Wollastonite from Hydrothermally Synthesized Calcium Hydrometasilicate Obtained from Various Modifications of Silica

A hydrothermally CaO-SiO2-H2O system was investigated at 150 degrees-200 degrees C, 2.5 h (CaO:SiO2 = 0.95) using various modifications Of SiO2 in the presence of a mineralizer. Synthetic (stabilized) gamma-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 degrees C. It is impossible to synthesize CSH free of C2SH on the basis of beta-cristobalite and beta-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.