Improvement of early-age properties for glass-cement mortar by adding nanosilica

Abstract

Poor early-age performance (e.g. lower early strength, longer setting time) is an important technical challenge for the application of blended cementitious materials containing low reactivity or high volumes of supplementary cementing materials. In this study, the mechanism of using nanosilica (NS) to improve the early-age properties for cement mortars blended with glass powder (GP) and glass aggregates has been investigated. The results indicate that the addition of NS into glass-based cement mortar largely improved the early stiffening which was dependent on high specify surface area of the NS rather than cement hydration. Combining the use of NS and GP was conducive to compensate the delayed setting times and the strength losses caused by the incorporation of GP. These beneficial behaviors were associated with the physical, acceleration, pozzolanic and pore refinement effects of NS. In terms of heat of hydration, the inclusion of NS intensified and accelerated the appearance of the third exothermic peak (AFt to AFm) due to the absorption of sulfate ions by the increased C-S-H formation. Also, the total hydration heat liberated was found to correlate linearly with the corresponding early-age compressive strength. Microstructural analysis suggest that NS significantly helped to densify the microstructure of the GP blended cement matrix and improved the interface between the GP particle and the binder matrix. This was verified by the contribution of NS on refining the coarse pore size caused by the use of GP as a replacement of cement.