Effects of Solid Solution of Magnesium on Zirconolite Ceramic Wasteforms

Zirconolite ceramic wasteforms with a stoichiometry of Ca_1-xZr_1+xTi_2-xMg_xO₇ (0≤_x≤0.20) were obtained by solid-state sintering of CaTiO₃, ZrO₂, TiO₂, and MgO at 1 350 ℃. The influences of the solid solubility of Mg²⁺ on the phase composition, microstructure, and crystal structure were investigated. The X-ray diffraction results demonstrate that the Ti site in Ca_1-xZr_1+xTi_2-xMg_xO₇ can accommodate Mg²⁺ in a range of x=0.05-0.15, and the main phase of the sample is determined to be a monoclinic-zirconolite structure (2M, space group: C2/c), as further confirmed by the selected area electron diffraction analysis. For the samples of x=0.05-0.15, the relative mass fraction of the zirconolite phase is higher than 97%; only a trace amount of the perovskite impurity can be found. When x increases to 0.20, the concentration of Mg²⁺ exceeds the limit of solid solubility of the structure, resulting in a significant increase in the impurity phase. Meanwhile, the dense morphology of the polished surface is observed by the scanning electron microscope, and the measured cation ratio is in good accordance with the designed stoichiometry when x≤0.15.