Portland cement is a fine powder, gray or white in color that consists of a combination of hydraulic cement materials comprising primarily calcium silicates, aluminates and alumino ferrites. More than 30 raw materials are known to be used in the manufacture of portland cement, and these materials can be divided into four distinct categories: calcareous, siliceous, argillaceous, and ferrifrous. These materials are chemically combined through pyro processing and subjected to subsequent mechanical processing operations to form gray and white portland cement. Gray Portland cement is used for structural applications and is the more common type of cement produced.

Wet Process- In India most of the cement factories used the wet process. In the wet process, the limestone brought from the quarries is first compressed to smaller particles. Then it is taken to a ball or tube mill where it is mixed with clay as the case may be and ground to a fine consistency of slurry with the addition of water. The slurry is a liquid of creamy consistency with water content of about 35 to 50 per cent, wherein particles, crushed to the fineness of Indian Standard Sieve number 9, are held in suspension. The slurry is pumped to slurry tanks or basins where it is kept in an agitated condition by means of rotating arms with chains or blowing compressed air from the bottom to prevent settling of limestone and clay particles. The mixture of the slurry is experienced to give the required chemical composition and corrected periodically in the tube mill and also in the slurry tank by blending slurry from different storage tanks. Finally, the corrected slurry is stored in the final storage tanks and kept in a homogeneous condition by the agitation of slurry. The corrected slurry is sprayed on to the upper end of a rotary kiln against hot heavy hanging chains. The rotary kiln is an important component of a cement factory. It is a thick steel cylinder of diameter anything from 3 meters to 8 meters, lined with refractory materials, mounted on roller bearings and capable of rotating about its own axis at a specified speed. The length of the rotary kiln may vary anything from 30 meters to 200 meters. The slurry on being sprayed against a hot surface of flexible chain loses moisture and becomes flakes. These flakes peel off and fall on the floor. The rotation of the rotary kiln causes the flakes to move from the upper end towards the lower end of the kiln subjecting itself to higher and higher temperature. The kiln is fired from the lower end.

The fuel is powered coal, oil or natural gas. By the time the material rolls down to the lower end of the rotary kiln, the dry material undergoes a series of chemical reactions until finally, in the hottest part of the kiln, where the temperature is in the order of 1500°C, about 20 to 30 per cent of the materials get fused. Lime, silica and alumina get recombined. The fused mass turns into nodular form of size 3 mm to 20 mm known as clinker. The clinker drops into a rotary cooler where it is cooled under controlled conditions. The clinker is stored in silos or bins. The clinker weighs about 1100 to 1300 gms per litre. The litre weight of clinker indicates the quality of clinker. The cooled clinker is then ground in a ball mill with the addition of 3 to 5 per cent of gypsum in order to prevent flash-setting of the cement. A ball mill consists of several compartments charged with progressively smaller hardened steel balls. The particles crushed to the required fineness are separated by currents of air and taken to storage silos from where the cement is bagged or filled into barrels for bulk supply to dams or other large work sites.

Chemical reactions involved are:-

$\begin{align} CaCO_3 &= CaO + CO_2\\ 2CaO + SiO_2 &= 2CaO.SiO_2\\ 3CaO + SiO_2 &= 3CaOSiO_2\\ 3CaO + Al_2 O_3 &= 3CaO.Al_2 O_3\\ 4CaO + Fe_2 O_3 + Al_2 O_3 &= 4CaO.Fe_2 O_3.Al_2 O_3 \end{align} $