Zeolites generally show pozzolanic activity due to their structural characteristics. The utilisation of pozzolans as additions to cements results in added technical advantages of the construction materials. In this study, the pozzolanic activity of a thermally treated natural mordenite-type zeolite from the Palmarito open-air deposit (Cuba) was evaluated with respect to a non-treated zeolite. Initially, a thermal treatment of the zeolite was performed at different temperatures within the range 300-1000 A degrees C for 5 h in order to evaluate the better temperature of treatment. Afterwards, the pozzolanic activity was determined for each temperature after 7 days of reaction with a saturated Ca(OH)(2) solution that simulates the release of lime from ordinary Portland cement during the hydration reaction. The higher pozzolanic activity was achieved with the thermal treatment performed at 300 A degrees C. Therefore, a further study studied the evolution of the mineralogical phases produced during pozzolanic reaction up to 90 days, carried out with the zeolite treated at that temperature. Consumption of Ca2+ in solution and formation of C-S-H-like phases with low Ca/Si ratio were experimentally observed as the main pozzolanic products. The thermodynamic study confirms high reactivity of the zeolite at short-term and chemical stability of the reaction products after 28 days. The zeolite thermally treated at 300 A degrees C confirmed an increase in the pozzolanic activity with respect to the non-treated zeolite.