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

Abstract

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.