## 2 Experimental Design
### Basic Mechanical Properties and Analysis Results of Filling Materials  
Taking the slump, uniaxial compressive strength, creep resistance, shrinkage resistance, and permeability as the main technical indicators, the performance of coal mine filling pasted was evaluated to determine the optimized coal mine filling paste in goat, accumulate experience in the development of coal mine paste filling technology, and promote the development of coal mine filling technology industry. (1) The slump is the main index used to evaluate the fluidity, water retention, and cohesiveness of the paste. The fluidity of the filling body is very different. Some have almost no deformation under the action of self-weight, and some show very high fluidity, like fluid; that is, they have self-flow characteristics. The slump experiment was carried out after a setting time of 15 min, 30 min, 45 min, and 60 min under room temperature (20 \({}^{\circ}\)C). The curing time of the filling body was 6-8 h in order to reflect its change rule conveniently and intuitively, as shown in Figure 1.  
According to the change trend of Figure 1, the slump of filling pastes with different ratios decreases significantly with the extension in setting time. The minimum value of slump of filling pastes is 152 mm, which meets the requirement that the slump of the paste should be greater than 150 mm [18]. The slump experimental values of the first, second, and fifth groups are larger in the early stage and change faster than other groups over time. After 30 min, there was no significant difference in the slump of the five groups, and the development of slump of each group as the extension of static time was basically the same. The slump values of groups 1, 2, and 4 in the later period are ideal, and the change in slump of the third group was slighter than that of the other groups. The analysis shows that the water-reducing agent has a great influence on the slump. With the increase in content of cementitious materials (cement and gypsum), the fluidity of the paste is improved. The greater the amount of cement, the thicker the slurry layer wrapped on the surface of the aggregate particles. This brings about better lubrication effect and reduces the friction  
Figure 1: The change in slump with time. \begin{table}
\begin{tabular}{c c c c c}
**Peer Group** & **Flyash:** & **Gypsum:** & **Cement:** & **Water Reducing Admixture:** & **Expansion** \\
**Coal Gagne** & **Coal Gagne** & **Coal Gagne** & **Coal Gagne** & **Cement, Gypsum** & **Agent: Cement** \\ \hline
1 & 3:5 & 1:10 & 4:25 & 1:13 & 7:8 \\
2 & 5:5 & 1:10 & 4:25 & 1:13 & 7:8 \\
3 & 3:5 & 2:10 & 4:25 & 1:13 & 7:8 \\
4 & 3:5 & 1:10 & 6:25 & 1:13 & 7:8 \\
5 & 3:5 & 1:10 & 4:25 & 2:13 & 7:8 \\ \end{tabular}
\end{table} 
Table 5: Paste filling body experimental material ratio table. between the aggregates, and thus the fluidity of paste becomes better. Gypsum is mainly used to adjust the setting time of paste. With the increase in gypsum, the fluidity of paste will decrease [19]. However, increasing the addition of fly ash has little effect on the slump of the paste. (2) Since the goat paste mainly bears the pressure of the overlying strata, the compressive strength is an important performance. The compressive strength of the filling paste was determined on a [20] 150 mm \(\times\) 150 mm cube. The test was carried out on the NYL-300 universal pressure tester with a maximum load of 300 kN and a precision of 0.1%. The loading speed was 0.3-0.5 MPa per second. The experimental results are shown in Figure 2.  
According to the results, the strength of the filling body will gradually increase with time, and the later strength will change more obviously than the early strength. The excessive fly ash will reduce the strength of the paste. Cement is the most important factor affecting the strength of the paste. Regardless of early strength or later strength, the strengths of group 4 were the largest, while the strengths of group 2 were the smallest.