Preparation of layered double hydroxides from coal gangue in a deep eutectic solvent system and their application in flame retardancy of coal gangue

2.1. Chemicals

Magnesium nitrate, (Mg(NO₃)₂·6H₂O), urea (CH₄N₂O, special grade), choline chloride (C₅H₁₄ClNO, special grade), sodium hydroxide (NaOH, special grade and potassium hydroxide (KOH, special grade) were purchased from Sinopharm Chemical Reagent Co. Ltd. and used as received without purification.

2.2. Preparation of C-LDH in ionothermal in a deep eutectic solvent

The coal gangue used in this study was obtained from the Si Jia zhuang Coal Mine of the Lu’an Group (Detailed chemical composition of the coal gangue can be found in Table S1). The raw samples were first calcined in a muffle furnace at 900°C, after which 100 g of the calcined product was subjected to ball milling for 1 h and sieved through a 100-mesh screen. A portion of 10 g of the sieved sample was dispersed in 50 mL of ultrapure water, and the pH was adjusted to 1 by dropwise addition of hydrochloric acid. After acid leaching, the concentrations of major elements in the leachate were determined by ICP-OES analysis. The results showed that the contents of magnesium, calcium, aluminum, and iron were 42.45 mg/L, 585.90 mg/L, 1187.32 mg/L, and 972.18 mg/L, respectively. The suspension was then heated in a water bath at 90°C under stirring for 2 h to extract available divalent and trivalent metal ions. The slurry was filtered to obtain the acid-leached solution, and the concentrations of the extracted divalent and trivalent metals were quantified using atomic absorption spectrophotometry. Mg(NO₃)₂·6H₂O was subsequently added to the leachate to adjust the molar ratio of divalent to trivalent cations to 2:1. The 2:1 ratio results in a larger interlayer space, allowing for the accommodation of more anions, which enhances the anion-exchange capacity. This increased capacity is beneficial for flame retardancy, as it facilitates the incorporation of thermal-stabilizing anions, improving the material’s ability to suppress combustion. In parallel, DES was prepared by mixing choline chloride (8.34 g) and urea (7.07g) in a 50 mL polytetrafluoroethylene vessel and heating the mixture to 80°C under continuous stirring until a homogeneous ionic liquid was formed. The acid-leached solution was then adjusted to pH 6 and combined with the prepared DES [17]. Afterwards, 1.69 g of KOH was added, and the resulting mixture was stirred at 100°C for 30 min. The reaction vessel was subsequently sealed and transferred to a constant-temperature oven, where it was maintained under static conditions at 100°C for 48 hrs (closed system). Upon completion, the product was washed thoroughly with deionized water and ethanol until neutral, and finally dried overnight at 50°C in a vacuum oven. The obtained material was identified as layered double hydroxides derived from coal gangue and is hereafter denoted as C-LDH.

2.3. Preparation of samples for flame-retardant testing

The coal gangue used in this study was obtained from the Sijiazhuang Coal Mine of the Lu’an Group and ground to a particle size of 150 mesh. The as-prepared C-LDH powder was mechanically mixed with coal gangue at predetermined mass ratios to obtain composite materials. The mixtures were thoroughly homogenized by grinding and subsequently stored in a drying chamber for further testing. According to the mass ratios of C-LDH to coal gangue (5%, 10%, 15%, and 20%), the samples were denoted as C-LDH-5, C-LDH-10, C-LDH-15, and C-LDH-20, respectively. To preserve the intrinsic structural and chemical characteristics of the coal gangue and to avoid the loss of small organic components that may influence the initial reactivity, no de-ashing treatment was applied. In addition, a 20 wt% MgCl₂ solution was prepared because 20 wt% was the highest C-LDH loading investigated in this study, allowing MgCl₂ to serve as a practical reference at the same nominal addition level. The MgCl₂ solution was mixed with raw coal gangue at a mass ratio of 1:1 and homogenized by stirring. Although MgCl₂ was initially added as an aqueous solution, whereas C-LDH was mechanically mixed as a dry powder, all treated samples were dried under identical conditions before the thermal and oxidation tests to minimize the influence of moisture. The dried samples were then sealed and stored in a cool, dry environment until further use. However, differences in the initial addition methods may still affect inhibitor distribution and should be considered when interpreting the comparison.

