Characterization and traditional ceramic application of clays from the Douiret region in South Tunisia

• The mineralogical, chemical and geotechnical characterization of the Tunisian clays
• The firing behavior of the Tunisian clays
• The ceramic application of the Tunisian clays

This study of the Aptian clays of Douiret (south-east of Tunisia) and their use in ceramic industry is original. At first, mineralogical, chemical, physical and thermal analyses of these clays are given. In terms of mineralogy, they can be considered as illitic clays (50–67%). However, other clay minerals, such as kaolinite, I/Sm mixed-layer, quartz, feldspar, dolomite and hematite, are present in small quantities. Next, this study reveals that the average amounts of silica and potassium are 53.9% and 3.3% respectively. The amount of alumina and iron oxide, with an average of 16.8% and 6.3% respectively, is variable. The plasticity test shows a high value (PI = 39–56%). The mineralogical changes during the firing process were recorded via the X-ray diffraction of the raw clays and subsequent firing at 300, 700, 800, 900 and 1150 °C for 3 h with heating rate of 10 °C/min. The main transformations were observed at 900 °C with the appearance of new crystalline phases especially mullite, anortite and spinel. Technical tests show that the properties fall within the ceramic International Standards (ISO). Finally, ceramic tiles and bricks prepared from these clays have suitable characteristics without defects and can be classified in group BIII according to the European Standard NF EN 159 (1991).