The ingredients used for making cement are limestone, that is calcium carbonate (CaCO3) and clay, which consists of silica (SiO2), alumina (Al2O3) and ferric oxide (Fe2O3).
Manufacture of cement
The following steps are involved in the manufacture of cement.
1. Limestone and clay are crushed to a fine powder.
2. They are blended in the desired proportion.
3. The mixture is poured into the upper end of a slow-turning cement kiln and heated to about 15000C.
4. The following reactions take place inside the kiln.
Calcium carbonate decomposes to form calcium oxide which combines with alumina and silica.
Note that chemically, cement is a mixture of calcium aluminate and calcium silicate (formed on adding water).
5. The mixture (cement) solidifies into limps about the size of a pea, called clinkers.
6. Clinkers are then mixed with gypsum (it delays the setting of cement).
7. These are then ground into a fine powder.