Binding of Cu(Ⅱ) and Zn(Ⅱ) in Portland cement immobilization systems: Effect of C-A-S-H composition

In this study, statistical EDS point analysis was conducted to understand the binding mechanisms of Cu²⁺ and Zn²⁺ in Portland cement immobilization systems based on their correlation with compositions of Al incorporated into calcium silicate hydrate (C-A-S-H). Cu²⁺ and Zn²⁺ significantly retarded early hydration and strength development within the first day but exerted an accelerating effect at later ages. The uptake of Cu²⁺ by C-A-S-H occurs through two types of binding forms, which are dominated by the adsorption of Cu²⁺ on the Al sites of substituted bridging silicate tetrahedra and on deprotonated silanol group sites. These binding forms result in competitive adsorption between Cu²⁺ and Ca²⁺. Zn²⁺ is primarily bound to the outer-product C-A-S-H. Zn anion species were adsorbed on the positively charged sites of the C-A-S-H surface, which were generated from the adsorption of Ca²⁺. Consequently, the uptake of Zn exhibits a noncompetitive relationship with Ca²⁺ and increased with Ca content in C-A-S-H.