<![CDATA[Developing sustainable stabilizers for zirconia extracted from natural zircon sand is essential to reduce reliance on conventional commercial additives while improving resource utilization. This study investigated the effects of calcium concentration and calcium source on the structural evolution and mechanical properties of zirconia synthesized via an alkali fusion–coprecipitation process. Seven samples were synthesized using eggshell-derived and commercial calcium dopants at concentrations of 0–1.5 mol%. Raman spectroscopy confirmed the dissociation and recombination of Si–O bonds in the ZrSiO4 lattice and the formation of ZrO2, while X-ray fluorescence verified a ZrO2 purity of 85.712 wt.%. X-ray diffraction revealed that calcium doping expanded the lattice parameter (c-axis: 5.100–5.180 Å), increased tetragonality (c/a up to 1.4420), reduced microstrain (109.2 × 10-4 to 51.8 × 10-4), increased crystallite size (7.4–8.3 nm), and nonlinearly enhanced the tetragonal phase fraction, reaching 7.2 wt.% at 1.0 mol% commercial calcium before declining at higher concentrations. Sintering at 1400 °C for 3 h completely transformed the tetragonal phase into the monoclinic phase. Eggshell-derived calcium at 0.5 mol% exhibited the best density (4.055 g/cm3), lowest porosity (19.447%), and greatest Vickers hardness (142.2 HV), comparable to commercial calcium. These findings suggest that calcium derived from eggshells is a viable and sustainable alternative stabilizer for zirconia ceramics, whilst also promoting the utilization of waste.]]>
