Insights into pore structure, carbon sequestration, and mechanical performance of CO2-foamed alkali-activated steel slag-blast furnace slag composite backfilling materials

To achieve synergistic utilization of industrial solid waste and CO 2 , this study employs CO 2 as a functional foaming agent to fabricate CO 2 -foamed alkali-activated steel slag-blast furnace slag composite backfilling materials (CFS), aiming to simultaneously achieve the construction of a porous structure and efficient CO 2 sequestration. The effects of CO 2 foam dosage on pore structure evolution, CO 2 sequestration, and mechanical performance were systematically evaluated, and the governing mechanisms were elucidated through multi-scale microstructural characterization. The results show that CO 2 foam dosage is a critical parameter controlling both sequestration efficiency and strength development via its influence on pore morphology. An optimal dosage of 4 L yields a uniform and compact pore structure, with a porosity of 16.12% and an average pore size of 0.04 mm. Under this condition, the CFS reaches a peak CO 2 sequestration amount of 2.78%, while retaining a 28-day compressive strength of 2.16 MPa, demonstrating that appropriate CO 2 foam dosage effectively balances pore refinement, enhanced CO 2 sequestration capacity, and mechanical stability. Microstructural analyses reveal that this phenomenon is attributed to the efficient encapsulation of CO 2 by stable bubble systems and the expansion of gas-liquid-solid reaction interfaces, which promote carbonate deposition at pore walls and strengthen the interfacial structure. However, when CO 2 foam dosage increases to 6 L, the paste's bubble-stabilizing capacity is exceeded, leading to bubble coalescence and rupture. The consequent development of coarser and more connected pores reduces CO 2 sequestration to 2.14% and significantly impairs strength, with the 28-day value decreasing to 0.71 MPa.