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Development and properties characterization of fly ash-based inorganic grouting materials for preventing coal spontaneous combustion
* Corresponding author: E-mail address: quanlin_shi@163.com (Q. Shi)
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Received: ,
Accepted: ,
Abstract
Air leakage in goafs is a major cause of spontaneous coal combustion, posing serious risks to mine safety. Traditional grouting materials often suffer from slow setting and insufficient bonding with fractured coal, limiting their effectiveness in emergency sealing applications. To address this challenge, this study aims to develop and optimize a novel fly ash-based inorganic grouting material with rapid setting, high early strength, and strong coal interfacial adhesion. A three-factor, three-level Box–Behnken design (BBD) within the framework of response surface methodology was employed to evaluate the effects of fly ash content, sodium silicate modulus, and water–cement ratio on three performance indicators: initial setting time, and 7-day and 28-day compressive strength. Regression modelling and ANOVA revealed that fly ash content had the most significant influence, followed by water-cement ratio and sodium silicate modulus. Interaction analysis further demonstrated that the combined effect of fly ash content and water-cement ratio played a critical role in governing early hydration kinetics and strength development. Scanning electron microscopy (SEM) confirmed that these interactions facilitated the formation of dense gel phases, resulting in refined pore structure and improved microstructural integrity. The optimal mix, 50% fly ash, 0.80 sodium silicate modulus, and 0.50 water-cement ratio, achieved an initial setting time of 2.17 min and a 7-day compressive strength of 4.18 MPa. This study provides a quantitative optimization strategy and microstructural understanding for the design of fast-setting, high-performance sealing materials, offering a practical solution for air leakage control and spontaneous combustion prevention in underground coal mines.
Keywords
Coal spontaneous combustion
Fly ash-based grouting materials
Plugging
Reinforcement
Response surface methodology
1. Introduction
Coal is one of the world’s major sources of energy, and as the amount of coal used increases, so does the production of fly ash, a byproduct of coal-fired power plants [1]. Globally, coal-fired power plants produce over 500 million tons of fly ash annually, yet only 20% to 30% is effectively utilized [2,3]. Fly ash contains significant amounts of organic pollutants and toxic metals [4], and its storage methods still primarily involve stacking. This practice not only occupies large tracts of land but also leads to the leaching of contaminants into surrounding soil and groundwater, altering their composition and ecological function, thereby posing long-term threats to environmental and public health [5-7]. Over the past few decades, a significant portion of the fly ash has been widely used in the construction industry [8]. In recent years, fly ash has been used in metal matrix composites (MMCs), geopolymers, and metal coatings [9]. Additionally, it has been used in the production of mine fire prevention and sealing materials.
Spontaneous combustion incidents in coal mines pose serious threats to coal production efficiency and operational safety [10]. The presence of fractures and airways in underground coal seams facilitates the infiltration of oxygen and the escape of gases, significantly increasing the risk of fire and hazardous gas emissions. Effective sealing is therefore essential for fire suppression, gas containment, and overall mine safety. Inadequate sealing can result in the migration of toxic gases, escalation of combustion, and, in severe cases, underground explosions. Consequently, the development of high-efficiency, environmentally benign sealing materials has emerged as a critical focus in contemporary mine safety engineering. In recent years, the types of sealing and fire prevention materials used in underground mining have diversified, encompassing organic polymers, inorganic mineral-based grouts, and organic-inorganic composites [11,12]. Zhang et al. prepared a polymer composite grouting material using diphenylmethane diisocyanate, the primary raw materials were polyether polyol and fly ash, and auxiliary reagents included a coupling agent and catalyst [13]. Li studied the production of ball-milled cement and combined it with water-based polyurethane (WPU) and oil-based polyurethane (OPU) to create a new polyurethane-cement composite grouting material [14]. Zhang investigated the preparation of polyurethane/sodium silicate (PU/WG) grouting materials with different proportions of catalysts [15]. Organic materials like polyurethane and phenolic resin offer strong adhesion and fast curing. However, these organic materials are prone to degradation or loss of adhesive properties in high-temperature environments, which affects their long-term sealing efficacy and raises environmental concerns. Yang et al. studied mine filling and sealing materials, mixing cement with aggregates and other additives to form a consolidated filling body with certain strength [16]. Xi et al. prepared cement-based foam materials (CBFMs) by mixing cement systems with foam systems [17]. However, as the use of fine aggregates increased, deficiencies in cement-based slurry became more apparent. Specifically, slurries with fine aggregates and low solid content often exhibit weak interfacial bonding, slow setting rates, high material consumption, and elevated costs [18]. Li et al. began adding other binders to cement-based slurries [6], but still faced challenges such as high costs and large cement consumption. Inorganic materials, particularly cement-based and bentonite-based grouts, are widely applied in coal mines due to their thermal stability, non-toxicity, and cost-effectiveness. However, conventional inorganic sealing materials tend to suffer strength degradation under elevated temperatures, and their long-term sealing performance remains suboptimal.
