# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 4 Results and Discussion
### Internal Structure and Porosity  
Figure 5 reports one sample for each composite and its internal structures, showing both transverse and longitudinal sections, acquired from ImageJ/Fiji software. For a more precise comparison, the same sections were considered for each sample. Among all the frames, the middle one (i.e., \(\
u_{2}\) of the total number of frames) and the quarter ones (\(\
u_{4}\) and \(\
u_{4}\) of the total number of frames) were reported. This allowed showing the successive sections of the internal structure, simplifying it. These results provide a visual evaluation of the homogeneity of the samples and the internal configurations of the aggregate-adhesive combinations. Indeed, Figure 5 shows the sizes and shapes of the voids, the possible connections between the internal voids, the orientations of the aggregates, and the distributions of the sodium silicate solution. In both the transverse and longitudinal sections, the white parts represent the sodium silicate; the grey parts represent the aggregates; and the black parts (inside the samples) represent the voids. \begin{table}
\begin{tabular}{l l l} \hline \hline
**Composite Samples** & **Apparent Density (kg/m\({}^{3}\))** & **Voids (\%) \({}^{*}\)** & **Solids (\%) \({}^{*}\)** \\ \hline _A.__donax_ & & \\ A1 & 416 & - & - \\ A2 & 438 & - & - \\ A3 & 555 & - & - \\ A4 & - & 26 & 74 \\ A5 & - & 35 & 65 \\ A6 & - & 33 & 67 \\ Average value \(\pm\) SD & 470 \(\pm\) 75 & 31 \(\pm\) 5 & 69 \(\pm\) 5 \\ \hline Hazelnut shells & & \\ H1 & 628 & - & - \\ H2 & 723 & - & - \\ H3 & 680 & - & - \\ H4 & - & 35 & 65 \\ H5 & - & 38 & 62 \\ H6 & - & 34 & 66 \\ Average value \(\pm\) SD & 677 \(\pm\) 47 & 36 \(\pm\) 2 & 64 \(\pm\) 2 \\ \hline _A.__donax_ + hazelnut shells & & \\ AH1 & 667 & - & - \\ AH2 & 576 & - & - \\ AH3 & 579 & - & - \\ AH4 & - & 21 & 79 \\ AH5 & - & 27 & 73 \\ AH6 & - & 29 & 71 \\ Average value \(\pm\) SD & 607 \(\pm\) 52 & 26 \(\pm\) 4 & 74 \(\pm\) 4 \\ \hline \hline \end{tabular} \({}^{*}\)Note: Percentages by total volume.  
\end{table} 
Table 3: Results of apparent densities at T = 50 \({}^{\circ}\)C and porosities: individual and average values and standard deviation (SD). Figure 5: Cylindrical samples of each composite and their corresponding internal structures (transverse and longitudinal sections): (**a**) _A. donax-_based composite; (**b**) hazelnut-shell-based composite; and (**c**) mixture-based composite. The differences between the internal structures of the composites were more pronounced along their thickness (h = 4 cm). The hazelnut-shell-based samples seemed more homogeneous than the _A.__donax_-based ones. For these, the sodium silicate (the white parts in Figure 5) was mainly on the bottom side (the one in contact with the mold during the production process). This is in line with the expectation, as the optimization of the production process was defined by considering hazelnut shells [31]. Furthermore, the differences between samples were coherent with the values of the standard deviation of the apparent density (Section 4.1). Evident differences were found between the shapes of voids and aggregates, which determined different internal connections. The contribution of the sodium silicate solution's distribution significantly affected the type of porosity. As represented in [PERSON] et al. 's study [68], the combination of the adhesive-aggregate produced solid, closed pores, open pores, dead-end pores, dead-end clusters, and, hence, kinematic porosity, and dead-end porosity (properties mainly considered in the acoustic field). Table 3 reports the results of the internal porosity of each composite sample, the average values, and the standard deviation (SD). The composites showed similar percentages of solids/voids, with a variation of only 5%. Hence, the porosity seemed to be determined by the mixture of the bio-aggregate-adhesive rather than by the type of bio-aggregate. The latter seemed to influence mainly the shape and size of voids, their connections, and the tortuosity.