Sustainable Graphene-Reinforced Geopolymer Composites Derived from Recycled Volcanic Ash: Matrix Optimization, Characterization, and Photoluminescent Proof-of-Concept

This study presents the development of sustainable and multifunctional geopolymer composites produced from recycled volcanic ash collected from Mount Etna (Italy), used as an aluminosilicate precursor for the fabrication of environmentally friendly materials. Geopolymerization was activated using a 6 M sodium hydroxide (NaOH) solution. The geopolymer matrix was optimized by investigating three liquid-to-solid (L/S) ratios (0.2, 0.3, and 0.4) in order to identify the most suitable formulation in terms of microstructure and porosity. SEM, EDS, BET, particle size distribution (PSD), and X-ray diffraction (XRD) analyses identified an L/S ratio of 0.3 as the optimal formulation. Based on this optimized matrix, graphene nanoparticles were incorporated at concentrations of 0.1, 0.2, and 0.5 wt.% to evaluate their influence on the microstructural, thermal, mechanical, and durability-related properties of the composites. The resulting materials were characterized by SEM, EDS, XRD, differential scanning calorimetry (DSC), mechanical testing, and water absorption measurements. Finally, hybrid prototypes consisting of geopolymer bars coated with a transparent epoxy resin containing photoluminescent powder were fabricated. Their photoluminescent performance was qualitatively evaluated following direct solar irradiation by monitoring the persistence of light emission over time. The results demonstrate the feasibility of integrating a geopolymer matrix derived from volcanic waste, graphene nanoparticles, and a photoluminescent coating into a single sustainable and multifunctional material. Although this work represents a preliminary proof-of-concept study, it provides a promising foundation for the development of smart geopolymer-based materials for applications in civil infrastructure, passive safety systems, and guidance devices operating under low-light conditions.