Fly ash, an industrial solid waste, has attracted increasing attention for use in underground sealing material development, owing to its abundant availability, low cost, and stable physicochemical properties. It offers favorable characteristics such as inherent flame retardancy, good mixability, and high pumpability, and can often be sourced locally near mining operations [19]. Doven studied the properties of fly ash cement composites formed by adding different proportions and types of chemical reagents to fly ash, finding that fly ash can serve as an effective compacting or flowable fill material [20]. Wang et al. optimized the fly ash grouting slurry mix ratio by orthogonal experiments and numerical simulations, characterizing its compressive strength and setting time [21]. However, fly ash as a sealing material still faces several challenges. When used alone, its strength may be insufficient, requiring modification or combination with other cementing materials to improve its performance. Additionally, variations in the source and composition of fly ash can impact its stability and applicability [22]. Soutsos investigated the factors influencing the compressive strength of geopolymers and highlighted that the optimal proportion of alkali activators in the activating solution requires further study [23]. Mehdizadeh studied the effect of water-cement ratio during cement hydration and analyzed the effect on the microstructure and hardness properties of cement paste [24]. Most existing studies adopt a single-factor approach, thereby neglecting the complex interactions among multiple influencing parameters. Therefore, a comprehensive investigation of the interactions among fly ash content, sodium silicate modulus, and water-cement ratio is essential for optimizing the material’s performance.
To address the limitations of previous mix design approaches, which often adjust one factor at a time or rely solely on simple orthogonal arrays, and considering that most existing studies focus primarily on bulk compressive strength while neglecting the critical bonding behavior at the coal-grout interface, the present study develops an activated fly ash-based inorganic sealing material optimized through a three-factor, three-level Box-Behnken design (BBD) of response surface methodology. The influencing factors include fly ash dosage, sodium silicate modulus, and water-cement ratio. This approach allows for rigorous modelling of both main and interaction effects on key performance indicators. The selected indicators are initial setting time, and 7- and 28-day compressive strengths. In addition, bonding reinforcement performance tests and scanning electron microscope (SEM) characterization were conducted to investigate the interfacial morphology and assess the mechanical properties of the grout-coal system.
2. Materials and Methods
2.1. Materials
The cement used in the study was 425 silicate cement produced by Jiangsu Xuzhou Zhonglian Cement Company. The fly ash is secondary low-calcium fly ash, with a calcium oxide content of 2.54%, sourced from Datun Power Plant. The sodium silicate was purchased from Jinan Maoxin Chemical Co., Ltd.
2.2. Experimental methods
2.2.1. Initial setting time measurement test
First, 15 groups of materials with different ratios were prepared. The cement and fly ash were mixed and poured into a beaker. Clean fresh water was added, and the mixture was stirred. After stirring evenly, the sodium silicate solution of the corresponding concentration was added and stirred for 2 min. The timer was started. The beaker was tilted every 2 min, and as the slurry’s fluidity decreased, the tilt angle was decreased, and the time intervals were shortened. The initial setting time is reached when the mixed slurry completely loses its fluidity when the test mold is tilted to 45°.
2.2.2. Compressive strength measurement test
Before measuring the various properties, samples need to be prepared. The 15 groups of samples were incubated for 7 and 28 days, with three samples in each group, totaling 90 samples. The samples were prepared under laboratory conditions with a temperature of 20 ± 3°C, water temperature of 20 ± 2°C, and relative humidity above 50%, using clean fresh water. The molds used were 40 mm × 40 mm × 40 mm triple molds. After demolding, the samples were sealed in a plastic bag and placed in a curing chamber with a relative humidity above 90% and a curing temperature of 20 ± 2°C. The compressive strength of the 15 groups of samples designed by the response surface methodology for two curing periods (7 days and 28 days) was tested using a laboratory microcomputer-controlled electronic universal testing machine, as shown in Figure 1. For three test samples as a group, the experimental results take the average value.
- Influence of each single factor on the strength of fly ash-based inorganic grouting material: (a) Fly ash dosage, (b) Sodium silicate modulus, (c) Water-cement ratio.
2.3. Experimental design using response surface methodology
Relevant studies and preliminary experiments have shown that the factors that have a significant effect on the properties of fly ash-based materials include factors such as fly ash content, sodium silicate modulus, and water-cement ratio. Therefore, the sodium silicate content in the mixture was set at 3% in this study. Fly ash content, sodium silicate modulus, and water-cement ratio were taken as influencing factors and denoted by A, B, and C, respectively. The levels of each factor were selected from the results of previous single-factor experiments, as shown in Table 1.
| Factors | Level | ||
|---|---|---|---|
| -1 | 0 | 1 | |
| Fly ash dosage (A) | 50.00% | 60.00% | 70.00% |
| Sodium silicate modulus(B) | 0.8 | 1.4 | 2.0 |
| Water-cement ratio(C) | 0.50 | 0.55 | 0.60 |
The BBD configuration with three factors and three levels was designed using the Design Expert 8 (DX8) software. To minimize the effect of error, three central points were set up, and a total of 15 groups of independent experiments were designed. To investigate the effects of the three factors on the material properties and develop a sealing material suitable for the needs of mines, the initial setting time and compressive strength of the material were used as the indexes for evaluating the performance of the fly ash-based material. Therefore, the initial setting time, 7-day compressive strength, and 28-day compressive strength of the fly ash-based materials were used as the response values in each set of experiments, which were labeled as R1, R2, and R3, respectively. The detailed response surface experimental design has been shown in Table 2.
| Number | Fly ash dosage (%) | Sodium silicate modulus | Water-cement ratio |
|---|---|---|---|
| 1 | 50.00 | 0.8 | 0.55 |
| 2 | 50.00 | 2.0 | 0.55 |
| 3 | 50.00 | 1.4 | 0.50 |
| 4 | 50.00 | 1.4 | 0.60 |
| 5 | 60.00 | 2.0 | 0.50 |
| 6 | 60.00 | 1.4 | 0.55 |
| 7 | 60.00 | 1.4 | 0.55 |
| 8 | 60.00 | 1.4 | 0.55 |
| 9 | 60.00 | 0.8 | 0.60 |
| 10 | 60.00 | 2.0 | 0.60 |
| 11 | 60.00 | 0.8 | 0.50 |
| 12 | 70.00 | 1.4 | 0.50 |
| 13 | 70.00 | 0.8 | 0.55 |
| 14 | 70.00 | 1.4 | 0.60 |
| 15 | 70.00 | 2.0 | 0.55 |
3. Results and Discussion
3.1. Experimental results
The experimental results have been summarized in Table 3.
