Effect of thermal processing on the structural characteristics of fly ashes

This work investigated the course of fly ash's glassy phase crystallization in terms of structural and thermal changes. On the basis of XRD studies the olivine and albite phases are formed after the second step of the heat treatment. XRD/Rietveld analysis confirmed increased amount of mullite. The occurrence of crystallizing phases depends on the chemical composition of fly ash. It was observed that under the heat treatment of fly ashes with a small amount of modifying oxides a larger mullite superstructure appeared, what favors the albite rich plagioclases formation. The structural changes upon the heat treatment of fly ashes were studied by means of 27Al MAS NMR and FTIR spectroscopy. On the basis of NMR studies it has been confirmed that aluminum ions in devitrified glassy phase occurred only in the tetrahedral positions.