## 3 Experimental Results and Discussion
### Influences of Mix Proportions on Slump of the Gangue-Based Shotcrete Material
#### 3.2.2 Influence of the Mass Concentration  
Table 4 displays the peak compressive strengths of gangue-based shotcrete specimens under schemes B\({}_{1}\)-B\({}_{4}\) with curing periods of 1 d, 3 d, 7 d, 14 d, and 28 d. Based on these data, Figure 10 shows the change curves of the peak compressive strengths of the specimens under schemes B\({}_{1}\)-B\({}_{4}\) with different curing periods.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Peak Compressive** & **1 d** & **3 d** & **7 d** & **14 d** & **28 d** \\ \hline B\({}_{1}\) & 0.31 & 0.64 & 1.15 & 1.48 & 1.81 \\ B\({}_{2}\) & 0.45 & 0.75 & 1.35 & 1.97 & 2.23 \\ B\({}_{3}\) & 0.51 & 1.11 & 1.69 & 2.26 & 3.34 \\ B\({}_{4}\) & 0.64 & 1.52 & 2.42 & 2.85 & 3.61 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Peak compressive strengths of gangue-based shotcrete specimens under schemes B\({}_{1}\)–B\({}_{4}\) with different curing periods.  
Figure 10: Influence of different mass concentrations on the compressive strength of the gangue-based shotcrete material applied to roadways.  
Table 4 and Figure 10 demonstrate that the cement hydration reaction continues, and cementing material fills the capillary pores, as the curing period is prolonged [40]. As a result, the compressive strengths of all mass concentration specimens increase. Additionally, an increase in mass concentration leads to an increase in compressive strength after the same curing period. Figure 10 lists various schemes in descending order after all curing periods. According to the peak compressive strengths of corresponding specimens, the compressive strengths of specimens under scheme B4 reach the maximum (3.61 MPa) when the curing period is 28 days. The other schemes, B3, B2, and B1, have lower compressive strengths.  
The increase in mass concentration is equivalent to a reduction in the water/cement ratio of the specimens during the initial period. The decrease in water content during the setting and curing processes reduces the number of pores and fractures in the specimens, thereby improving their compressive strength.  
An increase in the water/cement ratio results in a decrease in the dispersion performance of cement hydration products, making it difficult for them to form a strong skeleton structure. Analyzing the material's conveying properties, an increase in water/cement ratio (resulting in a reduced mass concentration) leads to the increased fluidity of the shotcrete slurry, which makes the aggregate of the shotcrete slurry easier to deposit, resulting in an uneven distribution of the aggregate and reduced mechanical properties in gangue-based shotcrete material specimens.  
Furthermore, it was observed that shotcrete specimens with different mass concentrations achieve comparable compressive strengths in a short curing period. However, as the curing period is prolonged, specimens with a higher mass concentration gradually demonstrate their superiority in terms of compressive strength. Figure 10 illustrates that, after 28 d curing, the compressive strengths of specimens under schemes B4 and B3 are higher than those subjected to schemes B1 and B2.