In tunnel engineering, when facing geological hazards such as water inrush and mud inflow, the performance of grouting materials directly affects construction safety and long-term stability. To address the limitations of traditional cement-based materials, such as slow setting and high cost of chemical grouts, this study utilizes fly ash (FA) and ground granulated blast furnace slag as the base materials, and sodium silicate combined with sodium hydroxide as alkali activators, to prepare geopolymer grouting materials with various mix proportions. The effects of hydrated lime, polyacrylamide (PAM), anhydrite, superplasticizer, sodium aluminate, and the dosage of alkali activators on material properties were systematically investigated. Results show that the incorporation of sodium aluminate significantly enhances the late-stage expansion rate, while alkali activators promote further expansion. A combination of low PAM dosage and high superplasticizer content effectively balances compressive strength and fluidity. All mix designs exhibited zero bleeding, indicating excellent material stability. Microstructural analysis further revealed that ettringite formed from the reaction of sodium aluminate, anhydrite, and Ca²⁺ under alkaline conditions provided a rigid skeleton that contributed to increased 7-d strength. Meanwhile, the formation of sodium aluminosilicate hydrate gels improved interfacial bonding and enhanced long-term durability. This study provides theoretical support and technical guidance for the application of FA–slag-based geopolymer grouting materials in complex geological environments.