| Number | Impact factors | Response value | ||||
|---|---|---|---|---|---|---|
| A | B | C | R1 | R2 | R3 | |
| Fly ash dosage (%) | Sodium silicate modulus | Water-cement ratio | Initial setting (time/min) | 7-day compressive strength (MPa) | 28-day compressive strength (MPa) | |
| 1 | 50 | 0.8 | 0.55 | 5.96 | 3.58 | 9.00 |
| 2 | 50 | 2.0 | 0.55 | 15.25 | 3.21 | 8.71 |
| 3 | 50 | 1.4 | 0.50 | 10.13 | 3.65 | 11.60 |
| 4 | 50 | 1.4 | 0.60 | 18.94 | 2.51 | 8.02 |
| 5 | 60 | 2.0 | 0.50 | 15.22 | 2.87 | 9.54 |
| 6 | 60 | 1.4 | 0.55 | 19.01 | 1.83 | 6.28 |
| 7 | 60 | 1.4 | 0.55 | 18.82 | 1.95 | 6.32 |
| 8 | 60 | 1.4 | 0.55 | 18.73 | 1.75 | 6.56 |
| 9 | 60 | 0.8 | 0.60 | 9.61 | 2.17 | 5.38 |
| 10 | 60 | 2.0 | 0.60 | 19.17 | 1.85 | 6.30 |
| 11 | 60 | 0.8 | 0.50 | 7.57 | 2.93 | 8.73 |
| 12 | 70 | 1.4 | 0.50 | 17.32 | 1.49 | 6.64 |
| 13 | 70 | 0.8 | 0.55 | 11.51 | 1.67 | 4.92 |
| 14 | 70 | 1.4 | 0.60 | 20.90 | 1.12 | 2.53 |
| 15 | 70 | 2.0 | 0.55 | 18.93 | 1.19 | 3.79 |
Basic statistics of the experimental data are shown in Table 4.
| Item | Unit | Minimum | Maximum | Mean | Standard deviation |
|---|---|---|---|---|---|
| A | % | 50 | 70 | 60 | 7.30 |
| B | 0.8 | 2.0 | 1.4 | 0.44 | |
| C | 0.50 | 0.60 | 0.55 | 0.04 | |
| R1 | min | 5.96 | 20.90 | 15.14 | 4.73 |
| R2 | MPa | 1.12 | 3.65 | 2.25 | 0.80 |
| R3 | MPa | 2.53 | 11.60 | 6.96 | 2.28 |
3.2. ANOVA and goodness of fit test
The experimental data from Table 3 were processed using DX8 software. Based on the sum of squares, mean square, and fitting error, secondary regression models for the response values were established, as shown in Eqs. (1-3).
The regression models for the initial setting time, 7-day compressive strength, and 28-day compressive strength were subjected to ANOVA. The analysis results have been shown in Tables 5-7.
| Source of variance | Degrees of freedom | Mean square | Sum of squares | F-value | P-value | P-value |
|---|---|---|---|---|---|---|
| Model | 9 | 39.33 | 353.96 | 43.57 | < 0.0001 | *** |
| A | 1 | 42.23 | 42.23 | 46.79 | 0.0002 | *** |
| B | 1 | 143.82 | 143.82 | 159.35 | < 0.0001 | *** |
| C | 1 | 42.23 | 42.23 | 46.79 | 0.0002 | *** |
| AB | 1 | 0.87 | 0.87 | 0.97 | 0.3578 | ⊙ |
| AC | 1 | 6.84 | 6.84 | 7.58 | 0.0284 | ⊙ |
| BC | 1 | 0.91 | 0.91 | 1.01 | 0.3483 | ⊙ |
| A2 | 1 | 4.25 | 4.25 | 4.71 | 0.0666 | ⊙ |
| B2 | 1 | 102.53 | 102.53 | 113.60 | < 0.0001 | *** |
| C2 | 1 | 4.42 | 4.42 | 4.90 | 0.0625 | ⊙ |
| Residual | 7 | 0.90 | 6.32 | |||
| Lack of fit | 3 | 1.72 | 5.15 | 5.89 | 0.0598 | ⊙ |
| Pure error | 4 | 0.29 | 1.17 | |||
| Total | 16 | 360.28 | ||||
| R2 | 0.9825 | |||||
| Adjusted R2 | 0.9599 | |||||
| Predicted R2 | 0.7661 | |||||
| SNR ratio | 19.492 | |||||
| C.V.% | 6.10 |
Note: SNR is signal-to-noise ratio; *** indicates extremely significant differences (P < 0.001); ** indicates highly significant differences (P < 0.01); * indicates significant differences (P < 0.05); ⊙ indicates non-significant differences (P > 0.05).
