Nanostructure of calcium-silicate-hydrates in fine recycled aggregate concrete

Abstract

Our research objective is to foster a multiscale understanding of fine recycled aggregate concrete using a bottom-up approach. We investigate the influence of fine recycled concrete aggregates on the structure of calcium-silicate-hydrates. To this end, we introduce a new thought model for fine recycled aggregate concrete with four levels of structural hierarchy: beam, mortar, binder, and C–S–H gel levels. We characterize the behavior of fine recycled aggregate concrete and natural sand concrete using analytical methods such as X-ray diffraction, and scanning electron microscopy, along with depth-based mechanical testing methods such as micro-indentation and nano-indentation. Micromechanical modeling integrated with statistical deconvolution methods is applied to quantify the phase distribution of natural sand mortar and fine recycled concrete aggregate mortar. We observe that fine recycled aggregate concrete exhibits a higher volume fraction of calcium silicate hydrates. Furthermore, the presence of fine recycled concrete aggregates leads to nanostructural changes at the C–S–H gel level. Fine recycled aggregate concrete exhibits a higher volume fraction of loosely-packed low-density calcium silicate hydrates compared to natural sand concrete. Calcium silicate hydrates in fine recycled aggregate concrete exhibit a higher C–S–H gel porosity. Finally, the C–S–H gel porosity in fine recycled aggregate concrete accounts for 56.2% of the total porosity. These results suggest that to improve the mechanical and transport properties of fine recycled aggregate concrete, it is essential to reduce the C–S–H gel porosity of calcium silicate hydrates and increase the fraction of optimally-packed high-density calcium silicate hydrates.