The reactivity of fly ash in geopolymer synthesis: a qualification study by FTIR

Geopolymers are a family of green cementitious materials synthesized by alkali activation of aluminosilicate precursors. The chemical composition (mainly Al and Si) and high amorphous content enables fly ash to be one of the most suitable candidate raw materials for geopolymer synthesis. However, the reactivity of fly ash varies from source to source, and even from time to time at the same power station. Therefore, analysis of the reactivity of fly ash becomes an essential work for the wide manufacturing and application of geopolymers. In this study, FTIR spectroscopy was applied to analyse the environments of Al-O and Si-O bonds in fly ash. Five fly ashes from different sources were activated with a sodium silicate activator. Compressive strength testing showed no clear relationship with the overall Si/Al ratio or the content of amorphous phases in fly ash. By comparing the FTIR spectra, it is noted that the relative intensities of the bands at around 1000, 910 and 700 cm-1are much higher in fly ash with higher reactivity. These three band locations are respectively assigned to the asymmetric stretching of (Si, Al)-O-Si, the stretching of Si-O- units and symmetric stretching of Al-O in isolated [AlO4], in accordance with the understanding of silicate glass chemistry. Deconvolution analysis of the band from 400 to 1400 cm-1shows that the cumulative area of these three resolved bands, together with the band at ~1090 cm-1, the asymmetric stretching of Si(Al)-O-Si, is proportional to the reactivity of fly ash, which strongly suggesting that these three bonds are more reactive. If it is assumed that the area of the resolved bands is proportional to the concentration of the corresponding bonds, a general indication is therefore that fly ash containing more reactive bonds will exhibit higher reactivity in geopolymer synthesis. FTIR spectroscopy provides a fast and reliable approach to predict the reactivity of fly ash, from the perspective of glass chemistry