CEMENT

in general, adhesive substances of all kinds, but, in a narrower sense, the binding materials used in building and civil engineering construction. Cements of this kind are finely ground powders that, when mixed with water, set to a hard mass. Setting and hardening result from hydration, which is a chemical combination of the cement compounds with water that yields submicroscopic crystals or a gel-like material with a high surface area. Because of their hydrating properties, constructional cements, which will even set and harden under water, are often called hydraulic cements. The most important of these is portland cement. This article surveys the historical development of cement, its manufacture from raw materials, its composition and properties, and the testing of those properties. The focus is on portland cement, but attention also is given to other types, such as slag-containing cement and high-alumina cement. Outline of Coverage Constructional cements share certain chemical constituents and processing techniques with industrial ceramic products such as brick and tile, abrasives, and refractories. For a directory to articles on these subjects, see Industrial Ceramics: Outline of Coverage. For detailed description of one of the principal applications of cement, see the article building construction. in building and engineering construction, finely ground powder that, when mixed with water, sets and hardens into a solid mass. Other types of cements are used as binding materials generally classed as adhesives. The ancient Romans made cement from lime and volcanic ash and used it extensively in building throughout their empire. In 1824 the British engineer Joseph Aspdin patented portland cement, naming it portland because, in colour, it resembled the limestone quarried on the Isle of Portland. This type proved stronger and more water-resistant for longer periods of time than previous forms and soon became the most widely used cement. Limestone and clay are the two most common raw materials from which portland cement is made. These contain the three basic ingredients of cement: lime (found in limestone) and silica and alumina (found in clay). In the manufacture of cement, the ingredients are crushed and ground to a fine powder, blended to the desired proportions, and burned in large rotating kilns. This last process forms fused lumps, called clinkers, which are then ground and mixed with gypsum to retard the setting of the cement. Special ingredients may be added to produce cement with special propertiesfor example, oil-well cement (designed to resist the high temperatures and pressures found in deep wells) and waterproof cements. The hardening of portland cement when it is mixed with water largely results from the hydration of the calcium silicates in it to form a colloidal gel, which eventually hardens to a solid mass. Other types of cement include slag cement, made with slag by-products of blast furnaces producing pig iron, and high alumina cement, made from a mixture of limestone and bauxite. These are often used for their ability to resist attack by chemicals such as the sulfates present in air pollution. Cements made from gypsum are used in plaster and plasterboard. Other types of cements have incorporated plastics and other natural and synthetic materials. Cement is used by itself for only a small number of purposes, such as for grout. It generally is mixed with a filler material, called an aggregate. Mixed with sand, cement forms mortar, used in masonry construction, and, with gravel or crushed stone, it forms concrete. Concrete, which is sturdy, flexible, and inexpensive, is the world's most widely used building material. For comparative statistical data on cement production, see mining (table). Additional reading A.C. Davis, A Hundred Years of Portland Cement, 18241924 (1924); and Robert W. Lesley, History of the Portland Cement Industry in the United States (1924, reissued 1974), give historical accounts. More recent works are H.F.W. Taylor, Cement Chemistry (1990); Sidney Mindess and J. Francis Young, Concrete (1981); and Sandor Popovics, Fundamentals of Portland Cement ConcreteA Quantitative Approach (1982 ), and Concrete Materials: Properties, Specifications, and Testing, 2nd ed. (1992). Sir Frederick M. Lea Thomas O. Mason