## 3 Results and Discussion
### Inorganic Polymer's Apparent Density and Thermal Conductivity  
The influence of the mixture design (e.g., foaming agent and cork amount) and the synthesis route (e.g., foams; cork-foams and bi-layered composites) on the inorganic polymer apparent density is illustrated in Figure 6. The density of the reference composition (prepared without cork or foaming agent) is 1.17 g/cm\({}^{3}\). As expected, the addition of foaming agent to the compositions strongly decreases the specimens' apparent density, reaching 0.45 g/cm\({}^{3}\) in the higher porosity foam (prepared with 0.1 wt.% Al). Surprisingly, the density of the cork foams (containing 75 vol.% of cork) is similar to that seen in the foams prepared using the same amount of foaming agent but without adding cork. This unexpected result might be due to the mixing protocol in which the cork granules were added after the foaming agent to the slurries possibly disrupting the formation of gas bubbles and hindering their entrapping in the mixture. Figure 5: SEM micrograph at two magnifications (**a**,**b**) and corresponding EDS elemental mapping (**c**) for the cork-foam prepared with 0.10 wt.% Al. As for the bi-layered composites, the porous/dense specimen prepared without adding cork to the bottom layer and adding 0.10 wt.% Al to the upper layer showed the highest density (0.87 g/cm\({}^{3}\)). Interestingly, the other bi-layered sample having a dense layer but with a cork-containing layer instead of a porous layer showed similar density (see insets in Figure 6b). The addition of a minor amount of foaming agent (0.05 wt.%) to the sample's upper layer reduced their apparent density by a factor of 1.7 compared to the dense/cork-containing specimen, while higher amounts induced only minor fluctuations. The thermal conductivity of the inorganic polymer specimens developed in this study is presented in Figure 7a. The matrix presents a fairly low thermal conductivity, and this is explained by the mixture composition which has been intentionally designed to produce low thermal conductivity materials. As expected, the addition of the foaming agent reduced the thermal conductivity of the foams, higher Al amounts inducing a stronger reduction. The cork foams show similar thermal conductivity in comparison with the foams, in line with their comparable apparent densities. The bi-layered composites showed the lowest thermal conductivity values demonstrating the interesting potential of this innovative material. The lowest thermal conductivity (94.9 mW/m K) was observed in the specimen containing the highest amount of foaming agent (0.1 wt.% Al) in its upper layer. The lowest thermal conductivity seen for the bi-layered composites (94.9 mW/m K) was compared with other literature studies addressing the synthesis of lightweight inorganic polymers (foams and composites) and the results are provided in Figure 7b. As observed, the thermal conductivity of the bi-layered composites is much lower than most of the studies dealing with the addition of lightweight aggregates [14; 20; 21; 34]; slightly lower than that seen for expanded waste glass [35] and expanded polystyrene waste composites [18]; being slightly superior to our previous works on cork-composites [24; 25] due to the much smaller amount of cork used in the present study (75 vol.% instead of 90 [25] and 92 vol.% [24]), and 2.8 times higher than that reported for polystyrene-containing composites [19]. Moreover, the thermal conductivity of the porous/cork-containing composites is also much smaller than those reported for several other foams [36; 37; 38; 39; 40]; comparable to [41; 42; 43; 44; 45; 46; 47]; slightly higher than [48; 49; 50; 51]; and much higher than that observed in [52; 53]. The bi-layered composites' thermal conductivity, amongst the lowest reported to date for inorganic polymers, validates their use as energy-saving building material. Figure 6: (**a**) Influence of the Al powder amount on the apparent density of the various compositions.