As the cost of grouting treatment for water control in coal roofs during underground coal mining continues to rise, coupled with the accumulation of industrial solid waste resulting from rapid economic development in China, the ecological environment is facing severe challenges. To address these issues, this study, based on a high water-to-cement ratio, uses mine overburden (OB) and furnace bottom ash (FBA) as the primary raw materials, with sodium silicate as the modifier, to develop a new type of geopolymer grouting material with high stability and compressive strength for coal roof water control. Additionally, COMSOL software was used to numerically simulate the diffusion process of the grout slurry in fractures under dynamic water flow. The results indicate that, with a sodium silicate modulus of 1.5 and a dosage of 4%, the stability of the slurry increased by 26.2%, and the 28-day compressive strength improved by 130.98%. Numerical simulations further show that the diffusion process of the slurry is closely related to slurry viscosity, grouting pressure, and grouting time and that the diffusion pattern in the fractures is similar to that of ultra-fine cement slurry. This study provides a theoretical basis for coal mine roof grouting water control projects.