Experimental Evaluation of the Mechanical Properties and Microstructure of Cemented Paste Backfill Containing Rheological Modifier

Abstract

The macroscopic mechanical properties of cemented paste backfill (CPB) are closely related to its microstructural evolution. However, the mechanism by which rheology modifier influences the mechanical properties of CPB while improving the flowability of the backfill slurry remains unclear. This study investigated the effects of different cementitious material-to-tailings ratios (c/t) (1:6, 1:8, 1:10) in CPB containing a rheological modifier on the mechanical properties and microstructure at various curing ages (7 d, 14 d, 28 d). Strength parameters and failure characteristics of CPB were determined through uniaxial compression tests and direct shear tests. Pore structure, crack analysis, and microstructural characterization were conducted using nuclear magnetic resonance (NMR), computed tomography (CT), and scanning electron microscopy (SEM/EDS). The results of the study indicate that both uniaxial compressive strength and shear strength increase with higher c/t ratios and longer curing periods. The addition of rheological modifier resulted in a 0.9% to 31.5% reduction in uniaxial compressive strength. This negative effect was more pronounced at low c/t ratios and during early-stage curing. However, the effect on shear strength is twofold. Cohesion decreased significantly, while the angle of internal friction increased slightly. Therefore, the strength-reducing effect of the rheological modifier is somewhat mitigated under high normal stress conditions. Analysis of the stress–strain curves indicates that the rheological modifier increased the deformation during the pore consolidation stage and reduced the slope during the linearly elastic stage. However, it slowed the post-peak failure rate, suggesting that it improved the ductility characteristics of CPB to a certain extent. CT analysis showed that the rheological modifier increased the extent of crack damage and the fractal dimension, but did not alter the overall tensile-shear composite failure mode. NMR and SEM analyses further confirmed that the rheological modifier increased the number of micropores, but did not alter the type or elemental composition of the hydration products. This study elucidates the mechanisms by which rheological modifier influences the macroscopic mechanical behavior, pore structure, crack evolution, and microstructure of CPB. It provides a theoretical basis for the optimization of CPB mix designs and the engineering application of rheological modifier.