Clinkers are nodules (diameters, 0. 2-1. 0 inch [525 mm]) of a sintered material that is produced when a raw mixture of predetermined composition is heated to high temperature. The low cost and widespread availability of the limestone, shales, and other naturally occurring materials make portland cement one of the lowest-cost materials widely used over the last century throughout the world. Concrete becomes one of the most versatile construction materials available in the world.
Cement kiln is used for making of cement clinker and there are dry and wet methods to make cement. Metallurgy chemical kiln is used in metallurgy industry and ironworks for lean iron ore, chromium ore and ferronickel ore calcimine. Rotary kiln is used for calcimine of high aluminum vandal ochre in refractory material industry; for calcimine of calotte and aluminum hydroxide in aluminum manufacturer; for claiming of chrome sand ore and chrome powder ore in chemical plant.
Lime kiln is used for baking active lime and dolomite in the steel factory and ferroalloy factory. Portland cement clinker is made by heating, in a kiln, a homogeneous mixture of raw materials to a sintering temperature, which is about 1450 °C for modern cements. The aluminium oxide and iron oxide are present as a flux and contribute little to the strength. For special cements, such as Low Heat (LH) and Sulfate Resistant (SR) types, it is necessary to limit the amount of tricalcium aluminate (3 CaO¢Al2O3) formed.
The major raw material for the clinker-making is usually limestone (CaCO3) mixed with a second material containing clay as source of alumino-silicate. Normally, an impure limestone which contains clay or SiO2 is used. The CaCO3 content of these limestones can be as low as 80%. Second raw materials (materials in the rawmix other than limestone) depend on the purity of the limestone. Some of the second raw materials used are clay, shale, sand, iron ore, bauxite, fly ash and slag.