Dicalcium silicate crystallizes in four polymorphic modifications which are known under the designations α, α', β, and γ. The β modification is metastable at all temperatures. It is converted to the less dense γ modification at below 500 ℃. This conversion is the reason why a cement clinker rich in C2S may disintegrate at low cooling rates. In industrial clinkers, however, burnt above 1200 ℃, disintegration generally does not take place because the different high temperature modifications of C2S are stabilized by incorporation of, e.g., Al3+, Mg2+, P3+, Na+, K+ etc. The type of modification present in industrial clinkers additionally depends on the cooling rate. Commonly, C2S exists in its β modification besides the α and α' modifications.
Dicalcium silicate occurs when cement clinker is not completely saturated with calcium oxide. Pure dicalcium silicate melts congruently at 2130 ℃. In lime-rich raw mixes, C2S forms already at temperatures below 1000 ℃ by solid-state reaction. During the sintering process in the presence of a melt above 1250 ℃, C2S incorporates additional Ca2+, resulting in the formation of C3S.
