## 3 Results and Discussion
Furthermore, in the bottom left corner of Figure 4a (point 1), the presence of a further dehydrated salt agglomerate can be identified. This salt cluster is totally embedded in the silicone matrix, as identifiable by the smooth surface binder that acts as a covering layer. Afterwards, by increasing the pressure up to 800 Pa, therefore increasing the relative humidity from 0.1% to 10.9%, no evidence of salt hydration was identified. Subsequently, by lowering the temperature to 5 \({}^{\circ}\)C and keeping the pressure constant at 800 Pa, the hydration process was stimulated, caused by the increase in humidity that reached 91%. In these conditions (Figure 4b), the salt volume increase was evident due to the acquisition of the water molecules. This volume increase, however, does not involve the salt detachment from the matrix, avoiding salt loss during the hydration/dehydration process. At the same time, no cracks or local damaged areas on the silicones embedding matrix were evidenced. This result suggests that the hydration kinetics are quite slow, as reported in [26]; in fact, the composite material does not undergo sudden transformations such as to cause superficial damages. The quite slow kinetics could be due to the transition from the amorphous phase (stable in dehydrated conditions, reported in XRD patterns in Figure 3) to the crystalline phase (salt hydrate), which takes place during the hydration process. Nevertheless, the hydration/dehydration of magnesium sulfate hexahydrate depends on both temperature and water vapor pressure showing that the equilibrium between water vapor and the salt is bivariant [11]. The volume increase in the salt hydrate is not regular and it is mainly located in edges and interstices of salt clusters (points 2 and 3 in Figure 4b). This behavior can be explained, considering that these regions are directly located at the edges of the foamed particle, thus being near to the surface from where the vapor enters the foam particle. Therefore, the initial reaction will occur here, resulting in a pronounced volume increase; thus, the water agglomeration is favored. Further interesting information can be also acquired by analyzing the morphology modification induced on the crystal of salt embedded in the silicone matrix. In particular, at the bottom of the micrograph, the presence of a salt hydrate agglomerate can be highlighted (Figure 4b, point 4). This provides a very  
Figure 4: SEM images of salt hydrate composite foam: (**a**) P = 10 Pa and T = 40 \({}^{\circ}\)C, (**b**) P = 800 Pa and T = 5 \({}^{\circ}\)C, (**c**) P = 10 Pa and T = 25 \({}^{\circ}\)C.  
important confirmation that salt crystals, although incorporated in the silicone matrix (as observed in Figure 4a, point 1), are able to complete their hydration and dehydration process. The matrix, thanks to the high vapor permeability of the PDMS [29], does not hinder the flow of water vapor and allows the interaction of the salt with the environment even if it is totally embedded in the silicone structure. Moreover, thanks to the flexibility of the silicone matrix, the volume expansion in the salt agglomerate induced by the hydration process does not imply the formation of defects or cracks on the cover layer of silicone. It deforms significantly without thereby implying a mechanical instability of the cover coating, offering an effective protective capability of the embedded salt hydrate. A detailed analysis of the hydration process will be deeply investigated in future work. Finally, the characterization cycle was completed by bringing the environmental chamber down to 10 Pa and 25 \({}^{\circ}\)C at a humidity of 0.3% (as starting conditions). In these conditions, the material was suitably dehydrated (Figure 4c), acquiring a structure morphology roughly compatible with the initial one (Figure 4a). As a consequence, no damages due to trapped water occurred, as expected by the XRD analysis of pure salt, preserving the reusability of the foam.