# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 1 Introduction
Indeed, they would be compatible with the traditional walls and roofs and could ensure their preservation. Sodium silicate, also known as \"water glass\", is a widely produced material, which has demonstrated several benefits. It is water-soluble, fire-resistant, non-toxic, and, hence, not dangerous to human health, environment-friendly, and economically convenient [38; 39; 40]. It has been employed in several sectors, such as to produce detergents and cleaning compounds [38], for material treatments [40], in ceramics [41], and in the production of geopolymers [42] and concretes [43]. Among its several uses, the employment of sodium silicate as a binder to produce bio-based building composites could be a promising possibility [44]. Indeed, in addition to guaranteeing the feasibility of their production, it can reduce some of the main weaknesses derived from their organic matrices, improving their resistance to fire [45] and preventing chemical decomposition [46; 47]. Some past studies have investigated the employment of sodium silicate as an adhesive for bio-based building boards. For example, [PERSON] et al. [39] evaluated the properties of reed and straw fiber-based composites bonded with sodium silicate and rosin, and [PERSON] et al. [46] produced composites made of olive fibers and straw and a sodium silicate solution as the adhesive. Both studies reported promising thermal insulation performance of the composites. [PERSON] et al. [21] considered some bio-basedboards bounded with different adhesives and evaluated their biological susceptibility without inoculation. The composites with sodium silicate demonstrated the highest resistance, avoiding mold growth at high relative humidity levels. Besides its several benefits, it has also shown some drawbacks, such as high hygroscopicity and low moisture resistance [45; 48], which could cause the degradation of the materials [49]. Furthermore, it could be less eco-efficient than other adhesives (i.e., bio-based ones or natural glues). Indeed, higher temperatures could be required for its production [38], even if lower than those required for other widely used adhesives (such as cement-based binders). As hazelnut-shell-based and _A. donax_-based boards have demonstrated interesting hygroscopic behavior [29; 30], which can be useful to passively contribute to indoor comfort [50], reducing energy consumption [51], the present study mainly considered this property. The contribution of each material was investigated by testing them individually and in composites. Furthermore, the materials' dispositions in the proposed boards were analyzed, assessing the correlation between the hygroscopicity and internal structure. X-ray computed tomography, which is a non-invasive 3D-imaging technique able to supply crucial information on the studied products [52], ensured the evaluation of the internal configurations of the boards. This technique has been employed in several fields, such as medical, industrial, and cultural heritage [53], as well as to investigate bio-based composites [54]. It provides information such as the pore size distribution, porosity, and tortuosity of the porous network [55]. In this work, X-ray computed tomography ensured the identification of the distributions of the aggregates and adhesive and the porosities of the bio-based boards. Finally, statistical analysis (ANOVA) was carried out for further consideration of the results. The present work aimed to analyze the composites' properties and define the influence of each material on the final performance, both in terms of the materials' properties and the aggregate/adhesive's internal distribution.