2.4. Characterization methods

3.1. Characterization

As shown in Figure S1(a), C-LDH consists of aggregated particles with irregular polyhedral morphology. The particles are closely stacked and form relatively compact aggregates. In contrast, coal gangue (Figure S1b) exhibits an irregular and heterogeneous morphology composed of coarse particles with rough surfaces. Since the SEM images showed severe stacking of particles, it was difficult to directly observe the sheet-like morphology. Therefore, AFM analysis was employed to further characterize the surface features of the synthesized C-LDH (Figure S2). The image clearly reveals the formation of nanosheets with lateral dimensions mostly below 1 μm, accompanied by some larger nanosheets exceeding 2μm. The nanosheets exhibit irregular morphologies and are evidently smaller than conventional LDH crystals, indicating that the coal gangue precursor favored the direct formation of fine, thin platelets [20]. The corresponding height distribution confirms the nanosheets possess relatively uniform thickness and flat surfaces. These results demonstrate the successful synthesis of nanosheet-like LDH structures from coal gangue, which provide abundant exposed surface areas and active sites beneficial for flame-retardant applications. Similar nanosheet morphologies have also been reported by Gao et al. [17], further supporting that such ultrathin platelet structures are intrinsic features of LDH synthesized under suitable conditions. XRF analysis was further conducted to determine the bulk elemental composition of the synthesized C-LDH (Table S2). The material contained Mg, Al, and Fe as the major metal components, with a calculated bulk Mg:(Al+Fe) molar ratio of approximately 2.3:1.

The formation of the relatively thin C-LDH nanosheets may be associated with the coordination and growth-regulation effects of the choline chloride/urea DES. This solvent contains an extensive hydrogen-bonding network involving chloride ions, urea, and choline species, producing a structured ionic environment that can alter metal-ion solvation and hydrolysis behavior. In the hydrated DES system, Mg²⁺ and Al³⁺ may undergo dynamic interactions with water molecules, the carbonyl oxygen of urea, and chloride ions, thereby reducing the effective activity of freely hydrated metal ions and moderating their hydrolysis after alkali addition. Temporary coordination of Al³⁺ may suppress excessively rapid Al hydroxide precipitation and favor more synchronized coprecipitation with Mg²⁺. Meanwhile, the relatively high viscosity of the DES can restrict long-range ion diffusion and particle aggregation, while transient adsorption of DES components on growing crystallites may inhibit stacking along the crystallographic c-axis and promote the formation of thin nanosheets [21,22].

The XRD patterns of the synthesized C-LDH and the raw coal gangue are shown in Figure 1. The coal gangue exhibited sharp reflections at 2θ = 20.8°, 26.6 and 50.1°, corresponding to SiO₂, and at 29.3° and 36.4°, which can be assigned to CaCO₃. These results indicate that the crystalline phases of the raw material are dominated by silicate and carbonate minerals. In contrast, the C-LDH sample displayed well-defined reflections at 2θ = 11.5° (003), 23.4° (006), and 34.8° (012), which match well with the standard diffraction pattern of layered double hydroxides [23]. The disappearance of the dominant quartz and calcite peaks together with the emergence of typical LDH reflections confirms the successful transformation of coal gangue into a crystalline LDH phase. The ordered layered structure provides abundant interlayer galleries and anion-exchange sites, which not only enhance the material’s surface reactivity but also contribute to its flame-retardant functionality by promoting char formation, heat insulation, and smoke suppression during combustion.

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Figure 1. XRD pattern of (a) C-LDH and (b) Coal gangue, Circles represent LDH, squares represent SiO₂, and triangles represent CaCO₃.

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Figure 2 shows the FTIR spectra of coal gangue and composites with different C-LDH loadings. The broad and intense band at 3200-3600 cm⁻¹ is attributed to O–H stretching vibrations [24], originating from adsorbed water, hydroxyl groups, and structural water in coal gangue, which are further enhanced after aqueous treatment. In C-LDH-containing samples, the contribution from layer hydroxyls is particularly evident. The band at 2972 cm⁻¹ corresponds to the stretching vibration of aliphatic C-H bonds derived from coal organic matter, and its intensity remains essentially unchanged after treatment [25]. The peak near 1622 cm⁻¹ is assigned to the H–O–H bending vibration of water molecules, mainly from adsorbed water, indicating high water content in all samples [26]. The bands at 1416 cm⁻¹ and 711 cm⁻¹ are characteristic of carbonate vibrations [27]. In coal gangue, these features arise from trace carbonates. For C-LDH composites, the carbonate band intensity increases progressively with C-LDH loading (from 5% to 20%), serving as direct evidence for the presence of C-LDH. All samples also display absorption peaks at 992, 871, 795, and 776 cm⁻¹, assigned to quartz and silicate minerals inherent in coal gangue; these inorganic frameworks remain unaffected by the treatments [28].

