Influence of Water-Cement Ratio on WPU-Modified Ultrafine Cement Grouting Materials and Reinforcement Mechanism for Crushed Sandstone.

To balance slurry workability for deep-mine grouting and the load-bearing capacity of grouted crushed-sandstone, a waterborne polyurethane (WPU)-modified ultrafine cement composite grouting material was successfully developed using ultrafine Portland cement blended with a polycarboxylate superplasticizer, an expansive agent, and an accelerator. Macro- and microscale characterizations were conducted to clarify the influence of w/c (0.40∼0.60) on slurry properties, mechanical performance, hydration kinetics, and microstructure. Load-bearing behavior and failure modes were further evaluated via crushed sandstone cementation tests. The results show that the addition of 0.5 wt % WPU shortens the initial setting time, reduces volumetric shrinkage, and enhances mechanical strength. With increasing w/c, fluidity and setting time increase markedly, whereas mechanical strength first increases and then decreases. Optimum performance is achieved at a w/c of 0.45, where the 1 d compressive and flexural strengths increase by 22.87 and 4.3%, respectively, relative to the control sample. In the grouting reinforcement test, the 28 d peak strength of the WPU-modified grouted crushed-sandstone specimen at a w/c of 0.45 reaches 20.09 MPa, which is 38.55% higher than that of the control sample, indicating improved load-bearing capacity and elastic deformation capability. XRD, FTIR, SEM, TG, and isothermal calorimetry collectively suggest that the w/c can modulate interfacial coverage and bridging effects of WPU by regulating the water state and pore structure, thereby improving matrix continuity. Accordingly, a triple-coupled mechanism involving hydration-kinetics regulation, interfacial cooperative enhancement, and constrained pore-structure evolution is proposed.