# Lightweight Insulating Geopolymer/Phase-Change Materials Applied Using an Innovative Spray Method
## 1 Introduction  
Geopolymers are inorganic polymers based on aluminosilicates and alkaline activators derived from these aluminosilicates. They are also referred to as polysililicates, polysilans, and alkaline-activated aluminosilicate binders (AAAS) [1, 2, 3, 4]. Geopolymers result from the well-known and widely described mineral polycondensation reaction known as geosynthesis [5, 6, 7]. They consist of copolymeric chains of silicon and aluminium oxides, stabilised by metal cations such as sodium, potassium, lithium, or calcium, along with bound water. The most commonly used activators are sodium hydroxide and aqueous sodium silicate solution (potassium water glass), due to their availability and cost-effectiveness. The properties of geopolymers are primarily determined by the precursor composition and keystructural parameters, including the Si/Al ratio, reaction temperature, and humidity [8; 9]. Consequently, differences in precursor composition can lead to differences in mechanical properties, even with similar mix designs. Recent studies demonstrate that employing machine learning can be beneficial in optimising mix designs and identifying influential factors affecting the performance of geopolymers, as shown by [PERSON] et al. [10]. Their study utilised machine learning models to predict compressive and flexural strength based on parameters such as the Si/Al ratio and water-to-binder ratio. This study highlighted the significant predictive potential of these models, indicating that they can effectively assist in the geopolymer design process for solid materials, and likely for foamed materials as well. The workability and setting time of geopolymers can be controlled using various additives and curing conditions. Geopolymers cure within a few hours at ambient temperature, while curing at higher temperatures accelerates their production and enhances their properties [11; 12; 13]. Geopolymers have garnered significant interest due to their unique properties and the ongoing pursuit of developing sustainable construction materials. Rising energy costs and the high environmental impact of Portland-cement-based concretes are leading to an intensified search for alternatives, with geopolymers being one of them [14; 15; 16; 17; 18]. Appropriately designed geopolymers are considered to be lower-emission materials than concrete and exhibit higher compressive strength and chemical resistance, as well as exceptional fire and temperature resistance (up to 1000 \({}^{\circ}\)C). Other notable properties include low shrinkage during curing, faster setting, high wear resistance, enhanced acid resistance, and good adhesion to different materials [19; 20; 21]. These characteristics make geopolymers promising materials for the future of the construction industry. Special attention is given to foamed geopolymers due to their fine thermal insulation properties and fire resistance at temperatures exceeding 1000 \({}^{\circ}\)C [22; 23; 24; 25; 26]. Traditional insulation materials include expanded or extruded polystyrene (EPS and XPS, respectively), mineral wool, and glass wool. In recent years, the popularity of polyurethane (PUR) and polyisocyanurate (PIR) foams has increased, particularly those applied by spraying onto various surfaces. Despite their effectiveness as thermal insulators, all of these materials have numerous drawbacks, including flammability, water absorption, low mechanical strength, and environmental or human toxicity [27; 28; 29; 30]. Undoubtedly, further efforts are needed to identify alternative materials to those currently in use. In this context, a promising approach involves geopolymers modified with phase-change materials (PCMs). In our previous studies, we explained the mechanism of synergistic heat management in such systems [31; 32; 33; 34]. The PCM, in the form of silicate microcapsules, can be uniformly distributed within the porous geopolymer matrix, forming an integrated baffle that is capable of effectively regulating heat flow.