Study on the penetration and diffusion of chloride ions in interface transition zone of recycled concrete prepared by modified recycled coarse aggregates

This work aims at exploring the penetration and diffusion behavior of chloride ions in interfacial transition zones (ITZs) of modified recycled aggregate concrete (RAC). For this purpose, RAC were prepared by using nano-SiO(2 )and carbonation treated recycled coarse aggregates (RCA, with different strength grades of C30 and C60) and immersed in 3.5% NaCl solution for 45 days and 90 days. The mechanical properties and free chloride ion content of the modified RAC were investigated and the topographic characteristics and element distribution patterns of Ca, Si, and Cl in ITZs of RAC were detailed characterized. The results show that the chloride ion will permeate into the interface in ITZs of RAC in the first instance, and then intrude into mortar matrix through the pores and fractures. In contrast, the internal microscopic structure of ITZs become more compact and the number of the cracks are decreased obviously after nano-SiO2 treatment, which can provide effective barrier and significantly reduce the penetration depth of chloride ions. In addition, the high strength RCA can also effectively reduce the penetration and diffusion of chloride ions to concrete matrix due to the compact and uniform microstructure. This work provides a certain theoretical guidance for further improving the chloride erosion resistance of RAC.