Chemical Composition of Raw Materials | CEMENT | Part 1.1 | BMC

Basic constituents of Hydraulic Cement: Lime CaO, Silica SiO₂, Alumina Al₂O₃, Iron Oxide Fe₂O₃

• The various proportions of these oxide compositions are mainly responsible for influencing the various properties of cement. For example an increase in Lime content beyond a certain value makes it difficult to combine completely with other compounds because, free lime or surplus lime will exists in the clinker and result in an unsound cement.

• Unsound Cement: Soundness of cements refers to the ability of cement paste to retrain its volume after it has get hardened. Unnecessary expansion and contraction occurring as a result of disintegration is called unsound cement. Normally its happen due to presence of free lime or magnesia in cement.

• An increase in Silica content at the expense of Alumina and Iron oxide makes the cement difficult to fuse and form the clinker.

Raw Materials of Portland Cement:

i). Lime (CaO)

Composition: 60% to 65% usually 63%

Function: to controls the strength and soundness of cement.

Deficiency: Reduces strength and setting time

ii). Silica (SiO₂)

Composition: 17% to 25% usually 20%

Function: Gives strength to cement

Excess: Slow setting time.

iii). Alumina (Al₂O₃)

Composition: 3% to 8% usually 6%

Function: responsible for quick setting time

Excess: reduces the strength

iv). Iron Oxide (Fe₂O₃)

Composition: 0.5% to 6% usually 2%

Function: gives colour and helps in fusion of different ingredients

v). Magnesia (MgO)

Composition: 0.4% to 4% usually 2%

Function: Imparts colour and hardness

Excess: cracks in mortar or crete.

vi). Soda and Potash (Na₂O + K₂O)

Composition: 0.5% to 4% usually 1%

Excess: eflorescene and cracking

vii). Sulphur tri-oxide (SO₃)

Composition: 1% to 2% usually 1.5%

Function: makes cement sound