Effect of firing atmosphere and soaking time on heavy clay ceramics with addition of Bayer's process bauxite residue

Abstract

Ceramics were developed using Bayer's process bauxite residue and a clay mixture in a weight ratio 1 : 1. Firing was performed in a propane firing kiln at 1000°C for 1 or 4 h soaking. For oxidising atmosphere, water absorption varies between 18⋅9 and 19⋅5% and bending strength between 25⋅9 and 27⋅9 MPa. The crystalline phases identified are haematite Fe₂O₃, quartz SiO₂, plagioclase [anorthite (Ca,Na)(Al,Si)₂Si₂O₈], clinopyroxene [augite Ca(Mg,Fe⁺³,Al)(Si,Al)₂O₆] and corundum Al₂O₃. The microstructure is comprised of Si rich vitrified areas that contain quartz and have closed, round pores and an assemblage of particles creating a mass with open and interlinked pores and no signs of glassy phase. For reducing atmosphere and 1 or 4 h soaking, water absorption varies between 18⋅2 and 18⋅7% and bending strength is ∼29⋅5 MPa. The crystalline phases identified are qualitatively the same as for the oxidising atmosphere. The microstructure is comprised of two concentric zones due to the reduced gas permeability in the core of the body. The outer shell is highly vitrified with closed and rounded pores. The microstructure in the core of the body resembles that of samples fired in oxidising atmosphere; however, the average pore size is higher and the vitrified regions are more extensively formed. The build-up of compressive stresses in the outer shell may also account for the increase in the bending strength; however, cracks were developed in few occasions. In all cases, quartz participates in the development of liquid phase and dissolves in the melt. Increase in the soaking time results in a decrease in the quartz content in the body and an increase in the vitreous phase.