Multiscale characterization of geopolymers modified with alkali-catalyzed nano-silica: Effects on dispersion and mechanical properties

Nano-silica (NS) is widely used to enhance geopolymers, yet the influence of synthesis conditions (acidic vs. alkaline) on its dispersion and reactivity remains underexplored. This study systematically evaluates the multiscale effects of acid- and alkali-catalyzed NS on the performance of fly ash–slag blended geopolymers. Factors investigated include the NS synthesis condition (acid vs. alkali), dosage (0.16 wt%, 0.32 wt%), gel network density, elastic modulus distribution, pore structure, and mechanical properties. Transmission Electron Microscopy (TEM), compressive strength testing, Scanning Electron Microscopy (SEM)/Energy-Dispersive X-ray Spectroscopy (EDS) elemental mapping, Atomic Force Microscopy (AFM) topography with modulus mapping, and Small-Angle X-ray Scattering (SAXS) with Guinier analysis were used to characterize structural and mechanical changes across scales. Results show that alkali-catalyzed NS exhibits superior dispersion, promoting denser and more homogeneous gel networks. At 0.16 wt%, it enhances compressive strength by 38.4 %, reduces surface roughness by ∼50 %, and lowers radius of gyration (R_g) by 18 %. In contrast, 0.32 wt% causes particle agglomeration, compromising microstructural integrity. SEM/EDS indicates that the alkali-catalyzed groups have pronounced Si and Al enrichment at interfaces, forming a dense, continuous C-(N)-A-S-H gel network. AFM modulus mapping and fitting reveal higher and more concentrated modulus peaks and longer correlation lengths, indicating a more uniform nanoscale structure. SAXS scattering curves and Guinier analysis results demonstrate stronger scattering intensity and smaller R_g, indicating improved porosity and significantly enhanced microstructural continuity. Overall, 0.16 wt% alkali-catalyzed NS presents a promising strategy for improving both strength and uniformity in geopolymers, offering guidance for the design of nano-modified sustainable binders.