Raw materials: The raw materials required for the manufacture of cement are lime stone, stone and clay. Lime stone in calcium carbonate, (CaCO3) and it provides calcium oxide. (CaO) Clay is hydrated aluminium silicate, (Al2O3.2SiO2.2H2O) and it provides alumina as well as silica. A small amount of gypsum, CaSO4.2H2O is also required. It is added in calculated quantity in order to adjust the rate of setting of cement.
Manufacture: Cement is made by strongly heating a mixture of lime stone and clay in a rotatory kiln. Lime stone and clay are finely powdered and a little water is added to get a thick paste called slurry. The slurry is fed into a rotatory kiln from the top through the hopper.
The hot gases produce a temperature of about 1770-1870 K in the kiln. At this high temperature at the lime stone and clay present in slimy combine to form cement in the form of small pieces called clinker. This clinker is mixed with 2-3% by weight of gypsum (CaSO4.2 H2O) to regulate the setting time and is then ground to an exceedingly fine powder.
When mixed with water the cement reacts to form gelatinous mass which sets to a hard mass when three .dimensional cross lines are formed between silica oxygen silica and silica oxygen aluminium as.
…….. Si – O – Si …….. and Si – O – Al ……….. chains
Composition of cement:
CaO = 50 - 60%
SiO2 = 20 - 25%
Al2O3 = 5 - 10%
MgO = 2 - 3%
Fe2G3 = 1 - 2%
SO3 = 1 - 2%
For a good quality cement the ratio of’silica (SiO2) and alumina (Al2O3) should be between 2.5 to 4.0. Similarly the ratio of lime (CaO) to the total oxide mixtures consisting of SiO2, Al2O3 and Fe2O3 should be roughly 2:1:1, If lime is in excess, the cement cracks during setting. On the other hand, if lime is less than the required, the cement is weak in strength. Therefore, a proper composition of cement must be maintained to get cement of good quality.
