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In the broadest sense, the term cement refers to materials which act as adhesives. However, in this context, its use is restricted tothat of a binding agent for sand, stone and other aggregates within the manufacture of mortar and concrete. Hydraulic cements and limes set and harden by internal chemical reactions when mixed with water. Non-hydraulic materials will harden slowly by absorption of carbon dioxide from the air.Lime was used as a binding agent for brick and stone by the ancient civilisations throughout the world. The concept was brought to Britain in the first century AD by the Romans, who used the material to produce lime mortar. Outside Britain, the Romans frequently mixed lime with volcanic ashes, such as pozzolana from Pozzuoli in Italy, to convert non-hydraulic lime into hydraulic cement suitable for use in constructing aqueducts, baths and other buildings. However, in Britain, lime was usually mixed with artificial pozzolanas, for example crushed burnt clay products, such as pottery, brick and tile.Lime is manufactured by calcining natural calcium carbonate, typically hard-rock carboniferous limestone. The mineral is quarried, crushed, ground, washed and screened to the required size range. The limestone is burnt at approximately 950 °C in either horizontal rotary kilns or vertical shaft kilns which drive off the carbon dioxide to produce the lime products.Portland cement is manufactured from calcium carbonate in the form of crushed limestone or chalk and an argillaceous material such as clay, marl or shale. Minor constituents such as iron oxide or sand may be added depending on the composition of the raw materials and the exact product required. In principle, the process involves the decarbonisation of calcium carbonate (chalk or limestone) by expulsion of the carbon dioxide, and sintering, at the point of incipient fusion, the resulting calcium oxide (lime) with the clay and iron oxide. Depending on the raw materials used and their water content at extraction, four key variations in the manufacturing process have been developed: the wet, semi-wet, semi-dry and the dry processes.Concrete is a mixture of cement, aggregates and water together with any other admixtures which may be added to modify the placing and curing processes or the ultimate physical properties. Initially, when mixed concrete is a plastic material, which takes the shape of the mould or formwork. When hardened it may be a dense, load-bearing material or a lightweight, thermally insulating material, depending largely on the aggregates used. It may be reinforced or prestressed by the incorporation of steel.Most concrete is crushed and recycled at the end of its useful life, frequently as hard core for new construction work. However, a growth in the use of recycled aggregates for new concrete can be anticipated, as this will have a significant environmental gain in reducing the demand on new aggregate extraction.Aggregates for concrete are normally classified as lightweight, dense or high-density. Standard dense aggregates are classified by size as fine (i.e. sand) or coarse (i.e. gravel). Additionally, steel or polypropylene fibres or gas bubbles may be incorporated into the mix for specialist purposes.Natural stone aggregate concretes typically have densities within the range 2200–2500 kg/m3, but where densities below 2000 kg/m3 are required, then an appropriate lightweight concrete must be used