Cementitious material with a high ettringite content can be considered effective in a long-term (seasonal) thermochemical energy storage (TCES) system, resolving the issue of intermittency between production and availability of renewable energy. However, to evaluate the behavior of the storage material, an experimental study of energy storage in a thermochemical reactor containing the proposed material was required. A new and innovative large-scale energy storage prototype based on ettringite material has been developed and tested. This prototype regulates the temperature, humidity, flow rate, and pressure of the flow circulating through a 50 L CSA (calcium sulfoaluminate) sample monolith. This CSA cement formed an ettringitic phase (3CaO⋅Al2O3⋅3CaSO4 ⋅32H2O) to store thermochemical energy at about 60 ◦C. After the storage cycle (high temperature, low relative humidity), the material was cooled down to room temperature and kept there until energy was needed. The stored energy was released on demand by the hydration of cement from a cold, humid nitrogen flow. About 50 MJ/m3 was discharged from the 50 L monolith during the hydration cycle. The prototype showed the monolith’s ability to store and release energy: the first discharge phase experimental test showed an energy release of about 10 % of the theoretical value (43 MJ/m3) for a temperature increase of 6 K. Complementary investigations have to be performed to understand the impact of the monolith cracks on the energy performances