Sulphate in clinker
The amount of sulphate (SO3) varies widely in cement clinker. Arthur Harrisson discusses the sources of SO3 and its impact on clinker composition.
Figure 1: relationship between the contents of sulphate and aluminate in alite crystals from two cement plants
The four main phases in Portland cement clinker are alite (C3S), belite (C2S), aluminate (C3A) and ferrite (C4AF). As has frequently been discussed, the chemical compositions suggested in brackets for these compounds using the cement chemists’ notation are idealised and are not generally produced in practice in a cement kiln. This is due partly to the presence of other elements in the raw materials that can be incorporated into the compositions of the compounds and because the chemical reactions that produce the compounds are not continued to equilibrium. This is largely because the process is stopped by rapid cooling of the clinker with the introduction of secondary air to the kiln, which is needed for combustion of the fuel.
