Microstructural Evolution on Firing Brazilian Clay-Based Redware

Extensive red clay deposits in Brazil have been used for many decades as raw materials for the production of low-quality structural bricks and roof tiles. This paper describes efforts to use these clays for improved quality redware such as porcelain stoneware floor tiles. Batches containing Brazilian clay, nepheline syenite and quartz with two different mean particle sizes (~2mm and ~18mm) were prepared. Dust-pressed pellets were fired at 1050, 1075, 1100, 1125, 1150 and 1175DGC, and linear shrinkage, water absorption and bulk density determined. Overfiring occurred above 1075DGC for compacts from both batches. The various quartz-containing batches had non-uniform characteristic properties, leading to different open porosities at high firing temperatures. Microstructural analysis was performed using SEM and TEM/EDS, which showed the presence of haematite, mullite and quartz embedded in a glassy matrix.