Cements

Cements, a class of substances used to rigidly connect two bodies by forming a firm layer between, which strongly adheres to both. They may be divided into two classes: - (1) Building cements and (2) adhesive materials. The second class of cements, adhesive materials, are very numerous, and different in their characters. They may be divided into (a) resinous cements, which soften by heat and harden on cooling, as sealing-wax, marine glue, etc.; (b) solutions of substances which harden on exposure, owing to the evaporation of the solvent, e.g. Canada balsam, caoutchouc dissolved in chloroform, etc.; (c) mixtures containing oils, which harden owing to oxidation, putty, white lead; (d) substances which soften by the action of water and harden owing to the absorption of the water by the cemented materials, e.g. paste, glue, liquid glues, etc.

Until the introduction of Roman cement, only well-burnt stone lime was employed. Thus the mortar in ordinary use is a paste formed by mixing water with one part of slaked lime and three or four parts of sand, the most suitable proportions of these constituents varying with their quality. The purer limes are not so efficient. The hardening is perhaps due to the evaporation or other loss of the water, and to the formation of calcium carbonate by combination of the lime with the carbonic oxide in the air.

Roman Cement is formed by calcining certain mixtures of lime and clay, which occur in the newer geological formations; the burnt material is ground and sifted, and used with about an equal weight of sand. It is generally quick-setting, and is therefore useful in tide-works or other cases where rapid hardening is necessary. Though not very strong, it may form a good hydraulic cement, that is, one which hardens under water, when clay makes up half its weight. All hydraulic cements require at least 10 per cent. of silica.

Portland Cement, named from its resemblance to Portland stone, is a most important manufacture. It is used in many directions, especially with sand or stones, in the construction of docks and harbours. Many failures have occurred in its use, but time is required to show whether these are due to bad manufacture or to unavoidable insufficiency of the Portland. It may be made by wet or dry processes. The former is conducted on the banks of the Thames or Medway in places where chalk and river clay are available. Water is mixed with three parts of chalk and one of clay. This is reduced to a thick, creamy mud by revolving cutters in a wash-mill. It is then allowed to settle, and the superfluous water removed. Then it is dried by heating, calcined, and ground to powder. By the dry process the cement is made from hard limestone and clay. These, in proper proportions, are crushed, burnt, and ground to powder. This is moistened with water, bricks are formed, and a further course of burning and grinding gone through before the cement is available for use. Portland is mixed with a variable quantity of sand, but is stronger without. It forms a good hydraulic cement, but sets slowly. Roughly speaking, it is five times as strong as good Roman cement, both in tension and compression.

Plaster of Paris, i.e. gypsum or calcium sulphate, is a convenient cement for forming casts, cementing marble, or fixing metal on glass.

Rust Cement is formed with iron parings and sal-ammoniac. It is useful for joints in ironwork.

Sulphur Cement is made of sulphur, resin, and brick-dust; it is best adapted for earthenware joints.

Sodium-silicate and fireclay or asbestos powder form a heat-proof cement. For other cements see White and Red Lead, Shell-lac, Glue, and Isinglass.