## 1 Introduction
Alternatives like azoles and quaternary ammonium salts are costly. Additionally, biocodes used in wood impregnation can exacerbate harmful gas release during fires, potentially increasing fire accident lethality [12; 13; 14]. Flame retardants enhance fire resistance in timber and other flammable materials through several mechanisms. Fusible salts form a protective coating, restricting oxygen access; noncombustible gas release dilutes flammable gases; endothermic reactions or phase changes dissipate flames through cooling; while certain flame retardants are engineered to conduct heat away rapidly [15]. Quenching reactive free radicals inhibits flame propagation in the gas phase, halting chain reactions and preventing further combustion. Strong acids, bases, and halogen-based compounds contribute to fire retardancy, with the latter acting as gas-phase free radical scavengers [16; 17]. Fireproofing through impregnation can be enhanced with ceramic coatings [18] or fire-resistant paints. Fire-resistant paints are commonly used due to their simplicity and affordability, even post-construction. However, they offer inferior fire protection compared to impregnation methods. These paints may crack when wooden details warp, shrink, or expand due to humidity changes and can be damaged by solar UV radiation. Consequently, international building regulations since 2018 prohibit sole reliance on fire-resistant paints, mandating additional fire safety measures for full compliance. Although not yet enforced in the European Union, these regulations are active in 51 countries globally [5; 19; 20]. Among green methods, treatments on _Picea abies_ involve thermal-hydromechanical (THM) densification, where low-molecular-weight resin is infused into the timber structure [21]. The relatively green THM treatment method employs water, heat, and mechanical means to enhance timber density. When combined with phenol impregnation, it leads to increased density, hardness, abrasion resistance, and certain strength properties. Set recovery, indicative of compression deformation recovery, is regarded as a crucial parameter for compressed wood [22]. Tests on _Picaa abies_ demonstrated higher set recovery values, while decreased values and increased mechanical strength were observed for poplar wood [21]. Additionally, these treatments enhanced fire resistance [23]. Previous studies have examined termite resistance to oil-heat treatment in Scots pine sapwood (_Pinus sylvestris_ L) and Norway spruce (_Picaa abies_ L.). Oil-heat-treated wood effectively combats damages inflicted by Formosan termites residing in subterranean environments. Combinations of thermo-hydro techniques with vacuum treatment (at 100 mm Hg) for wood densification have proven effective in providing stability against _Monochamus_ spp. [24]. Successful wood treatment has involved injecting phenol-based aqueous resins into timber using a vacuum system, resulting in reduced set recovery and subsequent densification of the treated wood, thereby enhancing most dimensional stability values [25]. Silica fume boosts the structural strength of wood-cement materials, displaying high pozzolanic activity [26]. Silica aerogels improve wood properties, enhancing thermal stability and fire resistance; supplementation with a silica mineralized layer further enhances these qualities [24; 27]. Pine wood achieves significant fire resistance through treatment with guanylurea phosphate, effectively embedded within the wooden structure [28]. Phosphate treatment enhances tensile and flexural strengths of poplar wood, forming composite materials, and modified wheat starch treatment enhances the flame retardancy of wood fibers [29; 30]. Waterglass, a soluble silicate salt with silicic acid-containing anions ((SiO\({}_{4}\))\({}^{4-}\), (Si\({}_{2}\)O\({}_{7}\))\({}^{6-}\), (Si\({}_{3}\)O\({}_{9}\))\({}^{6-}\) (Si\({}_{4}\)O\({}_{2}\))\({}^{8-}\), (Si\({}_{6}\)O\({}_{18}\))\({}^{12-}\), etc.