Durability evolution and phase transformation mechanisms of wood-fiber-reinforced magnesium oxysulfate composites under hydrothermal cycling

Xia, G., He, F., Chen, F., and Fang, Y. (2026). "Durability evolution and phase transformation mechanisms of wood-fiber-reinforced magnesium oxysulfate composites under hydrothermal cycling," BioResources 21(4), 10485–10503.

Abstract

To evaluate the long-term durability of environmentally friendly biomass composites under hydrothermal conditions, this study investigated the degradation behavior of wood-fiber-reinforced magnesium oxysulfate (MOS) panels subjected to accelerated hydrothermal cycling with alternating immersion at 60 and 20 °C. The results showed non-monotonic changes in mechanical properties and microstructure. After 7 cycles, the diffraction peak associated with the 517-phase (5Mg(OH)₂·MgSO₄·7H₂O) weakened, while residual MgO continued to hydrate. Meanwhile, the porosity increased to 23.1%, and the modulus of rupture (MOR) decreased from 25.3 to 15.4 MPa. At 14 cycles, the porosity decreased slightly to 21.1%, while the MOR increased numerically to 16.3 MPa, indicating a temporary stabilization tendency associated with continued hydration and evolution of MOS hydration products. After 21 cycles, the diffraction features of the 517-phase weakened further, MgCO₃-related peaks became more pronounced, and the porosity increased to 33.9%. Interfacial deterioration was also observed, accompanied by a decrease in MOR to 14.8 MPa. Taken together, the hydrothermal durability of the composites was closely associated with the degradation of MOS hydration products, continued hydration of residual reactive components, and pore-structure evolution. These findings provide useful evidence for improving the hydrothermal durability of MOS-based biomass composites.

Download PDF

Full Article

Durability Evolution and Phase Transformation Mechanisms of Wood-Fiber-Reinforced Magnesium Oxysulfate Composites under Hydrothermal Cycling

Guicheng Xia,^a Fuqiang He,^a,* Fajiang Chen,^b and Yu Fang^a

DOI: 10.15376/biores.21.4.10485-10503

Keywords: Biomass board; Magnesium oxysulfide material; Hydrothermal cycling; Performance evolution

Contact information: a: School of Mechanical Engineering, Guizhou University, Guiyang 550025, China; b: Guizhou Shixiang Technology Co., Ltd, Guiyang 550025, China;

* Corresponding author: [email protected]

Graphical Abstract

INTRODUCTION

Wood-based composites are pivotal to the global construction sector, with production projected to reach 420 million cubic meters by 2025 (Stark and Cai 2021; Neitzel et al. 2022; Ikubanni et al. 2024). Among these, high-density panels reinforced with inorganic binders have gained prominence because of their superior hardness and structural integrity (Vitrone et al. 2021; Sharma et al. 2024). Unlike traditional composites that rely on aldehyde-based resins, these composite materials utilize eco-friendly inorganic adhesives, providing non-combustible and formaldehyde-free properties that align with sustainable building trends (Aiken et al. 2021; Petrovic and Thomas 2024).

Among various inorganic binders, magnesium oxysulfate (MOS) cement has shown good potential in biomass composites due to its relatively low alkalinity, rapid hardening behavior, and good compatibility with lignocellulosic fibers. Wood-fiber-reinforced MOS panels usually exhibit good mechanical performance and environmental advantages. However, their long-term durability under humid and high-temperature conditions remains a major concern, especially in tropical and subtropical regions (Wang et al. 2021; Xin et al. 2024; Przybek 2025). Under hydrothermal cycling, repeated moisture absorption and temperature changes can accelerate the degradation of the cement matrix and fiber–matrix interface, resulting in strength loss and structural instability. Therefore, understanding the durability evolution of MOS-based biomass composites under hydrothermal conditions is important for their practical application.

