SOLUBILITY OF ETTRINGITE (CA6[AL(OH)6]2(SO4)3 . 26H2O) AT 5-75°C - CORRELATIONS AMONG STANDARD MOLAL THERMODYNAMIC PROPERTIES OF AQUEOUS IONS AND HYDROXIDE COMPLEXES

Abstract

The solubility of ettringite (Ca6[Al(OH)6]2(SO4)3 . 26H2O) was measured in a series of dissolution and precipitation experiments at 5-75°C and at pH between 10.5 and 13.0 using synthesized material. Equilibrium was established within 4 to 6 days, with samples collected between 10 and 36 days. The log KSP for the reaction Ca6[Al(OH)6]2(SO4)3 . 26H2O # 6Ca2+ + 2Al(OH)4- + 3SO42- + 4OH- + 26H2O at 25°C calculated for dissolution experiments (-45.0 +/- 0.2) is not significantly different from the log KSP calculated for precipitation experiments (-44.8 +/- 0.4) at the 95% confidence level. There is no apparent trend in log KSP with pH and the mean log KSP,298 is -44.9 +/- 0.3. The solubility product decreased linearly with the inverse of temperature indicating a constant enthalpy of reaction from 5 to 75°C. The enthalpy and entropy of reaction ΔH^or and ΔS^or, were determined from the linear regression to be 204.6 +/- 0.6 kJ mol-1 and 170 +/- 38 J mol-1 K-1. Using our values for log KSP, ΔH^or, and ΔS^or and published partial molal quantities for the constituent ions, we calculated the free energy of formation ΔG^of,298, the enthalpy of formation ΔH^of,298, and the entropy of formation ΔS^of,298 to be -15211 +/- 20, -17550 +/- 16 kJ mol-1, and 1867 +/- 59 J mol-1 K-1. Assuming ΔCP,r is zero, the heat capacity of ettringite is 590 +/- 140 J mol-1 K-1.