Composition Of Cement

Composition of Cement There are eight major ingredients of cement. The general percentage of these ingredients in cement is given below:

Ingredient

Percentage in cement

60-65 Lime 17-25 Silica 3-8 Alumina 1-3 Magnesia 0.5-6 Iron oxide Calcium Sulphate 0.1-0.5 Sulphur Trioxide 1-3 0-1 Alkaline

Functions of Cement Ingredients The main features of these cement ingredients along with their functions and usefulness or harmfulness are given below: 1. Lime: Lime is calcium oxide or calcium hydroxide. o

2.

3.

4.

5.

Presence of lime in a sufficient quantity is required to form silicates and aluminates of calcium. o Deficiency in lime reduces the strength of property to the cement. o Deficiency in lime causes cement to set quickly. o Excess lime makes cement unsound. o Excessive presence of lime cause cement to expand and disintegrate. Silica:Silicon dioxide is known as silica, chemical formula SiO2. o Sufficient quantity of silica should be present in cement to dicalcium and tricalcium silicate. o Silica imparts strength to cement. o Silica usually presents to the extent of about 30 percent cement. Alumina: Alumina is Aluminium oxide. The chemical formula is Al2O3. o Alumina imparts quick setting property to the cement. o Clinkering temperature is lowered by the presence of the requisite quantity of alumina. o Excess alumina weakens the cement. Magnesia: Magnesium Oxide. Chemical formula is MgO. o Magnesia should not be present more than 2% in cement. o Excess magnesia will reduce the strength of the cement. Iron oxide: Chemical formula is Fe2O3. o Iron oxide imparts color to cement.

o o

It acts as a flux. At a very high temperature, it imparts into the chemical reaction with calcium and aluminum to form tricalcium alumino-ferrite. o Tricalcium alumino-ferrite imparts hardness and strength to cement. 6. Calcium Sulfate: Chemical formula is CaSO4 o This is present in cement in the form of gypsum(CaSO4.2H2O) o It slows down or retards the setting action of cement. 7. Sulfur Trioxide: Chemical formula is SO3 o Should not be present more than 2%. o Excess Sulfur Trioxide causes cement to unsound. 8. Alkaline:

Alkalines are Basic salts of Alkali metals such as Sodium(Na) and Potassium(K) which are in form of NaOH & KOH o o o

Should not be present more than 1%. Excess Alkaline matter causes efflorescence.

INTERACTION OF H2SO4 WITH DIFFERENT COMPONENT OF CEMENT Magnesia is in form of MgO. Magnesium oxide react with sulfuric acid MgO + H2SO4 → MgSO4 + H2O MgSO4.7H2O + Ca(OH)2

CaSO4.2H2O+Mg(OH)2 + 5.H2O

Worst Effect: Attack to CSH besides C3A & Ca(OH)2 3CaO.2SiO2.aq + MgSO4.7H2O

CaSO4.2H2O +Mg(OH)2 + SiO2.aq

Alkalines are Basic salts of Alkali metals such as Sodium(Na) and Potassium(K) which are in form of NaOH & KOH

NaOH + H2SO4 → Na2SO4 + H2O

Na2SO4.10H2O + Ca(OH)2

CaSO4.2H2O + 2NaOH + 8.H2O Gypsum

2(3CaO.Al2O3.12H2O) + 3Na2SO4 . 10H2O 3CaO.AL2O3.3CaSO4.31H2O+ 2Al(OH)3 + 6NaOH + 17H2O