Use of nano-silica to reduce setting time and increase early strength of concretes with high volumes of fly ash or slag

This paper presents an experimental study to evaluate the effects of nano-silica (NS) on rate of cement hydration, setting time and strength development of concretes with about 50% fly ash or slag. Results indicate that length of dormant period was shortened, and rate of cement and slag hydration was accelerated with the incorporation of 1% NS in the cement pastes with high volumes of fly ash or slag. The incorporation of 2% NS by mass of cementitious materials reduced initial and final setting times by 90 and 100 min, and increased 3- and 7-day compressive strengths of high-volume fly ash concrete by 30% and 25%, respectively, in comparison to the reference concrete with 50% fly ash. Similar trends were observed in high-volume slag concrete. Nano-silica with mean particle size of 12 nm appears to be more effective in increasing the rate of cement hydration compared with silica fume with mean particle size of 150 nm.