Synthesis and stability of silica-substituted hydrogarnet Ca₃Al₂ Si_3-xO_12-4x(OH)_4x

A range of garnet structured compositions can be synthesized between the end members C₃AH₆ (hydrogarnet) and C₃AS₃ (grossularite) at different temperatures and pressures. The extent of this series at 95°C and a pressure of one bar has been investigated. The synthesis of a range of garnets from C₃AH₆ to C₃AS_1·5 H₃ has heen attempted hy refluxing reactants at the same temperature and pressure. Product analyses were carried out using X-ray diffraction, electron microscopy, chemical and differential thermal/thermogravimetric analyses. Three target compositions from C₃A H₆ to C₃AS_0·24H_5·52 gave one garnet phase plus C₄AH₁₃. Another three target compositions from C₃S_0·5H₅ to C₃AS_0·76H_4·48 gave two distinct Si-hydrogarnet phases: a low Si-hydrogarnet, approximately C₃AS_0·42H_5·16, and a higher Si-hydrogarnet, approximately C₃AS_0·76H_4·48. The remaining three target compositions, from C₃ASH₄ to C₃AS_1·5H₃, again showed the two garnets to be present but with increasing amounts of calcium-silicate-hydrate gel (Ca/Si = 1·0) as the hulk Si content increased. It was found that the maximum Si content ohtained in Si-hydrogarnet was C₃AS_0·99H_4·02. It is concluded that the 95°C, 1 bar range of garnet stahility is quite different from inferences drawn from studies made at higher temperatures and pressures.