| Source of variance | Degrees of freedom | Mean square | Sum of squares | F-value | P-value | P-value |
|---|---|---|---|---|---|---|
| Model | 9 | 1.06 | 9.58 | 56.35 | < 0.0001 | *** |
| A | 1 | 7.07 | 7.07 | 374.25 | < 0.0001 | *** |
| B | 1 | 0.18 | 0.18 | 9.68 | 0.0170 | * |
| C | 1 | 1.36 | 1.36 | 71.86 | < 0.0001 | *** |
| AB | 1 | 2.025E-03 | 2.025E-03 | 0.11 | 0.7530 | ⊙ |
| AC | 1 | 0.15 | 0.15 | 7.97 | 0.0257 | * |
| BC | 1 | 0.017 | 0.017 | 0.89 | 0.3758 | ⊙ |
| A2 | 1 | 0.049 | 0.049 | 2.59 | 0.1515 | ⊙ |
| B2 | 1 | 0.59 | 0.59 | 31.14 | 0.0008 | *** |
| C2 | 1 | 0.10 | 0.10 | 5.52 | 0.0512 | ⊙ |
| Residual | 7 | 0.019 | 0.13 | |||
| Lack of fit | 3 | 0.019 | 0.056 | 0.97 | 0.4909 | ⊙ |
| Pure error | 4 | 0.019 | 0.077 | |||
| Total | 16 | 9.72 | ||||
| R2 | 0.9864 | |||||
| Adjusted R2 | 0.9689 | |||||
| Predicted R2 | 0.8961 | |||||
| SNR ratio | 25.651 | |||||
| C.V.% | 6.17 |
| Source of variance | Degrees of freedom | Mean square | Sum of squares | F-value | P-value | P-value |
|---|---|---|---|---|---|---|
| Model | 9 | 8.74 | 78.63 | 40.95 | < 0.0001 | *** |
| A | 1 | 49.62 | 49.62 | 232.55 | < 0.0001 | *** |
| B | 1 | 3.613E-05 | 3.613E-05 | 1.693E-04 | 0.9900 | ⊙ |
| C | 1 | 26.03 | 26.03 | 122.00 | < 0.0001 | *** |
| AB | 1 | 0.25 | 0.25 | 1.19 | 0.3122 | ⊙ |
| AC | 1 | 0.013 | 0.013 | 0.059 | 0.8146 | ⊙ |
| BC | 1 | 0.029 | 0.029 | 0.13 | 0.7245 | ⊙ |
| A2 | 1 | 3.720E-04 | 3.720E-04 | 1.744E-03 | 0.9679 | ⊙ |
| B2 | 1 | 0.29 | 0.29 | 1.37 | 0.2802 | ⊙ |
| C2 | 1 | 2.29 | 2.29 | 10.72 | 0.0136 | * |
| Residual | 7 | 0.21 | 1.49 | |||
| Lack of fit | 3 | 0.38 | 1.14 | 4.32 | 0.0957 | ⊙ |
| Pure error | 4 | 0.088 | 0.35 | |||
| Total | 16 | 80.12 | ||||
| R2 | 0.9814 | |||||
| Adjusted R2 | 0.9574 | |||||
| Predicted R2 | 0.7652 | |||||
| SNR ratio | 24.243 | |||||
| C.V.% | 6.74 |
From Tables 5-7, it can be observed that the P-value for the regression equation model is less than 0.001, indicating that using a quadratic function to fit the initial setting time, 7-day compressive strength, and 28-day compressive strength is reliable and reasonable, with less than 0.01% of the results caused by random noise. As mentioned earlier, when the P-value of a factor is less than 0.05, it is considered to have a significant effect on the model. Therefore, in the R1 function, the regression coefficients of A, B, C, AC, and B2 are significant, and their influence ranks from greatest to least as B > B2 > A = C > AC. In the R2 function, the ranking of the influence factors on the 7-day compressive strength is A > C > B2 > B > AC. In the R3 function, the influence of the factors on the 28-day compressive strength is ranked as A > C > C2. The P-values in the model are 0.0598, 0.4909, and 0.0957, which are all greater than 0.05, indicating that the misfit due to pure error is insignificant and the fitted model works well. Some of the data in the three tables have p-values closer to the boundary values, indicating that these factors influence the characterization, but the effect is relatively small. As shown in Table 7, the secondary term C2 is closer to the boundary value, with a p-value of 0.0136, indicating an effect on intensity development. There are also several interactions whose p-values are closer to the boundary value, indicating that these interactions have an effect, but the effect is minimal. There are two main reasons for these appearing in the regression model, which are retaining the hierarchical structure of the BBD and considering the potential compounding effects that can occur with different combinations of factors. Although the individual effects of these factors are small, incorporating them into the regression model helps to maintain the predictive integrity of the overall model. The fit of the model to the sample data can be expressed as R2, with higher values suggesting a better fit. The R2 values for the models are 0.9825, 0.9864, and 0.9814, all close to 1, and the predicted R2 and differences between the adjusted R2 are less than 0.2, with CV (coefficient of variation) values all below 10%. The regression model’s adjusted and predicted R2 values are both high, and the small difference between the two indicates that the model has good internal consistency and reliable predictive ability. Whilst the small difference between Adjusted R2 and Predicted R2 suggests stable model performance, the model may not be able to fully account for extreme conditions or non-linear interactions outside of the tested range. Future validation in a wider range of operational scenarios is required. The signal-to-noise ratio (SNR) is another important indicator for evaluating the model’s fit, the higher the SNR, the better the model fit, the smaller the experimental error, and the more suitable the model. When the SNR is greater than 4, the model is considered ideal. The SNRs for the models of initial setting time, 7-day compressive strength, and 28-day compressive strength are all greater than 4, indicating a good fit. In summary, it can be concluded that this model is suitable for further analysis.
