Chemical, mineralogical, and optical characterization of a Nepheline-Combeite bioceramic based on coal gangue and limestone dust

Nepheline (NaAlSiO₄) and Combeite (Na₂Ca₂Si₃O₉) are silicates belonging to the alkaline silicate ceramic family, recognized for their remarkable structural and functional properties. The development of composites integrating these both phases makes it possible to combine the structural and chemical advantages of each, paving the way for the design of innovative biomaterials. In this study, a Nepheline-Combeite bioceramic was successfully synthesized using an alkali activation method from industrial by-products, derived from coal gangue (as the source of SiO₂) and limestone powder (as the source of CaCO 3 ). The bioceramic was synthesized at room temperature using sodium hydroxide (NaOH) as the alkaline activator at different concentrations (2, 3, and 4 M) to evaluate the influence of alkali concentration on bioceramic formation. The mixtures were activated with an alkaline activator (NaOH) under magnetic stirring for 6 h, then heat-treated at 700 °C. Furthermore, the bioactivity of the synthesized bioceramics was evaluated by immersing them in simulated body fluid (SBF) and artificial saliva (AS) at 37 °C for 48 h. The structural, morphological, and optical properties of the obtained bioceramics were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy (RS), diffuse reflectance UV–visible spectroscopy (DRS), transmission electron microscopy (TEM), and selected area electron diffraction (SAED). The XRD analysis confirmed that NaOH concentration significantly affected bioceramic formation, with 4 M identified as the optimal concentration for the formation of the Nepheline–Combeite bioceramic. Furthermore, SEM observations revealed lamellar nepheline particles, while UV-Vis analysis showed an optical band gap of 3.02 eV with maximum absorption at 304 nm. The XRD analyses after only 48 h immersion in simulated media, complemented by scanning electron microscopy (SEM) observations, showed that these bioceramics rapidly promote the formation of a bone-like hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) layer, demonstrating their potential as biomaterials for bone regeneration. This study contributes to the United Nations Sustainable Development Goal 12 (Responsible Consumption and Production) through the valorization of industrial by-products into high-value bioceramics.