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CEMENT (from Lat. caementum, rough pi...

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Originally appearing in Volume V05, Page 656 of the 1911 Encyclopedia Britannica.
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CEMENT (from Lat. caementum, rough pieces of stone, a shortened form of caedimentum, from caedere, to cut), apparently first used of a mixture of broken stone, tiles, &c., with some binding material, and hence of any material capable of adhering to, and uniting into a coherent mass, fragments of a substance not in itself adhesive. The term is often applied to adhesive mixtures employed to unite objects or parts of objects (see below), but in engineering, when used without qualification, it means Portland cement, its modifications and congeners; these are all hydraulic cements, i.e. when set they resist the action of water, and can, under favourable conditions, be allowed to set under water. Hydraulic Cements.—It was well known to builders in the earliest historic times that certain limes would, when set, resist the action of water, i.e. were hydraulic; it was also known that this property could be conferred on ordinary lime by admixture of silicious materials such as pozzuolana or tufa. We have here the two classes into which hydraulic cements are divided. When pure chalk or limestone is " burned," i.e. heated in a kiln until its carbonic acid has been driven off, it yields pure lime. This slakes violently with water, giving slaked lime, which can be made into a smooth paste with water and mixed with sand to form common mortar. The setting of the mortar is due to the drying of the lime (a purely physical phenomenon, no chemical action occurring between the lime and the sand). The function of the sand is simply that of a diluent to prevent undue shrinkage and cracking in drying. Subsequent hardening of the mortar is caused by the gradual absorption of carbonic acid from the air by the lime, a skin of carbonate of lime being formed; but the action is superficial. Mortar made from pure or " fat " lime cannot with-stand the action of water, and is only used for work done above water-level. If, however, such " fat " lime is mixed in the presence of water, not with sand but with silica in an active form, i.e. amorphous and (generally) hydrated, or with a silicate containing silica in an active condition, it will unite with the silica and form a silicate of lime capable of resisting the action of water. The mixture of the lime and active silica or silicate is a pozzuolanic cement. The simplest of all pozzuolanic cements would be a mixture of pure lime and hydrated silica, but though the latter is prepared artificially for various purposes, it is too expensive to be used as a cement material. A similar obstacle lies in the way of using a certain native form of active silica, viz. kieselguhr, for it is too valuable as an absorbent of nitro-glycerine, for the manufacture of dynamite, to be available for making pozzuolanic cement. There are, however, many siliceous Pozzuolank .cement. substances occurring abundantly in nature which can thus be I employed. This excess does no harm, for that part which fails used. They are mostly of volcanic origin, and include pumice, tufa, santorin earth, trass and pozzuolana itself. The following analyses show their general composition:- Neapolitan Roman Trass Pozzuo- Pozzuo- (per cent). lana lana (per cent). (per cent). Soluble silica (SiO2) . 27.80 32.64 19.32 Insoluble silicious residue 35.38 25'94 50'40 Alumina (Al2O3) 22 13.86 Ferric oxide (Fe203) 19-8o 74 3-To Lime (CaO) . . 5.68 4.06 Magnesia (MgO) . 0.35 1.37 0.13 Sulphuric anhydride (SO3) Trace Trace 7.57 Combined water (H30) . Carbonic anhydride (CO2) 4'27 8.92 { Moisture 5.04 Alkalis and loss 6.72 4'33 0.58 100.00 I00.00 100.00 An artificial product which serves perfectly as,a pozzuolana is granulated blast-furnace slag. The slag, which must contain a high percentage of lime, is granulated by being run while fused into abundance of water. This granulated slag differs from the same slag allowed to cool slowly, in that a portion of the energy which it possesses while fused is retained after it has solidified. It bears to ordinary slowly-cooled slag a similar relation to that borne by plastic sulphur to ordinary crystalline sulphur. This potential energy becomes kinetic when the slag is brought into contact with lime in the presence of water, and causes the formation of a true hydraulic silicate of lime. The following analysis shows the composition of a typical slag:- Per cent. Insoluble residue 1.04 Silica (SiO2) 31.50 Alumina (Al2O3) . 18.56 Manganous oxide (MnO) 0.44 Lime (CaO) 42.22 Magnesia (MgO) 3.18 Soda (Na2O) 0.70 Sulphuric anhydride (SO3) 0.45 Sulphur (S) . 2.21 100.30 Deduct oxygen equivalent to sulphur . 