Effect of synthesis parameters on the mechanical properties of fly ash-based geopolymers

In this paper the effect of the water, sodium hydroxide and sodium silicate contents in the synthesis of fly ash-based geopolymers on their compressive strength is investigated. X-ray diffractograms of geopolymers indicated the existence of the major crystalline phases of fly ash, as well as the formation of a new amorphous phase in the geopolymeric matrices. Fourier-transform infrared (FTIR) spectroscopy analysis revealed essential fly ash phase transformations within geopolymers that affected their mechanical strength. Compressive strength of the produced geopolymers is correlated with the role that the investigated parameters play in the geopolymerization process. The compressive strength is increased with the decrease of water content, as well as with the increase of sodium silicate in the synthesis of geopolymers. Concerning the sodium hydroxide content in the geopolymers synthesis compressive strength is observed to being optimized for a sodium hydroxide concentration in the aqueous phase equal to 6.6 M. In general, a compromise among the investigated parameters might lead to geopolymeric materials with a compressive strength higher than 40 MPa.