Compatibility of Hydrogarnet, Ca₃Al₂(SiO₄)ₓ(OH)₄₍₃₋ₓ₎, with Sulfate and Carbonate-Bearing Cement Phases: 5-85 °C

The stable existence of hydrogarnet in Portland cement compositions cured at temperatures below 55 °C has long been predicted from application of equilibrium thermodynamics. However hydrogarnet is not often reported in hydrated commercial Portland cements. The substitutions (SO ₄ -CO ₃ -OH) in AFm have previously been shown to stabilise AFm to higher temperatures and raise the temperature at which AFm converts to Si-free hydrogarnet, C ₃ AH ₆ . But unanswered question remains about the compatibility of AFm and AFm solid solutions with Si-substituted hydrogarnet, Ca ₃ Al ₂ (SiO ₄ ) _x (OH) _{4(3 - X)} . Phase relations of C ₃ AH ₆ and Ca ₃ Al ₂ (SiO ₄ ) _x (OH) _{4(3 - X)} at sulfate and carbonate activities conditioned respectively by (gypsum and SO ₄ -AFt) and (calcite and CO ₃ -AFt) have been determined experimentally in the range 5-85 °C. The results confirm the instability of Si-free hydrogarnet with carbonate and sulfate-bearing cement phases, but do indicate that a range of silica-substituted hydrogarnet solid solutions are stable under conditions likely to be encountered in blended cement systems.