# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 4 Results and Discussion
### Hygroscopicity Properties  
Figure 6 shows the moisture adsorption/desorption content curves of the materials for four adsorption/desorption cycles. Three samples for each aggregate (i.e., three samples of _A.__donax_ (AA), three of hazelnut shells (HH), and three of the mixture of _A.__donax_ and hazelnut shells (AAHH)), and four samples of sodium silicate (S) were considered and reported. To provide a clearer representation, the same color and style of lines were considered for the same tested material. The differences between the moisture content values for each material become more marked along the cycles. Indeed, at the beginning of the test (12 h and 24 h), the curves achieved values that varied within \(\pm\)0.03 kg/m\({}^{2}\); at the end (84 h and 96 h), the range increased (\(\pm\)0.06 kg/m\({}^{2}\)). As for the curves, all the materials showed similar behavior. _A.__donax_ (both alone and mixed with the hazelnut shells) stabilized earlier than the others, having similar values during the four testing days. Meanwhile, for both the hazelnut shells and the dried sodium silicate solution, the curve increased during the test, demonstrating a higher moisture storage capacity, more markedly for the sodium silicate. Indeed, the sodium silicate solution achieved the highest values of moisture adsorption and desorption content for all cycles, followed by the hazelnut shells, and then by _A.__donax_ and the mixture of the two aggregates. These results are in line with the expectation, as sodium silicate is known to be highly hygroscopic [44; 48]. Considering the values of moisture adsorption (\(\rho_{\text{A,ac}}\)) for each cycle (average values), the hazelnut shells achieved the highest values, with a \(\rho_{\text{A,ac}}\) between 0.048 kg/m\({}^{2}\) and 0.066 kg/m\({}^{2}\). _A. donax_ achieved the lowest values, with a \(\rho_{\text{A,ac}}\) between 0.024 kg/m\({}^{2}\) and 0.038 kg/m\({}^{2}\). The same was found for the moisture desorption content (\(\rho_{\text{A,dc}}\)): the hazelnut shells showed the highest values, with a \(\rho_{\text{A,dc}}\) between 0.057 kg/m\({}^{2}\) and 0.059 kg/m\({}^{2}\), and _A. donax_ showed the lowest ones, with a \(\rho_{\text{A,dc}}\) between 0.034 kg/m\({}^{2}\) and 0.037 kg/m\({}^{2}\). The moisture content difference, \(\rho_{\text{A,sc}}\), for the bio-aggregates decreased after the first cycle, differently from the sodium silicate solution, for which the \(\rho_{\text{A,sc}}\) increased during the test: 0.003 kg/m\({}^{2}\) after the first cycle, 0.006 kg/m\({}^{2}\) after the second, 0.009 kg/m\({}^{2}\) after the third, and, finally, 0.005 kg/m\({}^{2}\). This shows that the sodium silicate stored the moisture, and its sorption capacity increased. [PERSON] et al. [28] evaluated the hygroscopicity of some agro-industrial wastes, including hazelnut shells, using the same test method. Although the researchers achieved higher values for both the sorption and desorption moisture content, the trend of the curve was similar, and the hazelnut shells showed a moisture storage capacity. The differences between the values could be determined by the different types of hazelnut shells (their physical and chemical properties) as well as the differences in the employed equipment. Hygroscopicity is strictly related to the chemical and physical properties of the material, including the porosity [54, 62, 69], as previously reported (Section 4.2). The results indicate a greater porosity for the hazelnut shells than for _A. donax_, which is different from the expectation. Indeed, the results of the loose bulk density (469 kg/m\({}^{3}\) for hazelnut shells [29] and 181.3 kg/m\({}^{3}\)[30] for _A. donax_) would lead to hypothesize a greater porosity for _A. donax_, as the two properties are correlated [70]. The same for the thermal insulation performance, as _A. donax_ demonstrated a promising thermal insulation performance [35, 39], and the two properties are strictly connected [60, 71].