## 2 Specim Preparation and Experimental Schemes
### Experimental Materials and Specim Preparation  
The raw materials used to prepare the specimens included gangue, cement, sand, and water. The gangue used in the experiments was all separated gangue from the Hegang Coal Mine (Jining City, Shandong Province, China, as shown in Figure 2), with sizes of 0-50 mm. The gangue here was typical of sandstone, with medium hardness, and belonged to the harder type of gangue. When tested, the natural moisture content was determined to be less than 1% and the water absorption rate was 4.49%. A crusher and a sieving machine were used to obtain gangue with sizes of 0-2.5, 2.5-5, 5-10, and 10-15 mm. The cement used was PO42.5 standard Portland cement; its main components were 2 CaO-SiO2, 3 CaO-SiO2, 3 CaO-Al2O3, etc., and common river sand was used.  
The minerals in this material are mainly quartz, kaolin and normal feldspar. Quartz is a stable mineral that can enhance the material's strength and improve the bearing capacity between particles. The surface of the gangue material is uneven, with some tiny pores and cracks, but they do not form a complete network of cracks. The material as a whole is compact and has good integrity. The edges of the pores and cracks, as well as the surrounding particles, are well cemented.  
The particle size of a fine aggregate (sand) must not exceed 4.75 mm, the void rate must be less than 44%, and the loose bulk density must be less than 1400 kg/m\({}^{3}\).  
The specimen formation process included two processes: slurry preparation and curing. The specimen preparation process is shown in Figure 3.  
Slurry preparation: Based on pre-test results and the results of a survey on an underground mine site, the gangue size ranged 0-15 mm with a mass concentration of 69-78%, sand content of 15-45%, and cement content of 9-18%, as shown in Table 2. A total of 16 groups of the gangue-based shotcrete materials applied to roadways (A\({}_{1}\)\(\sim\)A\({}_{4}\), B\({}_{1}\)-B\({}_{4}\), C\({}_{1}\)-C\({}_{4}\), and D\({}_{1}\)-D\({}_{4}\)) were prepared using single-factor rotational experiments. The percentages in the table represent the mass percentages of raw materials.  
During the preparation process, the pre-weighed coarse aggregate, fine aggregate, and cement were first mixed and stirred for 5 min. After that, the evenly mixed accelerator as well as water were injected into the dry material and stirred for another 5 min at a speed of 15 turns per minute. This process should be carried out in accordance with the conditions in the laboratory and the technical specification for application of sprayed concrete [38]. The total mass of solid materials in each group of experiments, including gangue, sand, and cement, was set at 8 kg. The specific water consumption was calculated based on the mass concentration, as shown in Table 2. The density of the gangue-based shotcrete slurry prepared according to the schemes ranged from 2300 kg/m\({}^{3}\) to 2490 kg/m\({}^{3}\), with an average density of 2385 kg/m\({}^{3}\).  
Figure 4: Mineral composition of gangue.  
Figure 5: Gangue microstructure.  
Specimen curing: The gauge-based shotcrete material applied to roadways in schemes A\({}_{1}\) to D\({}_{4}\) was poured into plastic cube molds measuring 70.7 mm \(\times\) 70.7 mm \(\times\) 70.7 mm. The specimens were then cured in a wet curing box at a temperature of 23 \(\pm\) 2 \({}^{\circ}\)C and a relative humidity above 95%, as shown in Figure 6. The curing periods were set to 1, 3, 7, 14, and 28 d.