Effects of Different Aggregate Gradations and CO₂ Nanobubble Water Concentrations on Mechanical Properties and Damage Behavior of Cemented Backfill Materials

Against the backdrop of increasingly severe global climate challenges, various industries are in urgent need of developing materials that can both improve performance and reduce carbon emissions. In this study, carbon dioxide nanobubble water (CO₂NBW) was evaluated as an innovative additive for cemented backfill materials (CBMs), and its optimization effect on the mechanical properties and microstructure of the materials was explored. The effects of different concentrations of CO₂NBW on stress–strain behavior, compressive strength, and microstructure were studied by uniaxial compression tests and scanning electron microscopy (SEM) analysis. The results show that with changes in CO₂NBW concentration and fractal dimension, the uniaxial compressive strength (UCS), peak strain, and elastic modulus of the specimens first increase and then decrease. At the optimal concentration level (<i>C</i> = 3) and fractal dimension (2.4150–2.6084), UCS reaches a peak value of 24.88 MPa, which is significantly higher than the initial value (<i>C</i> = 1). The peak strain and elastic modulus also reach maximum values of 0.01231 and 3.005 GPa, respectively. When the fractal dimension was between 2.4150 and 2.6084, the microstructural optimization effect of CO₂NBW on CBM was most significant, which was reflected in the compactness of the internal pore structure and the thoroughness of the hydration degree. In addition, based on the close correlation between peak strain and elastic modulus and UCS, a damage constitutive model of CBM specimens considering the influence of CO₂NBW concentration and fractal dimension was constructed. The study also found that the damage of CBM specimens is normally distributed with strain, and the accumulated damage in the plastic deformation stage dominates the total damage.