Low-valued raw materials challenge the common eligibility criteria for triaxial ceramics

In this work, two low-valued natural raw materials were used, namely a low plasticity and high iron content clay and a powdered rock waste generated during crushing of igneous rocks to produce construction aggregates. After characterization (chemical, mineralogical, thermal and granulometric), mixtures containing up to 75 wt% of rock powder were prepared and extruded closely following industrial practice, and fired for 60 min from 900 to 1100 °C. Property development was evaluated on as-extruded, dried and fired pieces. The results obtained were interpreted based on chemical compositions, the estimates from the Al2O3–SiO2–CaO phase diagram and presumed reaction kinetics. Such interpretation showed that physical characteristics dominate not only during shaping and drying, but also during low temperature firing (slow reaction kinetics), when rock additions act as inert material; at high firing temperatures, however, the rock promotes liquid phase development after first melting (fluxing effect) and thermodynamics prevail. The dominant fluxing effect results in improved fired properties, which were shown to depend almost linearly on the liquid phase content, predicted by the phase diagram and determined by the chemical composition. These results enabled the identification of behavioural composition ranges to best exploit the materials’ industrial use potential and demonstrate that current paradigms in raw material evaluation and processing in traditional ceramics industries need a re-assessment.