Symmetry-Constrained Structural Maturation of Mullite Ceramics Derived from Waste Clay–Diatomite: Phase Evolution, Lattice Metric and Microstructure

In the production of refractory mullite ceramics, the sintering duration is essential not only from an economic but also from a safety standpoint. Mullite ceramics derived from waste clay–diatomite have previously been shown to exhibit promising compressive strength. Encouraged by these moderately high values, a detailed structural and microstructural analysis was performed, including the quantification of the absolute phase fractions, i.e., the amorphous/glassy content, using the ZnO internal-standard Rietveld method, and the determination of the mullite composition parameter x, the crystallite size, and the microstrain. All these crystallographic findings were linked to the microstructural development of the investigated samples sintered at 1400, 1450, and 1500 °C for 2 h, produced from clay–diatomite tailings. The 62 wt.% mullite content across the investigated sintering range (1400–1500 °C) indicates that secondary mullitization is not the dominant process. The volume-weighted mean crystallite size remains below 100 nm and reaches a plateau already at 1450 °C (97.8 nm), with no further growth up to 1500 °C (97.9 nm). A small quantity of corundum (2 wt.%) is no longer detectable at 1500 °C, indicating its dissolution in an alkali-rich silicate melt. The grain size distribution revealed Ostwald-type coarsening, a mechanism characteristic of the intermediate stages of liquid-phase sintering, shifting the modal grain size to 0.22–0.30 μm and developing a tail extending to approximately 0.70 μm. Notably, the mullite composition parameter x remains within 0.23–0.28 at all temperatures, while the unit cell contracts slightly but systematically (ΔV = −0.106 Å3), and the orthorhombic distortion e = (b − a)/(b + a) decreases monotonically from 0.01008 to 0.00988, indicating a systematic drift of the lattice metric towards the pseudotetragonal limit at constant composition.