Influence of sand-to-cement ratio on mechanical properties of grout–rock interface in cement-based grouting materials

Sand content in cement-based grouting materials (CBGMs) significantly influences the mechanical performance of anchorage structures. In this study, seven CBGM mix proportions with sand-to-cement ratios (S/C) of 0, 0.5, 1, 1.5, 2, 2.5, and 3 were prepared. The flowability, compressive strength, and drying shrinkage characteristics of cement-based grouts with different S/C values were systematically evaluated. Rock-bolt pull-out specimens were then prepared using CBGMs as anchoring agents, and pull-out tests were conducted using a self-developed pull-out testing apparatus to investigate the influence of sand content on the mechanical properties of anchorage structures. The test results indicate that sand content has a significant regulatory effect on the flowability and shrinkage behaviour of CBGMs, with both flowability and drying shrinkage gradually decreasing as sand content increases. The compressive strength of the grout first increases and then decreases with increasing sand content: the strength increases continuously when S/C is below 1.5, whereas a minor reduction occurs once this critical value is exceeded. Appropriate sand content suppresses the propagation of penetrating cracks within the cement paste, thereby enhancing the compressive strength and volumetric stability of CBGMs. In contrast, excessive sand content deteriorates the interfacial transition zone between the cement matrix and sand particles, reducing the overall compressive strength of the composite grout. Higher sand content strengthens the dilatancy effect at the grout–rock interface, generating greater radial confining stress and improving the ultimate pull-out capacity of anchorage structures. However, further increases in sand content beyond S/C = 1.5 provide only limited enhancement in radial stress and anchoring resistance. The outcomes of this study provide experimental references for the optimal mix design of CBGMs used in anchorage structures.