Gehlenite and anorthite crystallisation from kaolinite and calcite mix

Clay products can be reinforced by using clay and calcite mixes, even if sintering is at low temperature (1100 °C). The formation of a micro-composite microstructure favours a significant strength increase as fired materials contain anorthite grains embedded in a silico-aluminate matrix. The clay used, rich in kaolinite mineral, is originated from Burkina-Faso. It is contaminated by a moderate iron level, substituted to Al in octahedral sites of kaolinite, influencing the thermal comportment. An alternative approach was also used when substituting the common clay by an almost pure kaolinite mineral, to understand the iron role. In general, during thermal transformations, the reaction sequence of recrystallisation is metakaolinite—gehlenite—anorthite, but mullite was never detected. The interpretations of DTA spectrums indicate increasing activation energy for gehlenite and then anorthite formations. For mullite, although the corresponding activation energy is the lowest, it is not present. This energetically interpretation differs from the analysis of the SiO2–Al2O3–CaO ternary diagram, where transformation paths are not so clear. It is then proposed that the reaction sequence is favoured by the existence of structural similarities between metakaolinite, gehlenite and anorthite, which are pseudo-layered structures. For mullite, the 3D structure is very different, which explains its absence in the material.