The main constituents of ordinary portland cement and their respective functions.

The main constituents of OPC and their functions are as under:

1. Lime (CaO) (60% - 70%): This is an important ingredient of cement. Lime in excess makes cement unsound and causes the cement to expand and disintigrate. If there is a deficiency of lime, strength of cement decreases and it causes cement to set quickly.

2. Silica (SiO2): This is also an important ingredient of cement and it imparts strength to cement due to the function of dicalcium and tricalcium silicates. If silica is present in excess quanity, the strength of cement increases but at the same time, its setting time is prolonged.

3. Alumina (Al2O3): This ingredient imparts quick setting property to cement. It acts as a flux and lowers the clinkering temperature. However, high temperature is essential for the formation of a suitable type of cement and hence, alumina should not be present in excess amount as it weakens the cement.

4. Calcium Sulphate (CaSO4): This ingredient is in the form of gypsum and its function is to increase the initial setting time of cement.

5. Iron Oxide (Fe2O3): This ingredient imparts colour, hardness and strength to the cement.

6. Magnesia (MgO): This ingredient, if present in small amount, imparts hardness and colour to cement whereas its high content makes the cement unsound.

7. Sulphur (S): A very small amount of Sulphur is useful in making sound cement. If it is in excess, it causes cement to become unsound.

8. Alkalies: Most of the alkalies present in the raw materials are carried away by the flue gases during heating and cement contains only a amount of alkalies. If they are in excess in cement, they cause a number if troubles such as alkali-aggregate reaction, efflorescense and staining when used in concrete or brick work mortar. Increase in strength beyond age of 28 days is strongly affected by the alkali content. Gain in strength is inversaly proportional.