## 3 Characteristics of Gangue Powder and Slurry
### Gangue Slurry  
The experimental research indicates that gangue, when transformed into slurry, can be effectively utilized for overburden isolated grout filling. The stability and fluidity of the gangue slurry surpass those of fly ash slurry. Additionally, the compaction coefficient of the gangue slurry is lower, suggesting that, under identical filling conditions, a greater quantity of gangue is required. The sedimentation rate, plastic viscosity, and compaction characteristics of the gangue slurry remain consistent across different lithologies. The results from basic characterization tests on three distinct gangue slurries are provided as illustrative examples. In conditions where the water to solid ratio remains constant, there is a positive correlation between the sedimentation rate of the gangue slurry and the particle size of the gangue powder. Similarly, when the particle size of the gangue powder is held constant, the sedimentation rate of the slurry is positively correlated with the water to solid ratio. This can be attributed to the fact that larger gangue powder particles have a higher specific gravity, making them more likely to sink in water. When the particle size of the gangue powder is constant, a higher water to solid ratio results in a lower concentration of gangue particles in the slurry. This reduces the contact force between particles, significantly decreasing the likelihood of friction and thus reducing the particles  
Figure 3: Physical properties of gangue powder: (**a**) gangue powder’s true density and specific surface area; (**b**) gangue powder and fly ash oxide fraction content. resistance to sinking. Consequently, this leads to the accelerated precipitation speed of the gangue particles. Therefore, both the particle size of the gangue powder and the water to solid ratio significantly influence the slurry precipitation rate. The impact of these factors on the sedimentation rate of slurry is substantial. Furthermore, when the water to solid ratio of fly ash slurry ranges from 1:1 to 1:1.5, the water separation rate of the slurry is in the range of 0.1-1.8 mL/min. Under identical water to solid ratio conditions, the water separation rate of the gangue slurry falls within the range of 0.1-1.5 mL/min, indicating that it is lower than that of the fly ash slurry. Consequently, when the particle size of the filling material and the water to solid ratio remain constant, the stability of the gangue slurry is superior (Figure 4). Gangue and fly ash slurry are both characteristic of Bingham fluids. When the water to solid ratio is consistent, there is a direct correlation between the particle size of the dry material and the plastic viscosity of the slurry; smaller particles result in greater plastic viscosity. It is evident that the primary determinant of plastic viscosity is the content of solid-phase particles within the slurry. Consequently, larger gangue powder particle sizes lead to fewer solid-phase particles in the gangue slurry, resulting in reduced plastic viscosity (Figure 5a-c). Figure 4: Variation in the sedimentation rate of gangue powder slurry with the water to solid ratio: (a) gangue slurry #1; (b) gangue slurry #2; (c) gangue slurry #3. A comparison of the plastic viscosity of gangue and fly ash slurry under varying volume concentrations reveals that the plastic viscosity of fly ash slurry is in the range of 0.45-145.2 mPa-s, while that of the gangue slurry falls in the range of 2.1-56.4 mPa-s. Consequently, the plastic viscosity of the gangue slurry is lower than that of fly ash slurry, suggesting that the mobility of gangue slurry surpasses that of fly ash slurry (Figure 5d). The compaction coefficient of the slurry is defined as the ratio of the volume of compacted ash within the overburden fracture to the mass of raw ash. This study calculates the compaction coefficient of gangue slurry with a water to solid ratio of 1:1.1 under varying threshold pressures (4, 6, 8, 10, 12, 14, and 16 MPa).