The solubility of calcium zineate (CaZn2(OH)6•2H2O)

Ca zincate (CaZn(2)(OH)(6). 2H(2)O) plays an important role in the geochemical behaviour of Zn in cementitious systems. This paper describes the determination of a solubility product for Ca zincate and its range of application. Calcium zincate was synthesized and characterised by X-ray diffraction (XRD). Samples were then equilibrated at 25 degreesC and an ionic strength of 0.1 M in a pH range between 11.4 and 12.7. A solubility product, logK(s)0 = 43.9, was determined for infinite dilution using the Davies equation for the following reaction. CaZn(2)(OH)(6). 2H(2)O((s)) + 6H((aq))(+)<----> Ca((aq))(2+) + 2Zn((aq))(2+) + 8H(2)O The solubility product can be used to successfully interpret the results of experiments in which Zn is equilibrated with C-S-H(I) at pH values between 11.7 and 12.78 at 25 degreesC and an ionic strength of 0.1 M. However, the solubility product is not able to model both the Zn and Ca concentration at a given pH value because dissolution is incongruent. (C) 2001 Elsevier Science Ltd. All rights reserved.