# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 1 Introduction  
Research is nowadays increasingly focused on studying and proposing innovative and sustainable solutions practices. The environmental impact of the construction sector (for new construction, refurbishment, use, and deconstruction) is widely recognized [1, 2], as well as the worldwide interest in reducing the production of toxic and environmentally damaging substances [3]. Both the Agenda 2023 [4] and the European Green Deal [1], among several proposals, have reported the need for a reduction in energy demand and the encouragement of a circular economic system. Thus, among the investigated innovative possibilities, the use of bio-wastes and bio-resources seems to be a particularly attractive solution. It reduces energy consumption [5], ensures a circular economy, and moderates the problem of waste disposal [6, 7]. Furthermore, the use of these types of materials, whichdo not act as water vapor barriers, can guarantee cultural heritage preservation, namely, vernacular architecture. Past research has already considered bio-based building composites and assessed their properties. Both bio-wastes (such as agro-industrial wastes) and bio-resources have been investigated (such as straw [8], corn pith [9], rice husk [10], banana fibers [11], hemp shives [12], and reed [13]). Depending on the biomaterials used, the bio-based composites demonstrated different promising properties, namely, good thermal insulation performance, sound absorption capacity, high hygroscopicity, or adequate mechanical resistance. Several types of bio-based composites and products have been produced and studied, such as plastering mortars [14], glues [15], bricks [16], and boards [17; 18], which are the ones considered in this study. Bio-based boards are usually made of bio-aggregates (bio-wastes or bio-resources) and an adhesive (glue or binder). In addition to the bio-wastes/bio-resources, the employed adhesive also significantly influences the board's final properties, as well as its sustainability and toxicity [19; 20]. Considering the same bio-aggregates and different adhesives, the composites' behaviors deeply change [21]. Using adhesives that could be harmful to human health, such as formaldehyde-based ones [22; 23], reduces the non-toxicity and the eco-efficiency derived from the use of bio-based materials. The selected adhesive could also moderate some of the main drawbacks, such as low resistance to water, fire, and biological attack [24; 25; 26], improving the final composite's performance. Thus, knowing the properties of both the aggregates and the adhesive, individually and in composites, is crucial in building composites' production [27; 28]. The present work addresses the topic of materials' influence on the final performance of composite boards. In particular, this research study considered bio-based boards made of bio-aggregates, namely, hazelnut shells and giant reed (_Arundo donax_ L.), bounded by a sodium silicate solution [29; 30] (these two reference studies provided characterizations of the proposed composites, evaluating their physical and mechanical properties). Hazelnut shells are a widely produced bio-waste [29], available throughout the year due to hazelnut treatment before their use [31]. Some past studies have already considered the possibility of using hazelnut shells for building practices [32; 33]. Nevertheless, they have not been largely investigated as such [34]. _A. donax_ L., known as giant reed or giant cane, is an herbaceous, perennial, non-food, invasive, and largely available bio-resource [35]. It can be found worldwide due to its tolerance to different climates and soil conditions [5]. It has already been employed for construction practices--i.e., in panels, concrete, plasters, and new construction technologies--showing promising mechanical and thermal properties [13]. Being natural products, both bio-aggregates could be employed in innovative building products compatible with vernacular architectural elements [36; 37].