Sustainable Multifunctional Hydrogel Based on SA/GEL/PVA/GL for the Synergistic Inhibition of Coal Spontaneous Combustion

Abstract

To improve the prevention of coal spontaneous combustion (CSC) and overcome the poor durability of traditional inhibitors, a novel double-network (DN) biomass hydrogel, termed SGPGc, was developed. It was synthesized from sodium alginate (SA), gelatin (GEL), poly­(vinyl alcohol) (PVA), and glycerol (GL) via a CaCl₂-borax dual cross-linking system. The Ca²⁺–SA ionic network constituted a rigid skeleton, whereas the dynamic borate bonds between B­(OH)₄ ^– and PVA provided toughness and self-healing capability. This synergy endowed SGPGc with a high tensile strength of 16.25 MPa and a fracture elongation of 286.07%, ensuring tolerance to harsh underground mining environments. The hydrogel also exhibited excellent thermal stability and water retention, showing only 38.6% water loss at 90 °C, coupled with a high rehydration capacity of 37.73 g/g (dry weight basis) for the dried film, which enabled dynamic cooling and functional regeneration. Structural analysis revealed that SGPGc significantly reduced the total pore volume and specific surface area of coal by 47.23 and 50.25%, respectively, effectively blocking oxygen diffusion channels. Furthermore, its water retention capability enabled sustained cooling, while its high rehydration capacity facilitated secondary sealing. In comparison with raw coal, the SGPGc-treated samples showed significantly reduced CO generation and production rate, along with substantially increased crossing point temperatures (CPT, up by 33.7 and 29.0 °C). The demonstrated inhibition efficacy surpassed that of conventional calcium chloride, confirming the operational closed-loop synergistic mechanism of “barrier-cooling-functional regeneration” in effectively inhibiting CSC.

1. Introduction

Coal remains a dominant global primary energy source, occupying an irreplaceable status in the energy structure, especially in China. , However, CSC has evolved into an increasingly intractable hazard during mining, transportation, and storage, which can initiate catastrophic incidents including fires and explosions with heavy casualties, while also leading to substantial resource wastage and severe ecological degradation , Therefore, effective prevention and control of CSC are crucial for ensuring coal mine safety production.

To mitigate CSC disasters, scholars have proposed a range of mitigation technologies, including grouting, inert gas injection, , and inhibitor application. − However, the applications of grouting and inert gas injection are constrained by complex operations and limited coverage, restricting their scope. In comparison, inhibitors are particularly favored due to their superior environmental adaptability and remarkable suppression effectiveness. Based on the inhibition mechanism, inhibitors can be classified into physical and chemical inhibitors. Physical inhibitors (e.g., halogen salts, ammonium salts) primarily act via endothermic cooling and oxygen isolation. However, such inhibitors generally exhibit transient effectiveness: their moisture gradually evaporates as the coal temperature rises, causing diminishing inhibitory efficacy, shortened effective duration, and poor stability. Furthermore, certain physical inhibitors, particularly halogen salts, may decompose or react, producing toxic and hazardous gases that pose risks to miners’ health and safety. Chemical inhibitors (e.g., ionic liquids, antioxidants , ) primarily inhibit coal oxidation via disrupting its free-radical chain reaction. However, such inhibitors face multiple challenges: immature synthesis processes, excessive production costs, poor biodegradability, and susceptibility to deactivation, which constrain their large-scale and engineering-scale applications.

As a specialized class of physical inhibitors, gel materials exhibit substantial potential for CSC prevention owing to their superior water-retention and cooling properties; they efficiently encapsulate coal surfaces and penetrate pores and fractures to achieve oxygen isolation. , However, conventional gel materials exhibit multiple limitations, including poor flowability, restricted diffusion range, susceptibility to pulverization, and low mechanical strength coupled with a high rupture tendency, − alongside contamination of mining areas and adjacent ecosystems induced by residual monomers (e.g., acrylamide) in certain synthetic gels. Therefore, the rational design and fabrication of eco-friendly inhibitors featuring persistent cooling capability and robust adaptability to harsh underground conditions hold profound theoretical value and practical implications for the prevention of CSC.