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Figure 2. FTIR of coal gangue and C-LDH composites.

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3.2. TG/DSC analysis

As shown in Figure 3(a), the thermal behavior of coal gangue and C-LDH and coal composites can be divided into three main stages. The first stage (30–220/260°C) corresponds to the removal of physically adsorbed water and partial interlayer water from LDH, with a weight loss of 3-6%. The MgCl₂-treated sample shows the largest loss (5.6%) due to its hygroscopic nature, which is higher than that of raw coal gangue (4.1%) and the C-LDH-modified samples. The second stage (210-610°C) exhibits the most significant weight loss of 14-19%, arising from the oxidation of organic matter in coal gangue combined with LDH decomposition, which is consistent with the strong DSC exothermic peak. In high LDH-loading samples, a small shoulder at 140-170°C further indicates the onset of LDH decomposition. The third stage (610-800°C) accounts for only 2-3% mass loss, mainly attributed to the phase transitions of LDH-derived oxides and gangue minerals, corresponding to the DSC endothermic peak at 653-670°C. From a flame-retardant perspective, raw coal gangue shows a DTG peak at 479°C with concentrated combustion (Figures 3b and c). The incorporation of C-LDH effectively delays this process, with the DTG peak shifting to 491°C for C-LDH-5, 505°C for C-LDH-10 (26°C higher than the baseline), 503°C for C-LDH-15, and 507°C for C-LDH-20, demonstrating improved thermal stability. In contrast, the MgCl₂-treated sample shows a lower DTG peak temperature of 475°C, suggesting catalytic promotion of combustion. The DSC results further corroborate these findings: the strong exothermic peak at 470-510°C corresponds to the oxidation of carbonaceous organics, with raw coal gangue showing the lowest peak temperature (478°C) and the highest enthalpy release (3774 J/g). The addition of C-LDH shifts the peak to higher temperatures while suppressing heat release: 492°C and 3359 J/g for C-LDH-5, 503°C for C-LDH-10, 504°C and 3091 J/g for C-LDH-15, and 505°C and 2811 J/g for C-LDH-20, with the MgCl₂-treated sample again performing worse than the baseline. In addition, the similar DTG and DSC peak temperatures of C-LDH-15 and C-LDH-20 suggest that the combustion-delaying effect approaches a plateau at high C-LDH loadings. This may be related to near-saturation of surface coverage and partial particle aggregation, which reduce the marginal effect of further C-LDH addition on the peak temperature. In contrast, the total heat release reflects the integrated oxidation process. Increasing the C-LDH loading from 15% to 20% further increases the proportion of non-combustible and endothermic components and generates more inorganic residue, thereby reducing the overall extent of oxidation and decreasing the heat release from 3091 to 2811 J g⁻¹. An additional endothermic peak between 653 and 670°C is attributed to phase transitions of LDH-derived oxides (e.g., Spinel formation) and transformations of aluminosilicate minerals. Overall, the combined TG–DSC evidence demonstrates that the incorporation of C-LDH effectively delays combustion and suppresses heat release, with C-LDH-15 and C-LDH-20 providing the most favorable flame-retardant performance.

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Figure 3. Thermal analysis of coal gangue and C-LDH composites. (a) DSC curves, (b) DTG profiles, and (c) TG curves.

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To evaluate the effect of the DES-assisted synthesis route, LDH prepared by the conventional urea method was tested under the same conditions. At a loading of 20 wt%, the urea-derived LDH increased the DTG peak temperature from 479 to 492°C, whereas 20 wt% C-LDH shifted it to 507°C. This indicates that both materials delayed coal gangue oxidation, but C-LDH produced a stronger thermal-inhibition effect. The improved performance of C-LDH may be related to its thinner nanosheet morphology, better dispersion, and more effective contact with the coal gangue surface.

