CEMENTS. A cement is a substance used to make the surfaces of solid bodies adhere to one another; it is applied in a liquid or viscous state, and hardens after the surfaces are brought together. When fused metals or alloys are used in this manner, they are called solders. There is a great variety of C. derived from animal, vegetable, and mineral substances. The animal C. are chiefly composed of gelatine and albumen as their bases. Joiners' glue is an example. See GLUE. The binding materials of vege table C. are gums, resins, and wax. The mineral C. are chiefly of lime and its com pounds. In many C., animal, vegetable, and mineral substances are combined. The simplest of the mineral C. is plaster of Paris, which is used for uniting slabs of marble, alabaster, and many similar purposes. It is mixed with water to the consistence of thick cream, and then applied. This hardens rapidly, but is not very strong. Its hardening depends upon the true chemical combination of the water with anhydrous sulphate of lime, of which plaster of Paris is composed, and the formation thereby of a solid hydrate. The plaster of Paris may be mixed with thin glue. with diluted white of egg, or a solu tion of size or gum, instead of water, and is strengthened thereby.
Keene's marble cement is prepared by steeping plaster of Paris in a concentrated solution of alum, then recalcining and powdering. This powder is mixed with water in the same manner as plaster of Paris. It is used as a stucco for internal decorations, takes a high polish, and when colored, forms beautiful imitations of mosaic, marbles, seagliola, etc.
A mixtnre of paper pulp, size, and plaster of Paris in equal proportions, forms a useful cement, and is also used as a sort of papier-milche for casting into architectural ornaments, etc.
Common mortar is one of the most important of the lime cements. It is composed of slaked lime, or a mixture of this with sand; its hardening depends upon the slow formation of carbonate of lime by the absorption of carbonic acid from the atmosphere, and a partial combination with the silica of the sand. Cow-hair is sometimes mixed with it. to bind It when laid in masses. In order to obtain a fine smooth paste, which is required for good mortar, the lime should be slaked rapidly by adding about three parts of water to one of lime; if the quantity of water is too small, a coarser or semi crystalline hydrate of lime is produced by the slaking. For the mode of applying
mortar, see BRICKWORK.
Ordinary mortar, when exposed to the continuous action of water, softens and dis integrates, and some of the lime dissolves away. Lime which contains 20 or 30 per cent of clay. or finely divided silica, produces a mortar which is not liable to this softening, but possesses the property of hardening under water; such lime is called hydraulic, and the mortar made from it, hydraulic cement or mortar.
Puzzolana, a porous lava found at Puzzuoli, near Naples, has been long celebrated for its property of forming a hydraulic cement, when mixed with ordinary lime. It is mainly composed of silicates of alumina, lime, and soda. Portland cement, so named from its resemblance to Portland stone when dry,'is made from clay found in the valley of the Medway, which is intimately mixed with the neighboring chalk, and then burned. Roman cement is similar to the Portland, but of a darker color; it contains a larger pro portion of clay, and solidifies more rapidly.. These C. should be mixed with a sufficient quantity of water to form a moderately thick paste; the surfaces to which they are applied should quantity well wetted, and the cement kept slightly moist until it hardens. The solidifica tion of hydraulic C. depends upon the combination of the lime with the silica and alu mina forming, first a hydrated compound, and finally a true silicate. They expand slightly in solidifying.
The following receipts include some of the most useful and reliable C. applicable to the purposes specified: for water-tight joints, such as slate cisterns, aquaria, etc., and for uniting broken pieces of stone, and filling up metallic joints—take equal parts of red and white lead, and work them into a stiff paste with boiled linseed oil. When used for metal joints, it should rather thin, and both pieces of metal, as well as the washer, well smeared with it. This cement herdeus slowly, but becomes ultimately of almost flinty hardness. We have before us air aquarium, bolding fifteen gallons of water, made of plate-glass, cemented at the angles to mahogany columns with this composition. It has stood without leaking for above three years, in spite of much rough handling and moving about; and the cement is now so hard, that it is difficult to scratch it with a knife.