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

Abstract

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 CO₂ nanobubble water. Normal cement-fly ash based backfill and CO₂ 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 CO₂ 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 CO₂ 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 CO₂ 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.

Keywords

Low-carbon backfill / CO₂ nanobubble / High-volume fly ash / Carbon sequestration / Cost-benefit analysis

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