## 3 Experimental Results and Discussion
### Evolution of the Compressive Strength  
The uniaxial compressive strength of shotcrete refers to the ratio of the maximum failure load borne by standard specimens to the stressed area under uniaxial compression. In the experiments, a WAW-1000 testing machine was used to apply load to shotcrete specimens in the displacement-controlled mode. After tests, Equation (1) was used to calculate the compressive strength of the specimen.  
\[R_{\text{C}}=\frac{P}{S}\times 10^{-6} \tag{1}\]  
where \(R_{\text{c}}\) is the compressive strength of the shotcrete specimens (MPa); \(P\) is the failure load of shotcrete specimens (N); \(S\) is the initial stressed area (m\({}^{2}\)).  
For each test group, three specimens were evaluated, and the mean compressive strength was calculated from their averaged results. The test results of the compressive strengths of the shotcrete specimens with the partial replacement of cement with gangue powder are displayed in Figure 8.  
Figure 8: Change trends of compressive strengths of shotcrete with the partial replacement of cement with gangue powder.  
The analysis of Figure 8 reveals that relatively low early-stage compressive strength and high long-term compressive strength of the specimens are observed. As the ball-milling duration of gangue extends, the compressive strength of specimens is gradually improved. The compressive strength is low when the ball-milling duration is 1 h. For example, at the rate of cement replacement of 30%, as the ball-milling duration is prolonged from 1 h to 5 h, the compressive strength of shotcrete cured for 28 d increases by 12.8% from 18.68 MPa to 21.07 MPa; that of shotcrete for 7 d grows by 41.5% from 10.16 MPa to 14.38 MPa. The increment of compressive strength of specimens is because ball-milling can improve the structure of gangue particles, refine these particles, and increase the content of active components such as free SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\). After these active components are mixed with cement, minerals in cement begin to be hydrated to produce Ca(OH)\({}_{2}\), which undergoes secondary reactions with SiO\({}_{2}\) and Al\({}_{2}\)O\({}_{3}\), thus forming calcium silicate hydrate (C-S-H) and calcium aluminate hydrate (C-A-H) gels. These reactions are conducive to the formation of tighter structures, thereby improving the long-term compressive strength of shotcrete. However, the addition of gangue powder dilutes cement particle concentration, slowing early-stage heat release and compromising initial strength development.  
With the increase in the rate of cement replacement, the compressive strength of specimens gradually declines. The compressive strength is large at a rate of cement replacement of 30%. For instance, when the ball-milling duration is 3 h and the rate of cement replacement enlarges from 30% to 50%, the compressive strength of shotcrete cured for 28 d declines by 54.5% from 20.18 MPa to 9.18 MPa; it reduces by 28.0% from 12.72 MPa to 9.16 MPa for shotcrete cured for 7 d. The main cause of the reduction in the compressive strength is that as the proportion of cement replaced by gangue powder increases, the cement content in the shotcrete decreases accordingly. The ball-milled gangue particles are poorly cemented, which leads to an insufficiency of the cementitious material in the shotcrete, thus reducing its compressive strength.