For natural materials so heterogeneous from the mineralogical point of view as kaolinitic clays, it is important to understand the effect that mineralogical composition may have on its pozzolanic reactivity once calcined. In this paper, the pozzolanic reactivity of the calcination products at different temperatures of five kaolinitic clays is analyzed. Two kaolinitic clays were collected from clay deposits associated to weathering crusts, while the other three are associated to hydrothermal alteration phenomena. Analysis by XRD, TGA, and XRF of the kaolinitic clays indicates a kaolinite content ranging from 10 to 72 wt. %, with each clay showing a distinctive paragenesis. Raw clays were calcined at 750 and 850 ºC for 1 h in a box furnace under open atmosphere, and then ground in a ball mill using same grinding protocol for all batches. Morphological features of the calcined clays were assessed by BET and PSD, while pozzolanic reactivity was evaluated by alkali solubility of Al and Si and by R3 test using isothermal calorimetry. An integral methodology was established for the analysis of clay deposits as source of SCM. The general trend of pozzolanic reactivity is in agreement with the content of 1:1 clay mineral, but fluctuations in pozzolanic reactivity within the thermal activation window depend on the complex interaction between specific surface and development of Al-Si reactive content.