Preparation of Hydrated Strontium Silicates from Silica Hydrogel Isolated from Serpentines and Their Thermal Transformations

Abstract

Interaction between silica hydrogel isolated from serpentines (Mg(Fe))₆[Si₄O₁₀](OH)₈, sodium hydroxide NaOH, and strontium chloride SrCl₂ in aqueous medium was studied by DTA and X-ray powder diffraction (XRD). Stirring of the boiling aqueous suspension prepared from these reagents under the atmospheric pressure for 2 h yields hydrated strontium silicate species: Sr₃Si₂O₇ · 4H₂O and Sr₃Si₂O₇ · 3H₂O or their mixtures, whose heating to 810–825°C is accompanied by some phase transformations: strontium metasilicate Sr₃Si₃O₉ or strontium orthosilicate Sr₂SiO₄ is formed after bound water is removed in the range 250–350°C, and strontium silicate SrSiO₃ is formed at higher temperatures. Sr₂SiO₄ single phase is observed upon heat treatment to 700–750°C. The optimal molar ratio of the reagents was found to favor Sr₂SiO₄ formation.