Enhanced Fire Performance of Poplar Wood by Two-Step Silicate–Borate–Calcium Mineralisation

This study investigates a two-step inorganic mineralization treatment designed to improve the fire performance, mechanical properties, and leaching resistance of poplar wood. White poplar (Populus alba L.) was impregnated under vacuum–pressure conditions using sodium silicate, followed by boric acid and calcium chloride. The treatment resulted in a weight percent gain of approximately 51%, indicating substantial retention of treatment-derived inorganic material within the wood structure. Surface hardness increased significantly, by approximately 60%, whereas the increases in the modulus of rupture (18.6%) and modulus of elasticity (17.5%) were not statistically significant. Thermal analysis revealed increased residual mass after treatment under both inert and oxidative atmospheres. Fire performance was substantially enhanced, with the maximum heat release rate decreasing from 239 to 126 kW/m2 and the time to ignition increasing from 14 to 26 s. The limiting oxygen index increased from 18% for untreated wood to 44% after treatment and remained higher than that of untreated wood after leaching (23%). The treatment retained 64.2 ± 2.4% of the treatment-derived mass after leaching; however, increased equilibrium moisture content and partial loss of water-soluble components were observed. Overall, the results indicate that two-step silicate–borate–calcium mineralization is a promising approach for simultaneously improving the fire and selected mechanical properties of poplar wood, although further optimization is required to enhance moisture behaviour and long-term treatment durability.