CO₂ nanobubble-enhanced cement–fly ash backfill: Optimizing aggregate gradation and microstructure

Mine filling materials urgently need to improve mechanical properties and achieve low-carbon transformation. This study explores the mechanism of the synergistic effect of optimizing aggregate fractal grading and introducing CO₂ nanobubble technology to improve the performance of cement–fly ash-based backfill materials (CFB). The properties including fluidity, setting time, uniaxial compressive strength, elastic modulus, porosity, microstructure and CO₂ storage performance were systematically studied through methods such as fluidity evaluation, time test, uniaxial compression test, mercury intrusion porosimetry (MIP), scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS), and thermogravimetric-differential thermogravimetric analysis (TG-DTG). The experimental results show that the density and strength of the material are significantly improved under the synergistic effect of fractal dimension and CO₂ nanobubbles. When the fractal dimension reaches 2.65, the mass ratio of coarse and fine aggregates reaches the optimal balance, and the structural density is greatly improved at the same time. At this time, the uniaxial compressive strength and elastic modulus reach their peak values, with increases of up to 13.46% and 27.47%, respectively. CO₂ nanobubbles enhance the material properties by promoting hydration reaction and carbonization. At the microscopic level, CO₂ nanobubble water promotes the formation of C–S–H (hydrated calcium silicate), C–A–S–H (calcium-alumino-silicate hydrate) gel and CaCO₃, which is the main way to enhance the performance. Thermogravimetric studies have shown that when the fractal dimension is 2.65, the dehydration of hydration products and the decarbonization process of CaCO₃ are most obvious, and CO₂ nanobubble water promotes the carbonization reaction, making it surpass the natural state. The CO₂ sequestration quality of cement–fly ash-based materials treated with CO₂ nanobubble water at different fractal dimensions increased by 12.4wt% to 99.8wt%. The results not only provide scientific insights for the design and implementation of low-carbon filling materials, but also provide a solid theoretical basis for strengthening green mining practices and promoting sustainable resource utilization.