Composition
Clinker is produced[1] by heating the "rawmix" in oxidising conditions to around 1400-1450°C, at which temperature partial melting (sintering) takes place, producing hard spherical nodules around 5-20 mm in diameter. These are rapidly cooled (quenched) to preserve their reactive mineral constituents. The clinker is primarily composed of tricalcium silicate (alite), dicalcium silicate (belite), tricalcium aluminate (3CaO.Al2O3), and calcium aluminoferrite (2CaO.(Al,Fe)2O3). As defined by the European Standard EN197.1, Portland cement clinker is a hydraulic material which shall consist of at least two-thirds by mass of calcium silicates (3CaO.SiO2 and 2CaO.SiO2), the remainder consisting of aluminium- and iron-containing clinker phases and other compounds. The major raw material for making clinker is usually limestone(CaCO3).

Uses
Clinker is ground (usually with the addition of a little calcium sulfate) to become Portland cement. It may also to be interground with other active ingredients to produce:
blastfurnace slag cement
pozzolanic cement
silica fume cement

Clinker, if stored in dry conditions, can be kept for several months without appreciable loss of quality. Because of this, and because it can easily handled by ordinary mineral handling equipment, clinker is traded internationally in large quantities. The freight cost is much lower than that for the equivalent amount of finished cement. Cement manufacturers purchasing clinker grind it as an addition to their own clinker at their cement plants. Manufacturers also ship clinker to grinding plants in areas where cement-making raw materials are not available.