How Is Cement Manufactured?


Cement is the most important building construction material. It is fine grey powder which is made from limestone and clay. It has the property of setting into hard mass after mixing with water in required amount. The major cement producing countries in the world are China, Japan, United States of America and India. The main cement producing plants in our country are in Gujarat, Rajasthan, Orissa, Madhya Pradesh, and Tamil Nadu. The production of cement in our country started in 1904 at Chennai in Tamil Nadu.

Manufacture of Cement

The two important raw materials used for the preparation of cement are:

1. Limestone

The chemical formula of limestone is CaCO3 and it is used in the manufacture of cement just to provide CaO.

2. Clay

Clay is hydrated aluminium silicate and iron (iii) oxide having chemical formula Al2O3SiO2Fe2O3.2H2O. Thus clay consists of aluminium oxide (Al2O3), silicon dioxide (SiO2), and iron (iii) oxide (Fe2O3) along with water molecules.

In addition of limestone and clay one more substance which is also used during preparation of cement is gypsum which is added into the cement to slow down the initial setting (hardening) of cement when water is added into it.

Chemical Composition of Cement

The %age composition of different compounds present in cement is: 

1) Calcium oxide (CaO) 60-70 %
2) Silicon dioxide (SiO2) 20-25%
3) Aluminium oxide (Al2O3) 5-10%
4) Iron oxide (Fe2O3) 2-3%

Manufacture of Cement

To manufacture cement first of all limestone is added into the clay in proportion of 3:1 by weight. This mixture of limestone and clay is then dried and crushed to get a fine powder called 'raw meal'. The raw meal is then added into a rotary kiln through a hopper . The rotary kiln is actually a long kiln which is capable to rotate on its axis. It is kept slightly inclined downward so that when it rotates, the raw meal present in it slowly moves downward to the other end of the rotary kiln.
Then a very high temperature of about 1773 K is produced in the rotary kiln by passing hot gases in the rotary kiln from the bottom. At this high temperature limestone reacts with clay to form small pieces of cement called ‘clinkers’. 
The clinkers are then cooled down and a small amount of gypsum is added into it. This mixture of clinkers and gypsum is then ground into a fine powder which is called cement. The whole process of preparation of cement can be represented with the help of a simple equation as given below:

Limestone + Clay + Gypsum Cement