Hydration of the two calcium silicate clinker phases, C3S and C2S proceeds, after an initial induction period, to form a poorly-defined calcium silicate hydrate (C-S-H, where no particular composition is implied). The hydration of C3S may be considered thus, although the precise composition of the C-S-H phase is variable

2(3CaO.SiO2) + 11H2O —› 3CaO.2SiO2.8H2O + 3Ca(OH)2

The aluminium containing phases (C3A and C4AF) hydrate very rapidly and rapid hydration known as ‘flash-set’ can occur when substantial quantities are hydrated. Within seconds a poorly crystalline gel forms on the clinker grain surfaces, which then converts into a permeable network of needle-like hexagonal hydrates, C4AH19 and C2AH8. These phases in turn convert to the cubic hydrogarnet phase, C3AH6.

Gypsum (CaSO4.2H2O, CSH2) is often used to prevent flash set. C3A hydration in the presence of gypsum leads to the formation of a dense, coherent layer of ettringite (Ca6Al2(SO4)3(OH) 12*26H2O or C3A.3CS.H32). This layer retards diffusion of the dissolving ions and hinders the formation of the hexagonal hydrates, thus preventing flash-set. Figure 2 shows the Raman spectra recorded from a hydrating C3A-gypsum paste over a period of 28 days. The most intense band, at 1009 cm-1, is due to gypsum, and it is slowly replaced by a band at 989 cm-1 due to ettringite. Other changes are also visible in the spectra upon hydration.