## 2 Experimental Design
### Basic Mechanical Properties and Analysis Results of Filling Materials
The analysis shows that the fineness of fly ash is small, which is close to the fineness of slurry aggregate. The promotion effect of fly ash on cementitious materials is limited, which is the main reason for its failure to improve the strength index of slurry. The strengths of groups 3 and 4 are relatively higher. Therefore, the most important factor affecting the strength of the paste is the content of cementitious material [21]. Due to the particle size of the aggregate, the contribution of the aggregate to the strength is limited, and the strength of the paste is mainly determined by the characteristics of the cementitious material. (3) Creep refers to the trend of slow and permanent movement or deformation of solid materials under the influence of stress [22]. The creep of filling paste was tested on the electro-hydraulic servo rock triaxial testing machine XTR01-01 with a maximum load of 2000 KN. The applied loading method was uniaxial creep loading, and the loading strength was 1.2 MPa according to the actual engineering situation. The results are shown in Figure 3.  
The experimental results of the five groups of different components did not show unstable creep in the creep test, indicating that the five kinds of filling pastes can ensure the stability of the goat after filling. Among them, the creep value of group 5 is the smallest, and the creep value of group 3 is the largest. In terms of its creep resistance, group 5 has excellent creep resistance. Compared with other groups, it is a superior material for meeting the filling performance of paste in goat. Further analysis shows that cement has a great effect on the creep resistance of the paste. The addition of fly ash and gypsum will reduce the creep resistance of the paste, and the water-reducing agent has no effect on the creep resistance of the paste [23]. Figure 2: Early strength comparison curve. (4) Like concrete, the filling material of coal mine will produce certain shrinkage [24]. Shrinkage cracks not only affect the bearing capacity but also provide channels for the flow and infiltration of surface water. The shrinkage of the filling paste was thus tested by the concrete strain gauge with the data acquisition instrument. During the experiment, the heat source and cold source around the experimental environment were removed to reduce the slight change in the strain gauge caused by the vibration interference. The purpose of this experiment is to provide favorable suggestions and improvements for the selection of coal gangue backfill paste. The results are shown in Figure 4.  
The experimental results show that after 24 h, the shrinkage reaches equilibrium. Group 5 presents minimum shrinkage in the early and late stages. Group 1 has the greatest shrinkage and therefore has the highest possibility of cracking in practical engineering applications. The shrinkage of group 2 in the medium term is obviously larger than those of the other groups, which indicates that the contribution of fly ash to the shrinkage is not significant. This may be related to the fact that water-reducing agent in group 5 reduced water usage. The largest shrinkage of group 1 is the result of high content of coal gangue, which leads to serious water loss. The shrinkage performance of ratio 3 and ratio 4 is general. In comparison, the cement content of ratio 4 is slightly greater, and its shrinkage performance is more stable than that of ratio 3. (5) The existence of groundwater is a threat to the goal. If the filling paste is highly permeable, the invasion of groundwater may pose a threat to the stability of the goat after filling. Therefore, the permeability of the filling paste should be investigated to provide a  
Figure 4: Shrinkage diagram of paste material with time. Figure 3: Creep curve diagram. guarantee for the stability of the goaf [25].