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A hydrothermally CaO–SiO2–H2O system was investigated at 150°–200°C, 2.5 h (CaO:SiO2=0.95) using various modifications of SiO2 in the presence of a mineralizer. Synthetic (stabilized) γ‐tridymite is the most reactive among SiO2 modifications. In the reaction mixture, the optimal concentration of the mineralizer (KOH) is 2% (versus the solid phase). The binding degree of CaO with SiO2 practically is 100% at 150°C. It is impossible to synthesize CSH free of C2SH on the basis of β‐cristobalite and β‐quartz under the investigation conditions without the use of mineralizer. The calorimetric effects as well as heat of de‐hydration of hydrosilicates were determined during their transformation into wollastonite. The entropy change at the peaks has been calculated.

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