Understanding the processing of mullite-based ceramics from kaolin waste: a cautionary note on the inadvertent use of split-plot designs

Experimentation plays an important role in ceramic science and engineering, which involves the design of new products, development of new manufacturing processes, and also the optimization of processing. In this work, a cautionary note on the inadvertent use of split-plot designs was presented to understand the processing of mullite-based ceramics from kaolin waste. Samples were obtained by solid-state reactive sintering of kaolin waste following a 2² × 2 full factorial split-plot design. The influence of firing temperature, time and applied pressing pressure on the physical properties was evaluated. The structural and microstructural analyses of fired samples were evaluated by X-ray diffractometry (XRD) and field emission scanning electron microscopy (FESEM). Kaolin waste processed ceramics at 1400 ºC consist of elongated mullite crystals uniformly embedded into a glassy phase and minor amounts of quartz. Statistical models were constructed for open porosity and density at 1% level of significance. The estimated P-values indicate that the coefficients associated with the applied pressing pressure and its interaction with firing temperature and time (\(\:\gamma\:\) and \(\:{\delta\:}_{1}\)) were statistically significant for open porosity and density. The higher applied pressing pressure leads to lower open porosity. The densification process was influenced by different sintering mechanisms depending on temperature (solid phase sintering vs. liquid phase sintering). The lack of harmony between the planning and the execution of the experiment does not allow the testing of the main effects of firing temperature, time and the interaction between them (β₁, β₂ and β₃). Understanding the proper way to plan, execute and analyze experiments are of urgent interest in the field of ceramic processing, and that is the key message of this paper.