Sulfate resistance of high-volume fly ash/cement mortars hybrid containing 0~1.5 wt.% of nano-silica (Nano-SiO2, NS) and 0~1.0 vol.% of polyvinyl alcohol (PVA) fibers was investigated in this study. Fly ash was replaced with Portland cement at levels of 60% by weight. The resistance to sulfate attack was investigated by exposing the mortars to 10 wt.% sodium sulfate (Na2SO4) solutions for 72 days, after which change in mass, compressive, and flexural strengths were determined. For comparison, the compressive and flexural strengths of cement mortar after 100 days of curing in water were also investigated. Microstructural deteriorations caused by sulfate attack were analyzed by using scanning electron microscope (SEM). The test results showed that the combination of NS and PVA fibers was effective in enhancing the mechanical properties and the resistance to sulfate attack. After 28 days curing, the hybrid addition of 1.5 wt.% NS and 1.0 vol.% PVA fibers increased the flexural strength by 90% over the control one without NS and PVA fiber. Moreover, regardless of PVA fibers content, due to the crystal nucleus and pore-filling effects, the adding of 0.5 wt.% NS increased the compressive strength by 67.1~118.2%. Chemical reaction took place between fly ash and Na2SO4 as no un-hydration particles could be observed in the samples immersed in Na2SO4 solutions for 72 days, while a lot of un-hydration fly ash particles could be found in the SEM image of mortar after 100 days curing in water. The chemical reaction production could increase the adhesive property and fill the pores of cement mortar. As a result, the compressive and flexural strengths of cement mortars after immersed in Na2SO4 solution for 72 days, were much higher than that after 28 days curing. Moreover, the compressive strength of mortars incorporating 1.0~1.5 wt.% NS was even higher than that after 100 days curing in water, indicating the combination of Nano-SiO2/PVA fiber is effective in enhancing the resistance to sulfate attack.
Keywords: PVA fibers; high-volume fly ash cement mortar; mechanical properties; nano-SiO2; sulfate resistance.