A dual closed-loop CO2 mineralization system with multicycle stability: Sustainable production of CaCO3 and upgrading supplementary cementitious materials via amino acid selective leaching

The utilization of industrial waste as supplementary cementitious material (SCM) offers a promising approach to mitigate CO 2 emissions from the cement industry, in spite of ongoing challenges in the selective separation and utilization of individual components from complex wastes. In this study, a dual closed-loop CO 2 mineralization system was proposed to selectively recover Ca-bearing minerals as CaCO 3 from coal fly ash (CFA) and upgrade the residue into SCM. The amino acid leaching process achieved a Ca leaching efficiency of 31% to 41% with a high selectivity exceeding 96.7%, outperforming the 76.28% selectivity of HNO 3 . Theoretical speciation and stoichiometric analysis revealed a pH-dependent dual mechanism. Low-pH proton-driven and alkaline complexation-driven pathways activated selective Ca leaching while suppressing the impurity dissolution. The leached residue was depleted in Ca element by ~36% and enriched in pozzolanic components like SiO 2 . The integrated process demonstrated the stability over ten 1 L-scale operational cycles. The leachates maintained 83%–100% CO 2 mineralization efficiency, confirming efficient Gly regeneration. The precipitated CaCO 3 product exhibited a stable morphology and high-valued vaterite polymorph across all cycles. Mortars using the residues from cycles 1, 5, and 10 as SCM (10% replacement) achieved 28-day compressive strengths of 44.4 MPa, 47.6 MPa, and 45.6 MPa, respectively, an enhancement compared to the 39.1 MPa of the untreated CFA control. This enhancement might be related to the altered hydration process and accelerated C-S-H formation, driven by optimized inorganic phases and residual organic Gly resulting from the selective leaching process. This work provided a sustainable process, closing the solvent loop via amino acid regeneration and the material loop by reintroducing CaCO 3 as cement raw material or filler.