Nano-engineering the interfacial transition zone between recycled concrete aggregates and fresh paste with graphene oxide

The high water absorption and porosity of recycled concrete aggregates (RCA) typically result in a weak interface transition zone (ITZ), thus severely limiting the mechanical property and durability of the prepared concrete. This study proposes a targeted approach to improve the microstructure of ITZ through conformally coating graphene oxide (GO) on RCA (i.e., GO@RCA). The experimental results reveal that the 28-day compressive/ splitting tensile strength of concrete increases by 27%/40%, when GO@RCA replaces RCA. The GO coating significantly reduces the water absorption coefficient and the total charge passed by 15.1% and 63.4%, respectively. To further explore the practical application of such a strategy, instead of direct coating GO on RCA in the case of GO@RCA, RCA is pre-wetted with GO (termed WGO@RCA) before mixing with cement. Inter-estingly, the data show similar results as the case of concrete containing GO@RCA, strongly indicating the lo-cality of GO at ITZ for WGO@RCA. The enhancement of concrete's mechanical and durability properties resulted from the sub-nanometer thickness of GO coating and the hydrophilic functional groups to promote cement hy-dration at ITZ locally.