3.3. Results and analysis of single-factor experiments
The compressive strength of grouting materials is a key performance indicator for mine leakage sealing applications, as it directly determines their ability to resist in-situ pressures and maintain operational safety. Given the complex geological conditions and frequent air leakage in underground mines, the use of high-strength grouting materials is critical to maintaining structural integrity and ensuring safe production. As shown in Table 6, factors A (fly ash content) and C (water–cement ratio) had highly significant effects on the 7-day compressive strength (p < 0.01), while factor B (sodium silicate modulus) showed a statistically significant but less pronounced effect (p < 0.05). According to Table 7, the 28-day compressive strengths were significantly affected by factors A and C. The results have been summarized below, but factor B has no significant effect. In summary, the order of influence is: A > C > B. The effects of individual factors on compressive strength have been shown in Figure 1. Figure 1(a) shows the relationship between A and the compressive strength of the material when B is 1.4 and C is 0.55. It is evident that when factors B and C are held constant, the compressive strength decreases significantly with increasing fly ash content, indicating a strong sensitivity of the material to fly ash dosage. This phenomenon is primarily attributed to the excessive surface area generated by higher fly ash content, which exceeds the slurry’s wetting capacity. As a result, the fine particles tend to agglomerate rather than disperse uniformly, thereby weakening the material’s compactness and ultimately reducing its compressive strength [25,26]. Figure 1(b) illustrates the relationship between factor B and the compressive strength of fly ash-based inorganic grouting materials at different curing ages when A was 60% and C was 0.55. It shows that the material is significantly influenced by the modulus of sodium silicate. This behavior is primarily attributed to the fact that a lower sodium silicate modulus leads to a higher OH⁻ concentration, which promotes the activation of more reactive ions. This, in turn, facilitates the formation of additional hydration products and a denser microstructure, thereby enhancing the compressive strength. As the modulus increases, the concentration of OH⁻ gradually decreases, resulting in a reduction in compressive strength. However, beyond a critical modulus, the elevated SiO₂ content in the system contributes to increased reactive silica availability and gel volume, which compensates for the earlier decline and leads to a subsequent improvement in compressive strength [27]. Figure 1(c) shows the relationship between C and the compressive strength of fly ash-based inorganic grouting materials at different curing ages for A of 60% and B of 1.4. The water–cement ratio is shown to be a critical factor influencing compressive strength. Excess water remaining in the mixture after hardening either forms air bubbles or evaporates to create capillary voids, thereby increasing the overall porosity of the hardened matrix [28]. Higher porosity significantly reduces the effective load-bearing cross-sectional area, leading to a corresponding decrease in compressive strength [29,30]. It can be concluded that when A is kept constant, the effect of C on compressive strength is not as significant as that of A, indicating that A has a more prominent influence on the compressive strength of fly ash-based inorganic grouting materials at all curing ages.
3.4. Response surface experiment results and analysis
3.4.1. Initial setting time analysis
Based on the fitted equation R1, by setting the water-cement ratio C was 0.55, sodium silicate modulus B was 1.4, and fly ash content A was 60%, the response surface plots and contour maps illustrating the pairwise interaction effects on the initial setting time were drawn, as shown in Figures 2-4.
- Effects of fly ash dosage and sodium silicate modulus on initial setting time: (a) response surface plot; (b) contour plot.
- Effects of fly ash dosage-water-cement ratio on initial setting time: (a) response surface plot; (b) contour plot.
- Effects of sodium silicate modulus and water-cement ratio on initial setting time: (a) response surface plot; (b) contour plot.
Only by controlling the initial setting time within a reasonable range can efficiently filling and plugging be achieved. As shown in Figures 2(a,b), the response surfaces have low slopes and the contours are not dense, indicating that the synergistic effect of A and B on the initial setting time of the test blocks is not significant. Fly ash enhances the material properties primarily through physical filling effects and pozzolanic reactions, whereas sodium silicate accelerates the setting process via chemical activation mechanisms. These mechanisms operate independently and do not exhibit a strong synergistic interaction within the matrix. Furthermore, fly ash tends to retard the hydration process, while sodium silicate solution can partially enhance it. As a result, the opposing effects on hydration may offset each other with respect to initial setting time, resulting in statistically insignificant interaction effects and making the outcome more susceptible to external factors. From Figures 3(a,b), the response surface has a steeper slope and denser contour lines, indicating that the interaction between A and C significantly affects the initial setting time of the test block. When A is 50%, increasing C from 0.50 to 0.60 results in an 86.97% increase in the initial setting time of the grouting material. This indicates that at lower A values, the initial setting time increases as C increases. When A is 70%, increasing C from 0.50 to 0.60 leads to a 20.67% increase in the initial setting time. This suggests that at higher A values, the initial setting time increases more slowly with rising C. The synergistic effect of A and C on the initial setting time of the grouting material was shown to be significant. This phenomenon can be primarily attributed to the fine and spherical morphology of fly ash particles, which act as micro-lubricants in the cementitious slurry, thereby enhancing the overall fluidity of the mixture. However, this increased fluidity can simultaneously reduce the effective water–cement ratio, consequently influencing the rate of hydration. Moreover, at higher water–cement ratios, the diffusion paths of water molecules into cement particles become longer, which slows the diffusion rate and delays the hydration reaction. This effect is further amplified under high fly ash content conditions, where fly ash particles occupy the pore spaces that would otherwise facilitate the access of water to cement grains. This spatial hindrance exacerbates the reduction in diffusion rate and significantly delays the initial setting time [30]. Figures 4(a, b) show a low response surface slope and sparse contour lines, indicating that the interaction between B and C has an insignificant effect on the initial setting time of the test block. The sodium silicate modulus primarily determines the ratio of silicate to sodium ions in the solution, thereby modulating the chemical reactivity and setting behavior of geopolymer and cementitious systems [31]. Meanwhile, the water-cement ratio governs the amount of free water available in the system, which significantly impacts the dispersion of cement particles, the kinetics of the hydration reaction, and the overall workability of the mixture. As the two parameters influence different aspects of the hydration mechanism, their interaction effect is relatively weak and statistically insignificant. In summary, the response surface plots and contour maps in Figures (2-4) are consistent with the variance analysis results in Table 5, mutually verifying that the model R1 derived from experimental data can be used to analyze and optimize the initial setting time of inorganic materials.