1. To 99.20 Granulated slag of this character is ground with slaked lime until both materials are in a state of fine division and intimately mixed. The usual proportions are three of slag to one of slaked lime by weight. The product termed slag cement sets slowly, but ultimately attains a strength scarcely 'inferior to that of Portland cement. Although it is cheap and suitable for many purposes, its use is not large and tends to decrease. Pozzuolanic cements are little used in England. Generally speaking, they are only of local importance, their cheapness depending largely on the nearness and abundance of some suitable volcanic deposit of the trass or tufa class. They are not usually manufactured by the careful grinding together of the pozzuolana and the lime, but are mixed roughly, a great excess of pozzuolana beingto unite with the lime serves as a diluent, much as does sand in mortar. In fact, ordinary pozzuolanic cement made on the spot where it is to be used may be regarded as a better kind of common mortar having hydraulic qualities. Good hydraulic mortars may be made from lime mixed with furnace ashes or burnt clay as the pozzuolanic constituent. Cements of the Portland type differ in kind from those of the pozzuolanic class; they are not mechanical mixtures of lime and active silica ready to unite under suitable conditions, but consist of definite chemical compounds of lime and PoCemennlanta silica and lime and alumina, which, when mixed with water, combine therewith, forming crystalline substances of great mechanical strength, and capable of adhering firmly to clean inert material, such as stone and sand. They are made by heating to a high temperature an intimate mixture of a calcareous substance and an argillaceous substance. The commonest of suchsubstances in England are chalk and clay, but where local conditions demand it, limestone, marl, shale, slag or any similar material may be used, provided that the correct proportions of lime, silica and alumina are maintained. The earliest forms of cements of the Portland class were the hydraulic limes. These are still largely used, and are prepared by burning limestones containing clayey matter. Some of these naturally possess a composition differing but little from that of the mixture of raw materials artificially prepared for the manufacture of Portland cement itself. Although hydraulic limes have been in use from the most ancient times, their true nature and the reason of their resistance to water have only become known since 1791. Next in antiquity to hydraulic lime is Roman cement, prepared by heating an indurated marl occurring naturally in nodules. Its name must not be taken to imply that it was used by the ancients; in point of fact the manufacture of this substance dates back only to 1796. With the growth of engineering in the early part of the 19th century arose a great demand for hydraulic cement. The supply of materials containing naturally suitable proportions of calcium carbonate and clay being limited, attempts were made to produce artificial mixtures which would serve a similar end. Among those who experimented in this direction was Joseph Aspdin, of Leeds, who added clay to finely ground limestone, calcined the mixture, and ground the product, which he called Portland cement. The only connexion between Portland cement and the place Portland is that the cement when set somewhat resembles Portland stone in colour. True, it is possible to manufacture Portland cement from Portland stone (after adding a suitable quantity of clay), but this is merely because Portland stone is substantially carbonate of lime; any other limestone would serve equally well. Although Portland cement is later in date than either Roman cement or hydraulic lime, yet on account of its greater industrial importance, and of the fact that, being an artificial product, it is of approximately uniform composition and properties, it may conveniently be treated of first. The greater part of the Portland cement made in England is manufactured on the Thames and Medway. The materials are chalk and Medway mud; in a few works the latter is replaced by gault. The composition of typical samples of chalk and clay is shown in the following analyses: Chalk. Clay. Per cent. Per cent. Silica (SiO2) 0.92 Insoluble silicious matter 26.67 Consisting of Alumina + ferric oxide (Al203 + Silica (SiO2) 31.24 Quartz (SiO2) 19'33 Fe2O3) 0.24 Alumina (Al203) 16.60 Silica (SiO2) 5.19 Lime (CaO) 55.00 Ferric oxide (Fe2O3) 8.66 Alumina (Al2O3) . 1.47 Felspar Magnesia (MgO) 0.36 Lime (CaO) 0.25 Magnesia (MgO) 0.03 7.34% Carbonic anhydride (CO2) . 43.40 Magnesia (MgO) . 1.91 Soda (Na2O) . . 0.65 99'92 Soda (Na2O) 1•oo Potash (K2O) 0.45 26.67 Sodium chloride (NaCl) 1.86 Combined water, organic 11.36 matter, and loss Ioo•oo These materials are mixed in the proportion of about 3:1 by weight so that the dried mixture contains approximately 75 % of calcium Mtxleg carbonate, the balance being clay. The mixing may be effected• in several ways. The method once exclusively used consists in mixing the raw materials with a large quantity of water in a wash mill, a machine having radial horizontal arms driven from a central vertical spindle and carrying harrows which stir up and intermix any soft material placed in the pit in which the apparatus revolves. The raw materials in the correct proportion are fed into this mill together with a large quantity of water. The thin watery " slip " or slurry flows into large settling tanks (" backs ") where the solids in suspension are deposited; the water is drawn of{, leaving behind an intimate mixture of chalk and clay in the form of a wet paste. This is dug out, and after being dried on floors heated by flues is ready for burning. This process is now almost obsolete. According to present practice the raw materials are mixed in a wash mill with so much water that the resulting slurry contains 40 to 50% of water. The slurry, which is wet enough to flow, is ground between millstones so as to complete the process of comminution begun in the wash mill. Thorough grinding and mixing are of the utmost importance, as otherwise the cement ultimately produced will be unsound and of inferior quality. The drying of the slurry is generally effected by the waste heat of the kilns, so that while