Abstract:
The solubility and weathering reactions of ettringite, (Ca6Al2(SO4)3(OH)12#26H2O), were used to study the geochemical equilibria of the Ca(OH)2-Al2(SO4)3-H2O system at environmental pH conditions. Ettringite is a stable mineral above a pH of 10.7 and dissolved congruently with a log Ksp of -111.6 (±0.8). Between pH 10.7 and 9.5, ettringite underwent incongruent dissolution to gypsum and Al-hydroxides and controlled Ca2+, Al3+, and SO42- activities. At near neutral pH, Al-hydroxy sulfates precipitated in addition to gypsum and Al-hydroxide. These Al-hydroxy sulfate phases exhibited prismatic and anhedral shapes and had variable Al/S ratios. In addition, some new poorly crystalline Ca-Al-hydroxy sulfate phases were identified in microscopic studies when the pH was acidic (pH~5). The activities of Ca2+, Al3+, and SO42- suggest that the geochemistry of the Ca(OH)2-Al2(SO4)3-H2O system in the pH range of 7 to 10 is simple and its component Ca(OH)2-SO3-H2O and Al2(SO4)3-H2O systems behave independently of each other. The precipitation of Al-hydroxy sulfates below pH 7.0 significantly influenced Ca2+ and SO42- activities. This effect was pronounced when Ca-Al-hydroxy sulfate phases started precipitating (pH<5.0). The lack of thermodynamic data on the newly identified Al, and Ca-Al-hydroxy sulfates makes it difficult to interpret the geochemistry of Ca(OH)2-Al2(SO4)3-H2O system for pH≤5.0. Reaction path calculations conducted using the EQ6 computer code predicted ion activities close to the experimental values above pH 5.0. The observed differences between thermodynamic modelling and actual experimental data below this pH can be explained by the formation of Al-/Ca-Al-hydroxy sulfate phases in the system, as detected by electron microscopy and X-ray elemental analysis. These reactions are relevant and useful to the prediction of Al, and Ca geochemistry in natural systems.