ETTRINGITE SOLUBILITY AND GEOCHEMISTRY OF THE CA(OH)(2)-AL-2(SO4)(3)-H2O SYSTEM AT 1 ATM PRESSURE AND 298 K

The solubility and weathering reactions of ettringite, (Ca6Al2(SO4)(3) (OH)(12).26H(2)O), were used to study the geochemical equilibria of th e Ca(OH)(2)-Al-2(SO4)(3)-H2O system at environmental pH conditions. Et tringite is a stable mineral above a pH of 10.7 and dissolved congruen tly with a log K-sp of -111.6 (+/-0.8). Between pH 10.7 and 9.5, ettri ngite underwent incongruent dissolution to gypsum and Al-hydroxides an d controlled Ca2+, Al3+, and SO42- activities. At near neutral pH, Al- hydroxy sulfates precipitated in addition to gypsum and AI-hydroxide. These Al-hydroxy sulfate phases exhibited prismatic and anhedral shape s and had variable Al/S ratios. In addition, some new poorly crystalli ne Ca-Al-hydroxy sulfate phases were identified in microscopic studies when the pH was acidic (pH similar to 5). The activities of Ca2+, Al3 +, and SO42- suggest that the geochemistry of the Ca(OH)(2)-Al-2(SO4)( 3)-H2O system in the pH range of 7 to 10 is simple and its component C a(OH)(2)-SO3-H2O and Al-2(SO4)(3)-H2O systems behave independently of each other. The precipitation of Al-hydroxy sulfates below pH 7.0 sign ificantly influenced Ca2+ and SO42- activities. This effect was pronou nced when Ca-Al-hydroxy sulfate phases started precipitating (pH < 5.0 ). The lack of thermodynamic data on the newly identified Al, and Ca-A l-hydroxy sulfates makes it difficult to interpret the geochemistry of Ca(OH)(2)-Al-2(SO4)(3)-H2O system for pH 5.0. Reaction path calculati ons conducted using the EQ6 computer code predicted ion activities clo se to the experimental values above pH 5.0, The observed differences b etween thermodynamic modelling and actual experimental data below this pH can be explained by the formation of Al-/Ca-Al-hydroxy sulfate pha ses in the system, as detected by electron microscopy and X-ray elemen tal analysis. These reactions are relevant and useful to the predictio n of AI, and Ca geochemistry in natural systems. (C) 1998 Published by Elsevier Science B.V. All rights reserved.