This paper investigates the decomposition of three clayey structures (kaolinite, illite and montmorillonite) when thermally treated at 600 degrees C and 800 degrees C and the effect of this treatment on their pozzolanic activity in cementitious materials. Raw and calcined clay minerals were characterized by the XRF, XRD, Al-27 NMR, DTG and BET techniques. Cement pastes and mortars were produced with a 30% substitution by calcined clay minerals. The pozzolanic activity and the degree of hydration of the clinker component were monitored on pastes using DTG and BSE-IA, respectively. Compressive strength and sorptivity properties were assessed on standard mortars. It was shown that kaolinite, due to the amount and location of OH groups in its structure, has a different decomposition process than illite or montmorillonite, which results in an important loss of crystallinity. This explains its enhanced pozzolanic activity compared to other calcined clay cement blends. (C) 2010 Published by Elsevier Ltd.