Spatial distribution characteristics and microscopic mechanisms for enhancing mechanical properties of MWCNTs in recycled coarse aggregate shotcrete

This study aimed to investigate the spatial distribution characteristics of multi-walled carbon nanotubes (MWCNTs) in recycled coarse aggregate shotcrete (RCAS) and the influence of the dosage of MWCNTs on flow characteristic, mechanical properties and its microscopic mechanism. In this paper, 100% of natural coarse aggregate (NCA) was replaced by recycled coarse aggregate (RCA). Multiple nano-characterization techniques were adopted, and the wet-mix shotcrete technology and a combined device for variable addition of MWCNTs suspension were used. The experimental results showed that with the increase of the dosage of MWCNTs, the flow characteristic of RCAS in fresh state decreased. In terms of mechanical properties, the compressive strength, splitting tensile strength and flexural strength of RCAS all showed a trend of increasing first and then decreasing, when MWCNTs was 0.09 wt%, the flexural strength and splitting strength of RCAS reached the peak value. When the dosage of MWCNTs was 0.03 wt%, 0.06 wt% and 0.09 wt%, the quantity distribution of MWCNTs in the RCAS was average and centered. Appropriate dosage of MWCNTs (0.06 wt% or 0.09 wt%) had a positive effect on improving the compactness of interface transition zone (ITZ) of the RCA, optimizing the internal pore size distribution of RCAS, accelerating the hydration reaction rate and increasing the yield of hydration products.