Ordinary Portland Cement (OPC) is one of several types of cement being manufactured throughout the world. OPC is the general purpose cement used in concrete constructions. OPC is a compound of lime, silica, alumina, iron and sulphur trioxide. Magnesium (MgO) is present in small quantities as an impurity associated with limestone. SO3 is added at the grinding stage to retard the setting time of the finished cement. When cement raw materials containing the proper proportions of the essential oxides are ground to a suitable fineness and then burnt to incipient fusion in a kiln, chemical combination takes place, largely in the solid state resulting in a product aptly named clinker. This clinker, when ground to a suitable fineness, together with a small quantity of gypsum (SO3) is Portland Cement.