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.
Under dry conditions, clinker can be stored for several months without significant loss of quality. As a result, and considering its relative ease of handling, clinker is traded internationally in large quantities using mainly bulk carrier vessels.