<p>This study systematically investigated the effects of sodium silicate on the setting characteristics, expansion behavior, and mechanical properties of expansive slurry through setting time tests, expansion evolution monitoring, uniaxial compression experiments, X-ray diffraction, and scanning electron microscopy analyses. The main findings are as follows: (1) Sodium silicate significantly shortens the setting time of the expansive slurry, reducing the initial setting time from over 10 h to less than 1 h, representing a reduction of more than 90%. (2) Sodium silicate does not markedly influence the temporal progression of expansion evolution, it alters the final expansion ratio, which exhibits a nonlinear response characterized by an initial slight increase followed by a gradual decline with increasing sodium silicate content. (3) Sodium silicate substantially enhances the uniaxial compressive strength of the slurry, elevating the strength from below 5 MPa to over 17 MPa. (4) Mechanistic analysis reveals that sodium silicate accelerates the hydration reaction by rapidly generating C–S–H gel, while the abundant C–S–H products fill interparticle voids within hydration products, significantly reducing specimen porosity. This study reveals the dual mechanism by which sodium silicate enhances mechanical strength by accelerating the hydration process, providing a new theoretical basis for material design in the rapid reinforcement of fractured rock masses in underground engineering.</p>