Abstract: A novel fly ash (FA) modified ultrafine cement composite grouting material (FASC) was developed under the coupling effect of multicomponent admixtures, including an accelerator, a polycarboxylate superplasticizer, and an expansive agent, to address deficiencies in the mechanical performance and low industrial solid waste utilization of conventional cement-based grouting materials in deep soft rock mining. The water-to-cement ratio was fixed at 0.4, and the mass fraction of FA content was varied from 0% to 20 % (designated as S1–S5). All experiments were conducted at (20±2) °C. The workability parameters—setting time, fluidity, bleeding rate, and expansion—were measured according to Chinese standards. Mechanical properties, including compressive and flexural strengths, were evaluated at 1, 3, 7, and 28 d. Microstructural evolution was characterized by X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy with deconvolution fitting, scanning electron microscopy (SEM), and thermogravimetry (TG) analyses on samples cured for 7 and 28 d. Sandstone gravel cementation tests were also performed using 0% and 5% FA formulations to assess the practical reinforcement performance under uniaxial compression. The results indicated that increasing FA content prolonged both initial and final setting times, enhanced fluidity, and significantly reduced the bleeding rate. For the optimal 5% FA sample (S2), the initial and final setting times increased by 118.4% and 95.2%, respectively, compared to the control (S1), while the bleeding rate decreased by 25%. Compressive and flexural strengths first increased and then decreased with rising FA content. At 5% FA, the 28 d compressive strength reached 39.8 MPa, which was 2.6% higher than that of S1. The 1 and 28 d flexural strengths increased by 12.6% and 8.7%, respectively. Further increases in FA content by at least 10% led to gradual strength reduction, with S5 (20% FA) showing a 26.5% decrease in 28 d compressive strength relative to S1. The expansion ratio of hardened FASC peaked at 5% FA (+0.9% vs. S1). Microstructural analyses provided mechanistic insights. XRD patterns revealed that the principal hydration products were ettringite, C–S–H gel, and Ca(OH)₂. As the FA content increased up to 15%, the C–S–H peak intensified while the Ca(OH)₂ peak weakened, confirming pozzolanic consumption of Ca(OH)₂ to form additional C–S–H. FTIR deconvolution fitting in the 800–1300 cm^–1 region showed that the peak area of Q^2–3 (medium-to-low polymerized C–S–H) increased by 305%–424% for FA-containing samples relative to S1, indicating that FA promoted C–S–H formation. SEM observations demonstrated that the 5% FA sample possessed a dense microstructure with a well-defined C–S–H gel layer at the FA interface, effectively filling micropores. In contrast, higher FA content (≥10%) introduced unreacted glass beads and microcracks. Energy dispersive X-ray spectroscopy analysis revealed a decrease in Ca/Si atomic ratio from 4.44 (S1) to 2.14 (S2), confirming secondary hydration. TG/derivative thermogravimetry curves further showed that the 5% FA sample exhibited higher mass loss in the 0–150 °C range and stronger dehydration peaks at 150 °C and 400 °C, corresponding to increased C–S–H and ettringite, while the Ca(OH)₂ decomposition peak at 420 °C was weaker, indicating enhanced pozzolanic consumption. Sandstone gravel cementation tests confirmed the optimal performance at 5% FA. Compared to the control (J1), the 5% FA cemented specimen (J2) exhibited a 13.1% increase in uniaxial compressive strength (from 14.51 MPa to 16.41 MPa) and a 9.2% increase in elastic modulus, along with reduced elastic and plastic deformations (decreased by 22.2% and 28.6%, respectively), signifying improved deformation recovery. Failure mode analysis revealed that J2 developed a single dominant shear crack, while J1 exhibited severe splitting-shear failure with multiple cracks. SEM images of fracture surfaces further indicated that J2 possessed superior interfacial bonding. The reinforcement mechanism is attributed to the synergistic “physical filling-chemical bonding-crack passivation” effect of FA under the multicomponent admixture system. This study offers a green, high-performance grouting material for deep soft-rock mining applications.