Cement is either a natural or artificial mixture of limestone, clay, sand, and iron oxide. Limestone is an impure form of calcium carbonate mixed with silica (sand) and clay. When the limestone contains about 10% silica and clay it has the desired proportion to form a good mixture that hardens under water, as well as when exposed to the air. It is then good material for making cement and is called hydraulic lime. Portland cement is made by heating the powdered combination into a clinkered mass and then grinding it. A mixture of cement, sand, water, and crushed stone is called concrete.