# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 4 Results and Discussion
### Hygroscopicity Properties
However, considering only the porosity is too simplifying, as many other parameters affect both the hygroscopicity and the other properties. As for the dried sodium silicate solution, its high hygroscopicity is derived from its chemical composition. It ensures an easy reaction with water and makes the sodium silicate able to absorb moisture from the environment, as detailed in past research [38, 48]. Figure 6: Moisture adsorption/desorption contents for four cycles of three samples of each bio-aggregate and four samples of the sodium silicate solution, considered separately. Figure 7 shows the moisture adsorption/desorption content curves during the four cycles for the composite samples (i.e., three samples of the _A. donax_-based composite (A), three of the hazelnut-shell-based one (H), and three of the mixture-based one (AH)). Again, the same color and style of lines were considered for the samples of the same composite. The hazelnut-shell-based composite achieved the lowest values of moisture content in the four cycles. The composite made of _A. donax_ (both alone and combined) reached the highest values. The hygroscopicity depended on the aggregates, the adhesive, and their coupling [72], as well as the conformation of the composite, namely, the adhesive's distribution, the porosity, and the grain size of the aggregate [73]. For this reason, one of the samples of the _A. donax_-based composite showed different values: the exposed surface could have had a different distribution of the sodium silicate solution or different aggregate grain sizes. However, considering the average curves, the composites showed similar behavior (similar trends in the moisture adsorption/desorption curves). None of them reached stability after the four cycles, as the adsorption and desorption moisture contents continued to increase. Indeed, considering the average values, the moisture absorption content, \(\rho_{\rm A,ac}\), increased from 0.099 kg/m\({}^{2}\) to 0.110 kg/m\({}^{2}\) for the hazelnut-shell-based composite (increasing by 11%); from 0.186 kg/m\({}^{2}\) to 0.208 kg/m\({}^{2}\) for the _A. donax_-based one (increasing by 12%); and from 0.125 kg/m\({}^{2}\) to 0.139 kg/m\({}^{2}\) for the mixture-based one (increasing by 11%). The moisture desorption content, \(\rho_{\rm A,dc}\), increased from 0.076 kg/m\({}^{2}\) to 0.087 kg/m\({}^{2}\) for the hazelnut-shell-based composite (increasing by 13%); from 0.160 kg/m\({}^{2}\) to 0.173 kg/m\({}^{2}\) for the _A. donax_-based one (increasing by 8%); and from 0.094 kg/m\({}^{2}\) to 0.101 kg/m\({}^{2}\) for the mixture-based one (increasing by 7%). The moisture content difference, \(\rho_{\rm A,sc}\), was between 0.023 kg/m\({}^{2}\) and 0.037 kg/m\({}^{2}\) for the hazelnut-shell-based composite; between 0.026 kg/m\({}^{2}\) and 0.053 kg/m\({}^{2}\) for the _A. donax_-based one; and between 0.031 kg/m\({}^{2}\) and 0.045 kg/m\({}^{2}\) for the mixture-based one. The highest values of moisture content difference were achieved during the third cycle for all composites. After this cycle, the samples started to stabilize. To investigate in more detail the correlation between the materials and the composites, the average values for all were considered. They are reported in Figure 8.  
Figure 7: Moisture adsorption/desorption contents for four cycles of three samples for each composite. These results show the strong influence of the sodium silicate on the final performance. Indeed, by adding the sodium silicate solution, the hygroscopicity and the moisture storage capacity of the bio-aggregates strongly increased. For example, as Figure 8 shows, considering the last cycle, from values of moisture content between 0.03 kg/m\({}^{2}\) and 0.06 kg/m\({}^{2}\) after 84 h for the bio-aggregates, the sodium silicate allowed achieving values in the range of 0.20-0.33 kg/m\({}^{2}\) after 84 h. The moisture content difference, \(\rho_{\text{A,sc}}\), increased, too.