Porous acicular mullite was fabricated at 1300 °C starting from Al₂O₃ and mixture of SiO₂ and MoO₃ obtained by previous oxidation of waste MoSi₂. It was found that the presence of MoO₃ favors formation of acicular (prism-like) mullite grains with sharp edges. The effect of addition of Fe₂O₃ (4–12 wt.%) on phase composition, compressive strength, thermal conductivity and microstructure was studied. The addition of Fe₂O₃ improved the compressive strength from approximately 25 MPa in pure mullite to about 76 MPa in samples containing 12 wt.% Fe₂O₃, while the open porosity decreased from 55.4% to 51.8%. The presence of Fe₂O₃ caused a decrease in mullite formation temperature owing to the formation of liquid phase and accelerated diffusion. The solubility of iron oxide in mullite lattice was between 8 and 12 wt.% Fe₂O₃. The incorporated iron ions also promoted the rounding of sharp edges in prismatic mullite grains, leading to a reduced specific surface area of 0.55 m²/g in the sample with 12 wt.% Fe₂O₃. The thermal conductivity of mullite increased with addition of 12 wt.% Fe₂O₃ reaching value of 1.17 W/m·K.

A promising approach for the removal of Cs⁺ ions from aqueous solutions and their long-term immobilization was investigated, as the safe containment of radioactive cesium is essential due to its long half-life and environmental hazards. The approach involves ion exchange using zeolite, geopolymerization, and subsequent heat treatment of the geopolymer/zeolite composite. Cs⁺ ions were removed from water by ion exchange using Na-13X zeolite, in which Cs⁺ ions replaced Na⁺ ions within the zeolite framework. The resulting Cs-13X zeolite was subsequently combined with kaolinite to prepare geopolymer/Cs-13X zeolite composites containing 10–50 wt% Cs-13X zeolite. During geopolymerization, a portion of the Cs was released from Cs-13X zeolite and incorporated into the geopolymer matrix, while the remaining Cs remained immobilized within the zeolite structure. Heat treatment of the geopolymer/Cs-13X zeolite composites at 950 °C induced crystallization of two stable Cs-bearing phases. The Cs incorporated into the geopolymer matrix crystallized as the CAS phase, whereas the Cs retained in the Cs-13X zeolite crystallized as pollucite. The crystalline phase content in the heat-treated composite samples increased with Cs-13X zeolite loading, resulting in improved density and compressive strength. The highest values of crystallinity (72%), bulk density (2.42 g/cm³), and compressive strength (~53 MPa) were measured for samples containing 50 wt% Cs-13X zeolite. This level of compressive strength is sufficient to ensure safe handling and long-term containment of the immobilized Cs⁺ ions.

This study investigates the thermally induced phase transformation of Ni-exchanged LTA zeolite as a dual-purpose method for nickel immobilization and the synthesis of stable ceramic pigments. The process describes a cost-effective and sustainable alternative to conventional pigment production, aligning with circular economy principles. Upon thermal treatment at temperatures ranging between 900 °C and 1300 °C, Ni-exchanged LTA zeolite undergoes a transformation to NiAl₂O₄ spinel, confirmed by XRPD, FTIR, and thermal analysis. Initially, NiO is formed, but as the temperature increases, it dissolves and transforms into NiAl₂O₄. Colorimetric studies revealed intensified blue pigmentation with increasing temperature, correlating with crystallite growth and structural evolution. SEM analysis showed morphological changes from cubic particles to sintered agglomerates, enhancing pigment stability and hardness. The Ni-LTA sample calcined at 1300 °C showed the highest hue angle, which was consistent with the formation of over 99 wt.% of the nickel aluminate crystalline phase at this temperature. The results demonstrate that Ni-LTA zeolite can be effectively transformed into durable greenish-blue pigments suitable for application in porcelain. This transformation is especially evident at 1300 °C, where a spinel phase (NiAlSi₂O₄) forms, with colorimetric values: L = 58.94, a* = –16.08, and b* = –15.90.

Dense Ce,Pr-doped YAG transparent ceramics (YAG:Ce,Pr) with enhanced red emission were fabricated by pressureless sintering in air at 1600 °C. Fine YAG powders containing 0.2 mol% Ce and varying amounts of Pr (0.04–0.32 mol%) were synthesized using a glycine–nitrate combustion method. The emission and excitation spectra were analyzed to investigate the influence of Pr doping on red emission. The results indicate that YAG: Ce,Pr ceramics are a promising candidate for blue-to-yellow light conversion. The strongest red emission component was measured in samples containing 0.16 mol% Pr. It was confirmed that electron transfer from Ce to Pr enhances the red emission component (609 nm), primarily by increasing the population of electrons relaxing from the ¹D₂ excited state to the ³H₄ ground state. Further increasing the Pr concentration beyond 0.16 mol% resulted in a reduction in the red emission intensity due to concentration quenching. Microstructural analysis of YAG:Ce,Pr revealed a dense and homogeneous microstructure composed of equiaxed grains.