Dynamic hydrothermal synthesis of xonotlite from acid-extracting residues of circulating fluidized bed fly ash

This investigation was made to examine how the conditions of hydrothermal synthesis influence the crystal structure of xonotlite and its morphology. For synthesis, we used acid residues after extracting the alumina from circulating fluidized bed fly ash as raw material, as far as we know, that no one used before. Staring with Ca/Si = 1, hydrothermal temperatures were between 180 and 260 °C. The samples were characterized in terms of mineralogical composition (XRD), morphological analysis (SEM), and thermal gravimetric and differential thermal analyses. During the 72 h synthesis at 240 °C, well formed xonotlite fibers of 10–15 μm long were obtained. Under the same conditions, but at 200 °C, the obtained xonotlite was poorly formed. The results indicated that the residue could be used to prepare pure xonotlite. Different hydrothermal temperatures and holding times have a great impact on crystallinity and morphology of xonotlite.