Marble waste is produced on a large scale in many countries, resulting in serious pollution problems. This investigation aimed to study the valorization potential of marble waste from the ornamental rock industry used in the synthesis of a novel calcium niobate–magnesium niobate composite powder prepared by a solid-state reaction between 1000 °C and 1200 °C. The chemical and mineralogical characteristics of the marble waste were determined. Structural and morphological characterizations of the synthesized calcium niobate–magnesium niobate composite powders were conducted by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy (SEM). The thermophysical properties were measured using open photoacoustic cell and photothermal techniques. Structurally, at all synthesis temperatures, the calcium niobate–magnesium niobate powders were found to be composed of a complex mixture of CaNb₂O₆/Ca₂Nb₂O₇/MgNb₂O₆/CaMg_0.33Nb_0.67O₃. In addition, the calcium niobate–magnesium niobate composite powders exhibited low values of thermal diffusivity (1.88–2.15 × 10⁻⁷ m²/s) and thermal conductivity (0.12–0.16 W/mK). The findings of this investigation highlight the potential of marble waste as a promising sustainable source of carbonate for obtaining calcium niobate–magnesium niobate composite powder, which has thermophysical properties that should be explored in low-thermal-conductivity applications.

The present work has investigated the effects of tin doping on morphological and optical-electrical properties of zinc oxide films produced by spray-pyrolysis technique. The films exhibited droplet impact marks along the surface. Top-view micrographs disclosed black clusters of non-decomposed salts after tin doing. These morphological aspects have not compromised optical performance although they may have contributed to the rise in electrical resistivity due to charge scattering on the surface. Electrical conductivity was lowered as tin-doping concentration increased. Tin ions occupied interstitial sites in the host matrix and acted as conventional defects. The film transmittance at 550 nm was not significantly affected by tin doping, yet a slight loss in transparency was observed after 1 at. % Sn doping. Optical bandgap was reduced from 3.25 to 3.16 eV due to deep states when doping with 1 at. % Sn. The optical bandgap widened to 3.23 eV with incorporation of 2 at. % Sn. This behaviour is attributed to the Burstein-Moss effect.

In recent years, sustainable recycling approaches for chicken eggshell waste have increased significantly worldwide due to environmental and circular economy benefits. This work aimed to synthesize and characterize a new calcium aluminate powder using chicken eggshell waste as an alternative source of calcium carbonate through mechanical activation and subsequent calcination. The starting formulation consisting of the eggshell waste (CaCO₃):Al₂O₃ (1:1) ratio was subjected to a high-energy milling process for 0 h, 15 h and 30 h and subsequent calcination at 1200 °C for 4 h. The resulting calcium aluminate powders have been investigated using X-ray diffraction (XRD), differential thermal analysis (DTA), scanning electron microscopy (SEM), and photoluminescence techniques. After calcination, a calcium aluminate-based composite powder with an average crystallite size between 46.45 nm and 52.27 nm and a predominance of the CaAl₂O₄ phase was found. The calcium aluminate powders produced (milled for 15 h and 30 h and calcined at 1200 °C) showed a luminescent behavior, emitting characteristic violet light with a wavelength between 380 and 418 nm. Our findings may provide a novel technical pathway for recycling chicken eggshell waste into calcium aluminate powder with luminescent properties.

Calcium niobate ceramics were synthesized by the solid-state reaction of seashell-derived CaCO₃ and Nb₂O₅ at a molar ratio of 4:1 over the temperature range of 800–1200 °C for 8 h. This work provides a correlated structural, morphological, and thermophysical characterization of the resulting calcium niobate phases. X-ray diffraction coupled with Rietveld refinement revealed a temperature-dependent phase transformation from a CaNb₂O₆-rich composition at lower synthesis temperatures to a Ca₄Nb₂O₉-rich material at higher temperatures. At 1200 °C, Ca₄Nb₂O₉ was obtained as the predominant phase (96.16 ± 1.81%), with only a minor Ca₂Nb₂O₇ contribution. This phase evolution was accompanied by an increase in the Ca₄Nb₂O₉ crystallite size from 39.49 ± 0.19 nm at 1000 °C to 50.38 ± 0.24 nm at 1200 °C. SEM analysis showed a concurrent morphological transformation from agglomerated and irregular particles to well-defined elongated and platelet-like particles, indicating enhanced crystallization and grain growth. Thermophysical properties were determined using open photoacoustic cell and laser-flash techniques. Thermal diffusivity ranged from 1.86 to 3.12 × 10⁻⁷ m² s⁻¹, heat capacity from 0.79 to 1.30 × 10⁴ JK⁻¹ m⁻³, thermal conductivity from 0.15 to 0.41 Wm⁻¹ K⁻¹, and thermal effusivity from 0.36 to 0.73 kWs^1/2 m⁻² K⁻¹. The combined results establish a clear synthesis–structure–property relationship and identify the Ca₄Nb₂O₉-rich ceramic obtained at 1200 °C as a promising low-thermal-conductivity material. The use of seashell waste additionally provides a renewable calcium precursor for the synthesis.

In Brazil, the ornamental stone industry generates a significant volume of marble waste annually, which needs to be recycled using a sustainable value-added approach. In this context, this research focuses on the on the feasibility of synthesizing calcium and magnesium cobaltite via a solid-state reaction route, using marble waste as an alternative carbonate source. The starting formulation for obtaining calcium-magnesium cobaltite, based on a stoichiometric molar ratio of marble waste:Co3O4 (9:4), was calcined at temperatures between 850°C and 950°C for 20 h. The synthesized material was characterized by X-ray diffraction, differential thermal analysis and thermogravimetry, Fourier transform infrared spectroscopy and scanning electron microscopy. It was found that marble waste is essentially composed of the mineralogical phases calcite (CaCO3) and dolomite (CaMg(CO3)2). The results also showed the formation of a multiphasic cobaltite composed of calcium cobaltite (Ca3Co4O9 and Ca3Co2O6) and magnesium cobaltite (MgCO2O4) at different temperatures. Therefore, marble waste can be highly promising for the synthesis of multiphasic ceramics based on calcium cobaltite and magnesium cobaltite.