Beyond strength: long-term durability and aging of nanofiber-reinforced fly ash-based concrete

Concrete is one of the most widely used construction materials, but the environmental impacts associated with ordinary Portland cement (OPC) production and the durability limitations of conventional fly ash-based systems motivate the development of more sustainable and durable cementitious materials. This review systematically examines the role of nanofiber reinforcement including carbon-based, polymeric, ceramic, and metallic nanofibers in fly ash-based concrete, with emphasis on mechanical performance, hydration, microstructural development, fracture behaviour, durability, ageing, and sustainability. The examined research reveals that nanofibers may enhance compressive, tensile, and flexural performance via crack bridging, nucleation of hydration products, pore refinement, interfacial transition zone (ITZ) alteration, and stress redistribution. Nanofiber integration may also decrease permeability and enhance resistance to chloride penetration, carbonation, sulphate attack, freeze–thaw exposure, and shrinkage-related cracking. However, performance depends on fly ash chemistry, replacement amount, nanofiber type and dose, dispersion quality, curing circumstances, and testing technique. Key barriers include nanofiber agglomeration, high material cost, dispersion challenges, limited standardization, environmental and occupational considerations, and insufficient long-term field validation. Finally, a multiscale viewpoint is presented to correlate nanoscale fiber matrix interactions with microstructural development and macroscale structural performance, highlighting the need for standardized testing, multiscale modelling, long-term monitoring, and sustainability evaluation.