## 3 Results and Discussion
### Compressive and Flexural Strength Measurements  
The mechanical performance of the various inorganic polymers is shown in Figure 8. For comparison, the values observed for the bi-layered bodies prepared without adding foaming agent to the upper layer (left-side inset in Figure 8b) and for that prepared without adding cork to the bottom layer (right-side inset in Figure 8b) were included in Figure 8b,d. The reference composition (prepared without cork or foaming agent) reached a compressive strength of 24.8 MPa and a flexural strength of 4.1 MPa. All the lightweight specimens showed much lower strength. A strong decline (-eightfold reduction) in the samples' compressive strength was observed when adding 0.05 wt.% Al, and the strength further decreased when a higher level of foaming agent was added. Nevertheless, the higher porosity foam (produced with 0.10 wt.% Al) has a compressive strength of 1.2 MPa. Interestingly, the cork foams show higher strength (compressive and flexural) than the foams regardless of the Al content. The cork foams prepared with 0.05 and 0.10 wt.% have a compressive strength of 4.0 and 1.8 MPa, respectively, this being roughly 22% and 32% higher than their counterparts not containing cork. On the other hand, the bi-layered composites showed the lowest strength values. Nevertheless, the sample produced with 0.10 wt.% Al in the upper layer and 75 vol.% of cork in the bottom layer has a compressive strength of 0.93 MPa and a flexural strength of 0.84 MPa.  
The differences between the various compositions are further illustrated by plotting their compressive strength vs. apparent density, and results are shown in Figure 9. Not surprisingly, the decrease in the specimens' density, achieved by using a foaming agent, cork or a mixture between them, strongly reduces their compressive strength. The differences between the foams and the cork foams are also clearly illustrated, the cork foams showing higher strength and densities possibly due to their lower porosity caused by the addition of cork.  
Figure 8: Compressive (**a**,**b**) and flexural (**c**,**d**) strength of the various inorganic polymers (measured at the 28 th day).  
To further characterise the specimens' mechanical performance their specific strength was determined (ratio between compressive strength and density). The specific strength of the foams varied between 2.5 and 4.9 MPa cm\({}^{3}\)/g, while higher values were seen in the cork foams, ranging between 3.2 and 6.0 MPa cm\({}^{3}\)/g. The bi-layered composites showed a broader fluctuation, with values ranging from 2.0 (composition containing 0.10 wt.% Al) to 6.1 (composition containing 0.00 wt.% Al) MPa cm\({}^{3}\)/g.  
The compressive strength (0.93 MPa) of the lightest (461 kg/m\({}^{3}\)) bi-layered composite here reported is superior to values reported for other low thermal conductivity inorganic polymers, such as those prepared using hydrogen peroxide as foaming agent (0.26 MPa) [41] and (0.78 MPa) [53], hydrogen peroxide coupled with air entrapping (0.26 MPa) or a cationic surfactant (0.57 MPa) [58], hydrogen peroxide coupled with calcium stearate (0.68 MPa) [51], and to those produced following a saponification route (0.2-0.4 MPa) [59], among others [45; 47; 48]. The lowest compressive strength exhibited by the cork foams (1.8 MPa) obviously surpasses all these studies, but also others including coconut-based (1.3 MPa) [52] and metakaolin-based foams (1.1 MPa) [40], and in addition is vastly superior to the other lightweight cork-inorganic polymer composites reported to date, (0.23 MPa) [24] and (0.15 MPa) [25].