632
CEMENT
set cement. The other chief constituent of Portland cement is a calcium aluminate. According to Le Chatelier, this body is a tri-calcium aluminate and sets when mixed with water and lime derived from other constituents of the cement, such as unstable ferrites, or from the decomposition of the tri-calcium silicate mentioned above, according to the equation 3CaO A1203 + Ca(OH)2 + 11 H20 - 4CaO A1203 12 H,0. According to Messrs Newberry, however, the aluminate is di-calcium aluminate, 2 Ca0Al203 ; the mode of hydration and setting of this body has not been studied. Although the precise composition of the aluminate is still a matter for discussion, it is generally admitted that it is the chief agent in the first setting of the cement. The ultimate hardening and the attainment of strength are due to the tri-calcium silicate. The function of the ferric oxide present in ordinary cement is little more than that of a flux to aid the union of the silica, alumina, and lime in the clinker; its r61e in the setting of the cement is altogether secondary. Magnesia is present in most Portland cements in quantities varying from 1 to 5 per cent. Within this range it appears to be inert, forming no cementitious compound. There is evidence that in larger proportion, e.y., 10 to 15 per cent., it may hydrate and set after the general setting of the cement, and may give rise to disruptive strains and cause the cement to “ blow ” and fail. In so-called natural cement (p. 633) which is comparatively lightly burnt, the magnesia appears to be inert, and as much as 20 to 30 per cent, may be present. Another constituent of Portland cement which influences its setting time is calcium sulphate, naturally formed from the sulphur in the raw materials or fuel, or intentionally added to the finished cement as gypsum or plaster of Paris. It has a remarkable retarding effect on the hydration of the calcium aluminate, and consequently on the setting of the cement; thus it is that a little gypsum is often added to convert a naturally quick-setting cement into one which sets slowly. It will be observed that in the hydration of tri-calcium silicate, the main constituent of Portland cement, a large portion of the lime appears as calcium hydroxide, ie., slaked lime. It is evident that this will form a puzzuolanic cement (p. 628) if a suitable silicious material such as trass is added to the cement. The ultimate product when set may be regarded as a mixed Portland and puzzuolanic cement. The use of trass in this manner as an adjunct to Portland cement has been advocated by Michaelis, but has not become general. The quality of Portland cement is ascertained by its analysis and by determining its specific gravity, fineness, mechanical strength, and soundness. A good Testing. sampie wiii have a composition within the limits cited above and approximating to the typical figures given on p. 631. It will be ground so finely that not more than 10 per cent, will be left on a sieve of 76 x 76 meshes per square inch, the wires of the sieve being 0‘005 inch in diameter. It will have, when freshly burned, a specific gravity not lower than 3T5, and briquettes made from it and kept in water will possess a tensile strength of 400500 K) per square inch seven days after they are made, while briquettes made from a mixture of 3 parts by weight of sand and 1 of cement will give about 200 lb per square inch at twenty-eight days. The soundness of the cement is ascertained by keeping thin pats of cement in water for seven and twenty-eight days at the ordinary temperature. The test is accelerated so that a valid opinion may be pronounced in a short time by keeping the water hot, e.cf., at 110 Pahr. or 176° Fahr. In this case an unsound cement may generally be detected in twenty-four hours. A pat of sound cement under these conditions remains fiee from cracks
and does not buckle. A cement slightly unsound will show numerous small cracks, while one which is markedly unsound may even go to pieces. A rigorous and exact test for soundness is that formulated by Deval, who has shown that briquettes of 3 of sand and 1 of cement kept in water for two days at 80° C. = 176° Fahr. attain approximately the same strength as similar briquettes attain at seven days in water at the ordinary temperature. In like manner briquettes kept at 176° Fahr. for seven days are equal in strength to those kept at the ordinary temperature for twenty-eight days. A cement not perfectly sound will give low results in the hot test, and a cement of indifferent soundness will crack and go to pieces. The test is admittedly severe, but can be passed without difficulty by cement made with proper care and skill. There are many modifications and elaborations of all the tests which have been mentioned. Cement for all important work is submitted to a rigorous system of testing and analysis before it is accepted and used. Hydraulic Lime is a cement of the Portland as distinct from the puzzuolanic class. The most typical hydraulic lime is that known as Chaux du Theil, made from a limestone found at Ardeche in France. This limestone consists of calcium carbonate most intimately intermixed with very finely divided silica. It contains but little alumina and oxide of iron, which are the constituents generally necessary to bring about the union of silica and lime to form a cement, but in spite of this the silica is so finely divided and so well distributed that it unites readily with the lime when the limestone is burned at a sufficiently high temperature. English hydraulic limes are of a different class. They contain a good deal of alumina and ferric oxide, and in composition resemble somewhat irregular Portland cement. Analyses of the two classes of hydraulic lime are as follows :— Chaux de Theil. Blue Lias. Per cent. Per cent. 2-39 0-3 Insoluble silicious matter 14-17 217 Silica (Si02) 6-79 1-8 Alumina (A1203) 20-6 Ferric oxide (Fe^Og) 63-43 74-0 Lime (CaO) 1-54 0-7 Magnesia (MgO) 0-3 1Sulphuric anhydride (S03) 3Carbonic anhydride (SO.,) 0'6 2Water (H20) . 1-38 Alkalies and loss 100-0
100-0
Hydraulic lime contains a good deal of uncombined lime, and has to be slaked before it is used as a cement. In France this slaking is conducted systematically by the makers, the freshly burned lime being sprinkled with water and stored in large bins where slaking proceeds slowly and regularly until the whole of the surplus uncombined lime is slaked and rendered harmless, while the cementitious compounds, notably tri-calcium silicate, remain untouched. In English practice hydraulic lime is slaked by the user. Seeing that regular and perfect slaking is more easily attained when working systematically on a large scale and by storing the material for a long period, the French method is the better and more rational. The product may then be regarded as a cement of the Portland class mixed with slaked lime. When gauged with water and made into a mortar it sets slowly, but ultimately becomes almost as strong as Portland cement. Its slow setting is an advantage for some purposes, e.g., for foundations and abutments where settlements may occur. The structure is free to take its permanent position before the lime sets, and cracks are thus avoided. A case in point is the employment of hydraulic lime in place of Portland cement as grouting outside the cast-iron tubes
