# Lightweight Insulating Geopolymer/Phase-Change Materials Applied Using an Innovative Spray Method
## 1 Introduction
Various methods, such as manual application using rollers or brushes, centrifugal application, air spraying, and robotic systems, have been discussed in patent applications and scientific publications [37; 38; 39; 40; 33]. [PERSON] et al. studied fly-ash-based geopolymer coatings applied by brushing onto cleaned mild steel plates [41]. [PERSON] et al. described a manual spatula application method [42]. Geopolymer coatings applied by dipping or brushing result in relatively thick layers, which may limit their application in cases requiring thin coatings. [PERSON] et al. presented a centrifugal application method [43]. The centrifugal coating method allows for producing well-ordered, thin geopolymer coatings (13-20 \(\upmu\)m thick). However, in the literature, spray application is the most frequently described method for producing geopolymer coatings, particularly on vertical substrates, and is often considered to be the preferred technique. The key advantages of the spray method include low cost, attractive efficiency, the ability to control the thickness of the applied layer, and the possibility of adjusting the application pressure. The resulting coatings are uniform, exhibit good stability, and demonstrate strong adhesion to the substrate. It is essential to ensure that the coating suspension possesses adequate fluidity for practical spray application [44; 45]. Several solutions concerning equipment designed specifically for applying geopolymer materials have also been reported. In [PERSON] et al. 's work, a robot-assisted spray-coating process was implemented for geopolymers. By optimising the composition of the mixture, the robot's operating paths, and the parameters of the spraying device, the researchers demonstrated a reliable and automated method for applying coatings onto surfaces of approximately 250 mm\({}^{2}\) under laboratory conditions. The hardened geopolymer exhibited a compressive strength of 20 MPa and a bond strength with the concrete substrate of 0.5 MPa [46]. Several noteworthy solutions have also been submitted for patent protection or registered as utility models, particularly concerning devices used for the production of insulating and fire-resistant geopolymer materials. For instance, utility model CN218010702U describes equipment for spraying fire-resistant geopolymer materials onto structural surfaces. The equipment includes, among other components, a stand and an auxiliary mixing device [47]. Similarly, utility model CN217998810U describes a device for spraying fire-resistant geopolymer material, comprising a base, spraying machine, control panel, connecting pipe, spraying pipe, agitator, feed hopper, filtering screen, pendulum mechanism, and a clamping and fixing structure [48]. All solutions described thus far in the scientific literature and patent filings are not free from drawbacks and imperfections, which can lead to significant problems in their industrial application. One of the most common issues is the uneven foaming process. It often results in the collapse of the porous structure during spraying, particularly when the application parameters and mixture composition are not correctly selected. Another problem is the formation of so-called \"sinks\", i.e., local depressions in areas of the material lacking a porous structure. This is most often caused by excessive liquidity of the mixture or inadequate curing control. Another undesirable phenomenon is \"sagging\" of the foam, caused by the rapid escape of gas bubbles before the material sets, leading to distortion of the foam structure and a decrease in insulation properties. Controlling the thickness of the applied layer also remains a significant challenge, especially in manual applications, where uniform material distribution is difficult to achieve without advanced equipment and operator experience. A different concern is the need for elevated temperatures during the curing of the geopolymer coating. During prefabrication, geopolymeric elements can be heat-cured in furnaces.