Processing of Mullite based Ceramics Using Bauxite-Fly ash Mixture

Abstract

The present work discusses the preparation and characterization of mullite based ceramics from bauxite-fly ash mixture. Two different varieties of bauxites viz. Bauxite 1 (B1) and Bauxite 2 (B2) were used, and each bauxite variety was mixed with different weight percent (30, 40, 50 and 70) of same fly ash (FA). The B1-FA compositions were processed through two stage firing, and the B2-FA mixture was processed through single stage firing schedule. Both the B1- FA and B2 – FA compositions were sintered at 1400, 1450 and 1500oC.
The mullitization reaction in the bauxite-fly ash commenced with the dissolution quartz and corundum followed by the appearance of mullite and corundum. The fraction of quartz and alumina decreased from 900oC onwards while that of mullite and glass increased. The mullite percentage in the sintered bauxite-fly ash samples was dependent on the bauxite type and the higher percentage of mullite in B1 –FA compositions could be attributed to the finer crystallite size of B1. No difference in the mullite percentage was observed between single stage and two stage firing schedule. High bauxite compositions had corundum and mullite, whereas, high fly ash compositions had cristobalite and mullite. The microstructure of the sintered B1-FA samples showed dense crystalline microstructure for 30% FA samples and mullite grains and more glassy phase for 70% FA sample. The B2 microstructure showed the presence of glassy phase and porous microstructure both for 30% and 70% FA. Equiaxed primary mullite was present in 30% FA containing samples.
The bending strength and ISB fracture toughness was measured using Ring on Ring test method. A linear relationship was observed between the fracture strength and fracture toughness, particularly, for the compositions sintered at the highest temperature. In the B1-FA compositions, samples with predominantly mullite phase had low strength and toughness while the samples with a combination mullite and another phase had high strength, hardness, and toughness. In the B2-FA compositions, the strength and toughness were high in 40 and 50 % FA containing compositions. It was also observed that the samples that exhibited high strength and toughness had two different type of elongated mullite grains. Some of the selected samples were also subjected to thermal shock from 1250oC for 5 cycles. The samples did not show any visible crack but showed a change in the mullite and glass content after cycling.