As a natural polymer, SA can form stable hydrogels with an “egg-box” structure upon cross-linking with Ca²⁺. The 3D cross-linked structure endows SA hydrogels with excellent water-retention capacity (WRC), and coupled with their inherent advantages of favorable biocompatibility and environmental friendliness, , such hydrogels have been extensively applied in fields including medicine , and food. Given the above properties, SA shows remarkable application potential for CSC prevention. However, unmodified SA-based hydrogels have inherent drawbacks of susceptibility to drying-induced cracking and weak mechanical properties; they cannot withstand complex underground conditions and tend to fail prematurely. Thus, targeted modification is required to improve their mechanical performance as well as endow them with core functions (long-term oxygen barrier, crack resistance, and water retention) for this application, thereby significantly enhancing their inhibition efficiency. Herein, GEL was incorporated as a thickener to enhance the system viscosity; meanwhile, PVA and tetraborate ions (B­(OH)₄ ^–) derived from borax formed dynamic reversible B–O bonds, endowing the gel with self-healing properties. GL was further introduced as a plasticizer to improve the material's elasticity. Taking advantage of the synergistic effects of these components and using SA as the primary matrix, a novel biomass DN hydrogel was fabricated via a CaCl₂–borax dual cross-linking strategy. Subsequently, the physicochemical properties, inhibition efficacy, and component synergistic mechanisms of the hydrogel were systematically investigated.

2. Materials and Methods

2.1. Materials

2.1.1. Coal Samples

Two brown coal samples (referred to as 1# and 2#) were collected from a spontaneously combusting coal seam in the Inner Mongolia Autonomous Region. The samples were crushed by using a cylindrical crusher to obtain particles with a size range of 0.18–0.25 mm. Subsequently, the coal samples were dried in a vacuum oven at 40 °C for 24 h. Table presents the proximate and ultimate analysis results of the two coal samples.

1. Proximate and Ultimate Analysis.

	proximate analysis/wt %				ultimate analysis/wt % (daf)
coal sample	M ad	A ad	V ad	FCad	C	H	O	N
1#	8.42	47.69	21	22.89	44.75	4.82	22.1	0.62
2#	8.53	46.17	21.7	23.6	45.13	4.98	22.9	0.61

2.1.2. Main Components of the Novel Gel

The novel gel developed in this study is composed of SA, GEL, PVA, and GL as the main components, all of which are nontoxic and environmentally friendly. Calcium chloride and a saturated borax solution were used as the cross-linking agents.

2.2. Methods

2.2.1. Preparation of SGPGc

The gel was synthesized via a simple, eco-friendly method (Figure ). First, SA powder was dissolved in 300 mL of distilled water at 60 °C with stirring to form a homogeneous solution. GEL, PVA, and GL were then added sequentially at a predetermined mass ratio. The mixture was stirred continuously until all components were completely dissolved, yielding a uniform gel precursor, denoted as SGPG. Then, SGPG was cast into a mold and left quiescently to allow for bubble removal. This is the first step in the preparation process.

1.

Preparation process of SGPGc.

Saturated borax solutions containing 1, 2, 3, 4, and 5% CaCl₂ were prepared by dissolving 4 g of borax and the corresponding amount of anhydrous calcium chloride in 100 mL of distilled water. The SGPG gel was sprayed with the cross-linking solution and cross-linked for 2 h at 21 ± 1 °C and 14 ± 2% RH to obtain the final product, designated as SGPGc. All samples were prepared under identical conditions to ensure uniformity.

2.2.2. Viscosity of SGPG

In engineering practice, gel viscosity is a critical parameter governing its flow behavior during pipeline transportation to the goaf. Low viscosity may result in excessive flow and inadequate coverage, while excessively high viscosity can induce prohibitive frictional resistance. Both scenarios ultimately compromise the completeness of coal surface coverage. In this study, the viscosity of SGPG was measured by using a rotational rheometer. A single-factor experimental method was employed to investigate the effects of individual component contents, whereby one variable was altered at a time while the others were held constant.. Each formulation was tested in five replicates. Based on these results, the optimal formulation was determined.

