2.1. Preparation of Paste Filling Material
At present, there is no relevant experimental procedure standard for coal mine filling materials. Therefore, the experimental mainly referred to the ‘Mortar basic performance test method standard’ (JGJ/T70-2009) [
14] and the ‘Ordinary concrete mixture performance test method standard’ (GB/T50080-2002) [
15].
Coal mine filling paste is an ideal material with good fluidity, uniformity, and low water consumption. In the experiment, coal gangue was used as the main aggregate, which is rich in source, convenient for drawing materials, and low in price while reducing the harm caused by the stacking of gangue on the ground. Cement and gypsum are cementitious materials that can improve stability. They have a very important impact on the physical and mechanical properties of the paste before and after solidification. Water reducer, expansive agents, and fly ash are usually used as additives in experiments. In the experiment, coal gangue was used as the main aggregate, and cement and gypsum were used as cementitious materials to improve stability. Additives included the water reducer, expansion agent, and fly ash. The details of raw materials are as follows:
(1) The natural coal gangue of a coal mine was sampled on site, and the filling aggregate with particle size less than 25 mm was prepared by hammer crushing, jaw crusher crushing, and vibrating screen screening. Coal gangue with particle size less than 25 mm acts as coarse aggregate. The chemical composition of coal gangue included SiO
2, Al
2O
3, Fe
2O
3, CaO, MgO, Na
2O, K
2O, SO
3, and P
2O
5 [
16].
(2) Ordinary Portland cement (P-O) was used as cement; its main components are clinker, gypsum (80–95%), and granulated blast furnace slag (5–20%). The bulk density is 300 kg/m
3, and the specific surface area is generally 300 m
2/kg. The fineness requirement of cement is that the sieve residue of 80 μm square hole sieve is not more than 10%, and the sieve residue of 45 μm square hole sieve is not more than 30%. The initial setting time of ordinary Portland cement should not be less than 45 min, the final setting time should not be more than 600 min, and the water–cement ratio is 0.5. The main chemical compositions in cement included 3CaO·SiO
2 (C3S), 2CaO·SiO
2 (C2S), 3CaO·Al
2O
3 (C3A), and 4CaO·Al
2O
3·Fe
2O
3 (C4AF) [
17]. The measured 3 d strength was 18.96 MPa, and the 28 d strength was 44.59 MPa. The strength was higher than the theoretical strength.
(3) The relative density of gypsum was 2.96 g/cm
3. The main component was CaSO
4. The main parameters of gypsum are shown in
Table 1 [
17].
(4) Water reducer was an extremely necessary and important additive for the filling material. The main parameters are shown in
Table 2.
(5) The expansion agent was used to limit the formation and development of cracks, effectively improve the impermeability of the paste, and improve the stability of the entire goaf. The parameters are shown in
Table 3.
(6) The main chemical compositions of fly ash were SiO
2, Al
2O
3, FeO, Fe
2O
3, CaO, TiO
2, MgO, K
2O, Na
2O, SO
3, MnO
2, etc. [
17]. In this experiment, fly ash was used as additive, and the physical parameters are shown in
Table 4.
In this experiment, five groups of coal mine filling materials with different proportions were tested. The content of expansive agent in the filling materials was used as an invariant. Using the method of single variable control contrast test, the contents of coal gangue were the same in each group, but the contents of expansive agent, water, fly ash, gypsum, cement, and water-reducing agent were appropriately increased. The filling paste was prepared by manual mixing. The compositions of qualified filling materials are shown in
Table 5.
2.2. 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 goaf, 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 °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 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 goaf 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 × 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. 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 goaf 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 goaf. 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].
(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 goaf. If the filling paste is highly permeable, the invasion of groundwater may pose a threat to the stability of the goaf after filling. Therefore, the permeability of the filling paste should be investigated to provide a guarantee for the stability of the goaf [
25]. The experiment was carried out on a permeation test machine with a pressure of 1.2 MPa for 8 h. The experimental results were plotted, as shown in
Figure 5.
During the permeability test, the permeability coefficient of the five groups changed with time, and their ability to resist the deformation caused by water flow infiltration under the influence of groundwater surface water was analyzed. The permeation test shows that the permeation law of the filling paste meets the Darcy permeation law [
26], and its impermeability gradually increases with time. The analysis of the results shows that group 4 has the lowest permeability coefficient, and the permeability coefficients of group 1, group 3, and group 5 are basically at approximate level. The permeability coefficient of group 2 is much larger than those of other groups. Specifically, the permeability of group 2 becomes significantly larger after 170 min. Further analysis of the results shows that the content of cementitious material is the most important factor affecting the permeability of filling paste. The larger the content of cementitious material, the smaller the permeability of filling paste. Group 4 has the best permeability performance and is a superior material for meeting engineering needs.