Calcium carbonate demonstrates huge potential to be a part of thermal storage applications. Its decarbonation at high temperatures makes it favourite for integration in concentrated solar power plants for their smooth operation. In this paper, silicon dioxide (SiO2) has been doped into CaCO3 at different loadings and the thermal energy density of all the samples was determined. Afterwards the effect of dopant on the heat storage process and heat storage efficiency of the developed thermochemical composite were investigated. The enthalpy of reaction and the specific heat capacity were analysed in order to predict the heat storage capacity. Cyclic tests were performed to determine the reusability of the material. The samples were tested non-isothermally at different heating rates to get the thermogravimetric data for analysis. It was found that the loading of dopant helped to increase the rate of decarbonation reaction, thereby making the heat storage process efficient as compared with pure CaCO3. It was observed that the large quantity (15% and 30%) of dopant had introduced a new phase of Ca3SiO5 during the decomposition of CaCO3. In addition, volumetric as well as gravimetric energy storage densities decreased with the increase in amount of dopant.