Abstract
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+).
Highlights
Calcium silicate hydrate (C-S-H) is the main solid phase in hydrated Portland cements, which is a poorly crystalline, nano-particulate, porous material, sometimes referred to as a “gel-like phase”
Optimized values of the interaction parameters between vacancy, Na and Ca in the bridging tetrahedral site were close to zero (0 +/− 1 kJ⋅mol− 1). These results indicate that allowing the substitution of Na in the BT sites brings no improvement of the fit and is not necessary to reproduce the experi mental data
If the alkali metals are added as hydroxide solution, pH of the solution increases, silicon in the BT site becomes less stable at higher pH and is replaced by vacancies at low C/S and by Ca at high C/S
Summary
Calcium silicate hydrate (C-S-H) is the main solid phase in hydrated Portland cements, which is a poorly crystalline, nano-particulate, porous material, sometimes referred to as a “gel-like phase”. The acronym “C-SH” refers to the cement chemistry notation, where ‘C’ stands for CaO, ‘S’ for SiO2 and H for H2O, the hyphens denote the variable composition of C-S-H gel, which is usually expressed in terms of molar C/S ratio C/S from 0.7 to 1.7. Chemical thermodynamic modelling of equilibria involving C-S-H, alkalis and other elements is crucial for quantitative predictions of composition and properties of hydrated ce ments and their changes upon interaction with the environment. From the variety of C-S-H models, only a few reconciled some structural features such as the connectivity of silica chains [2,4], and even fewer accounted for the uptake of alkalis [5,6]
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