Biomimetic Mg-Regulated Mineralization of Aragonite Laths for High-Strength and Fire-Resistant Densified Wood Composites

Developing wood engineering materials that simultaneously satisfy flame retardancy and mechanical requirements remains a formidable challenge. Inspired by the formation of nacre and bone in natural biomineralization, this study proposes a biomimetic mineralization strategy based on interface engineering to overcome the inherent flammability and mechanical limitations inherited from the porous structure of natural wood. The in-situ mineralization on microfibril surfaces was facilitated by the hydroxyl-rich cellulose scaffold generated through delignification, which provided abundant interfacial sites for mineral nucleation and growth. Mg2+ was used to regulate the crystallization pathway of calcium carbonate. To establish an intimate interlocking for enhanced interfacial stress transfer, the hot-pressing densification process was employed to promote the deep compaction of the mineral phases and the cellulose skeleton. Consequently, the optimized sample with a Mg2+/Ca2+ molar ratio of 5:1 exhibited an LOI of 37.0%, the total heat release (THR) reached 21.46 MJ/m2, and the peak heat release rate (PHRR) suppressed to 133.94 kW/m2. Regarding mechanical performance, the flexural strength surged to 359.85 ± 10.93 MPa, representing approximately 4.6 times the flexural strength of natural wood, with the specific strength further elevated to 295.82 ± 8.99 MPa·cm3/g. Furthermore, the mineralized and hot-pressed composite exhibited a higher initial apparent water contact angle and improved dimensional stability under high-humidity conditions. Even under extreme humidity (95% RH), the optimized sample maintained a robust flexural strength of 210.39 ± 27.07 MPa, which is approximately three times that of wet natural wood. This study reveals the pivotal role of the Mg2+/Ca2+ molar ratio in balancing phase separation and atomic regulation, providing a novel pathway for the design of high-performance, fire-safe wood structural materials.