Chloride diffusion and binding in concrete containing NO₂ sequestered recycled concrete aggregates (NRCAs)

This study investigates the effect of NO₂ sequestered recycled concrete aggregates (NRCA) on chloride ion diffusion and chloride binding capacity of concrete. Two types of NRCAs were used as a partial replacement for natural fine aggregate in fresh OPC mixtures at 20% and 40% replacement levels by volume. Control mixtures with conventional recycled concrete aggregates (RCA) were also evaluated for comparison purposes. The water-permeable porosity, rapid chloride permeability, long-term chloride diffusivity, and chloride binding capacity of the test concrete mixtures were evaluated. Results showed that the presence of NRCA reduced porosity as well as both short-term and long-term chloride ion permeability compared to conventional RCA containing concrete. Most importantly, the addition of NRCAs enhanced the chloride binding capacity of concrete. Thermogravimetric and X-ray diffraction analyses demonstrated increased formation of Friedel's and Kuzel's salts in concrete due to the incorporation of NRCA. Based on the test results, a mechanism is proposed for the enhanced chloride resistance in NRCA incorporated concrete.