Catalytic heat treatment of wood for enhanced fire and leach resistance

Wood is an attractive renewable construction material, but its inherent flammability, hygroscopicity, and poor resistance to leaching limit its broader application in fire-safe structures. This study investigates a catalytic heat treatment (CHT) approach combining ammonium dihydrogen phosphate, citric acid, oxalic acid, and glyoxal to improve durable fire retardancy and moisture resistance in poplar wood. Several formulations were screened based on weight percentage gain (WPG), equilibrium moisture content (EMC), and leach resistance (EN 84), leading to the selection of an optimized formulation (S3) for detailed characterization. Chemical fixation was confirmed by FTIR spectroscopy, revealing esterification, acetal crosslinking, and phosphorus–oxygen bonding, which remained partially intact after leaching. SEM–EDX analysis confirmed the incorporation of phosphorus within the treated wood and its partial retention after leaching. Optical microscopy demonstrated lumen filling and pronounced cell-wall bulking, consistent with reduced hygroscopicity and increased hardness. Flexural testing indicated a decrease in bending strength (26% and 36%) while elastic stiffness remained largely unchanged before leaching. Thermogravimetric analysis showed enhanced char formation and altered degradation pathways, while cone calorimetry revealed pronounced reductions in heat release rate (≈60%), smoke production (≈75%), and total heat release (≈40%), alongside increased residue yield. The limiting oxygen index increased from 24% for raw wood to 42% for treated wood. Microscopic observations of combustion residues revealed a more cohesive char structure in the treated samples. Fire dynamics simulator (FDS) modelling further demonstrated reduced compartment temperatures for treated wood. Neural network models were developed to predict carbon monoxide and smoke production rates, achieving high predictive accuracy for CO production (R² ≈ 0.96). Overall, the catalytic heat-treatment system promotes chemical fixation of phosphorus-containing species and improves fire performance while retaining a relatively simple processing route, offering a favourable balance between fire performance, durability, and processing simplicity. • Catalytic heat treatment creates leach-resistant, fire-retardant wood • Synergistic acids, phosphate and glyoxal enable covalent cell-wall fixation • Peak heat release and smoke production reduced by up to 60% and 75% • Limiting oxygen index increased from 24% to 42% after treatment • Compartment fire simulations show significantly reduced temperature growth