abstract
Knowledge about the relationships between mineralogical and microstructural characteristics of clays and the transformations that occur during firing is of fundamental importance for various industries, enabling the manufacture of better quality products and widespread use of those raw materials. The aim of this study was to interpret phase development and morphological transformations in clays subjected to different heat treatments, with emphasis on the effect of impurities (iron and alkalis), temperature and heating rate. The experiments were carried out using conventional and microwave heating, with characterization by X-ray diffraction and scanning electron microscopy. The interpretation of the results based on the mullite-silica-leucite phase diagram showed that crystallization and grain growth are closely related to the amount and viscosity of the liquid phase present at the firing temperature, which vary with the composition and can be predicted by the phase diagram. Faster heating rates tend to slightly delay the establishment of equilibrium, which appears to have special bearing on the liquid phase viscosity, hence, on grain growth. As a consequence, the resulting microstructure contains mullite needles that are shorter and more uniform in size. (C) 2016 Elsevier B.V. All rights reserved.
keywords
MULLITE FORMATION; THERMAL-TREATMENT; RAW-MATERIAL; PORCELAIN; CERAMICS; KAOLINITE; MONTMORILLONITE; BENTONITES; MICROWAVE; EVOLUTION
subject category
Chemistry; Materials Science; Mineralogy
authors
Santana, LNL; Gomes, J; Menezes, RR; Neves, GA; Lira, HL; Segadaes, AM
our authors
Ana Maria B C Segadães
Retired Professoracknowledgements
The authors are thankful for the financial support received from CNPq (309646/2013-8) and CAPES (post-doctoral scholarship BEX 9532/11-0), Brazil. Thanks are also due to Tiago Santos (PhD student in Physics at the University of Aveiro) for carrying out the microwave heat treatment, and to Bruno Almeida and Maria Joao Bastos (technicians at the Department of Materials and Ceramics Engineering at the University of Aveiro) for the microstructural and mineralogical characterizations.