Mechanical and Microstructural Characteristics of Fly Ash-Nano-Silica Composites

An enhancement of the inherent strength and durability characteristics is required for the effective utilization of fly ash (FA) in geotechnical engineering applications. This study presents a novel technique employing nano-silica (NS) to stabilize FA deposits, focusing on its effects on the unconfined compressive strength (UCS) and durability. The effects of different NS dosages (0.25%, 0.50%, 0.75%, and 1%), curing periods (7, 14, 28, 60, and 90 days), mixing methods (dry and wet), cement (CEM) addition, and cyclic wetting-drying (C-W-D) on the mechanical characteristics of FA-NS composites were studied. The test results revealed that FA-NS composites exhibit superior performance compared to FA in terms of strength and durability. An optimal dosage of 0.50% NS resulted in a substantial increase of 1043% in 28-day UCS, with strength development most pronounced during the early curing stages (7 days). The type of mixing method plays a major role in the assessment of the UCS of FA-NS composites. The addition of CEM, alone or combined with NS in FA, increased the UCS but was less effective than the FA-NS composite. The durability against C-W-D was improved, with 50% and 20% increases in the 28-day UCS after the 1^st and 12^th cycles, respectively, maintaining a higher UCS than the actual 28-day UCS. Scanning electron microscopy images and Fourier transform infrared spectroscopy indicated the formation of primary and secondary pozzolanic compounds, contributing to a cohesive structure and enhancing the UCS and durability of the FA-NS composite. Thus, incorporating a small dosage of NS remarkably improves the inherent strength characteristics of FA and offers substantial benefits for long-term construction and geotechnical engineering applications.