CEMENT

A material used for cor bining particles in one ma . Leavil on one side the various prep a ratio! used for joining gla or porcelain, ar for other minor operations, build i cements may be divided into thr ment and plaster of Paris cement The two most important natural cemen are Roman and the so-called natu Portland. They are prepared by b ing limestone or chalky matter con ing a proportion of clay, after wh the fused product is ground. They very rapidly, but lack the strength Portland cement. They are chiefly ised for repair work in places where strength is of minor importance. _ Portland cement was invented by Joseph Aspden, of Leeds, England, in 1824. It sonsists of a mixture of calcium and aluminum silicates and the setting roce consists of the combination of hese silicates with water, with the formation of hydrated silicates. It is manufactured by mixing together, in proper proportions, either wet or dry, chalk and slay, or similar materials. ‘The mixture is then calcined in a rotary kiln, consisting of a long cast-iron tube lined with fre-brick and slightly inclined to the horizontal. The temperature is sufficiently high to cause the mixture to fuse, a clinker being produced, which must then be ground to a fine powder. In place of the chalk, limestone or marl are sometimes used, while shale or slate may replace the clay. Plaster cements, the principal constituent of which is plaster of Paris (partially dehydrated sulphate of lime) are chiefly used for internal work. The best known is Keene’s cement, which is manufactured by soaking plaster of Paris in a solution of alum and then re-calcining. The setting of these cements is also a proce of hydration, the partially calcined gypsum’ eagerly absorbing water with the re-formation of hydrated sulphate of calcium in the form of small hard crystals