Influence of nano-SiO2 addition on properties of hardened cement paste as compared with silica fume

The influence of nano-SiO₂ (NS) addition on properties of hardened cement paste (hcp) as compared with silica fume (SF) has been studied through measurement of compressive and bond strengths of hcp, and by XRD and SEM analysis. Results indicated that the influence of NS and SF on consistency and setting time of fresh cement paste showed different. NS made cement paste thicker and NS accelerated the cement hydration process. Compressive strengths of hcp and bond strengths of paste–aggregate interface incorporating NS were obviously higher than those incorporating SF, especially at early ages. And with increasing the NS content, the rate of bond strength increase was more than that of their compressive strength increase. With 3% NS added, NS digested calcium hydroxide (CH) crystals, decreased the orientation of CH crystals, reduced the crystal size of CH gathered at the interface and improved the interface more effectively than SF. The results suggest that with a small amount of added NS, the CH crystals at the interface between hcp and aggregate at early ages may be effectively absorbed in high performance concrete (HPC).