In the manufacture of Portland cement, clinker occurs as lumps or nodules, usually 3 millimeters (0.12 in) to 25 millimeters (0.98 in) in diameter, produced by sintering (fusing together without melting to the point of liquefaction) limestone and alum inosilicate materials such as clay during the cement kiln stage. Clinker consists of various calcium silicates, including alite and belite. Tricalcium aluminate and calcium aluminoferrite are other common components. These components are often generated in situ by heating various clays and limestone.
Portland cement clinker is made by heating a homogeneous mixture of raw materials in a rotary kiln at high temperature. The products of the chemical reaction aggregate together at their sintering temperature, about 1,450 °C (2,640 °F). Portland cement clinker is ground to a fine powder and used as the binder in many cement products. A little gypsum is sometimes added. It may also be combined with other active ingredients or chemical admixtures to produce other types of cement.