Previous studies on MOS cement mainly focused on improving water resistance and stabilizing the hydration products. Some researchers reported that additives such as sepiolite, flue-gas desulfurization (FGD) gypsum, organic phosphates, and silicic acid could regulate the hydration reaction, inhibit Mg(OH)₂ formation, and improve the stability of the 517-phase (Li et al. 2017; Gu et al. 2020; Zhang et al. 2024; Hou et al. 2026). Accelerated carbonation treatment was also found to densify the fiber–matrix interface and improve short-term performance (Azevedo et al. 2024, 2025). Although these studies improved the initial properties of MOS systems, most of them were carried out under static or relatively stable laboratory conditions. The degradation behavior of MOS-based biomass composites under cyclic hydrothermal environments is still not fully understood. In addition, most existing studies have concentrated on material optimization strategies, such as feedstock pretreatment (Song et al. 2018; Tupciauskas and Rizhikovs 2020; Yel 2022), binder system ratios (Bekhta et al. 2019; Zhang et al. 2020), and binder formulations (Theng et al. 2019; Sihag et al. 2022). Durability assessments have generally been limited to isolated evaluations of mechanical properties (Luthfi et al. 2020) and static microstructure observation (Acda 2022; Yan et al. 2024). Relatively little attention has been paid to the long-term degradation mechanism under coupled hydrothermal conditions. However, under actual hydrothermal conditions, these factors usually interact with each other. The decomposition and carbonation of the 517-phase can weaken the matrix skeleton, whereas the hygroscopic swelling of wood fibers may accelerate interfacial deterioration. Meanwhile, the continued hydration of residual MgO may promote the secondary formation of hydration products, leading to localized matrix densification. Consequently, degradation and reconstruction processes may coexist and jointly govern the long-term performance evolution of MOS-based biomass composites. However, the coupled evolution of phase composition, pore structure, and interfacial integrity during hydrothermal cycling remains largely unexplored.

Therefore, this study investigated the durability evolution of wood-fiber-reinforced MOS composites under accelerated hydrothermal cycling. The phase composition, pore structure, interfacial morphology, and mechanical properties were analyzed to reveal the relationship between phase evolution and performance degradation during hydrothermal cycling. The results are expected to provide useful evidence for improving the long-term durability of MOS-based biomass composites in humid environments.

EXPERIMENTAL

Raw Materials

The raw materials used in this study included poplar wood fibers, magnesium oxide (MgO), magnesium sulfate (MgSO₄), phosphoric acid (H₃PO₄), ammonium dihydrogen phosphate (NH₄H₂PO₄), silicon dioxide (SiO₂), and deionized water. The poplar wood fibers were supplied by Guizhou Jingmu Building Materials Co., Ltd. in China, and were derived from waste generated during solid wood panel processing. These wood waste materials undergo dry crushing, grinding, and screening. The resulting fibers had a diameter of approximately 0.10 to 0.50 mm and a length of approximately 0.20 to 5 mm; approximately 60 wt% of the fibers passed through a 50-mesh screen. MgO was supplied by a manufacturer in Xinjiang, China, and was produced via high-temperature calcination of magnesite. The MgO exhibited an activity index of approximately 63%.

Specimen Preparation

The fabrication process of the biomass composite panels is schematically illustrated in Fig. 1. Initially, the poplar wood fibers were oven-dried to remove residual moisture prior to mixing. Subsequently, the dried wood fibers and MgO were blended at a volume ratio of 9:1 using a mechanical mixer. Before mixing, a magnesium sulfate solution (26 to 30°Bé) containing a compound additive was prepared. The compound additive was prepared by mixing magnesium sulfate solution, ammonium dihydrogen phosphate, phosphoric acid and silicone oil at a mass ratio of 2:1:1:0.02, with a total dosage of 3 wt% relative to the mass of MgO. During stirring, the additive-containing magnesium sulfate mixture was gradually sprayed into the wood fiber-MgO mixture until a homogeneous slurry was obtained. The prepared mixture was then uniformly spread onto a 7075 aluminum alloy plate and subjected to hot pressing using a hydraulic press at 95 °C under a pressure of 12 MPa for 14 min. The hot-pressing process facilitated matrix densification and reduced internal voids within the composite. After demolding, the obtained panels were cured under ambient conditions (23 ± 2°C) for 48 h. Direct sunlight and airflow exposure were avoided during curing to minimize surface cracking. Subsequently, the cured panels were cut into standard specimens using a sawing machine, followed by surface polishing to remove edge burrs.

