Electrical and microstructural characterization of fly ash and pyrophyllite-based geopolymers for sustainable energy transition applications

This study investigates the microstructural and dielectric characteristics of geopolymer materials synthesized from fly ash (FA) and pyrophyllite (PG), with the objective of assessing their suitability for sustainable energy transition applications. Unlike most previous works, which have mainly focused on mechanical performance or room-temperature behavior, this research provides a comprehensive evaluation of the dielectric response up to 900 °C and establishes correlations with microstructural transformations. These findings help determine the potential of these materials for energy storage and insulation. The geopolymer samples were synthesized using 14 M NaOH and Na₂SiO₃ as activators and cured at 80 °C. Their mineralogical compositions and microstructures were characterized using XRD, SEM, and FTIR. The results revealed that geopolymerization led to the formation of new zeolite-like phases and calcium aluminum silicate hydrate. SEM analysis showed the presence of both reacted and unreacted spherulites, indicative of incomplete reaction in some regions. Dielectric testing demonstrated that the dielectric constant increased with temperature but decreased with frequency, while dielectric loss varied with temperature due to thermally activated charge carriers. The complex impedance analysis confirmed a temperature-related relaxation process, and the FA-based geopolymer exhibited higher electrical resistivity (1.4 × 10 4 Ω at 600 °C) compared to the PG-based geopolymer (16,663 Ω). Charge conduction mechanisms were identified as non-overlapping small polaron tunneling (200–600 °C) and correlated barrier hopping (600–900 °C). These findings highlight the potential of FA- and PG-based geopolymers as sustainable and efficient materials for high-temperature energy storage and insulation applications, contributing to the advancement of sustainable energy transition technologies.