The world ‘cement’ usually means Portland cement used in Civil engineering works which sets well under water, hardens quickly and attains strength.
MANUFACTURE OF CEMENT:-the manufacture of cement consists of grinding the raw materials (calcareous and argillaceous stones containing silica, alumina and iron oxide) and mixing them intimately in a certain proportion.
It is then burnt in a large rotary kiln at a temperature of about 1500 degree Celsius, when the material sinters and partially fuses into balls known as clinker. The clinker is cooled and ground to find powder with some gypsum added, and the resulting product is the commercial Portland cement.
FUNCTIONS OF INGREDIENTS OF CEMENT
The functions of various ingredients of an ordinary Portland cement as mentioned are as follows:-
1) lime (63%):-it makes the cement sound and strong. The lime in excess quantity makes the cement unsound and causes the cement to expand and disintegrate.
2) silica (22%):-it provides strength to the cement due to the formation of dicalcium and tricalcium silicates. The silica in excess quantity cause the cement to set slowly.
3) alumina (6%):-it provides quick setting property to the cement and lower the clickering temperature.
4) iron oxide (3%):-it provides colour , hardness and strength to the cement. It also helps in the fusion of raw materials during the manufacture of cement.
5) magnesium oxide (2.5%):-it provides hardness and colour to the cement, when present in small quantity. The magnesium oxide in excess quantity makes the cement unsound.
6) sulphur trioxide (1.5%):-it makes the cement sound, when present in very small quantity. The sulphur trioxide in excess quantity makes the cement unsound.
7) alkalies (0.5%):- These should be present in small quantities. The alkalies in excess quantity will cause efflorescence.
COMPOSITION OF CEMENT CLINKER:-
The cement clinkers (which are formed when calcareous and argillaceous raw materials are mixed and burned in rotary kilns) consists of the following major compounds-
1) tricalcium silicate (40%)-the presence of tricalcium silicate in cement hydrates more rapidly. It generates more heat of hydration.It develops high early strength and possesses less resistance to sulphate attack.
2) dicalcium silicate (32%)-the presence of dicalcium silicate in cement hydrate slowly. It generates less heat of hydration. It hardens more slowly and offers more resistance to sulphate attack. It provides good ultimate strength to the cement.
3) tricalcium aluminate (10.5%)-the presence of tricalcium aluminate causes initial setting of cement. It reacts fast with water and generates large amount of heat hydration. It is the first compound which reacts with water when mixed with cement.
4) tetra calcium alumino ferrite (9%)-the presence of tetracalcium aluminoferrite in cement has poor cementing value. It reacts slowly with water and generates small amount of heat hydration.
The high percentage of tricalcium silicate and low percentage of calcium silicate in cement results in rapid hardening, early strength, high heat of generation and less resistance to chemical attack.
The low percentage of tricalcium silicate and high percentage of dicalcium silicate in cement results in slow hardening, much more ultimate strength, less heat of generation and greater resistance to chemical attack.
HYDRATION OF CEMENT:-the chemical reaction between cement and water is called hydration of cement or simply cement hydration. The phenomenon by virtue of which the plastic cement changes into a solid mass is known as setting of cement. The phenomenon by virtue of which the cement paste sets and develops strength is known as hardening of cement.
The tricalcium silicate hydrates more rapidly than dicalcium silicate and develops strength in cement for the first 7 days. The tricalcium aluminate and tetracalcium aluminoferrite compounds are responsible for the initial setting of cement.