Enhancing the interfacial transition zone of coal gangue aggregate mortar through the synergistic effect of red mud substitution and CO₂ mineralization during mixing

To improve the weak interfacial transition zone (ITZ) associated with coal gangue (CG) aggregates while promoting industrial solid waste valorization and carbon reduction, a synergistic modification strategy combining red mud (RM) substitution (8–24%) and in-situ CO₂ mineralization during mixing was proposed. The coupled effects of RM dosage and CO₂ flow rate (250–750 mL/min) on mechanical properties, hydration products, pore structure, ITZ characteristics, and environmental performance were investigated. Results showed that appropriate RM incorporation enhanced matrix compactness through micro-filling and secondary hydration, while also promoting the heterogeneous nucleation of CaCO₃ during mineralization. The optimal combination of 16% RM and 250 mL/min CO₂ achieved a 28-day compressive strength of 24.78 MPa, 7.97% higher than the corresponding non-mineralized specimen. Microstructural analyses demonstrated that controlled mineralization refined the pore structure, reduced the volume of harmful pores (>50 nm), and decreased the ITZ width to 18μm. However, excessive mineralization increased harmful-pore volume and induced degradation of hydration products, including C-(A)-S-H decalcification and partial destabilization of ettringite, thereby weakening the cementitious skeleton and resulting in strength deterioration. Additionally, the optimal mixture exhibited a carbon emission intensity of 486.68 kg CO₂-eq/t, corresponding to a 24.12% reduction relative to the control mixture. These findings demonstrate that RM-assisted in-situ CO₂ mineralization is an effective strategy for simultaneously enhancing ITZ quality, mechanical performance, and carbon reduction, providing a promising route for the production of sustainable low-carbon construction materials.