During the backfilling mining, cemented backfilling materials must be self-supporting without collapse after backfilling into goaf. Therefore, SRGBM needs to have early strength, that is, the strength required for the filling body to maintain self-supporting at the early stage. Although the early strength requirement is not high, it is of great significance for backfilling mining. The early strength design methods include the empirical formula and Thomas model (Wang, 2019). Considering the actual backfilling technology in a coal mine, the unconfined compressive strength (UCS) of the filling body at the curing age of 1 day is selected as the early strength index. The relationship between the early strength and the backfilling height of the goaf is shown in the following equation: h 2 = k σ z 3 , (1) where h is the height of the filling body, m; k is the empirical coefficient of early strength, and k= 600 generally; and σ _z is the early strength of the cemented filling body, MPa.

With the internal hydration reaction of the cemented backfilling slurry, the strength of the cemented filling body gradually increases and finally tends to be stable. In the goaf, the filling body is used to support the overburden, and the strength at the curing age of 28 days is generally taken as the later strength in the research. In this study, the control of surface subsidence is taken as the backfilling target. If the strength of the filling body needs to control the main key strata of overburden without breaking, then the later strength of the filling body needs to control the weight of rock strata between the immediate roof and the main key strata. The later strength calculation is shown in the following equation: σ h = a ∑ i = 1 n r i h i , (2) where σ _h is the later strength of cemented filling body, MPa; n is the number of strata between the direct roof of overburden and the main key strata; h _i is the thickness of the ith layer, m; r _i is the unit weight of the ith layer, MN/m³; and a is the safety factor of later strength, ranging from 1.1 to 1.4.

The test soda residue was taken from Huai’an Soda Plant. The fresh soda residue was a gray sticky paste with a pungent smell and strong corrosion. The soda residue was dried by natural air drying, and the moisture content, pH value, liquid plastic limit, UCS, and other basic parameters of the air-dried soda residue were tested according to Test Methods of Soils for Highway Engineering (JTG E40-2007). The chemical composition of soda residue was analyzed by XRF tests. Table 1 shows basic physical and mechanical parameters and chemical components of soda residue.

The moisture content of soda residue was 89.95%, the pH value was 9.2, and it was alkaline. The plasticity index was 29.53, belonging to high liquid limit clay. The UCS value of soda residue was only 0.20 MPa, indicating the low strength of soda residue. The scanning electron microscope (SEM) was used to scan the internal structure of air-dried soda residue. Figure 3 shows the internal structure under different magnifications. Based on the analysis, the soda residue was a porous aggregate structure. It was inferred that the material skeleton of soda residue was mainly formed by CaCO₃, and the single-particle size was about 2–5 μm. The particles were cemented with each other to form aggregates. Due to the main point contact of particles, the cementation of the soda residue was weak. The surface of the aggregate structure was rough, and there were many pores of different sizes on the surface and inside of particles, resulting in the high water content and large bearing deformation in a natural state.

The test gangue was taken from the washing gangue of the coal preparation plant. The natural moisture content of the gangue was 6.7% and the density was 1.9 g/cm³. The gangue was ground into a powder specimen and tested by a D/max-3B X-ray diffractometer. The main components of gangue are quartz and kaolinite, and the total amount of both is more than 70%; there are also some illite and illite/smectite formations, amorphous materials, and a small number of other minerals.

Gangue has a high content of SiO₂ and can act as the material skeleton, contributing to better deformation resistance of backfilling materials. Besides, gangue also contains carbon, aluminum, and Cao, which is easy to react with fly ash. Figure 4 shows the SEM micrographs of the internal structure of gangue under different resolution conditions.

According to Figure 4A, there are many holes on the surface of gangue, the surface joints and fissures are developed, the coarse and fine particles are cemented with each other, there are obvious edges and corners at the end of the particles, and the fracture is flat. After magnification of 6000 times (Figure 4B), the micro-layered structure of fractures can be clearly found; the rock specimens are stacked in sheets, and the sheets are crisscrossed. After magnification of 12000 times (Figure 4C), the microscopic pores are distributed on the surface of gangue, and the diameter of pores is about 2 μm. SEM results show that gangue itself has good density and can be used as an aggregate component of cemented backfilling materials.

