Phase evolution and sintering characteristics of porous mullite ceramics produced from the flyash-Al(OH)3 coating powders

Mullite precursors, waste flyash coated with aluminum hydroxide, were used to prepare low cost porous mullite-based ceramics by reaction sintering. The samples with different alumina contents (0-41.20 wt.%) were sintered at several temperatures from 1000 to 1500 degrees C. Phase evolution, sintering characteristics and microstructures were investigated in terms of alumina coating content and heat treating temperature. The X-ray diffraction (XRD) results showed, for the 33.30 wt.% alumina coated samples, the mullitization began to occur at 1250 degrees C via the reaction of alpha-alumina coating and silica (cristobalite and silica-rich glassy phase) in flyash, and completed at around 1400 degrees C. With increasing alumina content, both the crystallinity of mullite phase and aspect ratio of mullite crystals were decreased. Our results also demonstrated that the introduction of aluminum hydroxide coating had a positive effect on improving open porosity by inhibiting sintering shrinkage. Compared with flyash, the aluminum hydroxide coated samples showed a more wide sintering temperature range and well-controlled open porosity. (c) 2007 Elsevier B.V. All rights reserved.