3.4.2. 7-day compressive strength analysis
Based on the fitted equation R2, by setting the water-cement ratio C was 0.55, sodium silicate modulus B was 1.4, and fly ash content A was 60%, the response surface plots and contour maps illustrating the pairwise interaction effects on 7-day compressive strength were drawn, as shown in Figures 5-7.
- Effects of fly ash dosage and sodium silicate modulus on 7-day compressive strength: (a) response surface plot; (b) contour plot.
- Effects of fly ash dosage and water-cement ratio on 7-day compressive strength: (a) response surface plot; (b) contour plot.
- Effects of sodium silicate modulus and water-cement ratio on 7-day compressive strength: (a) response surface plot; (b) contour plot.
As an inorganic plugging material, it needs to have a certain level of impact resistance and load-bearing capacity within a specific period. From Figure 5(a), it can be observed that as the sodium silicate modulus increases, the 7-day compressive strength decreases gradually with increasing fly ash content. Analyzing the contour densities in Figure 5(b), it can be concluded that the effect of fly ash content on the response values is greater than the effect of sodium silicate modulus. In Figure 6(a), it is shown that the water-cement ratio is inversely proportional to the 7-day compressive strength, with larger water-cement ratios resulting in lower strength. Figure 6(b) also shows that the 7-day compressive strength is more affected by the fly ash content than by the water-cement ratio. Figure 7(a) shows that the sodium silicate modulus is inversely related to the 7-day compressive strength when the water-cement ratio is insufficient. However, when the accelerator content is sufficient, the lime content is positively related to the 7-day compressive strength. The water-cement ratio of 0.55 is the equilibrium point of these two trends, indicating that the water-cement ratio and sodium silicate modulus synergistically affect the compressive strength. From Figure 7(b), the 7-day compressive strength is more significantly influenced by the water-cement ratio. In conclusion, the ranking of the three influencing factors is: fly ash content > water-cement ratio > sodium silicate modulus, which aligns with the analysis results in Tables 3 and 4, indicating that the model has a good fit. From Figure 5(a), the steep slope of the response surface indicates that the interaction between A and C significantly affects the 7-day compressive strength. When A is 50%, increasing C from 0.50 to 0.60 reduces the 7-day compressive strength of the grouting material by 31.23%. This indicates that when A is low, the 7-day compressive strength decreases rapidly with increasing C. When A is 70%, increasing C from 0.50 to 0.60 reduces the 7-day compressive strength by 24.83%. This suggests that when A is higher, the 7-day compressive strength gradually decreases as C increases, indicating a significant interaction effect between A and C on the compressive strength. When the fly ash content is high, a larger water-cement ratio may significantly increase the porosity in the concrete, resulting in lower strength. On the contrary, if the water-cement ratio is moderate or low, the secondary hydration reaction of fly ash may be more effective, thereby improving the 7-day compressive strength of the concrete. This enhancement effect becomes less pronounced when the fly ash content exceeds a threshold level, as excessive fly ash may dilute the cementitious phase and hinder the overall reactivity [32,33]. In summary, the trends observed in the response surface plots and contour diagrams align well with the ANOVA results presented in Table 6, collectively confirming that the fitted model with its high coefficient of determination is reliable for analyzing and optimizing the 7-day compressive strength of fly ash-based inorganic grouting materials.
3.4.3. 28-day compressive strength analysis
According to the fitting equation R3, the water-cement ratio C was set to 0.55, sodium silicate modulus B to 1.4, and fly ash content A to 60%, and the response surface plots and contour plots of the 28-day compressive strength were plotted as shown in Figures 8-10.
- Effects of fly ash dosage and sodium silicate modulus on 28-day compressive strength: (a) response surface plot; (b) contour plot.
- Effects of fly ash dosage and water-cement ratio on 28-day compressive strength: (a) response surface plot; (b) contour plot.
- Effects of sodium silicate modulus and water-cement ratio on 28-day compressive strength: (a) response surface plot; (b) contour plot.
Inorganic materials need to develop sufficient compressive strength after setting to ensure long-term leak sealing. From Figures 8-10, it can be observed that the response surface for the 28-day compressive strength is similar to that of the 7-day compressive strength, but the overall strength increases. Figures 8(a, b) show that the fly ash content continues to significantly impact the 28-day compressive strength, while the modulus of sodium silicate has little influence. Figures 9(a, b) reveal that the water-to-binder ratio has less effect on the 28-day compressive strength than the fly ash content. Figures 10(a, b) further confirm that the modulus of sodium silicate has no significant effect on the 28-day compressive strength. In summary, the influence of the three factors can be ranked as: fly ash content > water-to-binder ratio > sodium silicate modulus. This ranking matches the analysis in Table 7, indicating good model fit and consistency with the ranking for the 7-day compressive strength.
Figures 8(a), 9(a), and 10(a) show that the response surface is nearly flat, suggesting that the interactions AB, AC, and BC are insignificant. This is further confirmed in Table 7, where AB, AC, and BC have no significant effect on the 28-day compressive strength. The AC interaction significantly affects the 7-day compressive strength but not the 28-day strength because at 7 days, concrete is still undergoing initial hydration, and strength is developing rapidly. The hydration process and microstructure of concrete are altered by the addition of fly ash. At elevated fly ash contents, an increase in the water–cement ratio may lead to higher porosity within the cementitious matrix, thereby reducing the 7-day compressive strength. In contrast, the 28-day compressive strength increases with prolonged curing time due to the continued progress of the hydration reaction, eventually reaching a stable strength profile [29]. Overall, the response surface and contour plots presented in Figures 8-10 show strong agreement with the ANOVA results in Table 7, validating that the constructed regression model is reliable and effective for predicting and optimizing the 28-day compressive strength of the fly ash-based inorganic grouting material.
