Modern Portland cement is termed 'Ordinary Portland Cement' (OPC) and is made by combining limestone or chalk (CaCO3) with sand (SiO2), clay and other materials (eg: Al2O3, Fe2O3, TiO2) in a kiln at temperatures in the region of 1400-1500??C.15 The mixture first undergoes decarbonation in which CO2 is lost from the limestone to produce calcium oxide (CaO).