To ensure experimental consistency, all specimens were prepared from the same batch of panels. Specimens with dimensions of 210 mm × 65 mm × 8 mm were divided into four groups corresponding to 0, 7, 14, and 21 hydrothermal cycles, respectively. Each group contained three replicate specimens, and the reported results were obtained by averaging the values from the three replicate specimens.

Fig. 1. Preparation process of the plate

Design of Hydrothermal Cycling Test

The hydrothermal cycling test was conducted using an alternating-temperature water immersion method. Initially, the specimens assigned to the 21-cycle group were fully immersed in distilled water at 60 °C for 12 h, followed by immersion in distilled water at 20 °C for an additional 12 h. This procedure constituted one complete 24 h hydrothermal cycle. To ensure simultaneous completion of the cycling treatments, specimens designated for different cycle numbers were introduced into the test at different times. After the first seven cycles, the specimens designated for the 14-cycle group were introduced into the cycling system. Upon completion of 14 cumulative cycles, the specimens belonging to the 7-cycle group were subsequently added. Thereafter, all cycled specimens underwent the remaining cycles under identical test conditions. The selected temperatures (60 °C/20 °C) and exposure durations (12 h for each stage) were not intended to reproduce actual daily temperature fluctuations in a specific region, but were used as an accelerated hydrothermal aging protocol. The 60 °C stage was used to accelerate moisture- and temperature-induced degradation, while the 20 °C stage introduced cyclic temperature variation. Each stage lasted 12 h to allow sustained moisture transport and microstructural evolution. Because the specimens remained fully immersed throughout the test, the imposed conditions were more severe than typical service environments and were therefore used for comparative durability evaluation.

After completing their designated numbers of hydrothermal cycles (7, 14, and 21 cycles), the specimens were removed simultaneously for subsequent physical and mechanical characterization. The 0-cycle specimens served as the unexposed reference group. The reported results represent the average values obtained from three replicate specimens for each group.

Physical Testing

The density of the specimens was determined after hydrothermal cycling, given that density exerts a significant impact on the water resistance and fireproof performance of the biomass panels. Four specimen groups were prepared with consistent initial dimensions and mass, and these parameters were accurately recorded prior to the initiation of the cycling test. Quantitative measurements were performed on each group both before and after the completion of hydrothermal cycling treatments. Specifically, sample dimensions were measured using a vernier caliper with a resolution of 0.05 mm, and mass was measured using a balance with a sensitivity of 0.01 g, calculations were performed according to Eq. 1.

(1)

where m is the mass of the test specimen (g), and l, b, and h are the length, width, and thickness of the specimen (mm), respectively.

The expansion ratio of the specimens was assessed following hydrothermal cycling. The thickness swelling (TS) rate was defined as the percentage increment in specimen thickness after exposure to cyclic hydrothermal treatments. Thickness measurements were conducted using a micrometer with a graduation value of 0.01 mm, and the corresponding TS% values were computed in accordance with Eq. 2.

(2)

where d₁ and d₂represent the thicknesses (mm) of the biomass polycrystalline wood board samples before and after the experiment, respectively.

The water absorption (WA) rate was evaluated to characterize the water resistance of the specimens. It was defined as the percentage ratio of the moisture mass absorbed by the specimens after hydrothermal cycling to their initial mass before testing, which serves as a critical indicator for evaluating the hydrophilicity, porosity and water resistance of the biomass panels. Its mass was measured using a balance with a sensitivity of 0.01 g. Results were calculated using Eq. 3.

