What question did this study set out to answer?

This research aims to develop a comprehensive performance framework for assessing nano-silica-enhanced alkali-activated composites.

February 14, 2026Infrastructures0 citationsOpen Access

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

MEMahmoud Abo El-WafaKing Abdulaziz University

Key Points

This research aims to develop a comprehensive performance framework for assessing nano-silica-enhanced alkali-activated composites.
Integrated fresh state flowability with mechanical strength indices for evaluation.
Utilized high-calcium fly ash and low-calcium fly ash as binders.
Incorporated nano-silica at varying percentages (0, 1, 2, 3 wt%) into the mixtures.
Assessed performance using Initial Flow Index, Flow Retention Index, and compressive, tensile, and flexural strengths.
Conducted correlation analyses between different strength indices and mineralogical assessments.
Higher nano-silica content led to reduced flowability and workability retention.
LCFA-based mixtures exhibited better fresh-state performance compared to HCFA systems.
Optimal mechanical performance was seen at around 2 wt% nano-silica, with SAI reaching 120% for HCFA and 118% for LCFA.
Strong correlations were found between SAI and tensile/flexural indices, indicating broader performance metrics.
Mineralogical analysis confirmed that performance trends are influenced by calcium and amorphous aluminosilicate interactions.

Abstract

This study presents a multi-index performance approach that moves beyond the conventional reliance on compressive strength, offering a more holistic evaluation of nano-silica-enhanced binders in resource-efficient alkali-activated composites. Based on the Strength Activity Index (SAI) framework described in ASTM C618, the method integrates fresh state flowability with mechanical strength indices to capture the overall binder synergy. High-calcium fly ash (HCFA) and low-calcium fly ash (LCFA) were used with fine aggregate replacement, the level of which was 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 reduce 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. Therefore, the proposed multi-index framework offers a robust and practical tool for quantifying binder synergy and optimizing nano-silica dosage, advancing the understanding and development of sustainable alkali-activated composites for infrastructure applications.

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