2.2.3. Mechanical Property (MP) of SGPGc

The mechanical properties of SGPGc were determined using an electronic universal testing machine. SGPGc samples were cut into strips measuring 100 × 20 mm, with a gauge length of 50 mm between the jigs, and the drawing speed was 20 mm/min. The samples were clamped using rock clamps and stretched at a speed of 20 mm/min until final fracture. Each SGPGc sample was tested 5 times, and the average values were recorded.

Tensile strength (TS) and elongation at break (EAB) of the SGPGc samples were calculated using the following formulas:

$$\mathrm{TS}=\frac{F}{A}$$ 1

where F is the biggest tension of test SGPGc when it fractures finally, N, and A is the cross-sectional area of test SGPGc, mm².

$$\mathrm{EAB}=\frac{L-L_0}{L_0}\times 100\%$$ 2

In the formula: L ₀ is the length of SGPGc before test, mm, and L is the maximum length of the SGPGc before fracture, mm.

2.2.4. WRC of SGPGc

As a key physicochemical property, the high WRC of the gel contributes to CSC inhibition by enabling prolonged suppression of the coal temperature. The water loss rate (WLR) serves as a direct indicator of the gel’s WRC. In this experiment, gel samples were prepared based on the optimal SGPG formulation (Section ) and subsequently treated with cross-linking agents at varying concentrations. Each sample had an initial mass of 100 g. The temporal variation in the sample mass was first monitored at room temperature. Subsequently, the samples were subjected to a series of elevated temperatures (30, 40, 50, 60, 70, 80, and 90 °C) for 1 h. After the dehydration process, their masses were recorded. The WLR was then calculated both at room temperature and under each elevated temperature condition using the following formula:

$$r=\frac{M_1-M_2}{M_1}\times 100\%$$ 3

where: r is the relative water loss rate,%; M ₁ is the weight of SGPGc before losing water, g; and M ₂ is the weight of SGPGc after losing water, g.

2.2.5. Water Rehydration Capacity (WRAC) of SGPGc

The water retention and absorption capacity (WRAC) of the gel is a critical performance parameter that directly governs its inhibitory efficacy against CSC. This capacity is primarily governed by the cross-linking density within the gel network. In this study, the WRAC of SGPGc (prepared as described in Section ) was determined using the tea-bag method. Prior to testing, all samples were dried at room temperature until a constant weight was achieved. The experimental procedure was as follows: (1) the weight of each tea bag used in the experiment was recorded as W ₀. (2) The weight of the dried SGPGc was recorded as W _s. (3) The dried SGPGc was placed into a tea bag and immersed in excess water for spontaneous swelling and hydration. The tea bag was then placed on dry filter paper to remove excess water, weighed hourly until the mass no longer increased, and the final mass was recorded as W _t . (4) The WRAC of SGPGc was calculated using the following formula, denoted as Q:

$$Q=\frac{W_t-W_0-W_{\mathrm{s}}}{W_{\mathrm{s}}}$$ 4

where Q represents WRAC of SGPGc, g/g; W ₀ is the weight of a dry tea bag, g; W _s is the weight of dried SGPGc, g; and W _t is the weight of the tea bag that contains SGPGc after water swelling, g.

2.2.6. Microstructure of SGPGc

Conventional scanning electron microscopy (SEM) requires samples to be completely dry. However, the drying process may alter the microstructure of water-containing materials, such as emulsions and gels, making it difficult to observe their native state. In contrast, cryo-fracture SEM is advantageous for imaging such samples. In this technique, the sample is rapidly vitrified (ultrarapidly frozen) on the microscope‘s cryo-stage, forming amorphous ice that preserves the native hydrated microstructure. In this study, cryo-fracture SEM was employed to analyze the microstructures of SGPG and SGPGc.

2.2.7. Liquid Nitrogen Adsorption Experiment

The pore structures of the coal samples was characterized using an ASAP 2020 micropore system, with N₂ as the adsorbate gas. Static nitrogen adsorption isotherms were measured over a relative pressure (P/P ₀) range of 0.01–0.99. The specific surface area, pore volume, and pore size distribution of the coal samples were determined by using appropriate models based on the N₂ adsorption capacity.