(3)

where m₁ and m₂represent the mass of the fiberboard before and after immersion, respectively (g).

Mechanical Testing

The modulus of rupture (MOR) of the biomass polycrystalline panels was tested in accordance with GB/T 17657-2022 using a three-point bending method (Zhang et al. 2025). A universal testing machine specifically designed for wood-based panels (BOS-10KNW, Xiamen Boshi Testing Equipment Co., Ltd.) was used for the test. The span length between the two supports was set at 160 mm, and the crosshead speed was controlled at 10 mm/min. During the bending test, the load was applied steadily until the specimen fractured, and the corresponding maximum load was recorded in real time. The MOR values were subsequently calculated in accordance with Eq. 4.

(4)

where M is the maximum load (N), L is the span length (mm), b is the specimen width (mm), and h is the specimen thickness (mm).

Microstructural Characterization

X-ray diffraction (XRD) was used to examine phase changes in samples at different cycle counts and to investigate the effects of the degree of hydration and crystal structure on the mechanical properties of the panels. Biomass composite board specimens were analyzed using a benchtop X-ray diffractometer (Bruker-D8 ADVANCE, Germany). The XRD instrument employed a copper target, operating at 40 kV, with a 2θ range of 10° to 80°, a current of 40 mA, and a scanning speed of 10°/min.

Scanning electron microscopy (SEM) was used to examine the microstructure and elemental distribution of the cross-section of the panel, and to evaluate the interfacial bond strength and damage at the fiber-matrix interface. A scanning electron microscope (Zeiss Sigma 360, Germany) was used to observe the microstructure of sample surfaces after different numbers of cycles.

The Brunauer-Emmett-Teller (BET) method was employed to monitor changes in surface porosity. After mechanical testing, representative specimens from each hydrothermal-cycle group were pre-dried in a thermostatic oven at 45 °C for 36 h to reduce residual moisture. All groups were subjected to the same pre-drying procedure. Before nitrogen adsorption measurements, the samples were degassed at 120 °C for 8 h in a physical adsorption analyzer (Quantachrome Nova E, USA). Nitrogen adsorption–desorption measurements were then performed at 77.3 K to determine the specific surface area and pore-size distribution.

Statistical Analysis

All physical and mechanical data are presented as mean ± SD of three replicate specimens (n = 3). The experimental data were analyzed and plotted using Origin software. Analysis of variance (ANOVA) was used to assess statistical significance among the different hydrothermal cycling groups, followed by Tukey’s multiple comparison test.

RESULTS AND DISCUSSION

Evolution of Physical Properties

Figure 2(a) illustrates the density variation of the panels over the course of hydrothermal cycling. The density decreased from an initial value of 1.89 g/cm³ to 1.86 g/cm³ after 7 cycles, then slightly increased to 1.87 g/cm³ after 14 cycles, before decreasing to 1.82 g/cm³ after 21 cycles. No significant differences were observed among the 0, 7, and 14-cycle groups (p > 0.05), whereas the density after 21 cycles was significantly lower than that of the other groups (p < 0.05). These numerical variations suggest progressive microstructural evolution within the composite matrix as hydrothermal cycling progressed. The initial slight decrease in density may be associated with partial degradation of the 517-phase and the accompanying development of internal pores. The small numerical increase observed at 14 cycles may be related to continued hydration of residual reactive components and the associated evolution of MOS hydration products, consistent with the subsequent XRD observations. However, this change was not statistically significant, indicating that any temporary densification was limited. Upon further cycling to 21 cycles, prolonged hydrothermal exposure promoted further degradation of the hydration products and development of the pore structure, resulting in a significant decrease in panel density.

