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concrete (i)it is difficult to imagine the modern structure and the concrete. concrete is a building material which led to great structural innovations. the most important quality of concrete and its property to be formed into large and strong monolithic units. the basic materials for making concrete, cement, aggregate and water. cement is the most essential material and the most important one for making concrete of high quality. cement is made of limestone and clay. it is burnt at high temperature and ground up into powder. depending on the kind and composition of the raw materials different types of cement are obtained. portland cement, blast furnace cement are suitable for putting up marine structures.concrete is made by mixing cement, water, sand and gravel in the right amount. as soon as it is thoroughly and it is poured into forms that hold it in place until it hardens. the forming crystals in the process of making concrete stick together in a very hard artificial stone. cement starts hardening one hour after the water has been added and the process of hardening lasts for about 28 days. the process is called concrete curing.the characteristics of concrete depend on the quality of the materials used, grading of the aggregates, proportioning and amount of water. the most important requirements for the concrete are: it should be hard, strong, durable, fire resistant and economical. concrete can be divided into two classes: those of plain concrete and reinforced concrete where it is necessary to produce steel. plain or mass concrete can be used for almost all building purposes. ferro - concrete is used in building bridges and arcs, dams and dock walls, for structures under water, for foundations, columns, beams girders,. the use of concrete and ferro - concrete is almost universal.Builders now produce two types of new building materials, alkali - slag concrete and silica concrete. in alkali - slag concrete cement is replaced by a truly of granulated blast - furnace slag and sodium and potassium compounds. the fillers can be sand or sandy loams containing various amounts of clay which usually cannot be used with conventional cement. the new material has been tested successfully and is now being used for irrigation systems, roads, pavements and other structures. Silica concrete is widely used in aviation and in underwater work.
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