Adhesion between geopolymer matrix and vegetable fibers: effect of matrix composition and fiber treatment

Abstract

Seeking to promote sustainable composites composed of a low-carbon matrix and a renewable reinforcement, this study evaluated the fiber–matrix adhesion of sisal fibers used as reinforcement in geopolymeric matrices produced by the alkaline activation of fiber cement waste, in combination with metakaolin. Natural, hornified, and sodium hydroxide–treated fibers were exposed for 28 days to matrices with different free alkali contents and subsequently characterized by thermogravimetric analysis, electron microscopy, and tensile and pull-out tests. Thermogravimetric analysis revealed reductions in the cellulose decomposition peak for all exposed fibers, with more pronounced shifts in more aggressive matrices (up to − 63 °C), indicating structural degradation and decreased thermal stability. Microstructural observations showed increased surface roughness, partial fiber disintegration, and the precipitation of geopolymeric and carbonate phases, with severe alkaline hydrolysis identified in hornified fibers, particularly in potassium-activated systems. Mechanical tests indicated significant losses in tensile strength and reliability after matrix exposure, with hornified fibers exhibiting the highest susceptibility to degradation and, in extreme cases, complete loss of mechanical integrity. In contrast, sodium hydroxide–treated fibers demonstrated improved strength retention, especially when embedded in matrices with lower free alkali content. Pull-out results highlighted that fiber–matrix adhesion is strongly governed by matrix alkalinity and fiber treatment, showing apparent pull-out behavior associated with reduced fiber integrity in less aggressive matrices and premature fiber rupture in highly alkaline environments. Overall, the results demonstrate that careful design of waste-based geopolymer matrices, including matrix composition, associated physicochemical properties, and free alkali concentration, is essential to enhance interfacial bonding and durability in vegetable fiber–reinforced composites.