# Enhanced Resistance to Fire of the Bark-Based Panels Bonded with Clay
## 1 Introduction  
The susceptibility to fire of bio-composites has not been broadly researched yet. There is a lot of potential for the use of biomass in different engineered products for buildings where the main concern is fire hazards [1]. The natural wood fiber reinforcement starts decomposition at about 200 \({}^{\circ}\)C, compared to the hydrocarbon backbones of most polymeric adhesives, which degrade without leaving any residue [2]. Novel bio-based reinforcements should be developed, which are simultaneously fire resistant and mechanically sound [3].  
Bio-fiber composites are renewable, partially or completely recyclable, lightweight and relatively cheap [4, 5, 6, 7, 8, 9]. Their disadvantages include their hydrophilic nature and poor fire resistance [10, 11, 12]. The latter has remedies through physical or chemical modifications, such as the adding of flame-retardant additives [13, 14, 15, 16, 17, 18]. Flame retardancy is the capacity of a material to prevent or impede the propagation of fire [19]. The inhibition of chemical reactions responsible for the flame or the set-up of a surface protective layer are two solutions in this case [20]. The flame retardancydepends on the alterations that occur inside the material and on the substances identified after the exposure of an inflammable material to high temperatures [21].  
There are different techniques to test fire resistance. Flammability tests (small, medium and full-scale) are used in both academic and industrial laboratories. These methods are cone calorimetry, pyrolysis combustion flow calorimetry (PCFC) and the limiting oxygen index (LOI) [1].  
Various additives can be blended in the bio-composite matrix: phosphorus additives (e.g., intumescent systems) [22], halogen additives (e.g., organibromine) [23], silicon additives (e.g., silica) [24], nanometric particles (e.g., nanoclay) [25] and mineral-based additives (e.g., metal hydroxide) [26; 27; 28; 29]. Cement is an alternative to these costlier additives, in the case of wood-based composites [30; 31]. Wood wool panels, bonded with cement, were developed around the beginning of the 20 th century [32]. These composites are made of wood particles, fibers and excelsior wood and bonded with Portland cement, and they were developed as panels, bricks and other products for construction [33; 34; 35; 36]. Nowadays, wood wool cement boards are reinforced with smart materials (e.g., nano-minerals) [37]. An alternative to wood wool as a sustainable raw material can be tree bark, a by-product of the wood industry. The resistance to fire of bark-based composites was analyzed by [38; 39; 40]. The phenolic compounds in tree bark contribute to the fire-retardant properties of composites. It is worth mentioning the transfer potential provided for biomimetic heat insulation and fire-stopping behavior found in many species of tree bark [41].  
Clay has been studied recently, even if it has been used for centuries as construction material. It is considered a binder due to its cohesive behavior [42; 43] and can improve the mechanical and fire retardancy properties of the composite material [44].  
The aim of this study was to investigate the flammability and heat insulation properties of larch bark-based panels, with different grain sizes, bonded with clay. This new bark-based composite is ecologically friendly, 100% natural and recyclable. It can be mostly used with respect to its thermal insulation, acoustics and fire resistance properties.