# Preliminary Investigation of Geopolymer Foams as Coating Materials
The geopolymer mortar used in this previous study was based on metkaolin and slag with the addition of MgO and polypropylene fibers, while a sodium-based solution was used as an activator. Using a blading method, the geopolymer coating was manually applied to the previously cleaned concrete surfaces of an acropolis located along the coast. After about 30 min, wet straw mats were applied to the geopolymer layer and removed after the layer had completely solidified. The disadvantage of using this method in atmospheric conditions is the high rate of shrinkage in geopolymer paste and the formation of microcracks on the surface. However, it was found that the main factors influencing the quality and coherence of the coating were the ambient humidity and the thickness of the applied layer; the best results were obtained for a 5 mm thick layer in the tidal area (i.e., with periodic contact with seawater) [32]. [PERSON] et al. researched geopolymeric coatings based on fly ash with various silicate-to-alkali ratios ranging from 0.5 to 2.5. The studied geopolymer layers were applied with a brush to the surfaces of cleaned mild steel plates. The conducted tests showed that the tested material had promising mechanical properties, fire resistance, and corrosion resistance [33]. [PERSON] et al. researched geopolymer coatings based on fly ash-metakaolin-phosphate. The coating was deposited on a mild steel substrate using the spin-coating method to improve the substrate-matrix interaction. The spin-coating method produced a well-ordered thin geopolymer coating (thickness 13-20 \(\upmu\)m), despite the use of clearly thick materials. The maximum adhesion force of the material to the substrate was 2.5 MPa. This research also showed the influence of parameters such as spin speed and time. A low spin speed limits the homogeneous spread of the geopolymer, which leads to incomplete material coverage. On the other hand, too short or too long centrifugation time adversely affects the thickness and efficiency of the coating [34]. After conducting the literature review, the authors decided to apply a geopolymer layer using the spray application method, which shows the highest application potential for foamed geopolymers. This method, as mentioned earlier, allows for a more accurate application of the layer to the substrate, compared with other methods reported in the literature. However, it should be noted that the final properties of geopolymer coatings, mortars, and concrete, in addition to the method of application, are determined by other factors, including the adhesion of the geopolymer material, which in turn strongly depends on the chemical composition of the raw materials, geopolymer composition, type of substrate, surface roughness, or water content [35]. The purpose of the present study was to test the applicability of additive technology for applying foamed geopolymers to existing structures and vertical surfaces. Most applications of foamed geopolymers are based on the installation/application of molded slabs. In this case, difficulties arise in protecting surfaces with variable geometries and curvatures. Spray application technology eliminates the problem of securing irregular geometries and contributes to the use of geopolymers as an insulation material, replacing commonly used combustible insulation materials. This work focused on investigating the feasibility of using an air gun with variable nozzles to apply layers of foamed geopolymers and assessing the adhesion of the applied material to the substrate and the degree of foaming in the applied geopolymer layers on a vertical surface.