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 (0–41.20wt.%) were sintered at several temperatures from 1000 to 1500°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 33.30wt.% alumina coated samples, the mullitization began to occur at 1250°C via the reaction of α-alumina coating and silica (cristobalite and silica-rich glassy phase) in flyash, and completed at around 1400°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.