one charge is burning another is drying ready for the next loading of the kilns. The kilns commonly employed are " chamber kilns," circular Loading structures not unlike an ordinary running lime kiln, but ~eklln~ having the top closed and connected at the side with a wide flue in which the slurry is exposed to the hot products of combustion from the kiln. The farther ends of the flues of several such kilns are connected with a chimney shaft. The slurry, in drying on the floor of the flue, forms a fairly tough cake which cracks spontaneously in the process of drying into rough blocks suitable for loading into the kiln. At the bottom of the kiln is a grate of iron bars, and on this wood and coke are piled to start the fire. A layer of dried slurry is loaded on this, then a layer of coke, then a layer of slurry, and sdon until the kiln is filled with coke and slurry evenly distributed. Fresh slurry is run on to the drying floors, and the kiln is started. The construction of an ordinary chamber kiln may be gathered from the accompanying diagram (fig. I). The Chimney operation of burning. is a slow one. An ordinary kiln, which will contain about 50 tons of slurry and 12 tons of coke, will take two days to get fairly alight, and will be another two or three days in burning out. Therefore, allowing adequate time for loading and unloading, each kiln will require about one week for a complete run. The output will be about 30 tons of " clinker " ready to be ground into cement. The grinding of the hard rock-like masses of clinker is effected between millstones, or in modern plants in ball-mills, tube-mills and edge-runners. It is an important part of the manufacture, because the finished cement should be as fine and " floury " as possible. The foregoing description represents the procedure in use in many English factories. There are various modifications in practice according to local conditions: a few of these may be described. In all cases, however, the main operations are the same, viz. intimately mixing the raw materials, drying the mixture, if necessary, and burning it at a clinkering temperature (about 1500 C. =2732° F.). Thus when hard limestone is the form of calcium carbonate locally available, it is ground dry and mixed with the correct proportion of clay also dried and ground. The mixture is slightly damped, moulded into rough bricks, dried and burned. A possible alternative is to burn the limestone first and mix the resulting lime with clay, the mixture being burned as before. By this method grinding the hard limestone is avoided, but there is an extra expenditure of fuel in the double burning. Many different forms of kiln are used for burning Portland' cement. Besides the chamber kilns which have been described, Other there are the old-fashioned bottle kilns, which are similar Othe' to the chamber kilns, but are bottle-shaped and open ldla at the top; they do not dry the slurry for their next charge. Their use is becoming obsolete. There are also stage kilnsof the Dietzsch type, which consist of two vertical shafts, one above the other, but not in the same vertical line, connected by a horizontal channel. At this middle portion and in the upper part of the lower shaft the burning proper proceeds; the upper shaft is full of unburnt raw material which is heated by the hot gases coming from the burning zone, and the lower shaft contains clinker already burned and hot enough to heat the incoming air which supplies that necessary for combustion at the clinkering zone. A pair of Dietzsch kilns, built back to back, are shown in fig. 2. There are other forms of shaft kiln, such as the Schneider, in which there is a burning zone, a heating and cooling zone as in the Dietzsch, but no horizontal stage, the whole shaft being in the same vertical plane. Another form is the Hoffmann or ring kiln, made up of a number of compartments arranged in a ring and connected with a central chimney ; in these compartments rough brick-shaped masses of the raw materials are stacked, and between these bricks fuel is sprinkled. At a given moment one of these compartments is burning and at its full temperature; the air for. combustion is drawn in through one or more compartments behind it which have just finished burning, and is thereby strongly heated; the products of combustion pass away through one or more compartments in front of it and heat their contents before they are subjected to actual combustion. It will be seen that the principle of the ring kiln is similar to that of the stage kiln. In each case the clinker which has just been burned and is fully hot serves to heat the air-supply to the compartment where combustion is actually proceeding; in like manner the raw materials about to be burned are well heated by the waste gases from the compartment in full activity before they them-selves are burned. (It may be noted that here and generally in this article " burn " is used in the technical sense; it is technically correct to speak of cement clinker Burmn9_zone being " burned," although it is not a fuel; in accurate terms it is the fuel which is burned, and it is the heat it generates which raises the clinker to a high temperature, i.e. technically " burns " it.) By this de- vice a great part of the heat is regenerated and a saving of fuel is effected. The methods of burning cement described above are obsolescent. They are being replaced by the rotatory process, so called because the cement is burned in rotating cylinders instead of in Rot atory fixed kilns. These cylinders vary from 6o to 150 ft. in length, an