Particle shape, crystallinity, and degree of polymerization of fly ash via combined SEM-EDS and Raman spectroscopy

Coal fly ashes, long used as SCMs, are quite complex, consisting of a mixture of crystalline and amorphous phases. In this study, we aim to shed light on this complexity by fusing two highly complementary techniques: SEM-EDS and Raman spectroscopy. By closely analyzing hundreds of individual particles from 18 unique fly ashes, we report two major findings. Firstly, there is a distinct correlation between particle shape (roundness/circularity) and degree of crystallinity (FWHM of Raman peaks) where jagged particles happen to be almost always crystalline. Secondly, the mean position of the symmetric stretching Raman band of the silicate tetrahedra (between 600 and 1000 cm−1) is an indicator of the degree of polymerization of the glassy phase in any given particle. These results highlight the importance of understanding these complex systems at the individual particle level, where multiple phases intermixed at the micro-scale ultimately play a dominant role in governing macro-scale ash behavior.