Manufacture of cement

Cement is the most important building construction material. It has the property of setting into hard mass after mixing with water in required amount. It is also known as Portland cement because after setting into hard mass it resembles with the rocks found in Portland in United Kingdom. It is fine grey powder which is made from limestone and clay.

The chemical formula of cement is CaO.Al₂O₃.Fe₂O₃

Raw Material Required for Manufacturing 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:

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 as shown in figure. 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

Chemical Reactions Involved in the Manufacture of Cement

Following chemical reactions take place during the manufacture of cement:

1. First of all limestone decomposes into lime after getting heated to a high temperature.

2. The lime produced in first reaction reacts with silicon dioxide to produce dicalcium silicate.

3. Lime also reacts with silicon dioxide to produce one more compound tricalcium silicate.

4. Lime also reacts with aluminum oxide to form tricalcium aluminate.

5. In the last step calcium oxide, aluminium oxide and ferric oxide react together to form cement.

Purpose of Adding Gypsum to Cement

As we have already studied that the main purpose of adding gypsum in the cement is to slow down its initial setting when water is added into it. The slow setting of cement helps us to work conveniently and it also leads to more hardening of cement.

Test your understanding and answer these questions:

1. Explain the process of manufacture of cement.