## 1 Introduction
Pure (Na\({}_{2}\)O/K\({}_{2}\)O)-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) ternary systems are known to present excellent thermal resistance due to the crystallization of feldspar and feldsparoid phases, i.e., framework alumino-silicates, featuring various proportions of both SiO\({}_{4}\) and AlO\({}_{4}\) units, the latter being stabilized by alkali ions. These phases comprise albite (Na feldspar, NaAlSi\({}_{3}\)O\({}_{8}\) or Na\({}_{2}\)O\(\cdot\)Al\({}_{2}\)O\({}_{3}\cdot\)6 SiO\({}_{2}\), having a liquidus temperature, 1118 \({}^{\circ}\)C), nepeline (Na-K feldsparoid, Na\({}_{3}\)KAl\({}_{4}\)Si\({}_{4}\)O\({}_{16}\), stable up to 1256 \({}^{\circ}\)C), and carnegieite (Na feldspatiohoid, NaAlSiO\({}_{4}\) or Na\({}_{2}\)O\(\cdot\)Al\({}_{2}\)O\({}_{3}\cdot\)2 SiO\({}_{2}\), having a liquidus temperature, 1526 \({}^{\circ}\)C) in the case of sodium-based activators. In the case of potassium-based activators, the developed alumino-silicate compounds exhibit even higher melting points, as in the case of orthoclase (K feldspar, KAlSi\({}_{3}\)O\({}_{8}\) or K\({}_{2}\)O\(\cdot\)Al\({}_{2}\)O\({}_{3}\cdot\)6 SiO\({}_{2}\), having a liquidus temperature of 1250 \({}^{\circ}\)C), leucite (K feldspathoid, KAlSi\({}_{2}\)O\({}_{6}\) or Na\({}_{2}\)O\(\cdot\)Al\({}_{2}\)O\({}_{3}\cdot\)4 SiO\({}_{2}\), with a congruent melting point of 1693 \({}^{\circ}\)C), and kalsilite (K feldspathoid, KAlSiO\({}_{4}\) or K\({}_{2}\)O\(\cdot\)Al\({}_{2}\)O\({}_{3}\cdot\)2 SiO\({}_{2}\), having a liquidus temperature, 1750 \({}^{\circ}\)C) [10]. The refractory performance of potassium-based ternary systems further benefits from the higher viscosity of potassium melts. This reduces deformation due to flow and reinforces the advantageous character of potassium-based AAMs for high-temperature applications. Nevertheless, it should be mentioned that Na\({}_{2}\)O-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\) systems can also present excellent resistance to high temperature if a specific set of synthesis conditions that promote carnegieite formation (melting point, 1526 \({}^{\circ}\)C) are provided. On the other hand, K\({}_{2}\)O-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\) systems present a broader set of synthesis conditions where refractory phases are formed upon sintering [10], and this is particularly relevant in industrial contexts where versatile and resilient mixture designs are of utmost importance. From this point of view, the use of a potassium-based activating solution is advantageous both in terms of thermal performance and industrial processing. The chemical composition of the slag produced in thermochemical conversion processes is characterized by the presence of significant calcium and iron content, which, however, have a direct influence on the phase composition and morphology of the material after heating. Low calcium systems (i.e., geopolymers) are generally preferable for producing high-temperature resistance AAMs as calcium reduces the liquidus temperature of K\({}_{2}\)O-SiO\({}_{2}\)-Al\({}_{2}\)O\({}_{3}\) systems. Furthermore, the near absence of hydrated and chemically bonded water in those low-calcium structures [11; 12] reduces the deleterious microstructural effects of dehydration, which results in more thermally stable AAMs. The presence of iron can, however, be deleterious to K\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) systems, depending on the amount of alumina present in the systems [10]. If insufficient alumina is available, the iron compounds formed are characterized by much lower melting temperatures relative to their iron-free analogs. Hence, the thermal performance of AAMs made from FeO\({}_{\text{x}}\)-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) precursors with low-aluminum content is expected to be significantly impaired relative to their iron-free equivalents.