3.5. Optimization of mixture proportion using response surface methodology
The composition of fly ash-based materials, including cement and fly ash, is complex, with significant interactions between components, making traditional methods inadequate for optimizing the mixture proportions. By applying the response surface methodology and considering actual mining operations, the range of values for the three factors and the desired response values can be determined to optimize the proportion of fly ash-based materials. The optimal mixture proportions for the fly ash-based materials are shown in Table 8.
| Factor | Fly Ash content (%) | Sodium silicate modulus | Water-cement ratio | Initial setting time (min) | 7-day compressive strength (MPa) | 28-day compressive strength (MPa) |
|---|---|---|---|---|---|---|
| Optimal value | 50 | 0.80 | 0.50 | 2.17 | 4.18 | 11.30 |
3.6. Sample inorganic plugging material SEM characterization
Figure 11 shows the microstructure of inorganic grouting materials with varying fly ash contents at a sodium silicate modulus of 0.8 and a water-cement ratio of 0.55. As shown in the figure, when the fly ash content reaches 70%, more fly ash agglomerates appear in the inorganic material. When the hydration reaction occurs, the surface area of the fly ash water contact is not enough to fully participate in the hydration, after wetting, it will form agglomerates, leading to a reduction in compressive strength [34]. In the inorganic material with a fly ash content of 50%, the fly ash is relatively evenly distributed throughout the matrix. The material contains a certain number of pores, primarily due to the insufficient amount of fly ash participating in the physical filling process. However, the absence of fly ash clusters has a greater effect on the compressive strength of the material. SEM observations reveal that the number of spherical particles increases progressively with higher fly ash content. This morphological change contributes to modifications in the microstructure, which in turn affects the material’s mechanical behavior and durability [35]. These microstructural trends are consistent with earlier findings, confirming that fly ash content significantly influences the compressive strength of the material.
- Microscopic morphology of inorganic grouting material with different fly ash dosages: (a) 50%; (b) 70%.
Figure 12 shows the microstructure of inorganic grouting materials with 50% fly ash content and a water-cement ratio of 0.55 under different sodium silicate moduli. It is evident from the figure that when the sodium silicate modulus is 0.8, needle-like ettringite crystals form on the surface of the inorganic material, intertwining with each other. When the sodium silicate modulus increases to 2.0, the surface of the inorganic material exhibits a significant number of narrow cracks. This phenomenon can be attributed to the excess water introduced by the sodium silicate solution, which dilutes the reactive species in the system and inhibits further polymerization. Nevertheless, the calcium-silicate-hydrate gel generated during this stage coats the fly ash particles, promoting the formation of a relatively compact microstructure [36]. This observation is consistent with previous findings on the relationship between microstructure and compressive strength.
- Microscopic morphology of inorganic grouting material with different sodium silicate moduli: (a) 0.8; (b) 2.0.
Figure 13 shows the microstructure of inorganic grouting materials with 60% fly ash content and a sodium silicate modulus of 0.55 under different water-cement ratios. When the water-cement ratio is 0.50, the fly ash particles are relatively evenly distributed. However, when the ratio increases to 0.60, there is more residual water, causing the fly ash particles to clump together. During the subsequent hardening process, the evaporation of water leads to the formation of numerous tiny pores and cracks in the inorganic material, significantly reducing the load-bearing area. This also weakens the bonding between the cementitious material and the fly ash [37], thereby lowering the compressive strength of the inorganic material.
- Microscopic morphology of inorganic grouting material with different water-cement ratios: (a) 0.50; (b) 0.60.
Figure 14 shows the microstructure of an inorganic grout material with 50% fly ash content, 0.8 sodium silicate modulus, and 0.55 water/cement ratio at different curing ages. Small amounts of unreacted spherical fly ash particles were still visible in the inorganic material cured for 28 days. This phenomenon may be attributed to the partial dissolution of soluble silica and alumina on the surface of fly ash particles when exposed to the alkaline activator, leading to the rapid formation of hydration gels that encapsulate the particles and form micro-aggregates [38]. Compared to the specimens cured for 7 days, those cured for 28 days exhibit lower porosity and a denser microstructure, aligning well with the observed enhancement in compressive strength.
- Microstructure of inorganic grouting material at different curing ages: (a) 7-day curing ages; (b) 28-day curing ages.
3.7. Sample inorganic plugging material bonding and reinforcement performance test
To study the sealing effect of fly ash-based inorganic grouting materials injected into coal pillars in a mine, an investigation was first carried out on the reinforcement effect of these materials on fractured coal. The fly ash-based inorganic grouting material, prepared with the optimized mixture from the response surface methodology, was injected into fractured coal samples of varying masses. Test the uniaxial compressive strength of the test specimen as it cures. The optimized material mix consisted of 50% fly ash, a sodium silicate modulus of 0.80, and a water-cement ratio of 0.50. To assess the bonding effect between the fly ash-based material and the coal, four different mass gradients of fractured coal were mixed with the fly ash-based material for compressive strength testing. The compressive strength results of the samples with varying coal particle volumes have been shown in Figure 15.
- Compressive strength of inorganic grouting material with different volumes of coal particles.