The fly ash used in the test was from the spontaneous combustion coal power plant. According to the analysis, the main oxide components in fly ash are SiO₂, Al₂O₃, Fe₂O₃. The Al₂O₃, and SiO₂, which account for 74% are the favorable component of fly ash activity and are closely related to the later setting and hardening characteristics of cemented backfilling materials (Papadakis, 1999).

According to SEM micrographs of fly ash, fly ash particles have different particle sizes and shapes. There are more particles with small sizes and fewer particles with large sizes; most of the particle size is 10–20 μm, and the maximum particle size is about 170 μm. It is observed that many pores are evenly distributed on its surface, and there are small flake structures locally.

The cement used in this test was P.O.32.5 cement produced by China United Cement Company. The apparent density of the cement was 3.1 g/cm³, and 80 μm sieve residual was 6.4%. The analysis shows that there are a lot of tricalcium silicate (C₃S), dicalcium silicate (C₂S), part of tricalcium aluminate (C₃A), and tetracalcium ferroaluminate (C₄AF) in cement. After hydration of cement, calcium silicate hydrate and ettringite with cementing ability can be produced. The results of the mineral composition analysis are shown in Table 2.

The lime used in this test was quicklime produced by Huihui Industrial Company. Through the analysis, it is found that the main components of lime are calcium oxide and calcium hydroxide, accounting for 70%. Calcium oxide is a kind of nonhydraulic cementing material, which has poor cementing ability in the water environment. The addition of lime can stimulate the activity of fly ash, promote the hydration process of fly ash, and improve the strength of the filling body; Besides, the calcium oxide contained in lime can react with water, consume free water content, and reduce slurry bleeding rate.

Figure 6 shows the slump test results of the SRGBM slurry with different soda residue contents in Groups A0–A4.

As shown in Figure 6, when the soda residue content is 0, 3, 6, 9, and 12%, the slump values of slurry are 110, 115, 124, 135, and 152 mm, respectively. It indicates that the slump value of slurry gradually increases with the increase in soda residue content. Compared with the reference group (Group A0), the increase in the slump value in Groups A1–A4 is 4.5, 12.7, 22.7, and 38.2%, respectively, and the increasing amplitude gradually increases with the increase in soda residue content. The reason is as follows: there is a large number of fine particles of soda residue after grinding and the water content is high, which increases the slump value of slurry and enhances the fluidity of the backfilling material.

Figure 7 shows test results of bleeding and bleeding rate of slurry with different contents of soda residue.

As shown in Figure 7A, the bleeding law of the SRGBM slurry in each group presents two-stage characteristics of rapid bleeding (stage I) and slow bleeding (stage II). In stage I, the speed of bleeding is faster and the bleeding proportion is larger in the whole process, while in stage II, the speed of bleeding is slower and the bleeding proportion is smaller in the whole process. The bleeding proportion of stage I in the reference group (Group A0) is as high as 98% and that of stage I in Group A4 is 91%. With the increase in soda residue content, the final bleeding in stage II increases gradually, but the proportion of bleeding in stage I decreases gradually. It indicates that soda residue can delay the initial bleeding rate of free water, which is conducive to material transportation.

As presented in Figure 7B, when the soda residue contents are 0, 3, 6, 9, and 12%, the bleeding rate of slurry in Groups A0–A4 is 2.4, 3.0, 3.8, 4.7, and 5.3%, respectively. The bleeding rate gradually increases with the increase in soda residue content. The increasing amplitude of the bleeding rate of slurry in Groups A1–A4 is 25, 58, 95, and 120%, respectively. This increasing amplitude is significant, and the bleeding rate of slurry in Group A4 is too large, which has exceeded the recommended value. The analysis shows that the water absorption rate of fly ash with the same quality is higher than that of soda residue; the water absorbed by soda residue is less than the water demand of fly ash; the free water of slurry is more, which leads to the increase in bleeding rate.

One specimen was selected from three specimens at different curing ages in Groups A0–A4 and subject to the loading. The stress–strain curve of SRGBM specimens was drawn, as shown in Figure 8.