2.2.8. Programmed Temperature Rise Experiment

A temperature-programmed experiment was conducted to evaluate the inhibitory efficacy of SGPGc against CSC. For comparison, coal samples treated with a calcium chloride solution served as the reference. The experiment was performed at a heating rate of 0.5 °C/min under a dry air flow of 120 mL/min. Gaseous products evolved during heating were collected at 10 °C intervals, and their concentrations were quantified by gas chromatography. The sample temperature was simultaneously monitored. These data provide a basis for assessing the inhibition performance of SGPGc.

3. Results and Discussion

3.1. Viscosity of SGPG

In this experiment, a single-factor experimental design was employed to determine the optimal formulation. The mass gradients for each component were set as follows: SA: 1.5, 3.0, 4.5, 6.0, 7.5, and 9.0 g; GEL: 1, 2, 3, 4, and 5 g; PVA: 1, 2, 3, 4, and 5 g. GL: 10, 20, 30, 40, and 50 wt % relative to the total mass of SA, GEL, and PVA. Figure illustrates the variation of SGPG viscosity with the contents of (a) SA, (b) GEL, (c) PVA, and (d) GL. Data points represent the mean of five replicates, with error bars showing the standard deviation. The observed variation primarily arises from the inherent fluctuation of the rotational rheometer, a typical random error in testing high-viscosity fluids. However, the viscosity differences between formulations are significantly greater than the variation within replicates, and each component shows a consistent trend. These results demonstrate a clear and reliable systematic effect of composition. The experimental results indicated that the viscosity of SGPG increased with the content of SA, GEL, and PVA, with SA exhibiting the most significant effect.

2.

Viscosity of SGPG as a function of the contents of (a) SA, (b) GEL, (c)­PVA, and (d) GL. Data are presented as mean ± SD (n = 5).

This dominant effect of SA stems from its extended polymeric chain architecture and the abundant, regularly spaced sodium carboxylate (−COO^–) groups along the backbone. Electrostatic repulsion extends the molecular chains, imparting a denser entangled network and thereby increasing the viscosity. Higher SA concentrations enhance this effect by supplying a greater density of chains, which further strengthens the interconnectivity of the network. The effect of GL is dual and concentration-dependent: it increases viscosity at lower concentrations by strengthening the network via interchain hydrogen bonds, whereas at higher concentrations, the dominating dilution and plasticization effects weaken intermolecular interactions, resulting in a pronounced decrease in viscosity. Consequently, to achieve the target viscosity (1000–1500 mPa·s), the following optimal formulation was established: 6 g SA, 3 g GEL, 3 g PVA, and 30 wt % GL.

3.2. Mechanical Property of SGPGc

To ensure reliable performance in complex underground coal seams, gel materials for CSC prevention must possess two key mechanical properties: high tensile strength to withstand pumping and compressive stresses, and high elongation at break to accommodate strata movement without compromising coating integrity. Together, these properties determine the long-term reliability of the gel’s oxygen-barrier function in harsh mining environments.

Figure shows that the GL dosage significantly modulates the mechanical properties of SGPG. Tensile strength continuously decreased with increasing GL content, whereas the elongation at break first increased and then decreased, peaking at 40% GL. This mechanical trend aligns with the rheological data (Figure d), where the viscosity also peaked at 40% GL before a sharp decline. The inverse relationship suggests that the flexibility imparted by GL compromises mechanical strength and structural viscosity. Excessive plasticization beyond 30% GL induced structural loosening, detrimental to mechanical integrity. Therefore, a 30% GL dosage was identified as optimal for balancing flexibility with sufficient mechanical performance.

3.

MP of SGPG at a specific GL dosage. Data are presented as mean ± SD (n = 5).

