Characterization, thermal behaviour and firing properties of clay materials from Cap Bon Basin, north-east Tunisia, for ceramic applications

This study focuses on the characterization, thermal behaviour and firing properties of clays from Cap Bon Basin, Tunisia. Chemical (inductively coupled plasma atomic emission spectroscopy) and mineralogical (X-ray diffraction (XRD), Fourier-transform infrared (FTIR) spectroscopy) characterization of the clays was carried out. Physical properties were identified using particle-size distribution and Atterberg limits tests. Differential thermal analysis/thermogravimetry, dilatometry, XRD and FTIR analyses were conducted to assess the thermal behaviour, transformations and crystalline phase development during firing. The clays studied show large proportions of silt and clay fractions. They are rich in SiO2, Al2O3 and Fe2O3 and mostly consist of illite, kaolinite, smectite and minor mixed-layer illite-smectite. Associated minerals are mainly quartz, calcite and feldspar. To monitor the firing properties, three clay mixtures referred to as M1, M2 and M3 were prepared according to the physicochemical and mineralogical characteristics of the raw clays. These were sintered at 900 degrees C, 1000 degrees C and 1100 degrees C and tested for firing shrinkage, flexural strength and water absorption. As a result, various mineralogical transformations occurred at 900-1100 degrees C associated with the crystallization of new phases. The main transformations identified at >1000 degrees C resulted in the formation of anorthite and mullite, causing significant densification of the fired ceramic materials. Sintering at 1100 degrees C increased the densification and thus improved the flexural strength. At this temperature, the sintered ceramic tiles exhibited excellent properties, namely water absorption of 2-3% and flexural strength of 30-41 MPa, indicating that these studied clays might be used as raw materials for the production of stoneware tile ceramics.