As shown in Figure 11, the strength curves of SRGBM specimens with different soda residue contents and different ages show significant three-stage characteristics, namely, pre-peak bearing area (zone I), post-peak attenuation area (zone II), and residual bearing area (zone III). In zone I, the strength of the specimen almost increases linearly with the increase of load. At the peak point, the specimen is in the ultimate strength, and the bearing capacity is the strongest. Zone I belongs to the strain hardening stage. In zones II and III, with the continuous action of the load, the strength of the specimen decreases rapidly, the bearing capacity becomes weak, the cracks continue to develop and connect, and the residual bearing area shrinks. However, the specimen in zone III does not completely lose the bearing capacity. It indicates that the material still has a certain bearing capacity after failure, but the bearing capacity is weak. Zones II and III belong to the strain-softening stage.

The UCS values of SRGBM specimens with different soda residue contents and ages were summarized. To further analyze the influence of soda residue content and curing age on the UCS of SRGBM specimens, the data are plotted as shown in Figure 9.

The cured specimen was broken and a small rectangular specimen with a base area of 10 × 10 mm and a height of 2–10 mm was cut from the inside of the specimen. The conductive adhesive was glued to the base and sprayed with gold to improve the electrical conductivity. Finally, the specimen was observed in the electron microscope. Figure 10 shows the production process and the actual photos of the experiment. Figure 11 shows the micrographs of SRGBM specimens under different resolution conditions.

According to the analysis of Chapter 3.1.4, the main components of cement are C₃A and C₄AF. The pH value and temperature of the solution are increased by the Cao in lime and water heating, and then trisulfide hydrated calcium sulfate (3CaO·Al₂O₃·3CaSO₄·32H₂O, known as ettringite, expressed by Aft) is generated byC₃A. The reaction formula is shown in the following equations: 3CaO·A l 2 O 3 +Ca ( OH ) 2 → 4CaO·A l 2 O 3 ·13 H 2 O, (3) 4CaO·A l 2 O 3 ·13 H 2 O+4CaS O 4 ·2 H 2 O + H 2 O → 3CaO·A l 2 O 3 ·3CaS O 4 ·32 H 2 O . (4)

When the gypsum is exhausted, calcium aluminate hydrate (C₄AH₁₃), which is the hydration product of C₃A, reacts with AFt to form calcium sulphoaluminate hydrate (3CaO·Al₂O₃·CaSO₄·12H₂O, expressed as AFm). Both of these hydrates are needle-like crystals and are insoluble in water. The reaction equations are shown in the following equations: 3CaO·A l 2 O 3 +6 H 2 O → 3CaO·A l 2 O 3 ·6 H 2 O, (5) 4CaO·A l 2 O 3 ·13 H 2 O+ 3CaO·A l 2 O 3 ·3CaS O 4 ·32 H 2 O → 3CaO · Al 2 O 3 ·CaS O 4 ·12 H 2 O . (6)

At the same time, fly ash is activated in the sodane environment of lime, the hydration reaction of cement is very rapid, and the released substances make the pH value of the solution further increase. The substances in fly ash have the characteristics of modifying the system structure, such as Ca²⁺, K⁺, and Na⁺; silicate or aluminosilicate minerals are also dissolved in the solution. These substances with the ability of modifying the system structure contact with each other to form calcium silicate cement (C-S-H) and calcium aluminate hydrate (C₄AH₁₃). These hydration products can increase the structural density and strength of cemented backfilling materials and make the materials have a higher bearing capacity.

With the addition of soda residue, the active SiO₂ and Al₂O₃ contained in fly ash are excited by the OH⁻ contained in soda residue. After the dissolution and re-polymerization with different structures, aluminosilicate cementitious polymer (N-A-S-H) is formed. N-A-S-H has a stronger cementing performance. In the soda residue cementing slurry system, C-S-H gel and N-A-S-H gel coexist (Liu et al., 2017), which improves the cementing performance of the system, reduces the connectivity of pores, and improves the strength of the material. This is the main reason for the gradual increases in the strength of the material with the increase in soda residue content in the range of 0–6%. The reaction process is shown in the following equation: n ( Si O 2 ·A l 2 O 3 ) +nSi O 2 +4n H 2 O → O H - ( -Si-O-Al-O-Si-O ) n . (7) With the continuous increase in soda residue content, the degree of soda residue participating in the hydration reaction of materials is weakened, and the surplus soda residue no longer participates in the hydration reaction. The decrease of fly ash content reduces the content of the C-S-H gel polymer and N-A-S-H polymer. Since the strength of soda residue itself is low, the strength of SRGBM shows a downward trend with the continuous increase in soda residue content.