Similarly, Figure shows that both the tensile strength and elongation at break of SGPGc exhibited a nonmonotonic dependence on CaCl₂ concentration. Tensile strength peaked at 19.94 MPa (3% CaCl₂), while elongation at break reached a maximum of 286.07% (2% CaCl₂). The optimal performance at moderate concentrations (≤2%) is attributed to a well-balanced dual-network (DN) structure, where covalent B–O bonds (B­(OH)₄ ^–/PVA) and ionic “egg-box” junctions (Ca²⁺/SA) synergistically enhance the strength while maintaining chain mobility for toughness. However, excessive CaCl₂ (>2%) led to over-cross-linking, which shortens network chains, restricts chain orientation and sliding, and consequently causes a sharp decrease in elongation at break. Thus, a 2% CaCl₂ concentration was determined to be optimal.

4.

MP of SGPGc at different cross-linker concentrations. Data are presented as mean ± SD (n = 5).

The trends reported in both figures are statistically robust, as the systematic variations across compositions far exceeded the experimental margin of error, which was controlled through a quintuplicate tests and standardized protocols.

3.3. Water Retention Capacity (WRC) of SGPGc

3.3.1. Room Temperature

As illustrated in Figure , the evolution of the WLR for SGPGc over time exhibited a consistent pattern when cross-linked with saturated borax solutions containing 1–5% CaCl₂. The WLR of SGPGc exhibited a characteristic two-stage pattern: an initial rapid increase, attributed to the evaporation of free water from its surface and macropores, was followed by a slowdown corresponding to the removal of bound water from its network. A significant negative correlation was observed between the final water loss rate and CaCl₂ concentration. Specifically, the rate decreased from 34.1 (0% CaCl₂) to 24.1% and 15.6% with 2 and 5% CaCl₂, respectively. This confirms that higher Ca²⁺ concentrations promote a denser “egg-box” structure, which enhances water retention by strengthening the capillary forces and increasing the diffusion resistance.

5.

Water loss rate of SGPGc over time at room temperature.

3.3.2. Elevated Temperatures

Considering that the low-temperature stage (below 70 °C) of the CSC exothermic process is crucial for its initiation and progression, the WRC of SGPGc within the room temperature to 70 °C range becomes a key factor for its inhibitory function. As shown in Figure , SGPGc demonstrated significantly lower WLR than SGPG across all tested temperatures. A strong negative correlation was observed between WLR and CaCl₂ concentration. Specifically, at 70 °C, the WLR decreased from 26.3 to 12.2% as the CaCl₂ concentration increased from 1 to 5%. At the elevated temperature of 90 °C, the samples cross-linked with 2 and 5% CaCl₂ presented WLRs of 38.6 and 26.0%, respectively. These findings demonstrated that a higher cross-linking density could markedly enhance the thermal stability of the gel network. This enhanced performance is attributed to a dual-cross-linking mechanism. The DN architecture, co-constructed through the “egg-box” structure between Ca²⁺ and the −COO^– groups of SA and the covalent cross-linking between B­(OH)₄ ^– and the −OH groups of PVA, exhibits remarkable thermal stability. The abundant hydrophilic groups (−OH, −COOH) within this network immobilize water molecules via stable hydrogen bonding, thereby endowing the material with superior water retention.

6.

Water loss rate of SGPGc over time at an elevated temperature.

3.4. Water Rehydration Capacity (WRAC) of SGPGc

Figure depicts the rehydration process of a dried SGPGc film (cross-linked with a saturated borax solution containing 2% CaCl₂) over 5 h. The film underwent significant volumetric expansion without dissolution upon rehydration, with the most rapid expansion occurring within 1 h. This initial rapid phase resulted from the fast diffusion of water molecules into the highly porous architecture of the DN, facilitating rapid capillary intake.

7.

Water reabsorption process of dry SGPGc film.

The rehydration process of SGPGc exhibited a characteristic two-stage kinetic profile characterized by an initial rapid uptake, followed by a slow equilibration phase (Figure ). The rehydration capacity was found to be highly dependent on the CaCl₂ concentration, with an optimal performance (37.73 g/g) achieved at 2%. This value was markedly higher than that of SGPG (8.9 g/g). Beyond this point, a clear decline was observed, with values decreasing to 31.82, 21.67, and 16.51 g/g at concentrations of 3%, 4%, and 5%, respectively. The optimal rehydration at 2% CaCl₂ resulted from a well-balanced DN architecture that provided both rapid water pathways and adequate storage space, whereas higher concentrations (>2%) caused excessive densification, which hindered water transport and limited swelling.

