The pozzolanic reactivity of thermally treated zeolites was studied on the basis of the Chapelle test combined with X-ray diffraction (XRD) and Fourier Transform (FTIR) spectroscopy, as well as thermogravimetric analysis (TG/DTG) and differential thermal analysis (DTA). The raw zeolite samples are from the Pentalofos area, Thrace, NE Greece. Their main mineral constituent is 'heulandite type-II', an intermediate type of the heulandite-clinoptilolite isomorphous series. Calcination of the samples was carried out up to 400, 500, 600, 700 and 1000C for 15 h. The changes were recorded using the above methods. The deformation of the zeolite crystal lattice starts at about 400C and proceeds as the temperature of thermal treatment rises. The thermal treatment of zeolite at 400C improves its pozzolanic reactivity and accelerates the reaction with Ca(OH)2.