Hydrothermal reactions of γ-dicalcium silicate

Abstract

The action of water on γ-Ca₂SiO₄, either alone or in the presence of calcium hydroxide, quartz, or amorphous silica, has been studied mainly at 150–600° and 5–350 bars. A new compound, of probable composition Ca₈Si₅O₁₈, is formed over a wide range of conditions; its appears to have a regularly interstratified structure composed of alternate layers of γ-Ca₂SiO₄ and kilchoanite (Ca₃Si₂O₇). The supposed compound, “γ-dicalcium silicate hydrate” or “dicalcium silicate hydrate (C),” which has been widely reported as forming under saturated steam pressures around 200°, is a mixture consisting principally of a disordered form of Ca₃Si₅O₁₈ and calciochondrodite, Ca₅(SiO₄)₂(OH)₂. Reaction sequences and equilibria are discussed; at least up to 450°, the initial and early products are crystalline compounds (kilchoanite, calcio-chondrodite, Ca₈Si₅O₁₈) with structures related to that of γ-Ca₂SiO₄.