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CEMENT partment 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 themselves 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 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, t.e., technically “burns” it.) By this device a great part of the heat is regenerated and a saving of fuel is effected. A more modern method of burning cement is by means of rotatory kilns. These are largely used in the United States, and are being introduced into England and on the f t ry k?ln ° Continent. They consist of cylinders about 60 ft. long and 6 ft. in diameter placed at a slight angle with the horizontal, and carried on rollers so that they can be rotated by power. The cylinders are made of boiler plate and lined with refractory bricks. At the upper end the raw material is fed in either as a dry powder or as slurry ; at the lower end is a powerful burner. Most of the kilns used in the United States employ petroleum as a fuel, but more recently powdered coal injected by a blast of air has been adopted. 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 is not fully clinkered and would not make Portland cement. As the kiln continues to revolve, the partly burnt material reaches the lower end of the kiln which is nearest the burner, and where the temperature is highest, and is there heated so highly that the union of the lime, silica, and alumina is complete, and fully burned clinker falls out of the kiln. It is extremely hot, and passes down one or more inclined rotating cylinders

Fig, 3. similar to the first, but 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 Dietsch and ring kilns. The output of one of these kilns is about 200 tons per week, as against 30 tons for an ordinary chamber kiln. A large saving in labour is also secured. The system presents many advantages, and is likely to replace the older methods of cement making. Fig. 3 represents diagrammatically a rotatory cement plant on the Hurry and Seaman system. Portland cement clinker, however produced, is a hard,

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rock-like substance of semi-vitrified appearance and very dark colour. The product from a well-run rotaCement tory kiln is all evenly burnt and properly vitrified; clinker. that from an ordinary fixed kiln of whatever type is apt to contain a certain amount (5 to 15 per cent.) 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, wrell-picked clinker when ground yields good Portland cement. Nothing is added during or after grinding save a small amount (1 to 2 per cent.) of calcium sulphate in the form either of gypsum or of plaster of Paris, which is sometimes needed to make the cement slower-setting. 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 the variations are tending to become smaller as regularity and skill in Composimanufacture increase. The following analysis tion. may be taken as typical of cements made from chalk and clay on the Thames and Medway : — Per cent. Silica (Si02) . . . . . . .22'0 Insoluble residue 1-0 Alumina (AkOs) 7-5 Ferric oxide (Fe203) 3-5 Lime (CaO) 62-0 Magnesia (MgO) 1-0 Sulphuric anhydride (S03) 1-5 Carbonic anhydride (COo) 0-5 Water (H20) 0-5 Alkalies 0-5 100-0 There may be minor variations from this composition. Thus the silica may range from 20 per cent, to 24 per cent., the alumina and ferric oxide jointly from 9 per cent, to 13 per cent., the lime from 60 per cent, to 65 per cent. Other things being equal, the higher the percentage of lime within the limits indicated above the stronger is the cement, but such highly limed cement is less easy to burn than cement containing about 62 per cent, of lime; and unless the burning is thorough and the raw materials are intimately mixed, the cement is apt to be unsound. Although the ultimate composition of cement, that is, the percentage of each base and acid - present, can be accurately determined by analysis, its proximate composition, i.e., the nature and amount of the compounds formed from these acids and bases, can only be ascertained indirectly and with difficulty. The best and most recent investigation on this subject has been made by M. H. le Chatelier, whose work has since been supplemented by that of Messrs Spenser B. Newberry and W. B. Newberry. According to Le Chatelier, the chief constituent of Portland cement is tri-calcium silicate, 3CaO Si02, a substance which when mixed with water combines with it according to the equation 2 [3CaO Si02] + 9 H.20 = 2[CaO Si02] 5 H20 + 4Ca(OH)2. The products of this reaction, hydrated mono-calcium silicate and calcium hydroxide, crystallize and form a mass of interlocking crystals which constitutes the bulk of