Energy storage is necessary to increase the use of solar heat in buildings. Ettringite, a common hydrated phase found in cement-based materials, has the advantage of high energy storage density at low temperature (around 60 °C). The ettringite-based material was synthesized by hydration of calcium sulfoaluminate cement for heat storage. As durability is a key criterion for selecting a heat storage material, the aim of this paper is to assess the stability and durability of an ettringite matrix intended to be used for this purpose. Thermal stability, reversibility and carbonation tests were performed on the material produced. Its physicochemical stability over time was followed by XRD, TGA, SEM and IR. The dehydration and rehydration process in the ettringite-based material, leading to the heat charging and discharging process, was reversible during several cycles. An accelerated carbonation test with the material showed that its use as heat storage material requires protection against CO2.