Development of super dimensional stable poplar structure with fire and mildew resistance by delignification/densification of wood with highly aligned cellulose molecules

Wood is one of the most popular materials for construction purposes because of its environmentally friendly and sustainable characteristics. However, the use of wood is constrained by the lengthy time it takes for trees to mature. Consequently, fast-growing wood species have become popular as substitute options due to their ability to rapidly reach maturity and high yields. Although the problem of low density and strength has been effectively addressed in recent years by densifying wood, the problem of large thickness swelling due to moisture and water absorption has limited its application. Therefore, we reported an effective modification strategy to overcome the thickness swelling issue of densified wood by preparing a cellulosic reinforced material through the synergistic action of alkaline chemical pretreatment, multi-step cyclic impregnation and high-temperature densification. The results showed that the alkaline chemical pretreatment was effective for removing a large amount of lignin and hemicelluloses, creating a large number of hydrogen bonds among the remaining strong celluloses. The impregnated sodium silicate solution bonded celluloses tightly, and the densification treatment contributed to the production of Si-O-Si structure, forming the shuttle hybridized structure through Si-O-C bonds. The hardness, flexural strength, elastic modulus, and compressive strength of the modified wood increased by 3.9, 6.0, 3.4 and 28.2 times, respectively. In addition, 0 % thickness swelling for 30-day moisture absorption and 1.0 % thickness swelling for 72-hour water absorption were achieved, realizing super dimensional-stable poplar structures. Furthermore, the high-performance densified wood prepared by this method has excellent fire and mildew resistance properties, which lays the foundation for the application of fast-growing wood in outdoor engineering structures.