# Lightweight Insulating Geopolymer/Phase-Change Materials Applied Using an Innovative Spray Method
## 1 Introduction
Upon thermal exposure (e.g., due to solar radiation on an exterior wall), the PCM partially absorbs thermal energy and stores it through a phase-change process (melting). This delays the temperature increase within the partition, contributing to enhanced thermal comfort inside the building. Subsequently, after the cessation of the heat source, the PCM recrystallises (solidifies), releasing the stored energy. Due to the insulating nature of the foamed geopolymer matrix, the released energy is predominantly retained within the structure and dissipated locally rather than being transferred to the interior. This mitigates the secondary reheating of the indoor environment and minimises the reverse heat flow effect. Such a synergistic mechanism between the PCM and the geopolymer matrix enables the material to store thermal energy effectively and insulate against it. This combination is unattainable when PCMs or conventional thermal insulation materials are used independently. Consequently, this mechanism leads to temperature stabilisation indoors, reducing the amplitude of daily temperature fluctuations and diminishing the demand for mechanical cooling during warmer periods. As a result,partitions constructed using the described material significantly enhance buildings' energy efficiency and improve the occupants' thermal comfort. In practical applications of geopolymer composites incorporating PCMs, long-term stability and cost-effectiveness remain key issues. In traditional PCMs, environmental factors such as temperature fluctuations, humidity, and chemical exposure can lead to structural degradation and reduced material performance over time [35]. However, PCM-geopolymer composites present a promising alternative, thanks to the unique protective properties of the geopolymer matrix. Geopolymers are characterised by high resistance to corrosive environments, which effectively protects the phase-change materials that they house from degradation. This paper highlights the use of PCMs encapsulated with silicates that show robust resistance to alkaline environments typical of geopolymers. As a result, the PCM does not degrade even under prolonged exposure. Studies have confirmed the high multi-cycle stability of such materials [35], which significantly enhances their practical utility. Based on the findings of previous studies, it is expected that the geopolymer matrix will provide a protective function for the PCM, shielding it from external factors and, thus, improving the overall durability and cost-effectiveness of the system in engineering and construction applications. In this study, greater emphasis was placed on the impact of the manufacturing process on the functionality of the geopolymer/PCM foamed coating, rather than on material factors or variations in composition and structure. Therefore, potential methods for producing such coatings require more comprehensive discussion. For years, the technology of foaming geopolymer mixtures has been researched, developed, and described in the scientific literature. However, several technological challenges remain, including the low stability of foamed structures before curing. This often results in a lack of reproducibility and foam collapse, leading to higher material density and degraded insulation properties. Various stabilisers are used to address these issues, and foamed geopolymers are usually produced by casting into moulds, allowing for better control over the foaming process [31; 32; 33; 34; 35; 36]. However, the most significant benefits of foamed geopolymers can be achieved by applying them directly to existing structures. Several authors have described such techniques, but no simple, inexpensive, and effective method has yet been developed to apply foamed geopolymer layers with stable porous structures that can be cured directly on the object to be thermally insulated.