# Hygroscopicity and Morphology of Bio-Based Boards--The Influence of the Formulation
## 5 Conclusions  
The present study analyzed _A. donax_-based and hazelnut-shell-based boards produced using a sodium silicate solution as the adhesive. The performed laboratory tests considered their apparent density, internal structure, porosity, and hygroscopicity. The connection between the composites and the materials' properties and the influence of the distributions of the aggregates and the adhesive were considered, too. Indeed, this research work wanted to investigate the correlation between the final performance of the boards and the contribution of each composing material. The results allowed the achievement of the following conclusions:  
* The hazelnut-shell-based composite had a higher apparent density than the _A. donax_-based one. Therefore, the addition of _A. donax_ to the mixture of hazelnut shells and the sodium silicate solution lowered the apparent density. Hence, it could probably improve the thermal performance, too. The results of the replications and the values of standard deviation confirm the good homogeneity between the samples of each composite. * The tomography enabled knowing the internal structures of the composites. A qualitative evaluation suggested a better homogeneity in the hazelnut-shell-based composite. In the _A. donax_-based composite, the sodium silicate solution was deposited more on one side of the sample (the bottom face). This could affect the final performance, such as the moisture storage capacity. * The tomography allowed defining the porosity, too. The results demonstrate that this property was mainly determined by the contents of the selected materials (the bio-aggregates and adhesive) rather than by the type of bio-aggregate. The latter affected the type of voids (size and shape) and the sodium silicate distribution. * Both the materials and the composites were highly hygroscopic, as expected. The addition of the sodium silicate solution increased the hygroscopicity and the moisture storage capacity. Furthermore, the sodium silicate's distribution in the samples significantly influenced their final performance, and this caused a mismatching between the results of the materials and composites. These results were confirmed by the statistical analysis (ANOVA). The correlations between the properties of the materials (individually) and the composites were defined. Knowing which and how these parameters could influence the final performance is extremely helpful for their optimization and future research development. The strong influence of the adhesive content on the final properties was confirmed, as well as the high hygroscopicity of the considered composites. The results suggest that in the case of the employment of the composites as internal coating boards, they could passively control indoor hygothermal comfort. This could improve both indoor conditions and internal comfort and lower energy demands. Furthermore, when considered as bio-based boards, they could be compatible with the elements in vernacular architecture. Hence, starting from this research study, useful solutions for the preservation of cultural architectural heritage could be developed. **Author Contributions:** Conceptualization, E.C., L.N., L.M. and P.F. ; methodology, E.C. and M.B. ; software, R.B. ; validation, E.C. and M.B. ; formal analysis, E.C. and M.B. ; investigation, E.C. ; resources, M.B., R.B. and M.P.M. ; data curation, E.C. ; writing--original draft preparation, E.C. ; writing--review and editing, L.N., L.M., P.F. and M.B. ; visualization, E.C. ; supervision, L.N., L.M. and P.F. ; project administration, L.N. and P.F. ; funding acquisition, L.N. and P.F. All authors have read and agreed to the published version of the manuscript. **Funding:** This research was funded by the Portuguese Foundation for Science and Technology (FCT), with the Ph.D. grant PD/BD/150579/2020 of the first author, as part of the Eco-Construction and Rehabilitation Program (EcoCoRe).