Ca zincate (CaZn2(OH)6·2H2O) 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°C and an ionic strength of 0.1 M in a pH range between 11.4 and 12.7. A solubility product, logKs0=43.9, was determined for infinite dilution using the Davies equation for the following reaction. CaZn2(OH)6·2H2O(s) + 6H+(aq)↔Ca2+(aq) + 2Zn2+(aq) + 8H2O 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°C 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.