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Concrete is the name given to a mixture of particles of sand and gravel, theaggregate, bound together with cement, the matrix. Fine aggregate is naturalsand which has been washed and sieved to remove particles larger than5 mm, and coarse aggregate is gravel that has been crushed, washed andsieved so that the particles vary from 5mm up to 50mm in size. The fine andcoarse aggregate are delivered separately. By combining them in the correctproportions, a concrete with very few voids in it can be made.The cement most used is ordinary Portland cement. It is manufactured byheating a mixture of finely powdered clay and limestone with water to atemperature of about 1200ºC, at which the lime and clay fuse to form a clinker.This clinker is ground with the addition of a little gypsum to a fine powder ofcement. Cement powder reacts with water and its composition graduallychanges; the particles of cement bind together and adhere strongly to materialswith which they are mixed. Cement hardens gradually after it is mixed withwater. Some thirty minutes to an hour after mixing with water the cement isno longer plastic and it increasingly hardens to a dense solid mass after7 days.The materials used in reinforced concrete are weighed and mixed in largeconcrete mixers. It is not economical for builders to employ expensive mixingmachinery. Concrete for foundations and floors is usually delivered to siteready mixed. Ready-mixed concrete is carefully prepared and delivered tosite by lorries