ordinary length in modern practice being 100 to 120 ft.; their diameter correspondingly varies from 6 ft. to 7 ft. 6 in. The cylinders are made of steel plate, lined with refractory bricks, are carried on rollers at a slight angle with the horizontal, and are rotated by power. At the upper end the raw material is fed in either as a dry powder or as a slurry; at the lower end is a powerful burner. In the early days of rotatory kilns producer gas was used as a fuel, but with little success; about 1895 petroleum was used in the United States with complete success, but at a relatively heavy cost. At the present time, finely powdered coal injected by a blast of air is almost universally employed, petroleum being used only where it is actually cheaper than coal. In the working of this type of kiln the rotation and slight inclination of the cylinder cause the raw material to descend towards the lower end. At the upper end the raw material is dried and heated moderately. As it descends it reaches a part of the kiln where the temperature is higher; here the carbonic acid of the carbonate of lime, and the combined water of the clay are driven off, and the resulting lime begins to act chemically on the dehydrated clay. The material is then in a partially burnt and slightly sintered state, but it is not fully clinkered and would not make Portland cement. The material continues to descend by the rotation of the kiln and reaches the lower end nearest j /ii//~ I%~ /Z.V /, zzo /# iii/ Plan Kiln the burner where the temperature is highest, and is there heated so highly that the union of the lime, silica and alumina is complete, and fully burnt clinker falls out of the kiln. It is extremely hot, and is cooled usually by being passed down one or more rotating cylinders, similar to the first. but smaller, and acting as coolers instead of kilns. On its way down the cylinders the clinker meets a current of cold air and is cooled, the air being correspondingly warmed and passing on to aid in the combustion of the fuel used in heating the kiln. This regenerative heating is similar in principle and effect to that obtained by means of the shaft and ring kilns described above. The output of these kilns varies from 200 to 400 tons per kiln per week according to their size and the nature of the raw materials burned, as against 30 tons per week for an ordinary chamber kiln. A large saving in labour is also secured. The rotatory system presents many advantages and is rapidly replacing the older methods of cement making. Fig. 3 represents diagrammatically a rotatory cement plant on the Hurry & Seaman system, which was one of the first to make cement by the rotatory process successfully on a large scale, using powdered coal as fuel. Rotatory kilns of various other makes are now in use, but the same principles are embodied, namely, the employment of a rotating inclined cylinder for burning the raw materials, a burner fed with powdered coal and a blast of air, and some device such as a cooling cylinder or cooling tower by which the clinker may be cooled and the air correspondingly heated on its way to the burner. Another method of making .Portland cement which has been proposed and tried with some success consists in fusing the raw materials together in an apparatus of the type of a blast furnace. The high temperature necessary to fuse cement clinker makes this process difficult to accomplish commercially, but it has many inherent merits and may be the process of the future, displacing the rotatory method. Portland cement clinker, however produced, is a hard, rock-like substance of semi-vitrified appearance and very dark colour. The cement product from a well-run rotatory kiln is all evenly burnt clinker. and properly vitrified; that from an ordinary fixed kiln of whatever type is apt to contain a certain amount (5 to 15 %) of underburnt material, which is yellowish and friable and is not properly clinkered. This material must be picked out, as such underburnt stuff contains free lime or unsaturated lime compounds. These may slake slowly in the finished cement and cause such expansion as may destroy the work of which it forms part. Well-burnt, well-picked clinker when ground yields good Portland cement. Nothing is added during or after grinding save a small amount (1 to 2 %) of calcium sulphate in the form either of gypsum or of plaster of Paris, which is sometimes needed to make the cement slower-setting. For the same purpose a small quantity of water (up to 2 %) may be added either by moistening the clinker or by blowing steam into the mills in which the clinker is ground. This small addition for this specified purpose is recognized as legitimate, but the employment of various cheap materials such as ragstone and blast-furnace slag, sometimes added as diluents or make-weights, is adulteration and therefore fraudulent. The composition of Portland cement varies within comparatively narrow limits, and for given raw materials the variations are tending Compost- to become smaller as regularity and skill in manufacture tion. increase. The following analysis may be taken as typical of cements made from chalk and clay on the Thames and Silica (SiO2) Per cent. . 22.0 Insoluble residue . I.O Alumina (Al203) • 7'5 Ferric oxide (Fe203) . • 3.5 . 62•o Lime (CaO) Magnesia (MgO) I.O Sulphuric anhydride (SO3) . . 1.5 Carbonic anhydride (CO2) 0.5 Water (H2O) , 05 Alkalis . .
End of Article: CEMENT (from Lat. caementum, rough pieces of stone, a shortened form of caedimentum, from caedere, to cut)
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