Extensions of CASH+ thermodynamic solid solution model for the uptake of alkali metals and alkaline earth metals in C-S-H

The new CASH+ core sublattice solid solution model of calcium-silicate-hydrate (C-S-H) can describe calcium uptake, solubility, water content, and mean silicate chain length up to 90 °C. In this study, the model has been consistently extended to describe the equilibrium uptake of alkalis (Li, Na, K, Rb, Cs) and alkaline earths (Mg, Sr, Ba, Ra). The new endmembers for each cation were defined, and their properties, along with binary interaction parameters, were fitted against known aqueous and solid phase compositions. The CASH+ model with its extensions can be directly used in GEMS codes or discretized and exported for use with other geochemical speciation computer programs. In agreement with the experimental data, the model indicates that the uptake of alkalis and alkaline earth cations occurs mainly as a competitive (interlayer) ion exchange, uptake is favored at low C/S ratios, silicate chains get shorter at higher pH, and the uptake of bivalent cations (in the order Mg2+ < Ca2+ < Sr2+ < Ba2+ < Ra2+) is preferential over that of monovalent cations (Li+ ≈ Na+ < K+ ≈ Rb+ ≈ Cs+).