Growth of metastable phases during brick firing: mineralogical and microtextural changes induced by the composition of the raw material and the presence of additives

Research Project MAT2016-75889-R Junta de Andalucía Research Group RNM179 Departamento de Mineralogía y Petrología (Universidad de Granada) Departamento de Didáctica de las Ciencias Experimentales (Universidad de Granada) Instituto Andaluz de Ciencias de la Tierra (CSIC-UGR) Grupo de Investigación HUM613 (Didáctica de las Ciencias Experimentales y de la Sostenibilidad)We thank Cerámica Castillo Siles for providing the raw material used to prepare and fire the bricks and Cervezas Alhambra S.L. (Granada) for providing the calcined diatomite sludge with yeast. We are grateful to Nigel Walkington for his assistance in translating the original text.Mineralogical and textural changes experienced by solid bricks made without additives and others made with added halite or with added calcined diatomite sludge, and fired in an electric oven at 800, 950 and 1100°C have been studied. Different analytical techniques (x-ray fluorescence, thermogravimetric analysis, x-ray diffraction, polarized optical microscopy and scanning electron microscopy) were used to reconstruct the changes that took place inside the bricks from a mineralogical and textural point of view. These changes are similar to those that take place in nature during pyrometamorphism. Carbonates decompose and react with silicates to form gehlenite, diopside and wollastonite. Plagioclase enriches in calcium and quartz concentration diminishes. Clay minerals favour the melting of matrix and the appearance of mullite. K-Feldspar changes from microcline to sanidine. The extent of vitrification increases in line with the increase in the firing temperature. When halite is added, new silicates appear earlier at lower firing temperatures and molysite is formed. Calcined diatomite sludge contains cristobalite, which becomes part of the mineralogy of the bricks. The newly-formed phases contain certain chemical elements that are not normally found in their standard chemical composition.This study was funded by Junta de Andalucía Research Group RNM179 and by the Research Project MAT2016-75889-R