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.

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