Figures 2(b) and 2(c) show the evolution of thickness swelling and water absorption during hydrothermal cycling. The thickness swelling increased from 3.32% at 0 cycles to 3.51%, 6.41%, and 12.0% after 7, 14, and 21 cycles, respectively. No significant difference was observed between the 0-cycle and 7-cycle groups (p > 0.05), whereas significant increases occurred after 14 and 21 cycles (p < 0.05). In contrast, the water absorption increased significantly from 3.37% to 4.24%, 8.57%, and 13.7% with increasing cycle number (p < 0.05).

The different early-stage responses of these two parameters indicate that water uptake preceded pronounced dimensional swelling. During the initial cycling stage, the matrix retained sufficient integrity to partially restrict the moisture-induced expansion of the wood fibers, although water penetration had already increased. With further cycling, degradation of the MOS hydration products and the development of pores and interconnected transport pathways facilitated greater water penetration into the composite. Increased exposure of the wood fibers to moisture subsequently intensified their swelling, resulting in the marked increases in both water absorption and thickness swelling observed after 14 and 21 cycles.

Mechanical Property Response

Figure 2(d) shows the variation in modulus of rupture (MOR) of the panels after different numbers of hydrothermal cycles. After 7 cycles, the MOR decreased markedly from the initial value of 25.3 to 15.4 MPa. A slight numerical increase to 16.3 MPa was observed after 14 cycles; however, this increase was not statistically significant compared with the 7-cycle group (p > 0.05). After 21 cycles, the MOR decreased again to 14.8 MPa.

The non-monotonic variation in MOR may be associated with the progressive evolution of the internal microstructure during hydrothermal cycling. Possible contributing factors include continued hydration of residual MgO, changes in the stability of the 517-phase, and the accompanying evolution of the pore structure and the fiber–matrix interface. In particular, the slight numerical increase observed after 14 cycles may reflect a temporary stabilization of the microstructure rather than a significant recovery in mechanical performance. The following microstructural analyses therefore focus on changes in phase composition, pore structure, and interfacial morphology at different cycle counts to further clarify the mechanisms underlying the evolution of MOR.

Fig. 2. Changes in physical and mechanical properties at different hydrothermal cycle counts: (a) density; (b) thickness swelling; (c) water absorption; and (d) modulus of rupture. Data are presented as mean ± SD (n = 3). Different lowercase letters indicate significant differences among groups according to one-way ANOVA followed by Tukey’s post hoc test (p < 0.05).

Phase Composition Analysis

Figure 3 presents the evolution of crystalline phases in the panels subjected to different hydrothermal cycling conditions. After 7 cycles, the diffraction peaks associated with residual MgO weakened markedly, while those associated with Mg(OH)₂ became more pronounced, indicating that the residual MgO continued to hydrate during hydrothermal exposure. The diffraction features associated with silica-related phases weakened, which may be related to interfacial reactions between silica species and hydroxyl groups on the wood fiber surface. Such interactions could enhance interfacial compatibility between the inorganic matrix and wood fibers, thereby partially alleviating the early-stage strength degradation (Ding et al. 2022; Liu et al. 2022). At the same time, the characteristic diffraction peak of 517-phase weakened, indicating that the primary hydrated magnesium sulfate product had undergone partial degradation. This phase evolution was consistent with the significant decrease in MOR observed during the early stages of the hydrothermal cycling.

$X-[OO2.1]ray diffraction patterns$

Fig. 3. X-ray diffraction patterns

After 14 cycles, the MgO-related diffraction peaks weakened further, indicating continued consumption of residual reactive MgO. The diffraction features associated with the 517-phase showed a slight increase relative to those after 7 cycles. However, because the present XRD analysis was based on relative diffraction-peak changes rather than quantitative Rietveld refinement, this variation cannot be regarded as definitive evidence for the reformation of the 517-phase. The slight numerical increase in MOR observed after 14 cycles was also not statistically significant. Therefore, this stage is more appropriately interpreted as a tendency toward temporary stabilization, which may be associated with continued hydration of residual reactive components and further evolution of MOS hydration products.

