The influence of thermal activation temperature in the pozzolanic reactivity of low grade kaolinitic clay and the impact of calcination products in OPC hydration are assessed in this paper. The raw material, with approximately 40% kaolinite and 40% of 2:1 clay minerals, was calcined to temperatures ranging between 500 and 1000 degrees C. The clay and its calcined products were characterized by XRD, XRF, FTIR, TGA-DTA, Al-27 NMR, PSD and BET. The pozzolanic reactivity of the calcined products and its impact on hydration of blended systems was assessed by compressive strength in mortars with 30% substitution of OPC and heat output from lime-pozzolan pastes. The influence of calcined clays on OPC hydration was characterized by XRD, Al-27 NMR and Si-29 NMR, and Isothermal Calorimetry. The increase of pozzolanic reactivity with the increase of activation temperature is limited not only by recrystallization above 900 degrees C, but also by the decrease of the specific surface that occurs between 800 and 900 degrees C. Highest pozzolanic reactivity is reached when calcined at 800 degrees C, a temperature representing the best compromise between structural disorder of the clay fraction and its specific surface. The impact of calcined clays on the hydration of blended systems is a combination of a filler effect at early ages and the pozzolanic reaction. Calcined clays favor the formation of AFm-type phases and C-A-S-H with longer chains, in which Al is substituting for Si. The experimental results indicate that clays with moderate contents of kaolinite constitute a potential source of high reactivity pozzolanic materials. (C) 2015 Elsevier B.V. All rights reserved.