Effects of nanosilica on the hydration and hardening properties of slag cement

Abstract

Ground granulated blast furnace slag (GGBS) has commonly been used in concrete to reduce energy consumption and pollution emission by replacing cement, but the early-age performance of GGBS-cement samples was unsatisfactory. Although nanosilica (NS) can significantly improve the performance of GGBS-replaced cementitious materials, the understanding of the effect of NS on slag was not consistent and some contradictory conclusions have been reported. In this paper, qualitative and quantitative analysis were used to study the effects of NS on the evolution of hydration and phase assemblage of the GGBS-cement system to reveal the source of performance changes at GGBS substitution of up to 70 wt%. Studies have shown that the compressive strength of GGBS-added samples was lower than control sample at early ages. When the addition amount was not more than 50%, the later age strength was higher, but when the substitution fraction was 70%, negative influence was displayed. The addition of NS continuously improved the mechanical strength, especially at early ages. The hydration heat release rate and cumulative heat of samples decreased with the increase of GGBS content, and NS promoted both parameters. TG results demonstrated that NS consumed CH to generate additional C-S-H gel and promoted hydration. This effect was due to the simultaneous promotion of the hydration of clinker and slag as shown by the analysis of XRD and selective dissolution results. And the enhancement effect of NS was most significant in the early stages, thus compensating the disadvantage of the poor early performance of high-volume-GGBS-replacement cementitious materials.