Cement is a mixture of various chemical compounds. All ingredients have their own specific roles to play and impart different properties to cement. Ratio of all ingredients is required to be maintained to get the desired quality.
Chemical composition of Cement is described in the following table.
Chemical Composition of Cement
Variation in the proportion of these elements effects the properties of Cement.
|Lime (CaO)||60-65||Controls strength and soundness|
|Sillica (SiO2)||20-25||Gives strength, excess quantity causes slow setting|
|Alumina (Al2 O3)||4-8||Quick setting, excess lowers strength|
|Iron Oxide (Fe2 O3)||2-4||Imparts color, helps in fusion of ingredients|
|Magnesium Oxide (MgO)||1-3||Color and hardness, excess causes cracking|
|Na2 O||0.1-0.5||Controls residues, excess causes cracking|
|Sulphur Trioxide (SO3)||1-2||Makes cement sound|
Functions of Ingredients in Cement
1. Function of Lime in Cement
- It is the major constituent of cement . Its exact proportion is important.
- The excess makes the cement unsound and causes the cement to expand and disintegrate.
- In case of deficiency, the strength of cement is decreased and cement sets quickly.
- The right proportion makes cement sound and strong.
2. Function of Silica in Cement
- It imparts strength to the cement due to formation of di-calcium silicate (2CaO SiO2 or C2S) and tri-calcium silicate (3CaO SiO2 or C3S).
- Silica in excess provides greater strength to the cement but at the same time it prolongs its setting time.
3. Functions of Alumina in Cement
- It imparts quick setting quality to the cement.
- It acts as a flux (rate of flow of energy) and lowers the clinkering temperature.
- Alumina in excess reduces strength of cement.
4. Functions of Iron Oxide in Cement
- It provides color, hardness and strength.
- It also helps the fusion of raw materials during manufacture of cement.
5. Harmful Ingredients in Cement
- Alkali oxides (K2O and Na2O): if the amount of alkali oxides exceeds 1%, it leads to the failure of concrete made from that cement.
- Magnesium oxide (MgO): if the content of MgO exceeds 5%, it causes cracks after mortar or concrete hardness.