sulphate.
By adding
gum,
humus, starch or other suitable colloidal matter. The addition of
bentonite is
being increasingly made for this purpose in other branches of
clayworking.
The addition
of a very small proportion of barium chloride, previously dissolved in
water,
to some clay will increase the plasticity by making any soluble
sulphates
insoluble. At the same time this substance reduces the tendency to
efflorescence or scum. Barium hydrate is even more effective but is
less
soluble in water.
MOULDING MACHINES
The
machines for moulding bricks are always supplied with box moulds, each
capable
of holding four, six, or eight bricks. These moulds are filled by
forcing the
plastic paste into them by means of either a pug mill or specially
shaped
rotating knives. The mode of filling is crude, there is always a
tendency for
it to be incomplete and for the inclusion of air within the bricks, but
in the
machines described in the following pages these disadvantages have been
overcome sufficiently to make the manufacture of bricks by this method
satisfactory and simple, at any rate as far as certain mild clays are
concerned.
It is
possible
to prepare a plastic paste from almost every kind of clay but for
economic reasons
the use of moulding machines is largely restricted to mild clays which
need
little or no preparation.
The
machine moulding process is not as cheap as the wire cut process, but
it has
two advantages over the latter (i) it can use a much softer paste and
is,
therefore, available for some river muds for which the wire cut process
is
unsuitable and (ii) it produces bricks which more closely resemble hand
made
ones and, therefore, fetch better prices. The very low costs often
mentioned in
connection with the soft mud process in America are not due to superior
machines, but to the large extent to which mechanism replaces manual
handling.
This reduces the cost enormously so far as actual running expenses are
concerned, but the capital charges are so great that mechanical
handling would
not pay in this country, except on much larger outputs than are
customary here.
In
many districts the wire cut process of brickmaking is displacing
moulding
machines, though where a facing of sand on the bricks is demanded the
latter
machines, or hand moulding, must be used.
All
machines used for moulding sand faced bricks must be provided with a
safety
release which comes into operation when stones or other causes of
excessive
pressure occur. Otherwise, the machine will be damaged, and however
desirable a
machine may appear to be in other respects, the absence of some form of
effective relief escapement should be regarded as sufficient to condemn
it.
One
of the earliest machines to imitate hand moulding was patented by R. A,
Morris
in 1899. The first machine was in full work for more than 35 years
after its
installation, but many changes and improvements were made in later
machines of
this type. It was made for many years by the Brightside Foundry
&
Engineering Co., Ltd., but this firm no longer makes such a machine.
In
the Morris Machine, the clay is given a light pugging and is forced
below a
descending plunger which fills a mould or series of moulds at a stroke
and
compresses the paste. On the plunger rising the mould is pushed
forward, the
surplus paste is struck off, and the mould is bumped to loosen its
contents
which are then turned on to pallet boards.
Each
mould makes three bricks at a time, the patentee claiming that this is
better, with
his machine, than producing a .larger number simultaneously. Ample time
is
allowed for the operation of cleaning, sanding, and replacing the
moulds and
effectual means are adopted for preventing the clay from displacing the
sand as
the former enters the mould. The Morris machine requires about 3 h.p.
to drive
it and can make 8000 bricks per day under normal conditions. It is also
suitable for making lightly compressed fire clay bricks. It can be
worked by
either a horse or an engine.
For
many years a popular machine was the Monarch sand faced brick making
machine,
made by the Monarch Company, but not now obtainable. The upper part of
the
machine consists of a double pug mill, from which the clay is passed
down to a
series of knives (known as wiperx or presses), and delivered to the
moulds
immediately beneath it. The action of the presses is somewhat similar
to that
of the mans fingers and thumbs in hand moulding, and is reciprocating,
not
rotary. A lad takes the moulds out of a sanding tank, places them at
the back
of the machine, and after the clay has been mechanically pressed into
the
moulds in the front of the machine, the mechanism at the back brings
another
set into position under the die. A man standing in frontof the machine
takes
the mould, scrapes of the surplus material with a strike and hands it
to
another man, who inverts it on to a pallet board which has previously
been
placed ready and lifts the mould from the bricks. The man then
turns round,
puts the mould ready to be re sanded, and seizes another full mould.
The pallet
and bricks are taken by another man to a hack or dryer, two to six
bricks being
made at a time. The whole operation is very simple and requires no
skilled
labour.
The
amount of pressure exerted on the clay in the moulds can be instantly
regulated
by moving a small lever in the front of the machine. This regulation is
essential in order to prevent difficulties due to variations in the
stiffness
of the clay.
Another
machine for making bricks of the same type was invented by Berry
& Son and
is now supplied by John Hart & Co., Ltd.. London, N. 14 this
machine
consists of a horizontal cylinder with the driving gear at one end and
the
moulding table at the other. The clay is fed into a hopper near the
gear and
carried by large pugging knives to the moulds, into which it is forced
by
eccentrically shaped cams or wipers. These blades avoid a defect in the
shape
of the bricks, which often occurs when the moulds are filled by a
plunger. When
one set of bricks has been made a freshly sanded mould is fed into the
back of
the machine and is automatically pushed into the correct position
whilst the
full mould is forced out at the front of the machine and any surplus
clay is
removed by an automatic strike. The bricks are then turned out of the
mould on
to pallets on the turntable and the empty mould is returned to the
sanding box
for re sanding. Only two men are needed to operate the machine. The
normal
output is 1200 bricks per hour.
The
machine made by Ernest Hole &. Son, of Burgess Hill, Sussex,
forms one
brick at a time, and this makes the subsequent bumping of the mould
much less
laborious than when three or more bricks are made simultaneously. Holes
machine
consists of a horizontal pug mill, which delivers the clay paste into a
chamber
provided with a plunger. The mould is carried on a carriage on which is
a dummy
block, forming the bottom of the mould. The plunger remains stationary
whilst
the mould is drawn forward. The single machine is capable of producing
five
bricks per minute or 15000 bricks per week. A duplex machine produces
double
this quantity.
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