Thermodynamic modeling and mechanistic insights into Cu2+ sorption by C-S-H with different Ca/Si ratios

Calcium silicate hydrate (C-S-H), the principal binding phase in Portland cement, plays a critical role in the immobilization of heavy metals, yet a quantitative and mechanistic understanding of Cu²⁺ uptake by C-S-H remains incomplete. This study investigates the immobilization behavior of Cu²⁺ by C-S-H with systematically controlled Ca/Si ratios (0.6–1.6) and develops a thermodynamically consistent sublattice solid-solution model. The C-S-Hs were synthesized under sorption equilibrium conditions and characterized to determine their structural evolution with Ca/Si ratio. Cu²⁺ sorption was subsequently examined on pre-synthesized C-S-Hs, combined with spectroscopic analyses to elucidate Cu coordination environments. The results show that Cu²⁺ is immobilized predominantly through structural incorporation at low Cu²⁺ loadings rather than hydroxide precipitation. With increasing Cu²⁺ concentration, Cu²⁺ progressively intercalates into the interlayers of C-S-H, partially replacing interlayer Ca²⁺ and forming Si-O-Cu linkages associated with Q² silicate sites. The Ca/Si ratio strongly controls both uptake capacity and binding mechanism. High Ca/Si ratios promote silicate-chain polymerization through conversion of Q¹ to Q² units, whereas extremely low Ca/Si ratios induce redistribution between Q² and Q_b² species. At intermediate Ca/Si ratios, Cu²⁺ is mainly stabilized as hydroxyl-associated species. Once the sorption capacity is exceeded, excess Cu²⁺ transforms into CuO under alkaline conditions. Based on these observations, a CASH+Cu thermodynamic model incorporating five Cu-bearing endmembers was developed and calibrated using both aqueous and solid-phase data. The model quantitatively reproduces Cu uptake, solution chemistry, and phase assemblages, providing a predictive framework for Cu²⁺ retention in cementitious systems.