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

Interaction between silica hydrogel isolated from serpentines (Mg(Fe)) 6 [Si 4 O 10 ](OH) 8 , sodium hydroxide NaOH, and strontium chloride SrCl 2 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: Sr 3 Si 2 O 7 · 4H 2 O and Sr 3 Si 2 O 7 · 3H 2 O or their mixtures, whose heating to 810–825°C is accompanied by some phase transformations: strontium metasilicate Sr 3 Si 3 O 9 or strontium orthosilicate Sr 2 SiO 4 is formed after bound water is removed in the range 250–350°C, and strontium silicate SrSiO 3 is formed at higher temperatures. Sr 2 SiO 4 single phase is observed upon heat treatment to 700–750°C. The optimal molar ratio of the reagents was found to favor Sr 2 SiO 4 formation.