8.

Cumulative water absorption of SGPGc during rehydration.

The rehydration mechanism of SGPGc exhibited a sequential process: water molecules initially formed bound water through hydration with hydrophilic groups (−OH, −COOH), subsequently diffused into the DN architecture via capillary action to generate free water, and ultimately induced significant synergistic volumetric expansion. The structural integrity (nondissolution) of the dried SGPGc film during rehydration was attributed to its stable DN architecture. Notably, the dried film could rapidly rehydrate upon contact with water, restoring its DN architecture and water-retention capacity. The functional regeneration capability ensures that SGPGc can deliver long-lasting cooling performance in underground coal-mine environments, establishing a foundation for the long-term prevention and control of CSC.

3.5. Analysis of Pore Structure of Coal

A key determinant of the physical oxygen adsorption that initiates coal spontaneous combustion (CSC) is the pore structure of coal. Low-temperature nitrogen adsorption isotherms (P/P ₀ = 0.01–0.99) revealed that all samples exhibited type II isotherms with H3-type hysteresis loops under IUPAC classification, as depicted in Figure . Although the three coal samples displayed consistent adsorption–desorption isotherm patterns with a shared inflection point at a relative pressure of P/P ₀ = 0.5, their hysteresis loop areas varied remarkably. A smaller hysteresis loop area corresponds to a higher micropore proportion and a more compact, well-ordered pore structure of the coal sample. Conversely, a larger area indicates a lower proportion of micropores and mesopores, with the pore structure dominated by macropores. Analysis revealed that the pore structure of raw coal was predominantly microporous, while treatments with SGPG and SGPGc shifted the porosity toward meso- and macropores. This structural evolution confirms that the treatments, particularly SGPGc, effectively suppressed the coal spontaneous combustion propensity by reducing the coal-oxygen contact area.

9.

Adsorption/desorption isotherm curves of coal samples.

The pore structure parameters of the coal samples are presented in Table . According to B.B. Hodot’s pore classification standard, treatment with SGPGc reduced the total pore volume and specific surface area of the raw coal by 47.23 and 50.25%, respectively. Overall, both parameters followed the trend: raw coal > SGPG-treated coal > SGPGc-treated coal. More critically, the proportion of micropores in both volume and specific surface area decreased significantly in the treated coal samples.

2. Pore Structure Parameters of Coal Samples.

		proportions of pore volume in each pore size segment %				proportions of specific surface area in each pore size segment %
coal sample	total pore volume (cm3/g)	<10 nm	10–100 nm	>100 nm	total specific surface area (m2/g)	<10 nm	10–100 nm	>100 nm
raw coal	0.01755	58.90	30.52	10.58	12.04065	91.44	8.16	0.40
SGPG-treated	0.01244	58.29	30.73	10.98	8.67674	91.14	8.51	0.347
SGPGc-treated	0.00926	55.13	33.38	11.49	5.98974	89.74	9.74	0.52

As shown in Figures and , the pore distribution curves indicated that both the pore volume and specific surface area of the coal samples were predominantly concentrated in micropores, a characteristic fully consistent with the data presented in Table . For the transitional pores, the segmented pore volume increased initially and then decreased sharply with increasing pore diameter, which reflected a significant inhomogeneity in their distribution. Notably, all segmented specific surface area curves peaked around 2 nm, highlighting that the micropores within this range were the core contributors to the total specific surface area. From the perspective of inhibition mechanisms, micropores (particularly those with diameters <2 nm) served as the primary sites for physical oxygen adsorption in coal. The reduction in the proportion of micropores resulting from the gel treatment directly decreased the effective oxygen adsorption sites on the coal surface, thereby significantly impairing the coal’s oxygen adsorption capacity during the low-temperature oxidation stage and ultimately inhibiting CSC.

10.

Relationship between the pore volume and pore size of coal samples.

11.

Relationship between the pore-specific surface area and pore size of coal samples.

