Composition of cement | Functions – civil learners

Composition of cement:

The two basic ingredients of cement are argillaceous and calcareous.

The composition of ordinary cement and ingredients are as follows:



Ingredients Normal range in  % Typical Composition             %
1)Lime (Cao)    60-70% 62
2)Silica (SiO2)    17-25% 22
3)Alumina (Al2O3)    3-8% 5
4)Iron Oxide (Fe2O3)    0.5 – 6% 3
5)Magnesia (MgO)    0.1-4% 2
6)Sulphur Trioxide (SO3)    1-3% 1
7)Soda or potash    0.5 – 1.3% 1
8)Calcium sulphate (CaSo4)    3 – 5% 4
     Total 100
For better understanding watch the video:

Also Read: Interview question process of Manufacturing of cement Most repeated and imp Hydration of cement

Functions of ingredients of cement and average composition of cement:


1. Lime:

It is a major ingredient of cement and constitutes about two-thirds of cement

  • It makes the cement sound and strong.


  • The lime in excess quantity makes cement unsound and causes the cement to disintegrate.


  • If lime is low in quantity, the mix will be improper which will lead to low strength once as this cement will set quickly.


  • Lack of lime will reduce the strength of cement.


  • Its percentage is between 60-70%



2. Silica:

  • It is an important ingredient of cement and imparts strength to the cement due to the formation of dicalcium and tricalcium silicates.


  • Silica in excess provides greater strength to the cement but at the same time prolongs its setting time.


  • Its percentage is between 17 -25 %




3. Alumina:

  • It provides quick setting property to the cement.


  • It acts as a flux and lowers the clinkering temperature.


  • Alumina in excess reduces the strength of cement.


  • Its composition in cement is between 3-8%



4. Iron oxide:

  • It provides colour, hardness and strength to the cement.


  • It also helps the fusion of raw materials during the manufacture of cement.


  • Its composition in cement is between 0.5 – 6%


 5. Magnesium oxide:
  • It provides hardness and colour to the cement when present in small quantities.


  • The magnesium oxide in excess quantity makes the cement unsound.


 6. Sulphur trioxide:
  • It makes the cement sound, when present in very small quantities.


  • If it is used in excess it produces expansion of cement thus making it unsound and also an excess sulphur trioxide increases the setting time.



7. Alkalies(Soda and Potash):

  • These are used in the form of soda and potash.


  • These should be present in small quantities.


  • The alkalies in excess quantity will cause efflorescence.


  • The average composition is about 1%


8. Calcium sulphate:

  • It is about 3% of the cement and is added in the form of gypsum during the process of grinding of clinker
  • It is a retarder since it increases the initial setting time of cement.
  • If gypsum is not added the cement will set at the moment water is added to it, without giving any time for mixing and placement.


Harmful ingredients of cement:

The following are the two ingredients which affect the quality of cement:

1. Alkali oxides.

2. Magnesium oxide.

If the amount of alkali oxides exceeds 1%. it leads to the failure of concrete made from that cement. If the content of magnesium oxide exceeds 5%, it causes cracks after mortar or concrete hardens.


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