Enhanced microstructure and mechanical properties of cementless ultra-high-performance fiber-reinforced alkali-activated concrete with silicon dioxide nanoparticles

This study investigates the effect of silicon dioxide nanoparticles (nano-SiO2) on the microstructure and me-chanical properties of eco-friendly, cementless ultra-high-performance fiber-reinforced alkali-activated concrete (UHP-FRAAC). Various amounts of nano-SiO2 were adopted in a range of 0%-5% of the mass of silica fume (SF). The experimental results showed that the addition of nano-SiO2 improves the packing density of ultra -high-performance alkali-activated concrete (UHP-AAC) and generates abundant calcium (alumino)silicate hydrate (C-(A-)S-H) gels to increase its density. Therefore, the interfacial bond strength and pull-out energy of steel fibers from UHP-AAC could be enhanced. At 2% nano-SiO2 replacement, the compressive strength of UHP-FRAAC was the highest (184.2 MPa), and its total porosity was the lowest, decreasing from 9.72% to 7.53%. The highest equivalent bond strength and strain energy density of UHP-FRAAC occurred at 2% nano-SiO2 replacement, i.e., 10.9 MPa and 72.5 kJ/m3, respectively. The maximum tensile strength of 14.5 MPa was observed at a nano-SiO2 content of 3%. Higher dosages of nano-SiO2, 3% or more, negatively affected the interfacial bond and tensile properties of UHP-FRAAC due to its agglomeration, but still exhibited higher performance than that of the plain UHP-FRAAC up to the dosage of 5%. Therefore, the optimal dosage of nano-SiO2 in UHP-FRAAC was suggested to be 2% by mass of silica fume.