Fly ash, mechanically activated up to 120 min, has been used to synthesize geopolymers at ambient temperature. Fourier transform infrared spectroscopy (FTIR), quantitative X-ray powder diffractometry (XRD) and transmission electron microscopy (TEM-EDS) have been used for the structural characterization. The decrease in characteristic particle diameter up to 60 min and then increase is related to particle breakage and aggregation/agglomeration respectively. The manifestation of mechanical activation (MA) during geopolymerisation resulted in enhanced dissolution and precipitation of Si and Al. The appearance of a new FTIR band in 1080-1096 cm (1) region in MA geopolymers is attributed to SiQ(n) (n = 3-4) structural units of quartz. The increment in amorphization in MA fly ash resulted in an increase in crystallinity of geopolymers. The shift in characteristic XRD amorphous hump towards lower d(angstrom) value in geopolymers is associated with incorporation of OH and Na resulting in structural changes in amorphous phase. TEM-EDS revealed that geopolymers from as received fly ash exhibited predominance of primary structures such as nano-crystalline mullite, microcrystalline quartz and alumino-silicate glass. Whereas geopolymers from MA fly ash has shown secondary features mainly N-A-S-H gel, whose nano-porosity increased due to changes in gel characteristic. Finally, a conceptual mechanism of reaction is presented. (c) 2016 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.