3.3. Released amount of CO and the inhibitory rate

The curves of CO concentration as a function of temperature for coal gangue and coal samples with different C-LDH contents are shown in Figure 4. For the raw coal gangue, the curve remains nearly coincident with the baseline below 80°C, due to limited adsorption and slow oxidation occurring within the pore structure. Between 80°C and 120°C, a slight increase in CO concentration is observed, remaining below 100 ppm. Once the temperature exceeds 120°C, CO release rises rapidly, suggesting the onset of accelerated oxidation reactions. When the temperature is higher than 160°C, the CO concentration enters a second rapid growth stage, ultimately reaching nearly 2700 ppm at 200°C. This result is similar to Huang et al. [29]. In contrast, the samples containing C-LDH show markedly suppressed CO emissions throughout the heating process. With 10% C-LDH loading, the increase in CO concentration is significantly alleviated, particularly beyond 120°C, and the maximum value at 200°C is much lower than that of the raw coal gangue. Increasing the C-LDH content to 15% and 20% further enhances this inhibitory effect, with the 20% sample maintaining the lowest CO concentration (<1100 ppm at 200°C). The inhibition rate (E) was calculated according to Equation (1):

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Figure 4. CO emission-temperature curves of coal gangue and different C-LDH.

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where C_r (ppm) denotes the released amount of CO from the raw coal sample, and C_t (ppm) denotes the released amount of CO from the treated coal sample. At 100°C, the inhibition rates of C-LDH-10, C-LDH-15, and C-LDH-20 were 36.84%, 49.12%, and 59.51%, respectively. This suppression can be attributed to the catalytic role and layered structure of LDH: during heating, LDH decompose to release water and generate metal oxides, which dilute combustible gases, catalyze CO oxidation, and promote the formation of protective char layers. Consequently, the incorporation of C-LDH effectively reduces toxic CO emissions while improving the overall fire safety of coal gangue-based materials.

3.4. Analysis of the TG-FTIR spectra

From Figures 5(a-d), in the low-temperature range of 30-200°C, both samples exhibited the release of adsorbed and interlayer water, with the LDH-composited sample showing more pronounced signals, indicating higher contents of interlayer water and hydroxyl groups. This early gas release contributed to dilution and cooling at the initial stage of combustion. In the range of 200-400°C, the gas evolution of coal gangue remained limited, whereas the LDH-containing sample displayed a continuous dehydroxylation process. The most significant difference was observed at 400-550°C: while coal gangue exhibited almost no detectable CO₂ signals, the LDH-composited sample showed intense CO₂ and H₂O peaks, attributed to the decomposition of interlayer CO₃²⁻ and the further removal of –OH groups. This phenomenon is consistent with the established flame-retardant mechanism of LDH, in which the release of inert gases (CO₂ and H₂O) at intermediate temperatures dilutes combustible volatiles and absorbs decomposition heat, thereby retarding combustion reactions. Upon further heating to 600-700°C, coal gangue exhibited the strongest CO₂ peak, derived from the decomposition of carbonates and the pyrolysis/oxidation of associated organic matter. In contrast, the LDH-composited sample showed significantly weaker CO₂ intensity in this region, which is in line with the “high-temperature solid-phase effect” of LDH: the metal oxide residues generated from LDH decomposition form a dense protective layer on the surface, hindering the transfer of heat and oxygen and consequently suppressing further decomposition of organics and carbonates.

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Figure 5. TG-FTIR spectra of gaseous products during thermal decomposition of (a, b) raw coal gangue and (c, d) C-LDH-20.

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Meanwhile, in the temperature-programmed oxidation tests, CO can be observed, whereas no distinct CO characteristic peak (∼2143 cm⁻¹) was detected in the TG-IR spectra. This discrepancy mainly arises from two factors: first, the intrinsic infrared absorption of CO is relatively weak, and under TG-IR conditions the strong absorptions of H₂O and CO₂ cause baseline fluctuations and peak overlap, thereby masking the signals of low-concentration CO. Second, the oxides generated from LDH during heating, together with the oxygen-rich and high-temperature transfer environment, can further catalyze the oxidation of CO to CO₂, resulting in a CO content below the detection limit of the instrument when reaching the infrared gas cell. Overall, the decomposition behavior of LDH provides a dual-stage flame-retardant effect-gas release at intermediate temperatures and oxide barrier formation at high temperatures, that synergistically suppresses the thermal instability and concentrated gas release of coal gangue, thereby systematically enhancing its flame-retardant performance.

