Chemical Composition of Cement:
The relative proportion of three oxide compositions is responsible for influencing the various properties of cement, like lime, silica and alumina.
An increase in lime content beyond a certain value makes it difficult to combine completely with other compounds.
Consequently, free time will exist in the clinker and will result in unsound cement. An increase in silica content at the expense of alumina and ferric oxide makes the cement difficult to fuse and form clinker.
|Iron oxide, Fe2O3||0.5-6||3|
|Sode/or potash (Na2O + K2O)||0.5-1||1|
1. Lime (CaO):
If lime is provided in excess then the cement becomes unsound and if it is in deficiency then the strength is reduced therefore chances of quick setting will be enhanced.
2. Silica (SiO2):
It imparts strength to the cement due to the formation of di-calcium and tri-calcium silicates. If it is in excess then the strength of the cement would be enhanced therefore setting time gets prolonged, hence it prevents quick setting.
3. Alumina (Al2O3):
It imparts the quick setting property of cement. If it is in excess then the strength of cement is reduced and the chances of rapid hardening would be increased. It acts as a flux and it lower’s the clinker temperature
4. Calcium Sulphate (CaSO4):
It is a retarder (admixture). If it is in excess then it slows down the quick setting which dominates to increase the strength. It is a gypsum form.
5. Iron Oxide (Fe2SO4):
It imparts colour, hardness, and strength to the cement. If it is in excess, then it imparts more coloured to the cement (grey).
6. Magnesia (Mgo):
It imparts hardness and colour (yellow) to the cement, if it is in small quantity, and if it is in excess then it imparts unsoundness to the cement.
7. Sulphur (S):
If it is in reference quantity then it imparts strength to the cement and if it is in excess then the unsoundness is increased.
8. Alkalies (Soda and Potash) (Na2O + K20):
Most of the alkalies present in raw materials are carried away by the flue gases heating and the cement contents only a small amount of alkalies.
If they are in excess in cement then they cause a number of troubles such as alkali-aggregate reaction,
efflorescence and staining when used in concrete, brickwork or masonry mortar.