Optimizing High-Volume Fly Ash Mortar with Nano-SiO₂ and PVA Fibers: Performance and Microstructure

The mechanical properties, capillary water absorption, drying shrinkage, and morphology of high-volume fly ash mortar were investigated. The mortar contained 0~2.5 wt. % nano-silicon dioxide (nano-SiO₂, NS) and 0~1.5 vol.% polyvinyl alcohol (PVA) fiber, with fly ash (FA) replacing 50% of cement by weight. The experimental results demonstrated that the synergistic incorporation of NS and PVA fiber significantly improved the mortar's mechanical performance. At 7 days of age, the flexural and compressive strength of mortar containing 1.5% NS and 1.0% PVA fiber increased by 105.8% and 25.1%, respectively, compared to the control mortar (without NS or PVA fiber). Moreover, NS and PVA fiber significantly reduced the capillary absorption rate and drying shrinkage. The composite addition of 2.0% NS and 1.0% PVA fiber led to a notable reduction in drying shrinkage: at 7, 14, 28, 90, and 180 days, the drying shrinkage decreased by 38.3%, 33.3%, 30.0%, 31.6%, and 31.4%, respectively, relative to the control mortar. The scanning electron microscopy and mercury intrusion porosimetry results indicated that NS and PVA fibers effectively improved the micropore structure of the mortar.