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

Abstract

A range of garnet structured compositions can be synthesized between the end members C 3 AH 6 (hydrogarnet) and C 3 AS 3 (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 C 3 AH 6 to C 3 AS 1·5 H 3 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 C 3 A H 6 to C 3 AS 0·24 H 5·52 gave one garnet phase plus C 4 AH 13 . Another three target compositions from C 3 S 0·5 H 5 to C 3 AS 0·76 H 4·48 gave two distinct Si-hydrogarnet phases: a low Si-hydrogarnet, approximately C 3 AS 0·42 H 5·16 , and a higher Si-hydrogarnet, approximately C 3 AS 0·76 H 4·48 . The remaining three target compositions, from C 3 ASH 4 to C 3 AS 1·5 H 3 , 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 C 3 AS 0·99 H 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.