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Cement

Chemical Composition:-
Ordinary cement contains two basic ingredients, namely, argillaceous and calcareous. In argillaceous materials,
clay predominates and in calcareous calcium carbonate predominates.
Broadly, the raw materials used for manufacture of cement consist mainly of lime, silica, alumina and iron oxide. These oxides interact with one another at high temperature and form more complex compounds. The relative proportions of these oxide compounds are responsible for
influencing the various properties of cement, in addition to rate of cooling and fineness of grinding.

Functions Of Cement Ingredients
1. Lime (CaO): Excess lime makes the cement unsound and causes the cement to expand and disintegrate.
Deficiency in lime makes loss of strength and makes cement to set quickly.
2. Silica (SiO2): SiO2 imparts the strength to cement by forming the di-calcium and tri-calcium silicates.
Excess silica makes the strength of cement increases,but prolongs the setting time.
3. Alumina (Al2O3): Al2O3 imparts quick setting property to the cement. It acts as a flux and lowers the clinkering temperature. Excess alumina makes cement weaker.
4. Calcium Sulphate (CaSO4): It is in the form of gypsum and increases the initial setting time of cement.
5. Iron Oxide (Fe2O3): Fe2O3 imparts colour, hardness and strength to cement.
6. Magnesia (MgO): MgO imparts hardness and colour, if present in small amount. Excess MgO makes cement unsound.
7. Sulphur (S): It is useful in making sound cement.
8. Alkalis: Most of the alkalis present in raw materials are carried away by the flue gases during heating process. Presence of alkalis causes problems such as alkali-aggregate reaction, efflorescence and staining, etc.
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