## 3 Determination of the Optimal Ratio of Cemented Crushed-Stone Slurry
### Cementation Factors of Crushed Stone Inside the Goaf
Analysis on factors affecting UCS  
The UCS test of the GCB was carried out for three specimens in each group, and the average value of the three specimens was taken as the UCS of the GCB in that group. The results of the UCS of the GCB at different ratios are given in Table 3.  
The average value of the test results for each factor and level was taken. Subsequently, the sensitivity analysis curves of factors influencing the UCS of the GCB were plotted with the UCS as the vertical coordinate and the levels of different influence factors as the horizontal coordinate (Figure 8). \begin{table}
\begin{tabular}{c c c c c c c} \hline
**No.** & **UCS/MPa** & **No.** & **UCS/MPa** & **No.** & **UCS/MPa** & **No.** & **UCS/MPa** \\ \hline
1 & 6.32 & 5 & 5.43 & 9 & 6.18 & 13 & 3.61 \\
2 & 4.82 & 6 & 5.34 & 10 & 3.33 & 14 & 4.02 \\
3 & 6.21 & 7 & 2.83 & 11 & 2.72 & 15 & 5.40 \\
4 & 3.30 & 8 & 4.58 & 12 & 2.69 & 16 & 4.39 \\ \hline \end{tabular}
\end{table} 
Table 3: UCS results of GCB specimens. Figure 7: Test procedure of slurry fluidity. As displayed in Figure 8, within the range of experimental design indexes, the cement-sand ratio exerts the strongest influence on the UCS of the GCB specimens, followed by aggregate grading, and water-cement ratio has the weakest influence. At the same time, the optimal ratio of the slurry to achieve a high UCS is found to be a water-cement ratio of 2.5:1 and a cement-sand ratio of 1:8, and the second optimal ratio is a 2.5:1 water-cement ratio and 1:4 cement-sand ratio. In addition, it can be found from the range values of factors in Figure 8 that the aggregate grading notably influences the UCS of the GCB. Thus, attention should be paid to the distribution of crushed stone at the grouting location during on-site grouting, and reinforced grouting is required when possible. As can be seen from Figure 9, cement-sand ratio is the primary factor that affects slurry fluidity, and water-cement ratio is the secondary factor. Additionally, the optimal ratio of the slurry to achieve a high slurry fluidity is a water-cement ratio of 3.1:1 and a cement-sand ratio of 1:4. The range values of water-cement ratio and cement-sand ratio differ slightly (0.966) in Figure 8, whereas they differ hugely (13.775) in Figure 9. Such results imply that in the design of the UCS of the GCB, both cement-sand ratio and water-cement ratio need to be taken into account, while in the adjustment of the slurry fluidity, cement-sand ratio is the primary factor to be considered. Grounded in the above discussion, the optimal ratio of the slurry is analyzed by combining schemes of factors affecting the strength of the GCB (the optimal scheme is a water-cement ratio of 2.5:1 and a cement-sand ratio of 1:8; the second most optimal scheme is a water-cement ratio of 2.5:1 and a cement-sand ratio of 1:4) and the scheme of factors affecting the slurry fluidity (the optimal scheme is a water-cement ratio of 3.1:1 and a cement-sand ratio of 1:4). As can be observed from Figures 8 and 9, in the two situations where the water-cement ratio is 2.5:1 and the cement-sand ratio is 1:8 and 1:4, respectively, the UCS of the GCB is similar, but the slurry fluidity differs remarkably. Thereby, on the premise of ensuring a fine strength, the scheme with a better slurry fluidity is selected. Ultimately, the optimal ratio of the slurry was determined to be a water-cement ratio of 2.5:1 and a cement-sand ratio of 1:4.