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Chemical Composition of Raw Materials | CEMENT

The three basic constituents of hydraulic cements are Lime, Silica-alumina and Iron-oxide.
The relative proportions of these oxide compositions are responsible for influencing the various properties of cement.
An increase in lime content beyond a certain value makes it difficult to combine completely with other compounds.
Consequently, free lime will exist in the clinker and will result in the unsound cement. An increase in silica content at the expense of alumina and ferric -oxide makes the cement difficult to fuse and form clinker.
The approximate limits of chemical composition in cement are given in table.

Constituents of Portland cement (Raw Material)

Oxide	Function	Composition (%)	Average
Lime, CaO	Controls strength & soundness. Its deficiency reduces strength & setting time	60-65%	63%
Silica, SiO₂	Gives strength excess of it causes slow setting.	17-25	20
Alumina, Al₂O₃	Responsible for quick setting if in excess it lower the strength.	3-8	6
Iron Oxide, Fe₂O₃	Gives colour and helps in fusion of different ingredients.	0.5-6	3
Magnesia, MgO	Imparts colour and hardness. If in excess, it causes cracks in mortar and concrete and unsoundness.	0.5-4	2
Soda & Potash, Na₂ O+K₂O	These are residues, and if in excess cause efflorescence and cracking.	0.5-1	1
Sulphur trioxide, SO₃	Makes cement sound	1-2	1.5

Note:

Rate of setting of cement paste is controlled by regulating the ratio SiO₂/(Al₂O₃ + Fe₂O₃).
When development of heat of hydration is undesirable, the silica content is increased to about 21% & the alumina and iron oxide contents are limited to 6% each.
Resistance to the action of sulphate waters is increased by raising further the silica content to 24% and reducing the alumina and iron contents in 4% each.
The variation in composition depends largely on the ratio of CaO to SiO₂ in the raw materials.
Alkali's accelerate the setting of cement paste.
Composition of Cement Clinker and their Setting Action:
When these raw materials are put in kiln then it fuses and following four major compounds are formed and they are known as Bogue compound.

The principal mineral compounds in Portland Cement	Formula	Name	Symbol
Tricalcium silicate	3CaO.SiO₂	Alite	C₃S
Dicalcium silicate	2CaO.SiO₂	Belite	C₂S
Tricalcium aluminate	3CaO.Al₂O₃	Celite	C₃A
Tetracalcium alumino-ferrite	4CaO.Al₂.Fe₂O₃	Felite	C₄AF

The mixing of cement and water formed various complicated chemical compounds are not simultaneous.
It is found that ordinary cement achieves about 70% of its strength in 28 days and about 90% in 1 year.
Properties of Portland cement varies significantly with the proportions of the above four compounds, as substantial difference is observed in their individual behavior.

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