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

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.