The production of Portland cement involves the firing (burning) of calcareous material (normally limestone, sea shells etc.) with an argilla­ ceous material (clay, an aluminosilicate). The solid raw materials are crushed and mixed in ball mills, and then heated in a kiln to about 1500 °C. The firing results in the formation of a clinker which consists of a number of compounds which set or harden when the clinker is ground to a fine powder (cement) and then mixed with water.