3.6. Characteristic Analysis of CSC

To further evaluate the inhibitory efficacy of SGPGc on CSC, this study adopted the CO and crossing point temperature (CPT) as the key indicators. Following a standard method for CSC prevention, the inhibition rate is defined as the percentage ratio of the difference in CO concentration between raw coal and gel-treated coal to the CO concentration of raw coal, measured at a constant temperature of 100 °C. To optimize SGPGc dosage, preliminary experiments were conducted with SGPGc-to-1# raw coal mass ratios of 1:2, 1:4, and 1:6, resulting in inhibition rates of 65.1, 73.3, and 79.8%, respectively. The inhibition rate increased with SGPGc dosage but showed diminishing returns beyond the 1:4. Considering both inhibition efficiency and economic feasibility, the optimal SGPGc-to-coal mass ratio was determined to be 1:4. For comparison, a 20% CaCl₂ solutiona widely used traditional salt-based inhibitor in CSC preventionwas selected as a reference for parallel experiments.

3.6.1. CO Generation Trend

Relationship curves between the CO concentration and temperature for different coal samples are presented in Figure . The results indicated that, at each temperature, both inhibitors reduced CO emission compared to raw coal, with SGPGc exhibiting the most significant suppression. All CO release curves displayed a similar trend: a slow release phase at low temperatures (30–90 °C), a stable period due to heat accumulation, followed by an exponential growth phase, indicating intense autothermal reactions. The SGPGc-treated sample showed the shallowest curve slope, corresponding to the lowest CO formation rate throughout the heating process.

12.

Relationship curves between the CO concentration and temperature for different coal samples.

3.6.2. Crossing Point Temperature

The CPTs of raw coal samples (1#, 2#) are compared with their inhibitor-treated counterparts (SGPGc and CaCl₂, respectively) in Figures and . A higher CPT generally indicates a lower coal oxidation and spontaneous combustion propensity. The CPTs of coal samples treated with CaCl₂ and SGPGc were both higher than those of the raw coal. For the 1# raw coal sample, the CPT was 147.5 °C; CaCl₂ treatment increased this value to 158.7 °C, while SGPGc further elevated it to 181.2 °C. In the case of the 2# sample, SGPGc treatment raised the CPT from 147 to 176 °C. These results confirm that SGPGc delivers superior CSC inhibition performance.

13.

CPT of 1# raw coal sample treated with CaCl₂ and SGPGc.

14.

CPT of 2# raw coal sample and treated with CaCl₂ and SGPGc.

The evolution profile of CO serves as a direct indicator of the coal-oxygen reaction kinetics. SGPGc establishes a dense barrier layer on the coal surface and within its pore matrix, which significantly impedes the mass transfer of oxygen to the reactive sites. This suppresses the formation of peroxy complexes from the outset, leading to a marked decrease in CO emissions. Concurrently, the endothermic phase change of water within the gel (evaporation) effectively mitigates the temperature increase of the coal mass. The concurrent restriction of oxidant access and active cooling synergistically attenuates the net exothermic rate of the coal oxidation process. This dual mechanism operates at both thermodynamic (shifting the heat balance) and kinetic (lowering reaction rates) levels to achieve the effective suppression of CSC.

3.7. Microstructure and Inhibition Mechanism of SGPGc

Figure illustrates the composition of SGPGc, the formation mechanism of its DN, and the mechanism for inhibiting CSC. Figure a shows that all components of SGPGc are derived from biomass, reflecting a green and sustainable design concept. The synergistic interaction among these components lays the foundation for constructing a high-performance DN architecture.

15.

Fire-extinguishing mechanism of SGPGc: (a) sources of gel precursors; (b) DN architecture microstructure formation mechanism; and (c) inhibition mechanism of SGPGc gel.

The network of SGPGc is constructed via an “ionic–covalent synergistic” strategy (Figure b). The ionic cross-linked network acts as the rigid skeleton; the egg-box structure between the Ca²⁺ and G units of SA provides mechanical support and morphological stability. Its dense network imparts water retention, ensuring adaptability to the harsh underground environment. The dynamic covalent network serves as the flexible phase, formed via reversible borate ester bonds between B­(OH)₄ ^– and −OH groups of PVA. It endows excellent toughness and self-healing ability (Figure ). During drying, the dynamic network sustains basic skeleton integrity, laying a foundation for its rehydration and regeneration. The two networks intertwine to form an interpenetrating structure, synergistically enhancing the overall stability and functional sustainability.

