CEMENT

binding material used in construction and engineering, often called hydraulic cement, typically made by heating a mixture of limestone and clay until it almost fuses and then grinding it to a fine powder. When mixed with water, the silicates and aluminates in the cement undergo a chemical reaction; the resulting hardened mass is then impervious to water. It may also be mixed with water and aggregates (crushed stone, sand, and gravel) to form concrete.

A cement made by grinding together lime and a volcanic product found at Pozzuoli on the Bay of Naples (hence called pozzuolana) was used in ancient Roman construction works, notably the Pantheon. During the Middle Ages the secret of cement was lost. In the 18th cent. John Smeaton, an English engineer, rediscovered the correct proportions when he made up a batch of cement using clayey limestone while rebuilding the Eddystone lighthouse off the coast of Cornwall, England. In the United States, production of cement at first relied on processing cement rock from various deposits, such as those found in Rosendale, N.Y. In 1824, Joseph Aspdin, an English bricklayer, patented a process for making what he called portland cement, with properties superior to its predecessors; this is the cement used in most modern construction.

Modern portland cement is made by mixing substances containing lime, silica, alumina, and iron oxide and then heating the mixture until it almost fuses. During the heating process dicalcium and tricalcium silicate, tricalcium aluminate, and a solid solution containing iron are formed. Gypsum is later added to these products during a grinding process. Natural cement, although slower-setting and weaker than portland cement, is still employed to some extent and is occasionally blended with portland cement. Cement with a high aluminate content is used for fireproofing, because it is quick-setting and resistant to high temperatures; cement with a high sulfate content is used in complex castings, because it expands upon hardening, filling small spaces.

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