Clinker is the main ingredient used in the manufacture of Portland cement. Clinker is formed in lumps or nodules, usually 3-25mm in diameter, by sintering limestone and alumino-silicate (clay) during the cement kiln stage. Clinker is ground in a mill, usually with gypsum, to produce Portland cement. It can also be ground with slag, pozzolan, and silica to produce special types of cement.

Ordinary Portland Cement Clinker is the core component in producing cement which is formed by sintering limestone. In the manufacturing of Portland cement, clinker formed by sintering or fusing together without melting to the point of liquefaction, alumina-silicate materials such as clay and limestone during the cement kiln stage and happens as 3 millimeters (0.12 in) to 25 millimeters (0.98 in) in diameter,