Formation and Performance of a Polymer–Cement Composite Gel in Magnesium Phosphate Cement Grouting Materials Modified by Steel Slag and Latex Powder

Magnesium phosphate cement (MPC) shows great potential for complex underground environments due to its rapid-hardening and early-strength properties. However, its large-scale application is hindered by several drawbacks, including high hydration heat, rapid setting, and insufficient long-term durability. To address these limitations, this study developed a novel MPC grouting material modified with steel slag (SS) and redispersible latex powder (LP). We systematically investigated the workability, mechanical properties, durability, and microstructural evolution of this modified system. Results indicate that incorporating SS and LP decreases both the fluidity and setting time of the grout. An optimal SS dosage accelerates reaction kinetics and raises the peak hydration temperature. Conversely, the LP-induced polymer film suppresses the overall temperature rise, delaying the first exothermic peak and advancing the second. The incorporation of 5% steel slag increased the 28-day compressive strength of the MPC to 54.86 MPa. Building on this, the combined addition of 0.15% latex powder further elevated the strength to 58.82 MPa. Microstructural and pore analyses confirmed that the steel slag enhanced interfacial bonding through physical filling and the formation of calcium phosphate crystals. Meanwhile, the latex powder formed a continuous polymer film, which tightly wrapped and bridged the hydration products and unreacted particles. This synergistic mechanism effectively sealed the capillary pores and reduced the proportion of harmful pores by 15.99% compared to the control group. Consequently, the densified MPC matrix laid a solid microstructural foundation for the material’s excellent durability. It offers reliable, high-performance material for seepage control and strata reinforcement in complex environments.