Control of geopolymer properties by grinding of land filled fly ash

• The finest fly ash could be produced using the stirred media mill with the lowest energy demand.
• Grinding caused considerable change in the fly ash morphology and fineness.
• The compressive strength of the geopolymer can be reasonably controlled by grinding of fly ash.
• The increasing of grinding time resulted in higher ICC exotherm peak maxima.

This paper is focused on the investigation of a land filled Hungarian fly ash (Tiszaújváros 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.