# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 4 Results and Discussion
### Additional Discussion
Considering all the results, it was confirmed that the hygroscopicity depended on both the aggregates and the sodium silicate. The latter more significantly affected the moisture storage capacity. As the percentages of the aggregate-adhesive were the same for all samples, the differences in the results were mainly caused by the sodium silicate distribution. This changed due to the types of aggregates, namely, their physical and chemical properties and their reactions with the adhesive. The statistical analysis confirmed these considerations. For the MBV, the distinguishable groups (Table 6) were the composites with _A. donax_ and the sodium silicate solution. According to Figure 5, these composites showed less homogeneity and a higher amount of sodium silicate on one side of the samples (Section 4.2). Bio-aggregates are known to be highly porous and, hence, highly hygroscopic. Knowing their internal structures, too, is useful to understand their properties even better [54]. However, other types of analysis are more suitable for this purpose, such as scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and sorption techniques. Future studies could consider these laboratory tests to investigate the correlation between these properties on a micro-scale. High hygroscopicity could be both an advantage and a drawback. Indeed, hygroscopic building materials and products can passively contribute to the control of indoor conditions [61] and can secure indoor hygothermal comfort and better indoor air quality [78; 79], which are extremely important for human health and users' well-being [80]. On the other hand, a high moisture storage capacity and moisture content could determine the material's degradation [24]. Furthermore, bio-based composites can be easily attacked by microorganisms due to their chemical composition, their organic nature, and their pH [81; 82]. Apart from the aesthetic impact, a biological attack can lower the durability of composites, affect the materials' properties, and restrict their performance, modify their chemical compositions, and compromise their natural structures [83]. Furthermore, biological degradation can affect human health, causing diseases and lowering indoor air quality [84; 85]. However, sodium silicate has demonstrated good resistance to biological attack [21] and is known to avoid mold growth and material decomposition [45; 46; 47]. The present study confirmed this property, as no biological colonization was detected after the hygroscopicity test. Moreover, as moisture storage can be crucial, determining the critical moisture level [86; 87] could be useful to better define this possible drawback and moderate it. Furthermore, some strategies could improve low moisture resistance, such as the addition of additives, such as phosphorus, boron, or silica fume [48]. The results suggest that the hazelnut-shell-based and _A. donax_-based composites could be employed as indoor coating boards, passively regulating internal conditions, hygothermal comfort, and energy demand [73; 88; 89]. These types of building composites could also be useful for the preservation of architectural heritage, namely, vernacular buildings. Being bio-based, the employed materials would be compatible with vernacular architecture's typical materials. Their high hygroscopicity and moisture buffering capacity would avoid the creation of water vapor barriers, which is crucial to avoid wall and roof degradation. Moreover, by knowing these types of composites' behaviors, other possibilities could be proposed, which could be suitable for historic architectural heritage, too. A possible effective solution for the bio-based composites could be a superficial application of the sodium silicate solution to both protect the bio-based materials and secure the passive control of indoor hygroscopic conditions. However, this is an idea for future studies, suggested by the analysis of the present study, which should be investigated and confirmed.