## 4 Conclusions  
Experimental schemes for the performance of shotcrete prepared at different rates of cement replacement were designed. The mineral compositions and microscopic characteristics of the shotcrete with the partial replacement of cement with gangue powder were compared. The influences of the partial replacement of cement with gangue powder on the slump, tensile strength, and compressive strength of shotcrete were unveiled. The following key conclusions were obtained:  
Figure 12: Micro-morphologies of shotcrete without replacement of cement. Figure 13: Micro-morphologies of shotcrete with the partial replacement of cement with gangue powder. 1. By analyzing the experimental results of the conveying performance of shotcrete with the partial replacement of cement with gangue powder, the rate of cement replacement is found to be inversely proportional to the slump of the shotcrete. This is mainly because the ball-milled gangue particles show non-uniform shapes and particle sizes, enhanced water absorption, and raised the number of pores. The ball-milling duration of gangue is inversely proportional to the slump of shotcrete mainly because as the ball-milling duration is extended, the gangue particles are gradually refined; their specific surface area gradually increases, and particles are arranged in a less uniform manner;
2. The analysis of the experimental results pertaining to tests of the mechanical performance of the shotcrete with the partial replacement of cement with gangue powder reveals that the compressive and tensile strengths of the shotcrete are directly proportional to the ball-milling duration of gangue and inversely proportional to the rate of cement replacement. This is mainly because with the extension of ball-milling duration, the gangue particles are refined; the activity is improved, and the filling effect is enhanced. As the rate of cement replacement is increased, the amount of the cementitious material in the shotcrete decreases; the particles show poorer dispersion, and the porosity increases. Moreover, the shotcrete meets the strength requirements for engineering applications only when the cement replacement ratio is 30% with gangue ball-milling durations of 3 h and 5 h;
3. The mineral compositions and microscopic characteristics of the shotcrete with the partial replacement of cement with gangue powder are analyzed. The results show that, after partially replacing cement with gangue powder, the shotcrete contains more clay minerals such as kaolinite while having a lower cement content than the shotcrete without the replacement of cement. As a result, the content of the cement-based cementitious material reduces accordingly, leading to lower strengths of the shotcrete. Overall, partially replacing cement with ball-milled coal gangue to produce shotcrete is feasible for underground mine roadway applications, effectively utilizing solid waste gangue and reducing cement consumption. However, studies indicate that the shotcrete with the partial replacement of cement with gangue powder exhibits lower early-age strength compared to similar materials, potentially leading to inadequate initial support strength post-spraying. Moreover, the ball-milling process for gangue is relatively complex, requiring specialized equipment and extended durations (3-5 h), posing challenges for large-scale applications in mine roadways. Future research should focus on optimizing materials and processing techniques, alongside conducting pilot projects to accumulate data for broader implementation in tunnel engineering and slope protection. Formal analysis, X.W. ; Investigation, R.H.; Data curation, S.K. ; Writing--original draft, S.L., X.W. and R.H.; Writing--review & editing, Y.Z. ; Project administration, X.W. and R.H. All authors have read and agreed to the published version of the manuscript. This work is supported by Open Fund of State Key Laboratory of Water Resource Protection and Utilization in Coal Mining [Grant No.