Cement is a mixture of compounds, consisting mainly of silicates and aluminates of calcium, formed out of calcium oxide, silica, aluminium oxide and iron oxide.
Cement is manufactured by burning a mixture of limestone and clay at high temperatures in a kiln, and then finely grinding the resulting clinker along with gypsum. The end product thus obtained is called Ordinary Portland Cement (OPC).
In India, OPC is manufactured in three grades, viz. 33 grade, 43 grade and 53 grade, the numbers indicating the compressive strength obtained after 28 days, when tested as per the stipulated procedure.
Apart from OPC, there are several other types of cement, most of them meant for special purposes, e.g. sulphate resistant cement, coloured cement, oil well cement, etc. However, there are some general purpose cements, the commonest one being Portland Pozzolana Cement (PPC).