1. Moulded and Ornamental Bricks and Blocks, Including Copings and Lintels, Cutters and Rubbers, Fireplace Bricks, Etc.

2 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, Refractory Heat-insulating Bricks, Developments in Refractory Brick
3.The Stiff-plastic Process of Brickmaking
The Simple Stiff-plastic Process, Preliminary Processes, Feeding the Mills, Crushing, Grinding Mills,
Precautions With Edge-runner Mills, Selecting a Mill, Storage of Raw Clay, Elevating Ground Material, Screens, Sieves and Riddles, Tailings, Storage of Ground Clay, Mixers, Adding the Water, Stiff-plastic Process Brickmaking Machines, Precautions, Re-pressing, Transport, Drying, Kilns
4. Hand-moulding Processes
Hand-made Facing Bricks, Hand-made Fire-
bricks, Preparing Clay for Hand-moulding, Hand-
moulding, Slop-moulding, Sand-moulding, Semi-dry Hand-made Bricks, Transport, Drying, Pressing, Taking Bricks to the Kiln, Burning, Characteristics of Hand-made Bricks, Hand-made V. Machine-made Bricks.
5. Glazed Bricks
Glazes and Bodies, Enamelled Bricks , First Dip, White Body, Colourless Glaze (Cone 8), Opaque White Glaze, Storage, Applying the Body and Glaze, Salt-glazed Bricks
6. 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, Pyroprocessing: Physical and Chemical Processes Involved, Preheating, Calcining, Clinkering (Sintering In The Presence of a Liquid Phase) , Cooling, Thermal Efficiency of Pyroprocessing, Process Control, The Heat Balance — Process Efficiency , Electric Power Consumption
7. Grinding and Fineness of Cement
Cement Milling, Factors Influencing the Grindability of Clinker, Minor Additional Constituents, Addition of Gypsum, , Fineness of Cement, Determination of Surface Area, Particle Size Distribution
8. Tests of Cement Quality
Introduction, Chemical Composition, Setting Times, Compressive Strength, Workability, Soundness, Heat of Hydration, Concluding Remarks—Durability of Concrete,
9. Admixtures and Special Cements
Admixtures, Accelerators, Retarders, Water-reducing (Plasticising) Admixtures, Air Entrainment, Oilwell Cements, Calcium Aluminate Cement (Cac), Alkali-activated Slag and Aluminosilicate Cements, Calcium Sulfoaluminate Cements, Expansive and Shrinkage Compensated Cements, Sulfoaluminate-belite Cements, Practical Considerations
10. Characterisation of Portland Cement Clinker
Introduction, Chemical Analysis By Selective Dissolution, Optical Microscopy, Characteristics of The Principal Clinker Phases, Quantitative Deter-mination of Phase Composition, X-ray Diffraction, Quantitative X-ray Diffraction Analysis (Qxda) , Electron Microscopy, Backscattered Electron (Bse) Imaging,
X-ray Microanalysis, Concluding Remarks
11. The Mineralogy of Asbestos
Introduction, Definitions, Chemical Composition, Crystal Structures, Occurrences, Synthesis, Optical Properties, X-ray Diffraction Data, Electron Optical Characteristics, Non-asbestiform Amphibole and Serpentine Minerals
12. Monitoring and Identification of Airborne Asbestos (Synopsis)
Introduction, The Membrane Filter Method, Outline Of Technique, Definition of the Fibres which are Evaluated, The Membrane Filter, Sampling, Transportation of Filters, Mounting of the Filter, Microscopical Evaluation, Accuracy of the Membrane Filter Method, Recent Developments In Fibre Evaluation, Determination of very Low Asbestos Concentrations, Direct-reading Dust Monitoring Equipment, Miscellaneous Instruments, Introduction, The Thermal Precipitator, The Konimeter, The Owens Jet Counter, The Impinger, Identification of Airborne Asbestos Fibres
13. Alternatives to Asbestos in Industrial Application
Introduction, Industrial Applications of Asbestos Products, Thermal Insulation and High-temperature Applications, Industrial Applications of Asbestos-cement, Dry-rubbing Bearings, Substitutes for Asbestos-reinforced Thermosets in Bearing Applications, Electrical Insulation, Health Hazards of Substitute Materials,
14. Getting, Cleaning, and Delivering the Clay
Removal of Overburden, Digging and Excavating, Blasting, Digging By Hand, Mechanical Excavators, Choice of Excavators, Loading and Loaders, Expansion after Excavation, Clay Haulage and Transport, Haulage, Safety Devices, Belt Conveyors, Wagons and Tubs, Tramway Tracks, Clay Storage, Preparing the Clay, Improving Workability, Sorting or Picking, Weathering, Selecting and Blending Clays, etc., Cleaning Clays, Rendering Lime Harmless in Clay, Chemical Treatment if Clays
15. 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, Mixers, The Addition of Water, Pug-mills, a Mixer Followed by a Pug-mill, Compressing, Extruding, and Shaping, The Clay Paste, The Collar Spacer or Distance-piece, Dies or Mouthpieces, Defective Working of Mouthpiece, Expression Roller Machines, Cutting Tables, Precautions When Cutting by Wires, Precautions in Shaping Wire-cut Bricks, Re-pressing, Precautions in Re-pressing Bricks, Die-boxes for Presses, Transport, Drying, Application of Heat, Sources of Heat, Types of Dryers, Shed Dryers, Chamber-dryers With Hot Floors, Air-heated Chamber Dryers, Corridor Dryers, Tunnel Dryers, External Air-heaters, Direct V. Inverse Dryers, Multiple-chamber or Compound Dryers, Humidity Drying, Admission of Air Into Tunnel Dryers, Fuel Consumption and Time Of Drying, Precautions in Drying, Cars and Rails For Dryers, Selecting a Dryer, Relative Costs of Drying, Control of the Dryer

