## 1 Introduction
GPs are similar in processing to cements yet similar to glass in microstructure. Synthesis of GPs can be done at ambient temperatures and pressures by the addition and high shearing of aluminosilicate precursor, such as calcined kaolin (metakaolin), with an alkali activator solution. Acidic synthesis is an alternative method of producing similar phosphoric GPs, however, this route is not only relatively new compared to the alkali route but also utilizes phosphorous, which is much less common than elements used in the alkali synthesis route [25; 26]. Prospect aluminosilicate precursors include various industrial waste ashes from metal processing, coal-fired power plants, and incinerators [25]. However, these tend to have high amounts of calcium, which is detrimental to geopolymer formation and maximum operating temperatures/environments [27]. This is due to the formation of calcium silicate hydrate phases, which are less thermally stable than covalently bonded aluminosilicates with minimal hydrates [27; 28; 29]. The repeatability of industrial waste ash composition is another concern, requiring attention to the individual batch formulation. Common alkali activator solutions are aqueous G1 hydroxides such as sodium and potassium and amorphous fumed silica to produce a solution of G1 metasilicate, which is also needed to fabricate geopolymers. Other alkali activating sources include G1 metasilicate, which is formed from the fusion of G1 carbonates with silica sand at high temperatures. Geopolymers made from G1 hydroxides have higher degrees of polymerization and therefore higher mechanical properties [30]. Metakaolin-based geopolymer composites can be made into complex shapes, easily incorporate functional phases, are thermally and chemically stable, and are relatively inexpensive. Additionally, ceramics can be produced by heating geopolymers to 950 \({}^{\circ}\)C [31]. For these reasons, metakaolin-based geopolymers could be a potentially suitable containment material for molten chlorides [32; 33]. Two obstacles that need to be overcome are microcracking and significant shrinkage/warping of the geopolymer matrix after heating to 800 \({}^{\circ}\)C [34; 35]. One method to prevent significant shrinkage during initial heating is the addition of alumina platelets [36]. The homogenous dispersion of glass-forming material in the composite has also been shown to be a successful strategy for filling microcracks [37; 38]. The filling of microcracks by a uniformly distributed phase in the composite, upon heating, makes the composite self-healing [31]. Previous work on this conducted by [PERSON] et al. (2022) showed that heat-treated, potassium metakaolin geopolymer containing homogeneously mixed alumina platelets and System 96 \(\copyright\) glass particulates of size 10-250 \(\upmu\)m did not chemically interact among themselves and produced a self-healing composite with uniform glaze [31]. X-ray diffraction analysis of bulk powder samples showed no new crystal formation among the phases after heating to 900 \({}^{\circ}\)C.  
Therefore, the purpose of the current study is to investigate the compatibility and performance of a self-healing geopolymer composite exposed to either molten sodium chloride or potassium chloride to determine if any chemical degradation occurs and if this material can be used to contain molten salt at high temperatures. Potassium chloride was selected because of its similarity to sodium chloride, as well as the presence of potassium in the geopolymer composite. The aim of this research is to successfully contain molten chlorides for extended durations. If successful, the containment of molten salts by geopolymers can be used as an inexpensive solution to energy storage and fossil fuel use reduction [11].