Clarifying and quantifying the immobilization capacity of cement pastes on heavy metals

Heavy metals (HMs) in concrete pose great environmental risks. However, mechanism-based quantification for cement paste's immobilization on HMs was seldom reported. Using a broad experimental design, we investigated the binding behaviors of representative HMs in C3S, C3A and C4AF pastes. Specifically, sorption and leaching tests were conducted to reveal the sorption roles of C-S-H, AFt and AFm. Afterwards, a thermodynamic model was established to quantify cement pastes' immobilization capacity and verified by progressive leaching. According to the results, the HMs are divided into three categories. In the 7-day cement pastes, precipitation-type HM Mn2+ can be completely immobilized as Mn hydroxides; amphoteric-type HM Pb2+ is partly immobilized as hydroxides and sorbed by C-S-H, AFt and AFm; and anion-type HM CrO42− has difficulty precipitating or being sorbed by hydrates (except AFm), while AsO43− can be completely immobilized. The quantification provides a basis for the environmental assessment of HM leaching from concrete.