Ternary diagrams as a tool for developing ceramic materials from waste: relationship between technological properties and microstructure

The present work proposes the manufacture of ceramic construction materials using raw materials which are only industrial wastes from different production processes (electric arc furnace steel slag, coal bottom ash, carbon fly ash, and oil-filtering diatomaceous-earth). From them, the SiO2-Al2O3-CaO equilibrium phase diagram has been used to formulate two materials compositions with the objective of obtaining ceramics whose chemical composition is located in the same area as a traditional clay-based material, but with two different technological behaviors: refractoriness due to calcium phase’s development and densification due to a greater melting capacity. The waste-based pieces have been sintered at three firing temperatures (1000 °C, 1050 °C, and 1100 °C) and the mineralogical composition has been quantified by Rietveld refinement-XRD in order to establish the agreement between the phases foreseen by the formulations in the diagrams and the mineralogical compositions actually developed after the sintering treatment. In addition, fired materials have been subjected to a complete microstructural characterization by means of SEM-EDX and the pore size distribution has been determined by means of mercury intrusion porosimetry and helium pycnometry. In general, all obtained materials display adequate technological properties for their use as building materials so this characterization has allowed to show the suitability of the ternary diagram SiO2-Al2O3-CaO for the design of ceramic compositions from industrial wastes. Therefore, extending the use of phase diagrams as a design tool is still little explored for waste-based ceramics, and those that have been mainly used for studying Mg-based phases can have an important contribution to a more sustainable construction sector.