Release and re-immobilization of heavy metals during the enforced carbonation of recycled concrete paste

Recycled cement paste (RCP) from demolition or industrial waste contains residual portlandite, C-(A)-S-H, and other hydration products that can be valorized by mineral carbonation. Enforced carbonation converts these reactive phases into stable calcium carbonates and pozzolanic alumina-silica gels (Al-Si gels), capturing CO2 and producing carbonated RCP (cRCP) suitable as a supplementary cementitious material (SCM). However, the accompanying pH decreases and transformation of cement hydrates during enforced carbonation may destabilize previously immobilized heavy metals, and the mechanisms governing their release and re-immobilization remain insufficiently understood.In this study, the fate of heavy metals in synthetic, heavy metal-doped, and industrial RCPs during enforced carbonation was investigated. Overall, more than 90% of cationic heavy metals were retained in the carbonation products, whereas oxyanions such as Cr and Mo showed enhanced mobility during carbonation. Enforced carbonation of RCP proceeds in two kinetic stages: a rapid initial carbonation of portlandite and AFt/AFm phases, followed by slower decalcification of C-(A)-S-H phases under declining pH. Most cationic metals (Ba, Mn, Ni, Sr, and Zn) were largely retained during the first step and were only gradually released during C-(A)-S-H decalcification. In contrast, oxyanionic Cr and Mo released from AFt/AFm carbonation exhibited early, rapid mobilization and remained largely soluble. These findings demonstrate that enforced carbonation effectively re-immobilizes cationic heavy metals, driven by mineralogical evolution and phase-specific solubility, while simultaneously valorizing recycled materials and mitigating CO2 emissions.