Development of densified wood with high strength and excellent dimensional stability by impregnating delignified poplar by sodium silicate

In terms of outdoor building materials, during the service, drawbacks of fast-growing poplar (Populus tomentosa Carrie`re) are susceptible to mildew, and cracking or great loss of strength due to its inherent hydrophilic and porous structure. Here we propose an effective method to prepare dimensionally stable wood with high me-chanical strength and flame retardancy. This simple method consists of three main steps, i.e., chemical pre-treatment to remove lignin and hemicellulose, vacuum impregnation with inorganic sodium silicate solution, and high temperature densification. The flexural strength, modulus of elasticity and compressive strength of the modified wood were increased to 322.5 MPa, 24.6 GPa and 228.5 MPa, which were counted to be the increases of 5.0, 2.4 and 27.2 times, respectively, compared with the unmodified wood. High mechanical strength of wood probably is caused by the densification and Si-O-Si bonding between cellulose nanofibers. The thickness swelling of moisture absorption at 30 days and water absorption at 72 h were measured to be 0% and 1.1%, respectively. It was shown excellent mechanical strength and dimensional stability compared to the natural wood. Moreover, the chemically pretreated wood exhibited superior fire-resistant performance after the impregnation with sodium silicate and densification. This simple, extensible, and effective approach provides huge potentials for the development of materials with high mechanical properties, excellent dimensional stability, and flame retardancy to be used as outdoor structural building components.