## 3 Experimental Results and Discussion
### Influences of Mix Proportions on Slump of the Gangue-Based Shotcrete Material
#### 3.2.4 Influences of the Cement Content  
Table 6 displays the peak compressive strengths of the gangue-based shotcrete specimens under schemes D1-D4 with curing periods of 1 d, 3 d, 7 d, 14 d, and 28 d. Figure 12 shows the changes in the peak compressive strengths of specimens under schemes D1-D4 with various curing periods.  
Table 6 and Figure 12 show that the compressive strengths of specimens under schemes D1-D4 improve with prolonged curing, consistent with the effects of other mix-related factors on the mechanical properties of such specimens.  
Specimens cannot be formed under scheme D1 after being cured for 1 d, resulting in a compressive strength of 0. However, under schemes D1-D4, the compressive strength gradually increases with an increase in cement content after the same curing period. After curing for 28 d, the peak compressive strengths of specimens prepared under schemes D1-D4 are 2.32, 3.03, 3.35, and 3.52 MPa, respectively. Increasing the cement content results in a decrease in specimen porosity, an increase in density, and an increase in the area bound to the aggregate, as well as an increase in compressive strength.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Peak Compressive** & **1 d** & **3 d** & **7 d** & **14 d** & **28 d** \\
**Strength/MPa** & & & & & \\ \hline D1 & 0 & 0.26 & 0.89 & 1.62 & 2.32 \\ D2 & 0.31 & 0.56 & 1.49 & 2.26 & 3.03 \\ D3 & 0.35 & 0.67 & 1.71 & 2.57 & 3.35 \\ D4 & 0.46 & 1.16 & 2.48 & 2.89 & 3.52 \\ \hline \hline \end{tabular}
\end{table}
Table 6: Peak compressive strengths of the gangue-based shotcrete specimens under schemes D1-D4 with different curing periods.  
Figure 11: Influences of different sand contents on the compressive strength of the gangue-based shotcrete material applied to roadways.  
On the other hand, on-site experience indicates that cement content cannot be increased indefinitely. Excessive cement content leads to poorer plasticity and increases the likelihood of cracking during the later stages of curing. Correspondingly, the cost also increases. Excessive cement results in an uneven distribution of cement paste within concrete specimens, making it difficult to form a uniform and stable structure. This leads to the presence of voids and a decrease in the material's forming ability, ultimately affecting the mechanical properties of the specimens. Additionally, the hydration reaction of cement releases more heat. The formation of contractions and cracks within concrete specimens is exacerbated by the combined effects of hydration products and accumulated heat, particularly during the early stages of the curing period [41]. This phenomenon did not occur in this case, as the cement content did not exceed a reasonable value under schemes D1-D4.  
Additionally, the early strengths of specimens prepared under schemes D1-D4 grow much faster than the later strengths, as do the compressive strengths of specimens under schemes A1-A4, B1-B4, and C1-C4. This can be attributed to the fact that the presence of C3A and C3S in cement enhances the early-age strengths, while the late-age strengths exhibit a slower increase. Secondly, with the development of the cement-grinding process, cement clinkers can be ground to greater fineness. Fine particles, smaller than 1 \(\upmu\)m in diameter, accelerate the rate of the cement's hydration, improving the early-age strength while contributing little to late-age strength.