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 filling materials (CFB). The fluidity, mechanical properties, pore structure and microscopic hydration reaction characteristics of the material were systematically studied by methods such as fluidity evaluation, time test, uniaxial compression test, mercury intrusion porosimetry (MIP), energy dispersive spectroscopy scanning electron microscopy (SEM-EDS) and 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 is 2.65, the 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 also reach the maximum value. 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, C-A-S-H 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 carbon sequestration capacity of cement fly ash-based materials treated with CO2 nanobubble water at different fractal dimensions increased by 12.4% to 99.8%, respectively. This study clarifies the possibility of low-carbon modified filling materials and lays a theoretical foundation for sustainable mine construction and resource utilization of industrial waste.