| Portland cements are made by grinding a mixture of limestone, clay and other corrective materials, viz. Laterite, Bauxite,etc. Essential constituents mainly are Lime, Silica, Alumina and Iron Oxide. The process of manufacturing consists of grinding of raw materials into fine powder, mixing them intimately and burning in a kiln at about 1400 deg. C. The resultant product is called Clinker. Clinker is cooled, ground to fine powder with gypsum. The end product is cement.