Effect of nano-silica on the mechanical performance and microstructure of silicon-aluminum-based internal-cured concrete

Abstract

This study explores the effect of nano-silica on the performance of internal-cured concrete (ICC) with silicon-aluminum-based lightweight aggregate (LWA) by evaluating the mechanical behavior, microstructure of the matrix and the ITZ between LWA and the surrounding paste and phases of hydration products. Adding nano-silica significantly increased the compressive, split-tensile and flexural strength with a maximum increase of 63.7%, 17.6% and 11.3%, respectively, at a 1.0 wt% dosage. Although nano-silica presented a good filling, nucleation and pozzolanic effect, a higher dosage of 2.5 wt% resulted in a minor negative consequence on concrete microstructure given that a visible gap was identified between LWA and the neighboring paste, leading to a reduction in mechanical performance. The additional water provided by the pre-wetted LWA can compensate for the decreased water absorbed by nano-silica to prevent the decline of relative humidity and promote hydration reactions. In addition to the internal curing effect, the silicon-aluminum-based LWA also presented pozzolanic reactivity and exhibited a good coupling effect with nano-silica, contributing to a strengthened microstructure with a large amount of C–S–H gel tightly bonded and intertwined with other morphologies of hydration products. An increase in internal-water to binder ratio from 0.012 to 0.036 improved the densification of the paste and the composition of hydration products but the impact is less significant. With the good combined effect of nano-silica and internal curing, the mixture with 40% LWA presented excellent mechanical properties.