3.5. Characterizaiton of sample after flame-retardant testing

The SEM image of the C-LDH and coal gangue composite after combustion shows an irregular and compact morphology with aggregated particles (Figure S3). The surface appears rough and densely packed. Before combustion (Figure 6), the XRD patterns of the C-LDH-10 show prominent LDH characteristic peaks at 2θ ≈ 11-12°, 23-24°, and 34-35°, indicating the stable presence of the LDH layered structure on the coal gangue surface. These peaks also suggest that the interlayer water molecules and exchangeable anions of LDH remain intact, interacting with the oxygen-containing functional groups on the coal surface, which contributes to the barrier effect by effectively blocking oxygen diffusion and hindering direct contact between oxygen and the reactive sites on the coal. Furthermore, the SiO₂ peak in coal gangue (2θ ≈ 26.6°, 50.1°) remains largely unchanged, indicating the stability of the silicate minerals in the coal gangue during heating, and suggesting that they serve as a rigid framework supporting the LDH structure. At the same time, the CaCO₃ peak (2θ ≈ 29.4°, 39.4°) in the coal gangue is still visible, indicating its presence in the composite and setting the stage for its subsequent thermal decomposition. After combustion (Figure 6), the XRD patterns clearly show the complete disappearance of the LDH characteristic peaks, indicating the decomposition of the LDH structure into metal oxides (e.g., MgO, Al₂O₃, or mixed oxides), consistent with the oxide protection effect described in the proposed mechanism. The SiO₂ peaks remain unchanged, further confirming the stability of silicate minerals under high temperatures. The CaCO₃ peak, however, significantly weakens or disappears, suggesting that it has decomposed, releasing CO₂ and forming calcium-based oxides. This process is aligned with the endothermic dilution effect, which helps to dilute combustible gases and slow the temperature rise of the system.

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Figure 6. XRD pattern of (a) C-LDH-20 before and after combustion, Circles represent LDH, squares represent SiO₂, and triangles represent CaCO₃.

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The XPS spectra of the C-LDH and coal gangue composite before combustion reveal the coexistence of coal-derived oxygenated groups and LDH structural species (Figures 7a). The C 1s spectrum is predominantly composed of C–C (64%), with smaller contributions from C–O (25%) and C=C (9%), representing the coal carbon backbone and surface oxygen functionalities (Figure 7b). The O 1s spectrum can be deconvoluted into M–O (35%), C=O (37%), and C–O (26%), indicating the presence of LDH lattice oxygen as well as carbonyl and hydroxyl groups (Figure 7c). The Mg 1s (1304.9 eV) and Al 2p (74.8 eV) peaks further confirm the incorporation of LDH into the coal gangue matrix, establishing a stable organic–inorganic interface (Figures 7d and e).

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Figure 7. XPS spectra of the C-LDH-20 before combustion. (a) Survey spectrum showing the presence of Mg, Fe, Ca, Al, Si, O, and C. High-resolution spectra of (b) O 1s, (c) C 1s, (d) Mg 1s, and (e) Al 2p.

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The coexistence of coal-derived oxygen-containing groups and hydroxyl-rich Mg–Al species suggests close interfacial contact between C-LDH and the coal gangue surface. This interfacial association favors the retention and distribution of C-LDH around reactive gangue particles. During heating, the dehydration and dehydroxylation of LDH consume heat, as supported by the TG–DSC and TG–FTIR results, thereby reducing local heat accumulation and delaying the oxidation of coal gangue. After combustion (Figure 8a and b), the C 1s spectrum shifts slightly, with C–C increasing to 65%, and C–O (24%) and C=C (10%) showing minimal change, reflecting an enrichment of the carbon backbone. The relative increase in C–C and C=C, together with the slight decrease in C–O, indicates the formation of a more condensed and carbon-rich residue. Such a residue can cover reactive regions of coal gangue, reduce direct oxygen contact, and slow the transfer of heat and volatile oxidation products. In the O 1s region, the proportions shift to M–O (34%), C=O (38%), and C–O (26%), suggesting a slight increase in carbonyl species and a minor decrease in M–O contributions (Figure 8c).

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Figure 8. XPS spectra of the C-LDH-20 after combustion. (a) Survey spectrum showing the presence of Mg, Fe, Ca, Al, Si, O, and C. High-resolution spectra of (b) O 1s, (c) C 1s, (d) Mg 1s, and (e) Al 2p.

