Structure, morphology and mechanism research on synthesizing xonotlite fiber from acid-extracting residues of coal fly ash and carbide slag

Xonotlite fibers have attracted much attention in super thermal insulation and flame retardant fields due to its ultra-light property. In this article, well crystalline xonotlite fibers were successfully prepared from two kinds of industry wastes, carbide slag and acid residues of fly ash respectively, by hydrothermal method. Influences of synthesis parameters, such as hydrothermal temperature, hydrothermal time and Ca:Si ratio, on both crystalline structure and micro morphology of hydrothermal products have been carefully researched. The results show that the formation of xonotlite fiber is remarkably accelerated by alumina impurities in carbide slag. Xonotlite phase could be formed within 24 h, and pure xonotlite fibers with satisfied crystalline degree are obtained after 48 h in this system. It also means that much thinner fibers could be achieved under a certain time. The resultant xonotlite fibers are in 50-300 nm diameter and 10-15 μm length. Moreover, because of relatively large particle size of carbide slag, pectolite intermediate is generated due to the temporary deficient condition of free calcium species. Furthermore, the phase type of product could be effectively controlled by altering Ca:Si ratio under suitable hydrothermal system. And xonotlite phase could be obtained under a wide range of Ca:Si ratio.