Optimizing water–cement ratio for fly ash nano-silica mortar: Strength and microstructural insights

This study conducts a deeper analysis of mortar’s mechanical and microstructural characteristics by examining the influence of nano-silica derived from fly ash on mortar's compressive strength and microstructure using the ratios of water-to-cement (w/c) of 0.40, 0.45, and 0.485. Nano-silica was synthesized via the sol–gel method, yielding amorphous spherical particles ranging from 30 to 80 nm in size, with 98.37% SiO₂ content, as observed from SEM micrographs. XRD patterns revealed a broad peak at 2θ ≈ 22°, confirming the amorphous nature of the synthesized silica. FTIR spectra exhibited characteristic Si–O–Si and Si–OH vibrations at 1080 cm⁻¹ and 950 cm⁻¹, respectively, verifying the presence of silica networks. Mortar mixtures contained nano-silica ranging from 1%, 2%, 3% and 4% by cement weight and were tested after 28 days of curing. The highest compressive strength was achieved with 1% nano-silica at w/c = 0.40, showing more than a 29% increase compared to the control sample. Unlike prior studies, this work integrates compressive strength, SEM, XRD, and FTIR analyses in a single investigation of fly ash-derived nano-silica under varying w/c ratios. Additionally, XRD and FTIR a