Nano-Silica–Enhanced Binder Synergy and Multi-Index Performance of Resource-Efficient Alkali-Activated Composites for Sustainable Infrastructure Applications

This study presents a multi-index performance method to measure the synergy of nano-silica-enhanced binders in resource efficient alkali-activated composites based on the Strength Activity Index (SAI) as a reference index according to ASTM C618. High-calcium fly ash (HCFA) and low-calcium fly ash (LCFA) were used with fine aggregate replacement level is kept constant at 20% by mass and nano-silica was incorporated at 0, 1, 2, and 3 wt% of the binder to prepare alkali-activated slag fly ash composites. The fresh state performance was assessed using the Initial Flow Index (IFI) and Flow Retention Index (FRI), while the mechanical performance was evaluated using the compressive, tensile, and flexural indices (SAI, TSI, and FSI). These results indicate that with an increase in nano-silica content, flowability and workability retention reduces systematically, with LCFA-based mixtures always exhibiting higher fresh-state retention than HCFA systems. Optimal mechanical performance was achieved with an intermediate nano-silica concentration of about 2 wt%, with consequent maximum SAI performance of about 120% at 28 days with HCFA-based mixtures and 118% at 28 days with LCFA-based mixtures, as well as a uniform improvement in TSI and FSI. Correlation analyses between SAI and tensile and flexural indices revealed clear linearity (R2 of about 0.91-0.95) which indicated that compressive strength is not a sufficient measure of total mechanical performance. The mineralogical and microstructural analyses assisted by X-ray diffraction (XRD) and scanning electron microscopy (SEM) showed that the performance trends observed depend on the interactions of the calcium supply, amorphous aluminosilicate and the nucleation effects of nano-silica. The proposed solution provides a performance-based approach towards the optimal utilization of nano-silica-modified alkali-activated composites to be used in sustainable infrastructure applications.