16.

Self-healing process of SGPGc.

The SEM results in Figure c-2,3 verify the DN cross-linking mechanism from the perspective of micromorphology. The SGPG precursor exhibits a continuous and uniform structure, indicating excellent component compatibility. In contrast, SGPGc presents a typical 3D cross-linked network morphology with a clearer skeleton and more regular pore distribution, confirming that ionic–covalent synergistic cross-linking successfully constructs an interpenetrating network and lays a structural foundation for gel performance.

SGPGc achieves closed-loop inhibition of CSC via a “barrier-cooling-regeneration” synergistic mechanism. The underlying mechanism is elaborated as follows: the dense oxygen barrier layer formed on the coal surface effectively fills and seals pores and fissures, blocks oxygen diffusion, and thus inhibits coal-oxygen reactions. Benefiting from the cross-linked DN architecture, this oxygen-barrier layer remains intact under complex underground stress, ensuring long-term stability of the physical barrier effect. Meanwhile, relying on its water retention property, the gel enables water evaporation and heat absorption during coal heating, breaking the heat accumulation cycle of the CSC and retarding the oxidation process. Furthermore, based on the reversibility of dynamic borate ester bonds, SGPGc realizes structural and functional regeneration through rehydration after dehydration, reactivates the cooling and sealing functions, forms such a closed loop, and ultimately achieves the goal of synergistic and long-term CSC inhibition.

4. Conclusions

A single-factor experimental design determined the optimal SGPG formulation as 6 g of SA, 3 g of GEL, 3 g of PVA, and 30 wt % GL. The viscosity increased with the content of SA, GEL, and PVA, most significantly with SA. In contrast, the viscosity showed an initial increase followed by a decrease with increasing GL content.

As CaCl₂ concentration rose, SGPGc water retention improved with increasing cross-linking density, while rehydration capacity and mechanical properties showed optimal performance at 2% concentration (37.73 g/g of rehydration capacity and 286.07% elongation). Further concentration increase to 3% enhanced water retention but caused excessive cross-linking, reducing both rehydration capacity and elasticity. These findings confirmed that moderate cross-linking is essential for balancing water absorption-retention capacity with mechanical performance.

SGPGc exhibited multisynergistic inhibition of CSC through its DN architecture. The Ca²⁺–SA and PVA–borax networks demonstrated synergistic effects, providing exceptional mechanical properties, self-healing capability, and functional sustainability. The inhibition mechanism involved a coordinated “barrier-cooling-regeneration” process: a dense coating reduced coal pore volume and specific surface area by 47.23 and 50.25%, respectively, blocking oxygen diffusion; water retention enabled persistent cooling; while the stable DN architecture provided high rehydration capacity (37.73 g/g) for long-term inhibition. Tests showed treated coal samples had significantly reduced CO generation and increased CPT (up by 33.7 and 29.0 °C). This synergistic mechanism enhanced the suppression durability, providing a theoretical basis for advanced mine fire prevention materials.

Future research should focus on several key areas: establishing quantitative relationships between the cross-linking degree and water content of SGPGc and its thermal conductivity, mechanical strength, and water-retention capacity to optimize formulation design; investigating the infiltration and film-forming behavior of SGPGc in coal fractures under different conditions through combined experimental and simulation approaches; and elucidating molecular-level interactions between SGPGc components and coal surfaces to clarify inhibition mechanisms. Additionally, developing geology–formulation–grouting coupling models will enable customized application strategies, while assessing the biodegradability of SGPGc and its environmental impact will ensure safe implementation in mining operations. These efforts will bridge laboratory research with practical applications, while addressing environmental sustainability requirements.

The authors do not have permission to share data.

X.H.: conceptualization, funding acquisition, and writingoriginal draft. F.W.: project administration and resources. J.Z.: funding acquisition and supervision. L.G.: conceptualization. J.S.: formal analysis, investigation, and validation. X.L.: funding acquisition, methodology, and writingreview and editing.

The authors declare no competing financial interest.