Natural vs. Synthetic Fibres in Volcanic Ash-Based AAMs: A Comparative Study

This study evaluates the influence of natural and synthetic fibres on the physical–mechanical performance of volcanic ash-based alkali-activated materials (AAMs). Volcanic ash from Mt. Etna (Italy) blended with metakaolin was used as a precursor and activated with 8 M sodium hydroxide and sodium silicate solutions. Prickly pear, palm trees and carbon fibres were added up to 1 wt.% in short (~10 mm) and long (~40–70 mm) configurations. FTIR analysis showed chemical modifications of the natural fibres after alkaline exposure, including the reduction or disappearance of bands associated with lignin, hemicellulose, and some cellulose-related functional groups. Mechanical tests demonstrated a dependence on fibre type and geometry. Short carbon fibres provided the highest flexural strength (23.19 MPa), corresponding to an increase of about 197% compared with the unreinforced binder. Long palm fibres yielded the highest stiffness, with a Young’s modulus of 12.16 GPa. In contrast, prickly pear fibres reduced flexural strength due to weak fibre–matrix adhesion and fibre pull-out. Water absorption tests showed that palm fibres exhibited the highest absorption rate (~20%) and mass loss (~1.9%), while prickly pear fibres proved to be less prone to absorb water. Microstructural analysis demonstrated good fibre–matrix adhesion, with localised interfacial gaps. Overall, carbon fibres ensure the best mechanical performance, while natural fibres remain a promising alternative for non-structural and restoration applications.