Physicochemical modification and mould growth response of Populus deltoides wood treated with kraft black liquor and alkyl ketene dimer

Evidence linking wood-modifier uptake and moisture behaviour to mould resistance remains limited in this area. This study compared KBL and AKD treatments of Populus deltoides using diffusion and vacuum-pressure impregnation (VPI), assessing uptake, moisture behaviour, hardness, and Aspergillus niger mould growth. At 5% concentration, KBL diffusion increased weight percent gain (WPG) from 6% at 24 h to nearly 10% at 120 h, whereas VPI produced WPG above 25%, density increase above 20%, and bulking ratio near 4%. Diffusion of 5% AKD produced WPG below 2%, while VPI increased WPG to 7–8%, bulking ratio to 6–7%, and density by nearly 10%. Equilibrium moisture content differed significantly among treatments (p = 0.033), with AKD-treated specimens showing lower values than the control. Hardness was higher in treated groups, but differences were not significant (p = 0.073). Mould-covered area differed significantly among treatments (p < 0.001), with KBL treatments forming the lowest homogeneous group in Duncan’s test, whereas AKD showed no consistent reduction in mould growth. FTIR and NIR showed spectral differences, while PCA explained 97% of spectral variance in the first two components. Overall, impregnation route influenced uptake, whereas moisture resistance and mould response were not directly coupled.