The impact of common fire retardancy salts and minerals on further properties of wood

Wood has become an increasingly important construction material. To ensure fire safety, fire retardants are widely applied. Recent research focuses on environmentally friendly fire retardants with strong fixation. Phosphate-based salts like diammonium hydrogen phosphate show high fire protection potential but are highly leachable due to their good water solubility. As a newer trend mineralisation using slightly water soluble minerals such as calcium carbonate, calcium oxalate, magnesium phosphate, and calcium phosphate offers good fixation. While fire resistance of mineralised wood has been extensively studied, other key properties, such as fixation durability, colour stability, and mechanical properties, require further investigation. This study examines these aspects in mineralised fir (Abies alba), mineralised European oak (Quercus spp.), and mineralised European beech (Fagus sylvatica). Additionally, the wood was treated with diammonium hydrogen phosphate and potassium oxalate in a single-step process. For mineralisation, a two-step process was applied: potassium oxalate followed by calcium acetate for calcium oxalate, and ammonium hydrogen phosphate followed by calcium acetate or magnesium chloride for calcium/magnesium phosphates. Colour stability under the influence of UV light was evaluated using a xenon tester and colour changes were evaluated according to the CIELab colour analysis. Additionally, Fourier-transform infrared spectroscopy was used to investigate the potential of the minerals to protect the wood structural elements due to UV irradiaton. Natural weathering was carried out over 16 months on the outdoor test field in Biel/Bienne (Switzerland). Specimens were analysed via colour measurements, microscopic investigations and energydispersive X-ray analysis. Mechanical properties were assessed through modulus of elasticity and modulus of rupture via three-point bending tests. Results indicate that mineralizsed wood shows reduction of its minerals after 16 months of outdoor exposure on the surface, but showed reduced blue-stain fungal infestation, improving colour stability. However, beech and oak exhibited colour changes indoors, with potassium oxalate improving stability. Furthermore, the minerals cannot protect wood structural compounds from degradation due to UV light. Mechanical tests revealed no significant impact on mechanical properties, even at high concentrations, suggesting that these treatments do not compromise the structural integrity of wood.