Preparation and Characterization of Sintered Mullite Based Ceramics Using Alumina-Fly Ash Mixture

Abstract

The present work discusses the preparation and characterization of mullite based ceramics from fly ash-alumina mixture. Two different varieties of aluminas namely, White tubular alumina (WTA) and Technical alumina fines (TA) were used, and each variety was mixed with different weight percent of fly ash (FA) giving 3 batches in molar ratios of 1.72 (excess alumina batch), 1.5(stoichiometric batch) and 1.28 (lean alumina batch). All the batches were mixed in solid condition, pressed and fired in a single stage firing process at temperatures of 1400, 1450, 1500 and 1550oC, each having a soaking time of 2 hours. The mullitization reaction commenced with the dissolution of quartz, cristobalite and corundum followed by the appearance of mullite, sillimanite and pyrope (for MgO addition). Quartz,cristobalite and corundum were found to decrease from 1400oC onwards, while the fraction of mullite and glass increased. At 1550 oC high compositions of alumina (1.72) had corundum,sillimanite and mullite, whereas, lower compositions (1.28) had more cristobalite, sillimanite and mullite. The stoichiometric ratio (1.50) showed mainly mullite and sillimanite phases.
The diametral strength was measured using the Brazilian Disc test method. The batches that were both milled and added with MgO which showed higher densification values (~3gm/cc), also showed good strength values (>45 MPa) as compared to the other batches. Only milled batches showed more promise in terms of density and strength as compared to MgO added batch and the batch having just a mixture of the precursor powders. It was observed that finer particle size lead
to a better densified matrix, seconded by the microstructure analysis which showed reducing porosities and higher packing of the mullite crystals. Addition of MgO to and/or milling of the precursor powders changed the acicular structure of mullite to a rod shaped structure having flat faces thereby increasing the packing of the structure.