A low-temperature method of the [beta]-wollastonite synthesis

The two-step procedure of [beta]-wollastonite production includes (1) hydrothermal treatment of the suspension prepared from silicon dioxide (SiO.sub.2) and calcium hydroxide [Ca(OH).sub.2] and (2) solid calcination of the Ca-containing silicate compounds produced by hydrothermal synthesis. This method suggests autoclave treatment lasting for hours which inevitably leads to a high expenditure of energy. However, the latter fact can be avoided if the amorphous silica extracted from serpentines [(Mg(Fe)).sub.6[Si.sub.4O.sub.10](OH).sub.8] is used as a raw material. Calcium hydro- and hydroxosilicates can be produced from this silica, calcium oxide (CaO) and water (H.sub.2O) by stirring at the temperature of 95 °C in air at ambient pressure within 2 h. Moreover, on heating these compounds begin transforming into [beta]-wollastonite at the temperatures of 800-810 °C. The central point of the present study is to reveal those structural peculiarities of the considered silica which account for such an intriguing energy effect. The temperature-induced processes and phase transformations in the calcium hydro- and hydroxosilicates were investigated by thermal and X-ray diffraction analyses. The synthesized [beta]-wollastonite was studied by infrared spectroscopy and electron microscopy. The data derived from our experimental investigations allow concluding that those peculiarities are the chain-like fragments of (SiO.sub.3).sub.n intrinsic in that form of silica.