## 1 Introduction  
Global energy consumption in 2021 was 595 exajoules according to statistical data provided by British Petroleum [1]. Of this energy consumed, 82% came from coal, natural gas, and oil, which equates to 36.3 gigatons of carbon dioxide released in 2021 alone [1, 2]. Climate change caused by the release of greenhouse gases, as well as the high cost of fossil fuels, have made renewable energies and technologies an attractive option [1, 3, 4]. A major drawback of renewable energy sources is the inconsistent supply compared to the grid demand. Energy storage systems coupled with renewable energy would greatly lower the cost of energy as well as reduce the amount of CO\({}_{2}\) released. There are several ways to store energy such as electrochemically, in the form of batteries, mechanically, in the form of pumped hydro, or thermally, in the form of hot or cold masses. Commonly deployed batteries (e.g., lithium-ion) require rare earth elements that are expensive and in limited supply [5], while hydroelectric batteries require natural or artificial height differences to take advantage of gravity, which limits locations where they can be placed [6, 7]. Alternatively, thermal energy storage systems can be made from abundant materials and placed in a variety of locations [8]. In general, thermal energy storage (TES) can be classified into three broad categories: sensible, latent, and thermochemical. It can be further characterized based on temperature, with applications ranging from sub-zero (e.g., cool rooms), to low- to medium-temperature (e.g., HVAC), or high-temperature (e.g., concentrated solar power). Of the stated categories, sensible is the most often deployed owing to its relative simplicity [9]. In sensible TES, the energy stored relies on the total mass of the storage material, the specific heat of the storage material, and the temperature difference of the storage material. A major challenge of these systems is the footprint required to house the storage material, which in many low-temperature cases is water. To reduce the footprint of thermal energy storage systems, a higher temperature difference, and therefore a higher temperature material, can be used. At higher temperatures, molten salts such as nitrates, chlorides, or carbonates can be used owing to their favorable melting points (for latent heat applications) and low cost [10; 11]. Of particular interest is the use of chloride materials as they are very abundant and low-cost [12]. For example, chlorine and sodium are the third and fourth, respectively, most abundant elements in Earth's oceans behind hydrogen and oxygen which makes sodium chloride salt abundant and relatively easy to process [13]. Sodium chloride, NaCl, is solid at ambient conditions and has a relatively high thermal conductivity, specific heat, and latent heat of fusion upon melting at 801 \({}^{\circ}\)C. These properties are ideal for storing thermal energy [14]. The major drawbacks of using NaCl and other chlorides for thermal energy storage are corrosivity, chemical degradation, and operating temperatures. Molten salts have also been used to quench alloys, extract minerals from ore, and are found in potential nuclear reactors [15; 16; 17], making their containment an important research topic. Container material for molten salts can be metal alloys, ceramics, or refractory composites. Common metal alloys such as stainless steel 304 and 316 have shown substantial levels of corrosion and require high-purity inert atmospheres [18; 19; 20]. Suitable alloys are not entirely corrosion-proof, use rare metals, and are expensive [21]. Ceramics require high amounts of time and energy by comparison [22]. Refractory bricks are affordable, and chemically and thermally stable but require joins and mortars which can be a major point of failure [23]. An alternative to these proposed solutions is geopolymer composites. Geopolymers (GP) are a class of X-ray amorphous, nanoporous, nanoparticulate, inorganic covalently bonded material typically comprised of Group 1 (G1) aluminiosilicates [24].