## 3 Experimental Results and Discussion
### Influences of Mix Proportions on Slump of the Gangue-Based Shotcrete Material  
According to the Technical Specification for Application of Sprayed Concrete and Standard for Quality Control of Concrete [38; 39], the optimum range of shotcrete slump is 80-220 mm under the condition of ensuring the concrete's compatibility and pumping ability. The slump of the gangue-based shotcrete material was recorded in each of the 16 single-factor rotational experiments. The sensitivity of the slump to changes in each proportioning factor is plotted in Figure 8.  
1. When changing only the gangue size, the slurry slump tends to decrease initially and then increase as the gangue size enlarges. In the experimental range of 0-10 mm, the slump decreased from 265 mm to 201 mm as the gangue size increased. However, it increased again to 245 mm when the gangue size was in the range of 10-15 mm. The increase in the gangue size enhanced the plasticity of aggregates in the slurry. Fine aggregates, including cement, can fill in the gaps between gangue and sand particles, causing the slump to gradually decrease. However, if the gangue size increases further, gaps in the slurry gradually widen, and the slurry cannot accumulate. This results in limited adhesion between particles, poor flow plasticity, and increased slump. 2. The slump is negatively correlated with the mass concentration, meaning that as the mass concentration increases, the slump decreases. The slump ranges from 272 to 267 mm when the mass concentration is in the range of 69-72%. If the mass concentration increases beyond this range, the slump reduces significantly, reaching 224 to 201 mm when the mass concentration is 75-78%. A high-concentration slurry results in poorer fluidity in the gangue-based shotcrete material when applied to roadways, which can cause it to set and accumulate into blocks, leading to a decrease in slump. Figure 7: Slump testing. 3. After altering the sand content, the slump initially decreases, and it then increases as the sand content increases. This trend is similar to the effect of the gangue size. At a sand content of 35%, the slump reaches its minimum at 200 mm. This is because as the sand content increases, the plasticity of the slurry gradually increases while the slump decreases. If the sand content increases further, the plasticity of the slurry deteriorates due to the low bonding capacity of sand. As a result, the shotcrete slurry cannot remain intact and the slump gradually increases. 4. Cement is an inorganic cementing material used to bond other raw materials. Increasing the cement content in the gangue-based shotcrete material applied to roadways within the range of 9% to 12% results in an increase in slump from 212 to 224 mm. However, further increasing the cement concentration to 18% leads to a decline in slump to 204 mm. The increment of cement content increases the amount of cementing materials as well as the cohesiveness, which results in a reduction in slump. Overall, the effects of cement concentration on the slump are less significant than other material proportioning factors. This is due to the fact that cement does not contribute to hardening in the initial stage of curing. However, increasing the cement content appropriately can improve the flow plasticity of the specimens. Influences of Mix Proportions on Mechanical Properties of the Gangue-Based Shotcrete Material Applied to Roadways  
The Technical Code for Engineering of Ground Anchorages and Shotcrete Support [40] provides a range of average compressive strength values for wet shotcrete at 1 d and 28 d. On this basis, the results of the experiment have been analyzed. During the early curing period, there were only minor differences in the compressive strengths of the specimens. However, after being cured for 28 d, the compressive strength reached its maximum value, and the difference became significant.