After 21 cycles, the MgCO₃-related diffraction peaks became more pronounced, accompanied by a marked weakening of the diffraction features associated with the 517-phase. This phenomenon suggests that prolonged hydrothermal cycling promoted further degradation of magnesium oxysulfate (MOS) hydration products, generating Mg(OH)₂and soluble sulfate species. The formation of the carbonate-containing phases may be related to a reaction with inorganic carbon dissolved in the immersion water, or to reactions triggered by exposure to atmospheric carbon dioxide during sample preparation and pretreatment prior to characterization. Because the present experiments cannot distinguish the relative contributions of these pathways, the exact stage at which carbonation occurred remains uncertain. The progressive decomposition of the 517-phase and the accumulation of carbonation products weakened the integrity of the inorganic matrix and increased internal porosity, ultimately leading to substantial deterioration of the panel strength (Dong et al. 2025). Overall, the continuous degradation of the MOS hydration products, along with the resulting changes in pore structure and matrix integrity, is consistent with the further decline in MOR observed after prolonged hydrothermal cycling.

Surface Morphology Observation

Figure 4 presents the macroscopic morphological evolution of the specimens subjected to different numbers of hydrothermal cycles. The untreated specimen shown in Fig. 4(a) exhibits a relatively dense and intact structure with smooth surfaces and well-preserved edge morphology. In contrast, the cross-sectional morphologies after hydrothermal cycling, shown in Fig. 4(b), reveal progressively greater structural deterioration with increasing cycle number.

Fig. 4. Cross-sectional views: (a) Specimen before treatment; (b) Cross-sectional morphology after treatment; (c) Cross-section of the 7-cycle group; (d) Cross-section of the 14-cycle group; and (e) Cross-section of the 21-cycle group

The 0-cycle specimen displayed a dense cross-sectional morphology with well-preserved matrix integrity. After 7 cycles, localized microcracks were visible near the edges of the cross-section. At 14 cycles, these cracks extended further toward the center of the cross-section. By 21 cycles, the cracks became wider and more pronounced, accompanied by discoloration and visible structural deterioration. Figure 4(c) presents the surface morphology of the 7-cycle specimen. The surface remains relatively smooth and dense, with clear and regular edge contours. Only a small number of white crystalline deposits are sparsely distributed on the surface, suggesting that the overall structure remains relatively intact at this stage and that moisture absorption and expansion effects are not yet significant. As the cycle number increases to 14 cycles (Fig. 4d), noticeable deformation has appeared along the edges of the panels, with localized areas of the surface becoming raised and rough. The amount of white crystalline deposits also increases slightly, indicating progressive deterioration of the surface structure during repeated hydrothermal treatment. By the 21^st cycle (Fig. 4e), white crystalline deposits become more abundant and cover a larger portion of the specimen surface. Severe edge deformation and fiber separation are observed. Simultaneously, surface wood fibers exhibit pronounced loosening and bulging, accompanied by local fiber disintegration, resulting in a substantial decline in the overall structural integrity of the specimen.

The macroscopic morphological evolution is consistent with the observed changes in physical and mechanical properties. During the early cycling stage, the specimens retained relatively good structural integrity, consistent with the relatively low water absorption and thickness swelling observed at this stage. However, prolonged hydrothermal exposure progressively weakened the matrix structure and increased defect development, thereby facilitating water penetration and accelerating the deterioration of dimensional stability and mechanical performance.

Processing of the SEM images shown in Fig. 5 yielded the pore distribution maps presented in Fig. 6, from which the pore parameters summarized in Table 1 were obtained. The quantified pore features revealed a general increase in pore development with increasing hydrothermal cycles. The detected pore count increased from 763 in the untreated specimen to 1390 after 21 cycles. Meanwhile, both the average pore size and porosity exhibited non-monotonic variation trends. The average pore size increased from 4.59 μm to 15.7 μm, while the porosity increased from 5.3% to 33.9%.

