Abstract The treatment of the Debye-Hückel theory as applied to the solubility of calcium hydroxide by Greenberg and Copeland is extended to permit calculation of the ion concentration solubility product in solutions of high ionic strength and high alkalinity, such as are present in the pores of hardened cement paste and concrete. Analytical data on such solutions expressed from hardened pastes after periods of up to a year provided by Longuet et al. are interpreted according to the equations developed. The solutions are initially supersaturated with respect to calcium hydroxide, as expected, but after a few days these solutions are in fact saturated with respect to calcium hydroxide. This concentration level (and saturation) is maintained for several months. The data indicate that subsequently the content of calcium in the pore solutions drops to undetectable amounts. Therappear to be no concurrent changes in other solution parameters to explain such a behavior; if confirmed, it would indicate that the pore solutions after a few months are no longer in effective contact with calcium hydroxide.