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See also:CEMENT (from See also:Lat. caementum, rough pieces of See also: 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 See also: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 See also:part which fails used. They are mostly of volcanic origin, and include See also:pumice, tufa, See also:santorin See also:earth, See also:trass and pozzuolana itself. The following analyses show their See also:general See also:composition:- Neapolitan See also:Roman Trass Pozzuo- Pozzuo- (per cent). lana lana (per cent). (per cent). Soluble silica (SiO2) . 27.80 32.64 19.32 Insoluble silicious See also:residue 35.38 25'94 50'40 Alumina (Al2O3) 22 13.86 Ferric See also:oxide (Fe203) 19-8o 74 3-To Lime (CaO) . . 5.68 4.06 See also:Magnesia (MgO) . 0.35 1.37 0.13 Sulphuric anhydride (SO3) Trace Trace 7.57 Combined water (H30) . Carbonic anhydride (See also:CO2) 4'27 8.92 { Moisture 5.04 Alkalis and loss 6.72 4'33 0.58 100.00 See also:I00.00 100.00 An artificial product which serves perfectly as,a pozzuolana is granulated blast-See also: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 See also:energy which it possesses while fused is retained after it has solidified. It bears to ordinary slowly-cooled slag a similar relation to that See also:borne by plastic See also: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 See also: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 See also:oxygen See also:equivalent to sulphur . 1. To 99.20 Granulated slag of this See also:character is ground with slaked lime until both materials are in a See also:state of See also:fine See also:division and intimately mixed. The usual proportions are three of slag to one of slaked lime by See also: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 See also:England. Generally speaking, they are only of See also:local importance, their cheapness depending largely on the nearness and abundance of some suitable volcanic See also: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 See also: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 See also:kind of common mortar having hydraulic qualities. See also:Good hydraulic mortars may be made from lime mixed with furnace ashes or burnt See also:clay as the pozzuolanic constituent. Cements of the Portland type differ in kind from those of the pozzuolanic class; they are not See also: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 See also: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, See also: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 See also: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 See also:ancient times, their true nature and the See also: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 See also:dates back only to 1796. With the growth of engineering in the See also:early part of the 19th See also:century arose a great demand for hydraulic cement. The See also:supply of materials containing naturally suitable proportions of See also: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 See also:Joseph Aspdin, of See also: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 See also:place Portland is that the cement when set somewhat resembles Portland stone in See also: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 See also:account of its greater See also:industrial importance, and of the fact that, being an artificial product, it is of approximately See also: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 See also:Thames and See also:Medway. The materials are chalk and Medway mud; in a few See also:works the latter is replaced by See also: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 See also: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 See also: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
See also: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 See also: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 See also:wash See also: 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 See also: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 See also:process is now almost obsolete. According to See also: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 See also: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 See also:waste See also:heat of the kilns, so that while one See also: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 See also:running lime kiln, but ~eklln~ having the See also:top closed and connected at the See also:side with a wide flue in which the slurry is exposed to the hot products of See also:combustion from the kiln. The farther ends of the flues of several such kilns are connected with a See also:chimney See also:shaft. The slurry, in drying on the See also: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 See also:grate of See also:iron bars, and on this See also:wood and See also:coke are piled to start the See also: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 See also: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 See also:time for loading and unloading, each kiln will require about one See also:week for a complete run. The output will be about 30 tons of " See also:clinker " ready to be ground into cement. The grinding of the hard See also:rock-like masses of clinker is effected between millstones, or in See also:modern See also:plants in See also:ball-See also:mills, See also: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 See also:procedure in use in many See also:English factories. There are various modifications in practice according to local conditions: a few of these may be described. In all cases, however, the See also: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 See also: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 See also:expenditure of See also:fuel in the See also: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 See also: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 See also:stage kilnsof the Dietzsch type, which consist of two vertical shafts, one above the other, but not in the same vertical See also:line, connected by a horizontal channel. At this See also:middle portion and in the upper part of the See also: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 See also: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 See also: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 See also:plane. Another form is the See also:Hoffmann or See also:ring kiln, made up of a number of compartments arranged in a ring and connected with a central chimney ; in these compartments rough See also: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 See also: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 See also: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 " See also:burns " it.) By this de- See also: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 See also:diameter correspondingly varies from 6 ft. to 7 ft. 6 in. The cylinders are made of See also:steel See also:plate, lined with refractory bricks, are carried on rollers at a slight See also:angle with the horizontal, and are rotated by See also:power. At the upper end the raw material is fed in either as a dry See also:powder or as a slurry; at the lower end is a powerful burner. In the early days of rotatory kilns producer See also:gas was used as a fuel, but with little success; about 1895 See also:petroleum was used in the See also:United States with complete success, but at a relatively heavy cost. At the present time, finely powdered See also: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 See also: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 See also: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/ See also:Plan Kiln the burner where the temperature is highest, and is there heated so highly that the See also: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 See also: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 See also: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 See also:system presents many advantages and is rapidly replacing the older methods of cement making. Fig. 3 represents diagrammatically a rotatory cement plant on the See also:Hurry & See also:Seaman system, which was one of the first to make cement by the rotatory process successfully on a large See also: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 See also:device such as a cooling cylinder or cooling See also: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 See also: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 See also: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 See also: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 See also:save a small amount (1 to 2 %) of calcium sulphate in the form either of See also:gypsum or of See also:plaster of See also: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 See also: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 See also:adulteration and therefore fraudulent. The composition of Portland cement varies within comparatively narrow limits, and for given raw materials the See also: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 (See also:H2O) , 05 Alkalis . Additional information and CommentsThere are no comments yet for this article.
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