Developing low-carbon mine backfill: Performance optimization and carbon storage using CO2 nanobubble and high-volume fly ash

Achieving carbon neutrality and large-scale industrial waste utilization requires low-carbon mine backfill materials. This study investigates a strategy to enhance cement-fly ash based composites using CO 2 nanobubble water. Normal cement-fly ash based backfill and CO 2 nanobubble-modified cement-fly ash based backfill were compared through mechanical and microstructural analyses, including uniaxial compression, mercury intrusion porosimetry, scanning electron microscopy and thermogravimetric analysis. The results demonstrate that CO 2 nanobubbles effectively mitigate the strength degradation induced by high fly ash replacement. Compared with normal backfill, the uniaxial compressive strength and elastic modulus of modified samples increased by 6.5–13.4% and 14.8–59.1%, respectively, enabling high-volume fly ash utilization without compromising mechanical integrity. Microstructural analyses reveal that CO 2 nanobubble water promotes hydration and in-situ carbonation reactions, leading to the formation of uniformly distributed C-S-H gels and calcium carbonate crystals that refine the pore structure and reduce total porosity by approximately 20%. Thermogravimetric results further confirm that CO 2 nanobubble significantly enhance carbonation efficiency, with the maximum carbonation degree reaching 13.07% at a fly ash content of 60%. Balancing performance and cost, the optimal fly ash content is identified within 20–60%, providing a green pathway for mining waste valorization.