Transforming MSWI fly ash into low-carbon and high-compatibility composites via designed calcium sulfoaluminate-based binders

Municipal solid waste incineration fly ash (MSWI FA) is a globally recognized hazardous waste. This study developed a low-carbon and high-compatibility calcium sulfoaluminate-enhanced ternary binders (PCG) composed of calcium sulfoaluminate cement, ordinary Portland cement and granulated blast-furnace slag for the stabilization/solidification (S/S) of MSWI FA. The complex interaction mechanisms between PCG binders and typical potentially toxic elements (PTEs, e.g., Pb and Cr) were investigated. Experimental results revealed that the various components in PCG synergically changed the content, type, and structure of calcium-aluminumsilicate-hydrate (C-A-S-H) phase and aluminate products, enhancing their immobilization efficiency to various PTEs. X-ray diffraction results revealed PTEs were captured by C-A-S-H via surface complexation effect, and Cr was further immobilized by aluminate products due to the ion substitution of CrO42- to SO42-. Mercury intrusion tests demonstrated the generation of hydration products exhibited positive effects on the physical coating effect of designed binders to PTEs. Subsequent practical S/S experiments demonstrated satisfactory mechanical properties (over 20 MPa) and acceptable PTEs leachability of PCG-based S/S blocks, fulfilling the criteria of lowcarbon construction materials. Life cycle assessment results revealed, compared to single ordinary Portland cement, the adoption of PCG binders for S/S of MSWI FA achieved a 60% reduction in carbon emissions. Overall, this study contributed to the advancement of S/S methodologies for MSWI FA and underscored the environmental advantages in employing low-carbon PCG binders in hazardous waste treatment.