## 3 Experimental Results and Discussion
### Influences of Mix Proportions on Slump of the Gangue-Based Shotcrete Material
#### 3.2.1 Influence of the Gangue Size  
Table 3 lists the peak compressive strengths of the gangue-based shotcrete specimens prepared following schemes A\({}_{1}\)-A\({}_{4}\) with curing periods of 1 d, 3 d, 7 d, 14 d, and 28 d. On this basis, changes in the curves of the specimens under schemes A\({}_{1}\)-A\({}_{4}\) with different curing periods are plotted in Figure 9.  
Table 3 and Figure 9 demonstrate that the compressive strength increases with the lengthening of the curing period for specimens of all gangue sizes. This is due to the gradual reduction in water content in the specimens and the more complete cementation of the aggregates. Among the specimens subjected to the same curing period, those under scheme A\({}_{2}\) (gangue size: 2.5-5 mm) exhibit significantly higher compressive strength than those under schemes. After 28 d of curing, the compressive strength of specimens in scheme A\({}_{2}\) reached 3.54 MPa. This is, respectively, 1.02, 1.27, and 1.43 times higher than the compressive strengths of specimens under schemes A\({}_{1}\), A\({}_{3}\), and A\({}_{4}\).  
Increasing the size of the gangue can improve the strength of the specimens in three ways: by compacting the aggregate arrangement, enhancing the aggregate skeleton effect, and increasing the coverage of water cement hydration products.  
A reasonable particle size can improve the uniform arrangement of gangue in concrete voids and enhance the compactness of concrete specimens. When compared to gangue with  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Peak Compressive** & **1 d** & **3 d** & **7 d** & **14 d** & **28 d** \\ \hline A\({}_{1}\) & 0.52 & 0.68 & 1.4 & 2.32 & 3.47 \\ A\({}_{2}\) & 0.76 & 1.02 & 1.79 & 2.5 & 3.54 \\ A\({}_{3}\) & 0.7 & 0.89 & 1.71 & 2.06 & 2.79 \\ A\({}_{4}\) & 0.62 & 0.82 & 1.56 & 1.91 & 2.48 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Peak compressive strengths of the gangue-based shotcrete specimens prepared under schemes A\({}_{1}\)-A\({}_{4}\) with different curing periods.  
Figure 9: Influence of the gangue size on the compressive strength of the gangue-based shotcrete material applied to roadways.  
larger particle sizes (schemes A\({}_{3}\) and A\({}_{4}\)), the filling capacity of gangue-based shotcrete material is insufficient, resulting in poor concrete compactness. Simultaneously, homogenizing the material specimens will stabilize their aggregate skeleton structure. This is the reason why the overall compressive strength of small-particle-size gangue-based shotcrete material specimens is significantly higher. Reducing the gangue particle size increases its surface area, allowing water cement hydration products to cover the surface more easily.  
The curing period of 14 d was taken as a boundary. The results show that specimens under scheme A\({}_{1}\) have lower compressive strengths than those under schemes A\({}_{3}\) and A\({}_{4}\) when the curing period is shorter than 14 d. Conversely, when the period is prolonged, the compressive strengths of specimens under scheme A\({}_{1}\) exceed those of specimens under schemes A\({}_{3}\) and A\({}_{4}\). Finally, specimens under scheme A\({}_{1}\) exhibit compressive strengths similar to those of specimens under scheme A\({}_{2}\), reaching 3.47 MPa. This phenomenon occurs because, during the early curing period, the compressive strength of specimens is mainly attributed to the compressive capacity of the aggregates themselves. Therefore, specimens' compressive capacity increases with larger gangue size during a short curing period. Conversely, shotcrete materials prepared with smaller gangue sizes have more uniformly distributed internal components, resulting in a better bearing capacity after a long curing period.