^ Top

 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 is 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 dying 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 brick makers 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 appear 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 Brick making

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 King's 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 are 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-brick making 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 summarized 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, depend 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 grind ability 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 utilization 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 (manufacturer’s 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 to’establish 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.

Fig. 1. Apparatus for the determination of setting times and standard consistence (BS 4550): A— holder for Vicat needles and consistence plunger. B—final set needle, C—standard mould. 

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 compressive strength developed in a mortar (or a concrete) depends on the materials used, the mix proportions, the procedure employed in mixing and the efficiency with which the mix is compacted into a mould, as well as the temperature, humidity and time of curing. Materials, procedures and equipment to be used are, therefore, described in detail in standards. The sand specified in EN 196 is not limited to that extracted from one source, but it must be CEN certified and give results equivalent to those obtained with a reference sand which is defined as consisting of rounded particles in five specified size ranges between 0.08 and 2 mm, with 99 ± 1% retained on 0.08 mm sieve. In addition, the silica content must be not less than 98% and moisture no greater than 0.2%. Samples of the reference sand may be obtained by contacting the German Standards Institute (DIN).

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 Feret’s 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. Curing is carried out at 20° ± 1°, in a mist room for 24 h and then, after demoulding, under water. 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.

Precision estimates in EN 196 are given as coefficients of variation for 28-day strength only. For reproducibility ‘between well-experienced laboratories’, a CV of ‘less than 6% may be expected’. For repeatability ‘within a well-experienced laboratory’, a CV ‘may be expected to lie between 1% and 3%’. Taylor pointed out that, expressed as 95% confidence limits, these CVs imply ± 10.1 N/mm2 and ± 5.0 N/mm2, respectively, for a mean 28-day strength of 60N/mm2. Continuing cooperative testing between laboratories is expected to improve precision. Compressive strength testing of cements is undertaken primarily to demonstrate the quality and consistency of the product. In addition, it gives the user some limited information on its likely performance in a ‘production’ concrete. Harrison gave a formula correlating EN 196 mortar prism and BS 4550 concrete cube compressive strengths for same cement:

ln(p/c) = 0.28/d + 0.25 (2)

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 is 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 t