Belite in Portland clinker

Published 22 July 2019

While belite is often represented as a pure dicalcium silicate, in practice the phase contains a range of impurities. Arthur Harrisson highlights the differences in belite composition and examines the causes of such impurities.

Type I belite crystals with dark laminations containing exsolved matter

The belite phase in Portland cement clinker is often characterised as dicalcium silicate or C2S, implying a lime-to-silica ratio of 2:1. The ratio refers to the number of CaO units to SiO2 units in the phase and because of the different atomic weights of the elements present, the weight ratio as determined by chemical analysis is slightly different. CaO atomic weight is 56 and SiO2 is 60, so two CaO is 112 and one SiO2 is 60. The CaO:SiO2 weight ratio (C:S) is 112/60 = 1.86.

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