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The weak increase in C=O reflects limited oxidation of the residual carbonaceous phase, while the small change in M–O suggests that the inorganic component is largely retained during combustion rather than being removed from the solid residue. The Mg 1s and Al 2p peaks remain evident, indicating the retention of LDH-derived metal oxides within the residue (Figures 8c and d). The persistence of Mg, Al, and M–O species confirms that LDH-derived Mg–Al oxide phases remain in the condensed phase after the layered structure collapses. Together with the carbonaceous framework, these oxide-containing residues can form a more stable organic–inorganic barrier, increase the diffusion path for oxygen, and limit heat transfer into the inner reactive regions. This interpretation is consistent with the disappearance of the LDH reflections and the retention of oxide-related phases in the post-combustion XRD patterns.

The retained Mg–Al oxide phases may also contribute to CO suppression by providing surface sites for the adsorption and further oxidation of CO under oxygen-rich conditions. Nevertheless, XPS alone cannot directly verify catalytic CO conversion. Therefore, catalytic oxidation is considered a possible complementary mechanism, whereas the reduced CO release is more directly attributed to the combined effects of delayed gangue oxidation, heat absorption, and condensed-phase barrier formation. The relative areas of the components show minor changes before and after combustion: C–C/C=C fractions increase slightly, C–O decreases marginally, and C=O shows a weak increase, while M–O decreases by approximately 1%. These changes indicate that the chemical environment is largely preserved, with minimal transformation of functional groups. Instead, combustion leads to the formation of a char-dominated carbonaceous framework coupled with LDH-derived metal oxides, which together provide a protective barrier and catalytic functionality. Thus, the changes in C 1s mainly support the enrichment and stabilization of the carbonaceous framework, whereas the O 1s, Mg 1s, and Al 2p spectra support the retention of thermally stable inorganic residues. The two components act cooperatively: LDH decomposition absorbs heat at the early and intermediate stages, while the carbon–oxide residue provides physical protection at higher temperatures and may assist CO oxidation. This stable organic–inorganic residue facilitates the suppression of CO emission and enhances the flame-retardant properties of the C-LDH and coal gangue composite. Therefore, the combined XPS and XRD data before and after combustion show that the C-LDH and coal gangue composite undergoes transformations, including the stabilization of functional groups, the release of water and CO₂, and the formation of metal oxides. These changes effectively suppress CO release and improve the flame-retardant properties, demonstrating the potential of C-LDH as a flame-retardant material for coal gangue-based systems.

3.7. Flame-retardant mechanism

Based on the characterization results and previous reports [30-32], the inhibitory mechanism of C-LDH on coal gangue oxidation can be summarized as follows (Figure 9). LDH contains interlayer water molecules and exchangeable anions and, after being distributed on the coal gangue surface, can act as a physical barrier to oxygen diffusion and reduce direct contact between oxygen and reactive sites. In addition, the surface –OH groups of LDH may interact with oxygen-containing functional groups, such as –COOH and –COO⁻, in coal through weak hydrogen bonding, thereby stabilizing these active groups and suppressing their low-temperature oxidation.

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Figure 9. Flame-retardant mechanism.

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During heating, C-LDH undergoes endothermic dehydration, dehydroxylation, and carbonate decomposition, releasing H₂O and CO₂. These gases absorb part of the heat generated by coal oxidation, dilute gaseous products, and slow local heat accumulation. This interpretation is consistent with recent studies of layered mineral flame retardants, in which the gradual release of interlayer water prolonged gas-phase inhibition rather than producing only an instantaneous cooling effect. The progressive release of H₂O and CO₂ from C-LDH may therefore extend the effective inhibition period across a broader temperature range.

