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

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%, respec-tively, 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 sig-nificant. With the good combined effect of nano-silica and internal curing, the mixture with 40% LWA presented excellent mechanical properties.