Porous acicular mullite ceramics produced from well and poorly crystallized kaolinite

This work studies how the crystallinity of a raw kaolinite clay affects the properties of mullite ceramics produced from it. To this purpose, two well and poorly crystallized kaolinite samples from the repository of the Clay Mineral Society were assessed. Also, other two samples were produced in which the growth of mullite acicular grains were promoted by addition of a MoO3 precursor. The role of the kaolinite crystallinity and the MoO3 precursor on the properties of the final ceramics was studied. For each sample, the microstructure and the textural, crystalline, and mechanical properties were analyzed. The poorly crystallized kaolinite is more reactive than the well crystallized one and promotes better mullitization. In addition, when the MoO3 precursor is used, the formation of grains with acicular shape inhibits the ceramic shrinkage and promotes higher porosities. On the other hand, the flexural strength is slightly lower for the mullite produced from poorly crystallized kaolinite. Nevertheless, when this property is extrapolated to the case of zero porosity it is found that in such cases the ceramic solid fraction presents higher flexural strength, indicating that the porous mullite with the acicular grain morphology is a reinforced lightweight ceramic. The obtained results show that the crystallinity of the starting clay should be considered in the ceramic fabrication process due to its effects on the properties of the final product.