Synergistic mitigation of brittleness in nanosilica-modified concrete through fly ash substitution

The incorporation of synthetic nanosilica (NS) into cementitious composites accelerates early-age hydration and significantly enhances mechanical strength but inherently induces extreme structural brittleness. This study investigates the synergistic mitigation of NS-induced brittleness through the incorporation of Class F fly ash (FA) acting as a delayed pozzolanic buffer. A comprehensive experimental program was conducted on four distinct concrete mixtures, evaluating both fundamental static mechanical properties and advanced non-linear fracture mechanics parameters derived from experimental load–crack mouth opening displacement ( F–CMOD ) envelope curves. The multi-criteria evaluation revealed significant behavioral transitions governed by the hybrid binder composition. The ternary mixture containing 5% NS and 15% FA achieved the highest absolute mechanical performance, increasing the compressive and splitting tensile strengths to 137% and 136% of the reference values, respectively. Conversely, increasing the FA substitution to 25% optimally maximized the damage tolerance and post-peak energy dissipation capacity, leading to a 207% increase in the total fracture Energy ( G f ) and a 149% enhancement of the characteristic length ( l ch ). Furthermore, Pearson correlation analysis established a highly robust positive linear relationship ( r = 0.9258) between the G f and the l ch . This provides definitive statistical evidence that the overall structural ductility of the investigated hybrid nanocomposites is intrinsically driven by their energetic fracture resistance during the post-peak softening phase.