At higher temperatures, C-LDH is converted into Mg–Al-containing oxide residues. These inorganic products can accumulate among coal gangue particles and contribute to the formation of a compact condensed-phase barrier, thereby restricting oxygen penetration, heat transfer, and volatile-product escape. Previous work has shown that thermally stable layered mineral residues can coat carbonaceous precursors and isolate them from air, while interactions between inorganic phases and oxygen-containing carbon structures can suppress excessive condensation and promote the retention of a stable solid carbon framework. By analogy, the LDH-derived metal oxides in the present system may cooperate with the carbonaceous residue of coal gangue to form an inorganic–carbon protective layer. This combined residue may improve structural integrity, reduce exposure of unreacted organic components, and limit the outward transport of CO and other volatile products. The retained metal-oxide surfaces may also provide adsorption or reaction sites for CO under oxygen-rich conditions, potentially promoting its further oxidation to CO₂. However, this catalytic contribution should be considered complementary to the more directly supported effects of heat absorption, gas dilution, and condensed-phase barrier formation. Therefore, the incorporation of C-LDH suppresses CO release and delays coal gangue oxidation through the combined effects of interfacial protection, endothermic gas release, and high-temperature inorganic–carbon barrier formation.

Coal gangue contains various impurities, including metal ions such as Fe³⁺, Ti⁴⁺, and Mn²⁺, as well as anions such as SO₄²⁻ and PO₄³⁻, which may influence the structure and performance of LDH materials. These metal ions can partially substitute for Mg²⁺ or Al³⁺ within the LDH layers, thereby modifying the layer charge density and improving thermal stability during combustion. In addition, high-valence anions such as phosphate and sulfate may enter the interlayer region, reduce anion exchangeability, and enhance the structural integrity of LDH. These interactions can promote the formation of stable oxide layers during combustion, thereby suppressing heat release, delaying ignition, and improving flame-retardant performance.

3.8. Regeneration of C-LDH for flame-retardant

It is generally recognized that LDH undergo spinel-type transformations when heated above 800°C. At relatively lower temperatures, however, LDH can be regenerated through rehydration or exposure to humid environments, which partially restores their layered structure. This regeneration behavior not only enhances the economic feasibility of LDH but also supports their potential for cyclic utilization in flame-retardant systems. Upon heating, LDH typically experience a series of sequential transformations. Specifically, the desorption of surface and interlayer water occurs below 200°C, followed by dehydroxylation and anion release between 200 and 450°C. These processes lead to the collapse of the layered structure and the formation of amorphous oxides, eventually culminating in structural reorganization into spinel-type phases such as MgO and MgAl₂O₄ above 800°C [15,33].

To investigate the flame-retardant performance of regenerated C-LDH, the material was calcined at 600°C, followed by hydration for 24 hrs to facilitate its regeneration. The regenerated C-LDH was then mixed with coal gangue at a 20% ratio to prepare the samples for TG-DSC. The overall XRD (Figure S4a) patterns show that the characteristic LDH reflections reappeared after rehydration, indicating partial reconstruction of the layered structure. However, the enlarged view of the main LDH reflection shows a clear decrease in peak intensity after regeneration, suggesting incomplete structural recovery and reduced crystallinity (Figure S4b). Consistently, the specific surface area of the C-LDH/coal gangue composite decreased slightly from 10.2 to 8.9 m² g⁻¹. The limited change may be partly masked by the predominance of coal gangue in the composite. These results indicate that the regeneration process achieved only partial structural reconstruction. The flame-retardant performance of the regenerated C-LDH was further evaluated after hydration treatment (Figure 10). The DTG peak temperature of the regenerated C-LDH was near 500°C, slightly lower than that of the pristine LDH. However, its DTG curve exhibited six distinct peaks, indicating a multi-stage inhibition effect throughout the entire combustion process. In the case of C-LDH-20, the exothermic enthalpy was the lowest among all samples (2534 J/g), and the peak temperature was 20°C higher than that of raw coal gangue, confirming a significant enhancement in flame retardancy while maintaining better cost efficiency (Figures 10a-c). Although the regenerated C-LDH partially recovered its original layered structure, the phase transition enthalpy remained relatively small, suggesting a change in thermal stability compared to the pristine LDH. These results emphasize that, despite partial structural alterations, regenerated LDH still provide substantial flame-retardant protection through multi-stage inhibition, while also offering advantages in sustainability and economic feasibility (Figure 10d). Although LDHs demonstrate regeneration capabilities, repeated calcination cycles lead to the formation of metal spinel oxides. These spinel oxides cannot be converted back into the LDH structure, causing a gradual loss of flame-retardant performance due to the deterioration of structural integrity and reduced anion-exchange capacity.

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Figure 10. Thermal analysis of C-LDH after regeneration for coal gangue flame retardant. (a) DSC curves, (b) DTG profiles, (c) TG curves and (d) Schematic diagram of the regenerated structure.

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