It is well known that kaolinite dehydroxylate between 600 and 850 °C, and the maximum reactivity is achieved for calcination temperatures of about 800 °C. From an industrial perspective, controlling undercalcination is an easy task, as XRD or TGA could be used to assess the presence of uncalcined kaolinite. However, the problem of overcalcination is more complicated, as the products remain (pseudo)amorphous up to recrystallization of mullite, which occurs above 1200 °C. However, between 850 and 1200 °C, a significant decrease in reactivity is observed. This study explored the transformations that occur in this range, leading to the decrease in reactivity of the calcined clay. A fundamental approach was used to characterize the surface, porosity and reactivity of the material. In addition, simple experimental procedures were tested to assess their suitability to distinguish properly calcined from overcalcined clay, which is critical for design and implementation of reliable quality control protocols for the industry.