Impact of heavy metals on the enforced carbonation of C-A-S-H

Enforced carbonation of recycled concrete paste (RCP) is a promising approach for CO 2 reduction, producing carbonated RCP (cRCP) that can serve as an effective supplementary cementitious material (SCM). While RCPs are generally environmentally safe, they may contain varying levels of heavy metals depending on their origin. This study investigates the behavior and retention of five representative metals-Ba 2+ , Cr 3+ , Cu 2+ , Mn 2+ , and Sr 2+ -during enforced carbonation, using synthetic C-A-S-H gel as a model for hydrated cement phases. Results showed that Ba 2+ , Mn 2+ , and Sr 2+ formed metal carbonates or incorporated into calcite, achieving carbonation efficiencies above 90%. In contrast, Cr 3+ and Cu 2+ are mainly immobilized via precipitation, resulting in slightly reduced carbonation efficiencies. However, both metals were effectively immobilized through sorption and precipitation on the carbonation products, with retention capacities exceeding 99% upon enforced carbonation. These results confirmed the robustness of enforced carbonation for both CO 2 sequestration and heavy metal immobilization, supporting the safe and sustainable use of cRCP in real-world cementitious applications.