Solubility of C-A-S-H phases with high degree of heavy metal ion substitution

Knowledge about interactions between cementitious materials and dissolved metal (Me) ions is key for environmental remediation and waste management, but thermodynamic data on this topic are still limited. We present solubility data for nanocrystalline C-A-S-H phases substituted by distinct Me ions with (Ca + Me)/(Si + Al) molar ratios from 0.86 to 1.04 and Ca²⁺ substitution of ∼ 48 mol% for Zn²⁺, ∼29 mol% for Co²⁺ and ∼ 27 mol% for Cu²⁺ in tobermorite-type Me-C-A-S-H. The solubility constants of the Me-C-A-S-H phases are significantly lower compared to pure C-A-S-H (logK = -8.3), ranging from logK = -8.6 to −11.3 for Co-C-A-S-H, logK = -9.1 to −11.6 for Cu-C-A-S-H and logK = -8.9 to −14.1 for Zn-C-A-S-H, as the Me content increases according to the expression: logK_Me-C-A-S-H=0.11±0.03·%Me_substitution-8.33±0.12;R²=0.89 independent on the type of incorporated Me ion. Thus, newly-forming C-A-S-H phases have a high immobilization potential for aqueous Me ions. The novel solubility data open the door for thermodynamic modelling of Me ion mobility in aquatic media.