Phase Transformation, Technological Properties and Microstructure of Fired Products Based on Clay-Dolomite Mixtures

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The present work reports the effect of carbonate addition to iron rich kaolinite on the phase transformation during firing. Mineralogical, thermal (including DTA-TGA and dilatometric) and physico-mechanical analyses were performed on fired product made from a mixture of kaolinite and 8% by mass of dolomite. The firing temperatures used are 920°C, 1050°C, 1150°C, 1200°C and 1250°C. The major mineral phases formed during firing are mullite, cristobalite and hematite associated to gehlenite and anorthite at temperature >1150°C. Their presence induces increased densification of the product that results in an increased in the flexural strength up to 1150°C. Beyond this temperature, increase glassy phase formation together with hematite development, induce a decrease of the flexural strength as well as the porosity and water adsorption. The SEM observations show that primary mullite is formed as from 1150°C. The EDS analysis from SEM is coherent with the chemical and the mineralogical analyses from XRD. The overall analyses indicate that the addition of dolomite at 8% by mass is favorable to vitreous ceramic formation as from 1050°C.

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