Cement Composition || Chemical Composition of Cement

 The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina and iron oxide. These oxides interact with one another in the kiln at high temperature to form more complex compounds. The relative proportions of these oxide compositions are responsible for influencing the various properties of cement; in addition to rate of cooling and fineness of grinding. Table-1 shows the approximate oxide composition limits of ordinary Portland cement.

Table-1. Approximate Oxide Composition Limits of Ordinary Portland Cement

Oxide	% Content
CaO	60-67
SiO2	17-25
AL2O3	3-8
Fe2O3	0.5-6
MgO	0.1-4
Alkalies (K2O, Na2O)	0.4-1.3
SO3	1.3-3

AS per Indian standard specification for 33 grade cement, IS 269-1989, specifies the following chemical requirements:

•  Ratio of percentage of lime to percentage of silica, alumina and iron oxide; known as Lime Saturation Factor, when calculated by the formula

Not greater than 1.02 and not less than 0.66.

• Ratio of percentage of alumina to that of iron oxide Not less tan 0.66.
• Weight of insoluble residue Not more than 4 per cent.
• Weight of magnesia Not more than 6 per cent.
• Total sulphur content, calculated as sulphuric not more than 2.5%.
•  Total loss on ignition Not more than 5 per cent.                          

As mentioned earlier the oxides persent in the raw materials when subjected to high clinkering temperature combine with each other to form complex compounds. The identification of the major compounds is largely based on R.H. Bogue’s work and hence it is called “Bogue’s Compounds”. The four compounds usually regarded as major compounds are listed in table-1.2

Table-1.2. Bogue’s Compounds

Name of Compound	Formula	Abbreviated Formula
Tricalcium silicate	3CaO.SiO2	C3S
Dicalcium silicate	2CaO. SiO2	C2S
Tricalcium aluminate	3 CaO.Al2O3	C3A
Tetracalcium aluminoferrite	4 CaO.Al2O3.Fe2O3	C4AF

It is to be noted that for simplicity’s sake abbreviated notations are used. C stands for CaO, S stands for SiO₂, A for Al₂O₃, F for Fe₂O₃ and H for H₂O.

The equations suggested by Bogue for calculating the percentages of major compounds are given below.

C₃S= 4.07 (CaO) – 7.60 (SiO₂) – 6.72 (Al₂O₃) – 1.43 (Fe₂O₃) – 2.85 (SO₃) C₂S = 2.87 (SiO₂) – 0.754 (3CaO.SiO₂) C3A = 2.65 (Al₂O₃) – 1.69 (Fe₂O₃) C₄AF= 3.04 (Fe₂O₃)

The oxide shown within the brackets represents the percentage of the same in the raw materials.