Use of thermogravimetric and differential scanning technique for determining the quality of calcination of kaolinite clays for cement production

To ensure the efficient production of calcined clays at an industrial scale, rapid testing method is required to prevent under or over- calcination and guarantee proper quality control. This study investigates the phase transformation processes of six kaolinitic clays calcined between 400 and 1000 °C, using X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. The results confirm that the formation of spinel phase indicates over-calcination as approximately 50% reduction was observed in pozzolanic reactivity at 1000 °C. The influence of various common impurities such as quartz, iron and 2:1 clay mineral on the onset of over- calcination has been studied. The impurities and crystallinity of kaolinite were found to influence only the temperature at which spinel forms and not the quantity. Highly disordered iron rich clays showed approximately 50 °C lower temperature than ordered quartz rich kaolinite clay. DSC proved effective in detecting the presence of spinel, which is not easily identified in other techniques. The combination of TGA and DSC can therefore be used not only to assess and quantify if a clay is properly calcined or not, but also to identify the optimal calcination temperature. Furthermore, practical guidelines for implementing DSC as a quality control tool for calcination are provided that would offer valuable insights for industrial applications.