High temperature mullite dissolution in ceramic bodies derived from Al-rich sludge

Mullite-based refractory ceramic materials were produced from an industrial Al-rich sludge derived from wastewater treatment of aluminium anodising or surface coating industrial processes, and other low cost raw materials (ball clay, kaolin and diatomite). Cylindrical samples processed by uniaxial dry pressing (32 MPa) were sintered at various temperatures (1250-1650 °C) to study the mullitisation process. The performance of the materials at high temperature (1650 °C) was evaluated through different techniques (XRD, SEM/EDS, optical microscopy, and impedance spectroscopy) to access the microstructural changes occurring under prolonged tests (dwell times up to 100 h). For dwell times <80 h, a preferential dissolution of the smaller mullite grains in the glassy phase and its partial re-precipitation onto the coarser ones, leading to an overall coarsening of the mullite crystals. For dwell times >80 h, coarse α-alumina and Cr-doped alumina developed at the surface of the specimens, being accompanied by the formation of pores in the vicinity of alumina grains. Near alumina grains, additional relevant features include the increase of surface roughness, the appearance of concentration gradients within the glassy phase, which became almost depleted in Al and enriched in alkalines. The continuity of the glassy phase and its enrichment in alkaline species enhanced the electrical conductivity of the material, enabling the use of impedance spectroscopy to access the microstructural changes occurring during prolonged heat treatment.