Effects of nano-silica and nano-limestone on flowability and mechanical properties of ultra-high-performance concrete matrix

The effects of nano-silica/SiO₂ (NS) and nano-limestone/CaCO₃ (NC) on the flowability, strengths and microstructure of ultra-high-performance concrete (UHPC) matrix under different curing conditions were investigated in this study. The NS and NC were incorporated at different ratios as partial mass replacements for cement. On the microstructure aspect, the results verify that the NS acts as an effective filling material, which reduced porous areas and accelerated the cement hydration process by pozzolanic effect. On the other hand, the NC acts mainly as an inert filler material that created a denser microstructure, but accelerated the cement hydration process through boundary nucleation growth effect. On the mechanical properties aspect, a threshold value of the NS and NC contents were found so that the compressive, flexural strengths and flexural to compressive strength ratio of the UHPC matrix were found to increase as the NS and NC contents increased towards the threshold content, and then to decrease with the increase of NS and NC contents when the threshold was surpassed. Corresponding to the highest measured mechanical strengths of UHPC matrix, the optimal contents of NS and NC are around 1.0% and 3.0%, respectively. The research concluded that the NS and NC contents are critical to the performance of UHPC matrix.