Microstructural Evolution in Clay-Based Ceramics II: Ternary and Quaternary Mixtures of Clay, Flux, and Quartz Filler
The complex microstructural evolution in mixtures of kaolinite clay, quartz, nepheline syenite, and soda–lime–silica (SLS) glass has been revisited. Detailed descriptions of reactions leading to dense whiteware bodies backed by semi-quantitative X-ray diffraction analysis reveal that ternary mixtures containing nepheline syenite differ from those containing SLS glass, as new crystalline phases develop inside and at the interfaces among SLS glass particles and decomposed clay and quartz. In SLS glass-fluxed mixtures, tridymite formation was hindered and cristobalite formed at ≥750°C followed by wollastonite at ≥800°C. Albite and plagioclase formed from interaction between clay and molten SLS glass above 800°C. Wollastonite was not present ≥1100°C, leaving only cristobalite in the surrounding regions. Quartz partially dissolves at temperatures ≥1000°C after interacting with molten SLS glass. In quaternary mixtures containing 6.25 wt% SLS glass and 17.25 wt% nepheline syenite fluxes, formation of types II and III secondary mullite was more pronounced than in the fully SLS glass-fluxed mixture. The more fluid liquid from nepheline syenite enhances the growth kinetics of mullite. The body fired at the optimum firing temperature (1100°C) has a microstructure containing primary type I and secondary types II and III mullites, remnant cristobalite, plagioclase, and partially dissolved quartz embedded in the glassy phase. Providing a roadmap for microstructural design in clay-based systems may have significant commercial impact in the emerging technology of use of waste materials in clay-based ceramics.
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