Specifically, the porosity increased markedly from 5.3% to 23.1% after 7 cycles, followed by a slight decrease to 21.1% after 14 cycles, before reaching a maximum value of 33.9% after 21 cycles. The temporary reduction in porosity observed at the intermediate stage suggests that continued hydration of residual reactive components and the associated evolution of MOS hydration products may have partially offset pore development. This trend corresponds to the slight numerical increase in MOR after 14 cycles.

Figure 7 presents representative SEM micrographs and corresponding pore distribution maps illustrating the interfacial evolution of the specimens during hydrothermal cycling. In the processed pore maps, red, yellow, and green regions represent relatively large, medium-sized, and small pores, respectively, while the remaining dark regions correspond to dense matrix areas. The results indicate that both pore size and pore connectivity increased significantly with increasing cycle number. At 0 cycles, the pore structure remained relatively compact, with only limited large pores observed. After prolonged hydrothermal cycling, pore coalescence and crack propagation became increasingly pronounced. By 21 cycles, many medium-sized and small pores had interconnected to form larger void regions, resulting in severe interfacial deterioration and matrix fragmentation.

Combined with the phase evolution results, these observations suggest that prolonged hydrothermal exposure progressively destabilized the MOS hydration products. During the intermediate cycling stage, residual reactive MgO may have continued to hydrate, and the associated evolution of hydration products may have partially compensated for pore development. However, after extensive cycling, the reduced amount of residual reactive MgO may have limited further evolution of MOS hydration products capable of partially compensating for internal damage. At the same time, the more pronounced carbonate-related diffraction features observed after 21 cycles suggest that carbonation-related reactions may also have occurred. These reactions may have involved inorganic carbon dissolved in the immersion water and/or atmospheric CO₂during subsequent sample handling and preparation. Overall, the continued degradation of MOS hydration products, together with pore enlargement and interfacial deterioration, is consistent with the reductions in density and MOR observed after prolonged hydrothermal cycling.

Table 1. Statistical Analysis of Image Porosity

Fig. 5. Original SEM image of the interface

Fig. 6. Pore distribution of the processed image

Fig. 7. Evolution of pore structure in samples during hydrothermal cycling: (a, c, e, g, i, k, m, o) SEM images, (b, d, f, h, j, l, n, p) corresponding pore distribution maps

Pore Structure Characterization

Figure 8 presents the nitrogen adsorption–desorption isotherms and corresponding BJH pore size distribution curves of the specimens subjected to different hydrothermal cycles. All samples exhibited typical Type-IV isotherms with distinct hysteresis loops, indicating the presence of mesoporous structures within the composite (Zhang et al. 2026). The adsorption capacity gradually increased with increasing relative pressure, while a pronounced separation between the adsorption and desorption branches was observed in the high-pressure region, suggesting the existence of interconnected pore structures (Calzaferri et al. 2023). Among all groups, the untreated specimen exhibited the lowest adsorption capacity and smallest pore volume, indicating a relatively compact internal structure. After hydrothermal cycling, the adsorption capacity increased progressively, reflecting the development of additional pores and internal defects. The 21-cycle specimen showed the highest adsorption capacity and broadest pore distribution, indicating severe structural deterioration after prolonged hydrothermal exposure.

The BJH pore size distribution curves further revealed the evolution of the pore structure during cycling. The untreated specimen displayed a relatively concentrated pore size distribution, primarily within the range of 2 to 10 nm, indicating a comparatively uniform pore structure. After 7 cycles, the pore size distribution broadened considerably, and a secondary peak appeared near 30 nm, suggesting the formation of larger mesopores. After 14 cycles, the pore distribution became relatively more uniform, which may be associated with continued hydration of residual reactive components and the associated evolution of MOS hydration products. However, after 21 cycles, the pore distribution became highly dispersed, accompanied by the appearance of multiple characteristic peaks, indicating severe pore coalescence and structural degradation.