constituents of cement

Name the four important constituents of cement and state the role of each in achieving its properties.

The four important constituents of cement are :-

(i) Lime (CaO) – 60 to 67%
(ii) Silica (SiO2) – 17 to 25%

(iii) Alumina (Al2O3) – 3 to 8%
(iv) Iron oxide (Fe2O3) – 0.5 to 6%

All these oxides interact with one another in the kiln at high temperature to form more complex compounds. The relative proportions of these oxides compositions are responsible for influencing the various properties of cement in addition to rate of cooling and fineness of grinding. The complex compounds which are formed due to the combination of these oxides are called Bogue’s compounds and four of them are usually regarded as major compounds. 

They are 

tricalcium silicate (C3S), 

tricalcium aluminate (C3A) and

tetra calcium aluminoferrite (C4AF).

The two silicates namely C3S and C2S which together constitute about 70 to 80 per cent of the cement control the most of the strength giving properties.
 Upon hydration, both C3S and C2S give the same product called calcium silicate hydrate (C3S2H3) and calcium hydroxide. 
C3S giving a faster rate of reaction accompanied by a greater heat evolution develops early strength. 
On the other hand, C2S liberates nearly three times are much calcium hydroxide as compared to C2S. That’s why C2S provides more resistance to chemical attack.The compound tricalcium aluminate (C3A) is characteristically fast reacting with water and may lead to an immediate stiffening of paste, and this process is termed as flash set. The role of gypsum added in the manufacture of cement is to prevent such a fast reaction. The hydrated aluminates do not contribute anything to the strength of concrete. On the other hand, their presence is harmful to the durability of concrete particularly where the concrete is likely to be attacked by suplhates. As it hydrates fast it may contribute a little to the early strength.

On hydration, C4AF is believed to form a system of the form CaO-Fe2O3-H2O. A hydrated calcium ferrite of the form C3FH6 is comparatively more stable. This hydrated product also does not contribute anything to the strength.The hydrates of C4AF show a comparatively higher resistance to the attack of sulplhates than the hydrates of calcium aluminates.

important constituents of cement