# Lightweight Insulating Geopolymer/Phase-Change Materials Applied Using an Innovative Spray Method
## 3 Results and Discussion
Figure 8: Close-up view of a cut-out section of the gypsum board with the applied geopolymer foam layer (**a**), and a side view of the boards with the applied coating (**b**). Layer thickness approx. 1.5 cm. Figure 7: Examples of a single layer of geopolymer material applied by spraying, which underwent foaming and curing on the substrate. The microstructure shown in Figure 9 provides valuable information on the efficiency of the geopolymer foaming process and the homogeneity of the resulting porous structure. It can be assessed that the pore size distribution is relatively uniform throughout the entire volume of the analysed material, with predominant pore diameters ranging from 500 to 2000 \(\upmu\)m. Although some deviations from ideal uniformity are natural and expected for materials of this type, the absence of clear clusters of excessively large or small pores confirms effective control over the foaming process. It should be emphasised that the pore size distribution itself was not the key objective of this study, since these parameters can easily be modified, if necessary, by appropriate selection of the blowing agent or adjustment of its concentration. The more important aspect was to confirm whether the process of foaming proceeded uniformly and efficiently, directly translating into the material's final thermal and mechanical properties. In the analysed sample, massive, unfoamed material fragments or excessively thick pore walls, which could indicate an incomplete or uncontrolled gas release reaction, were not observed. The tested geopolymer/PCM coating exhibited a typical open and well-developed pore system, which clearly demonstrated the efficiency of foaming and the appropriate choice of the technological parameters of the process, such as mixing time, temperature, and the activity of the blowing agent. A homogeneous microstructure contributes to stable performance and high quality of the final product. In the conducted tests, a crucial aspect of evaluating the performance of the foamed geopolymer/PCM coating was its adhesion to various types of substrates. Table 3 presents the results of adhesion tests for the foamed geopolymer/PCM coating applied by spraying onto steel, geopolymer, and gypsum plasterboard. The tests were performed using the PosiTest(r) AT (DeFelsko Corporation, Ogdensburg, NY, USA). automatic pull-off adhesion tester, with five measurements taken per substrate; the values shown in the table represent the average. Visual inspection after the tests showed that the adhesion was sufficiently strong for steel and geopolymer substrates, with fracture occurring within the foamed geopolymer layer. No coating detachment from the cardboard surface was observed for the gypsum board substrate, but the cardboard layer was torn from the gypsum core of the board. The adhesion strength between the coating and the cardboard exceeded the bond strength between the cardboard and the gypsum core. The high level of adhesion testifies to the strong interaction between the geopolymer/PCM and the cellulose surface of the cardboard, which may result from both the chemical properties (reactivity of the basic functional groups) and the microporous structure that allows for penetration and mechanical anchoring in the substrate. This phenomenon was confirmed not only by the  
Figure 9: Microstructure of the foamed geopolymer layer: optical microscopy (**a**); scanning electron microscopy (SEM) (**b**). experimental observation of damage but also by the determined peel strength values, which exceeded the internal adhesion limit of the gypsum board. The literature [52] indicates that the typical bond strength of cardboard with gypsum is approximately 0.2-0.3 MPa. In all cases, the pull-off strength exceeded 1 MPa, demonstrating good adhesion of the foamed coating to the substrate. These results are particularly significant because they surpass the standard adhesion values achieved for conventional coatings used in construction, such as plaster or protective coatings, which often reach values below 1 MPa [53].