## 3 Experimental Results and Discussion
### Changes in the Tensile Strength  
The tensile strength of shotcrete refers to the maximum tensile stress that can be borne by specimens at failure under uniaxial tensile stress. The tensile strength is generally evaluated through Brazilian splitting tests. In the method, a solid cylindrical specimen was placed in the testing machine, which applied the radial compressive load until fracturing of the specimen. In this way, the maximum tensile stress was ascertained. Data were computed after each experiment, and the tensile strength of specimens was calculated using Equation (2):  
\[R_{\text{t}}=\frac{2P}{\pi DL} \tag{2}\]  
where \(R_{\text{t}}\) is the tensile strength of the shotcrete specimens (MPa); \(P\) is the failure load of the shotcrete specimens (N); \(D\) is the diameter (mm); \(L\) is the thickness (mm). The average tensile strength of the specimens in each group was computed using Equation (3)  
\[R_{\text{ts}}=\frac{1}{n}\sum_{i=1}^{n}R_{\text{ti}} \tag{3}\]  
where \(R_{\text{ts}}\) is the average tensile strength of the shotcrete specimens (MPa); \(R_{\text{ti}}\) is the tensile strength of the \(i\)th shotcrete specimen (MPa); \(n\) is the number of specimens.  
For each test group, three specimens were evaluated, and the mean tensile strength was calculated from their averaged results. Test results pertaining to the tensile strength of shotcrete with the partial replacement of cement with gangue powder are illustrated in Figure 9.  
According to the analysis of Figure 9, the tensile strength of specimens is gradually improved with the prolongation of the ball-milling of gangue. The tensile strength is low when the ball-milling duration is 1 h. Taking shotcrete with a rate of cement replacement of 30% as an example, when the ball-milling duration is extended from 1 h to 5 h, the tensile strength of the shotcrete cured for 28 d is improved by 34.1% from 1.26 MPa to 1.69 MPa; that of the shotcrete cured for 7 d increases by 35.4% from 0.82 MPa to 1.11 MPa. The tensile strength of shotcrete is improved mainly because the prolonging ball-milling duration of gangue promotes the further activation and dispersion of gangue particles. As a result, more surfaces of gangue particles are exposed to the cement matrix, which enhances the combination of gangue particles and the cement matrix, thereby improving the overall tensile strength of the shotcrete.  
With the increase in the rate of cement replacement, the tensile strength of specimens gradually declines. The shotcrete exhibits a high tensile strength when the rate of cement replacement is 30%. For instance, when the ball-milling duration is 3 h and the rate of cement replacement grows from 30% to 50%, the tensile strength of shotcrete cured for 28 d declines by 40.4% from 1.56 MPa to 0.93 MPa; for the shotcrete cured for 7 d, its tensile strength reduces by 61.9% from 1.13 MPa to 0.43 MPa. The reduction in the tensile strength of shotcrete is mainly because as the rate of cement replacement is enlarged, the cement content in shotcrete decreases, leading to a reduction in cementitious materials in the shotcrete. Considering that cement is the main cementitious material in shotcrete and the ball-milled gangue exhibits a poorer cementation effect, the shotcrete has a low tensile strength overall.  
To meet practical engineering requirements, the shotcrete cured for 28 d needs to achieve compressive strengths exceeding 20 MPa and a tensile strength above 1.5 MPa. The shotcrete meets the strength requirements only when the cement replacement ratio is 30% with gangue ball-milling durations of 3 h and 5 h. Under these conditions, the compressive strengths are 20.18 MPa and 21.07 MPa, and tensile strengths reach 1.56 MPa and 1.69 MPa, respectively.  
Figure 9: Changes in the tensile strength of shotcrete with the partial replacement of cement with gangue powder.