# Preliminary Investigation of Geopolymer Foams as Coating Materials
## 3 Results and Discussion

Table 6: Working pressure range of the \(6\times 10^{-3}\) m spray nozzle for geopolymer materials. A geopolymer base without foaming agents was then sprayed onto the film. The layer was still relatively thin, but it was possible to achieve a foaming effect after spraying, as shown in Figure 4.  
The produced layer did not meet expectations, as cracks appeared after the layer dried. Therefore, as part of this research, it was decided to use the manual method of applying the layer. The manual technique allows for the application of a denser geopolymer, which reduces the number of cycles required. The proportions of the ingredients were the same as before (Mix 2). The addition of aluminum powder was 1 g, which did not cause excessive brittleness and was evenly distributed during the mixing process. This was the only test that was entirely successful, giving a foam layer that was still completely free of cracks and brittleness after 24 h. The resulting layer was subjected to thermal conductivity tests. The test results are presented in Table 7. Due to the application of the layer on a cardboard substrate, a thermal conductivity test was conducted on both the cardboard and the layer applied to the cardboard. Thermal conductivity tests showed that the foamed material applied to the pad had a thermal conductivity of 0.155 (W/(m \(\times\) K)). Figure 4: Foamed geopolymer layer sprayed on an aluminum foil: (**a**) foaming effect after partial drying, (**b**) foaming effect after complete drying with visible cracks. Figure 3: Scheme of applied material on aluminum foil. Porosity plays a critical role in the conductivity of materials. The greater the number of pores in materials, the higher the amount of enclosed air in the structure. This relation causes lower thermal conductivity. Research on geopolymeric materials foamed with aluminum powder or hydrogen peroxide showed that the conductivity of this type of material ranges from 0.07 to 0.22 (W/(m \(\times\) K)) [44; 45; 46; 47]. The obtained results were within this given range, but it should be emphasized that the produced layer was very thin, and the test results indicate the potential of this type of application. The results presented here show that the air spraying method of geopolymers using a cup gun is potentially applicable. However, the focus should be on the selection of a suitable mixture of foamed geopolymer and additives to stabilize the foaming process after the layer is applied to the substrate. In this paper, basic blends of foamed geopolymers were presented, without any hydraulic additives or stabilizers that could affect the foaming and setting of the geopolymer. The problem of the application of foamed geopolymers is very important, as the development of an application method for such material would allow the industrial use of geopolymers as insulation materials. Currently, it is only possible to produce these materials in molds. The authors decided to carry out tests on foamed geopolymers obtained by using the mold casting method, the material composition of which was the same as that of the geopolymers applied by spraying (Mix 1 and Mix 2). The performance of the foaming process obtained by casting the foamed geopolymer masses into molds was significantly better than that of the spraying method. The thermal conductivity values of the foamed geopolymers (Mix 1 and Mix 2) cured in the mold are shown in Table 8.  
The thermal conductivity coefficient for the foamed geopolymer, Mix 1, was 0.12493 (W/(m\(\cdot\)K)). In contrast, the foamed geopolymer Mix 2 achieved a slightly higher thermal conductivity value of 0.12581 (W/(m\(\cdot\)K)). For the geopolymer foams based on fly ash and metakaolin, [PERSON] et al. obtained thermal conductivity coefficient results oscillating in the range of 0.06-0.09 (W/(m \(\times\) K)) [48]. In turn, in their work, [PERSON] et al. obtained the thermal conductivity results of 0.08 (W/(m \(\times\) K)) for foamed geopolymers based on fly ash and marshallow, modified with gypsum and the stabilizing agent Mn 575 [44].