The results indicate that the mechanical properties of the samples containing coal particles were lower than those without coal particles. This is because the strength of coal particles is lower than that of the inorganic grouting material. However, as the volume of coal particles increased, the initial compressive strength of the grouting material also increased, with early strengths all exceeding 2.0 MPa. This demonstrates that the developed grouting material has good bonding and reinforcement effects on fractured coal. The material can diffuse into the gaps between coal particles, bonding the fractured coal into a solid mass, enhancing the bonding strength of the coal, and preventing the formation of air leakage pathways. This helps to suppress spontaneous combustion of coal and makes the material suitable for sealing and filling in underground mines.
A stereo microscope was used to analyze the microscopic interface morphology between the inorganic grouting material and the fractured coal blocks, as shown in Figure 16. The results show that the inorganic grouting material can penetrate well into the cracks between the coal blocks, providing bonding and reinforcement. The bonding forces between the two materials mainly consist of physical adhesion and chemical bonding.
- Microscopic interface morphology between inorganic grouting material and fractured coal.
The physical adhesion of the inorganic sealing material primarily arises from the interaction between its surface morphology and that of the coal block. As illustrated in Figure 17, during the curing process, the high fluidity of the cementitious slurry enables it to infiltrate microcracks and pores within the coal matrix. This infiltration establishes mechanical interlocking, thereby increasing the effective contact area. The inherent surface roughness of the coal further enhances interfacial bonding through additional mechanical interlock. Moreover, capillary action contributes to bonding as shrinkage during drying and curing generates tensile stresses at the interface, enhancing the overall adhesive strength. In the absence of coal blocks, the inorganic sealing material relies solely on its intrinsic mechanical strength to resist external loads. Without the coal acting as a reinforcing phase, the overall compressive strength of the material tends to decrease. This reduction can be attributed to weaker interparticle bonding within the matrix, resulting in lower toughness under stress and an increased tendency toward brittle failure.
- Microscopic mechanism analysis of inorganic grouting material.
The chemical adhesion of the inorganic sealing material primarily originates from hydration reactions within the cementitious matrix. Hydration products, particularly calcium silicate hydrate gel, can chemically interact with both organic components and mineral phases on the coal surface. These interactions generate hydrogen bonds, ionic bonds, and other chemical linkages, alongside chemical adsorption, thereby enhancing the interfacial bonding strength between the sealing material and the coal substrate [39]. In systems lacking coal block inclusion, bonding primarily depends on hydration products formed within the cement matrix. Without coal block filling and structural reinforcement, the available interfacial area for reaction is limited, thereby reducing opportunities for chemical bonding and ultimately diminishing chemical adhesion.
The physical adhesion and chemical bonding forces enable the coal blocks, when used as fillers, to form a denser structure within the material matrix, enhancing some of the characterization properties of the material. This enhanced mechanical performance allows the material to effectively bond with broken coal bodies, forming an integrated and solid structure, which prevents the formation of airflow leakage channels between coal bodies, effectively reduces the coal void adsorption of oxygen, to inhibit the occurrence of spontaneous coal combustion disaster.
4. Conclusions
Based on the experimental results, theoretical analysis, and microstructural observations, the following conclusions can be drawn:
Using response surface methodology, quantitative models were developed to describe the relationships between fly ash content, sodium silicate modulus, water–cement ratio, and three key performance indicators: initial setting time, 7-day compressive strength, and 28-day compressive strength. The regression models exhibited strong fitting accuracy, and ANOVA results confirmed that fly ash content and water–cement ratio had significant effects on both early- and late-age strength, while sodium silicate modulus significantly affected 7-day strength but showed limited influence on 28-day performance. Through a central composite design and multi-factor optimization, the optimal mix ratio was determined to consist of 50 % fly ash, a sodium silicate modulus of 0.80, and a water–cement ratio of 0.50. This formulation achieved a rapid initial setting time of 2.17 minutes, a 7-day compressive strength of 4.17 MPa, and a 28-day compressive strength of 11.30 MPa, satisfying engineering requirements for rapid plugging and long-term structural stability in coal mine leakage control applications. Furthermore, SEM analysis revealed that variations in fly ash content, sodium silicate modulus, and water–cement ratio significantly influenced the internal microstructure of the hardened grout. Higher fly ash content promoted the formation of secondary hydration products such as N–A–S–H gels, which filled the internal pores and refined the pore structure; an appropriate sodium silicate modulus enhanced early-stage gel polymerisation; and a lower water–cement ratio reduced capillary porosity and improved particle packing. These microstructural changes contributed to a denser matrix, an improved interfacial transition zone, and stronger bonding with fractured coal surfaces, while at later curing ages the continued development of interconnected gel networks enhanced load transfer and crack resistance. However, this study did not assess how environmental factors, such as underground temperature fluctuations, coal seam humidity, or cyclic loading, may affect microstructural stability and long-term sealing performance, which should be addressed in future work through field-scale validation and durability testing under realistic conditions.
Acknowledgment
This work was supported by the Jiangsu Science and Technology Association Youth Science and Technology Talent Support Program (JSTJ-2023-XH002), and the Key Laboratory of Mine Thermodynamic Disasters and Control of Ministry of Education (Liaoning Technical University) (JSK202202).
CRediT authorship contribution statement
Qingjie Zhang: Writing, Methodology, Conceptualization. Quanlin Shi: Writing, Revision, Supervision. Lihua Long: Investigation, Revision. Hemeng Zhang: Resources, Investigation.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Declaration of Generative AI and AI-assisted technologies in the writing process
The authors confirm that there was no use of artificial intelligence (AI)-assisted technology for assisting in the writing or editing of the manuscript and no images were manipulated using AI.
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