This paper is focused on the investigation of a land filled Hungarian fly ash (Tiszaujvaros dumpsite) as a main component of geopolymer. After determination of the fly ash properties such as particle size distribution, moisture content, real and bulk density and specific surface area, mechanical activation by fine grinding of the fly ash sample was performed in laboratory scale ball mill, vibratory mill and stirred media mill in order to improve its reactivity. Grinding kinetics was determined in each mill. The geopolymer cylindrical specimens were prepared and their uniaxial compressive strength was measured. The structure of the fly ash and the geopolymer was monitored by Fourier Transform Infrared Spectroscopy (FTIR) and X-ray diffraction (XRD); furthermore, the morphology was examined using optical microscopy and Scanning Electron Microscopy (SEM). Finally, Isothermal Conduction Calorimetry (ICC) measurements were carried out in order to determine the reactivity of the mechanically activated fly ash samples. As a result of the investigation, the relationship between the grinding process, the ground material properties and the geopolymer characteristics was established. It was found that the strength of the specimen strongly depends on the grinding conditions, i.e. type of mill, residence time. (C) 2015 Elsevier B.V. All rights reserved.