Calcium looping technology could be one of the most efficient ways to drastically reduce the carbon footprint of cement manufacture since CaCO3 is a major component of the raw meal used to produce clinker. However, calcined raw meal can be a poor CO2 sorbent due to the fact that the formation of Ca2SiO4 (belite) causes a reduction in the amount of free CaO. Effective reaction rates for the formation of belite from both CaCO3 and CaO (i.e., after calcination) were obtained in this work for different raw meals with similar compositions but very different levels of Ca-Si aggregation. Tests carried out in thermogravimetric analyzers revealed that belite can be formed quickly, even with calcination periods of about 1 min. The 3D-diffusion model proposed by Jander and Hoffmann [Jander, W.; Hoffmann, E. Reaktionen im festen Zustande bei hoheren Temperaturen. XI. Mitteilung. Die Reaktion zwischen Calciumoxyd and Siliciumdioxyd. Z. Anorg. Allg. Chem. 1934, 218, 211-223] represents reasonably well the conversion of the solids to belite in marl-type raw meals at temperatures between 800 and 900 degrees C. The activation energy calculated in this temperature range (i.e., 325 kJ/mol) is consistent with the data reported in the literature on belite formation in CaO/SiO2 materials at higher temperatures. The differences in the reaction rates between the materials are due to the pre-exponential factors, related to the level of aggregation of Ca and Si in the materials. The information on this topic will help to predict the decrease in the CO2 sorption capacity of the calcined raw meals used in calcium looping systems integrated in cement plants.