## 3 Results and Discussion  
### Microstructural Characterisation of the Aggregate and the Inorganic Polymers  
Figure 2 presents SEM micrographs of the cork granules used as light aggregates. As depicted, cork has a highly porous microstructure made by closed cells which explain their low apparent density (64 kg/m\({}^{3}\)) and low water permeability. Representative photographs of the various inorganic polymers (foams, cork foams and bi-layered composites) are provided in Figure 3. Figure 3a illustrates the influence of the foaming agent in the specimens' pore size, pore size distribution and volume of pores. As expected, increasing the foaming agent amount enhances the number and size of the produced pores, which will affect the thermal and mechanical properties of the foams (discussed in Section 3.2). The cork foams, shown in Figure 3b, present a homogeneous distribution of the cork granules in the binder regardless of the foaming agent content, showing that no segregation took place despite the much lower apparent density of the aggregates (64 kg/m\({}^{3}\)) in comparison with the inorganic polymer paste (\(\sim\)1600 kg/m\({}^{3}\)). Figure 3c shows the bi-layered composites formed by a porous upper layer, and by a cork-containing bottom layer, which is expected to endow thermal and acoustic insulation properties to the specimens [25]. The digital photographs of the bi-layered samples show a good interfacial connection between the layers demonstrating that the synthesis protocol is efficient, and that the amount of foaming agent in the top layer does not affect the adhesion between the two layers. The interface between the top and bottom layers was further evaluated using SEM and EDS line profile analysis and results for one selected sample (the higher porosity composition) are presented in Figure 4. The SEM micrograph shown in Figure 4a validates the macroscopic features depicted from the digital photographs, since a perfect adhesion between the layers is observed. The EDS line profile provided in Figure 4c shows the distribution of Si, Al and C within the layers. As expected, the Si and Al content in the upper layer is reasonably stable, the sudden drops are attributed to the presence of pores, while in the bottom layer the presence of C is associated with the presence of cork granules. The elements distribution in the sample is further illustrated by the EDS maps shown in Figure 4d,e. Figure 2: Scanning electron microscope (SEM) micrographs at two different magnifications (**a**,**b**) showing the microstructure of the cork granules. Figure 4: SEM micrographs (**a**,**b**), energy-dispersion spectroscopy (EDS) line profile (**c**) and elemental mapping (**d**,**e**) of the bi-layered composite prepared with 0.1 wt.% Al. In Figure 4b the two layers are identified using colors (porous upper layer—highlighted in light green; cork-containing (bottom) layer—highlighted in light blue) and the location and direction of EDS line profile identified by an arrow. Figure 3: Digital photographs of the (**a**) inorganic polymer foams, (**b**) cork-containing foams, and (**c**) bi-layered composites containing varying amounts of Al. (**d**) shows a comparison between the three set of samples prepared using the same amount of Al (0.05 wt.%). The interface between the aggregate and the matrix is a very important feature known to strongly affect the composites' mechanical properties [30; 31]. The interface between cork and the inorganic polymer matrix was studied using SEM and EDS mapping and the results are presented in Figure 5. The SEM micrographs show the expected dense microstructure of the matrix, remaining equally dense at the vicinity of the aggregate. In fact, in our cork-composites no interfacial transition zone between the aggregate and the matrix was observed and this is corroborated by the higher magnification SEM micrograph given in Figure 5b showing a smooth connection between cork and the inorganic polymer matrix. The presence of cracks or voids was not observed in the areas surrounding the aggregates.