Ettringite is a mineral compound and one of the products of cement hydration. The Ettringite/Meta-ettringite couple is also an interesting candidate for thermochemical heat storage. A thermochemical storage solution has an interest in a heating system for network application if its storage capacity it is higher than that of sensitive water, within the same temperature range. Given that water has a storage capacity of 4.18 kJ/kg.K, it can be deduced that the thermochemical material must be capable of storing more than 50 kWh/m3 for a ΔT of 40 °C. These storage materials link the chemistry structure to a thermal storage performance. Several studies have found cementitious materials with a maximum ettringite content of 72 wt%. In this study, new cement mixes formulations were tested that could theoretically contain between 80 wt% and 100 wt% ettringite. The objective of this study is to quantify the ettringite content in cement pastes through thermogravimetric analysis (TGA) and to evaluate its correlation with the theoretically calculated value. Additionally, other thermochemical parameters were conducted to assess the potential of cement pastes as a thermochemical storage material. The newly formulated solutions have been demonstrated to yield a matrix with 93 wt% of ettringite. However, thermochemical cycling analyses, which are employed to study the heat storage and release capacity, do not show a significantly higher result than the reference material “C80P20”. The correlation between ettringite chemistry and its capacity as a heat storage agent is not contingent upon the quantity of this agent within the cement matrix.