## 1 Introduction  
The great changes that are affecting our planet have undeniably led to a significant and radical transformation of the energy system, which needs to be remodeled on the basis of better efficiency, especially in regards to the industrial sector. From this point of view, improving energy efficiency means principally acting on the recovery of low-grade (100-200 \({}^{\circ}\)C) and medium-grade (200-300 \({}^{\circ}\)C) waste heat, which represent about 90% of the thermal energy lost in industrial processes [1]. To this purpose, in the last twenty years, the awareness of an urgent upgrade of thermal energy storage systems has led to the development of chemical heat pumps (CHPs), devices whose operation is based on a thermochemical storage material (TCM) capable of accumulating and releasing energy through a reversible reaction, schematized in Equation (1). \[\text{A}+\text{heat}\rightleftarrows\text{B}+\text{C} \tag{1}\]  
where the direct process (endothermic) represents the thermal charge, in which the waste heat allows the reaction to take place and the energy is therefore recovered. On the contrary,the reverse (exothermic) process represents the thermal discharge, in which the products (B and C), when mixed together, regenerate the initial compound (A), releasing the previously stored energy. Thermochemical storage promises to offer numerous advantages over latent heat and sensible heat technologies, despite these technologies currently being in an advanced stage of development [2; 3]. Firstly, the conversion of thermal energy into chemical energy ensures a long-term durability; in addition, the typical high-storage densities allow for significantly reduced volumes, making the storage systems easier to handle [4; 5]. Although it is relatively easier with sorption materials (which involve intermolecular bonding) to set up a system that operates cyclically with high performances, thermochemical storage materials (TCMs), which operate through proper chemical reactions, present many issues related to several factors, like chemical and structural stability of each reagent and product [6; 7; 8]. Within this research field, materials based on solid-gas reactions are the most investigated, ascribed to the possibility of collecting, in an isolated tank, the formed gas when the thermal charge ends, subsequently keeping the products separated until the thermal discharge is needed [9; 10; 11; 12]. In recent years, many charge/discharge working pairs operating by reduction/oxidation [13; 14; 15; 16] (oxides, salts), decarbonation/carbonation [17] (carbonates) and dehydration/hydration (hydroxides) [18] have been assessed. However, among these materials, it could be noted that the range between 200-250 \({}^{\circ}\)C has the greatest lack of developments; in fact, there is no material operating in this range that can be competitive, in relation to the storage density, with pairs operating at higher temperature and which are currently in an advanced stage of application (magnesium hydroxide, calcium hydroxide [19; 20; 21; 22; 23; 24]). As mentioned above, chemical and structural stability play a fundamental role, and it is not always easy to find systems capable of providing a good compromise between a high storage capacity and a long-term durability. Hence, the development of new TCMs operating in the 200-250 \({}^{\circ}\)C temperature range is still an open challenge. The purpose is to identify new materials that possibly meet the Thermal Energy Storage (TES) technological expectations in terms of performances, stability and economic industrial applicability [25]. Indeed, the heat storage at medium temperature represents a crucial technology that still requires an improvement of knowledge. In this context, the aim of this work is to investigate the calcium aluminate compounds as potentially effective and practical alternatives to conventional TES materials.