Investigation of diffusion behavior, stability of testing ions, interconversion feasibility and microstructures of iodide and chloride in concrete

Abstract

In coastal areas, the interiors of reinforced concrete (RC) structures are often contaminated or eroded by chloride intrusion. The natural chloride diffusion test method does not accurately assess the service performance status of these structures in terms of their resistance to chloride penetration during maintenance or repair of RC structures. By conducting natural chloride diffusion test and natural iodide diffusion test, the stability of iodide in different environments was analyzed, and the diffusing depth, ion content and diffusing coefficient of chloride and iodide (D(chloride) and D(iodide)) in concrete at different water-cement ratios (w/c) (0.38, 0.47 and 0.53) were determined. The ion-binding capacity of the two ions at different w/c was also analyzed, and the relationship between w/c and D(chloride) and D(iodide) was analyzed by data regression method (DRA), and the accuracy and applicability of the natural iodide diffusion test method were discussed. The microscopic morphology, pore characteristics and chemical phase of matrix were also studied using SEM-EDS, MIP and XRD techniques. The findings show that natural iodide diffusion testing should be performed in dark and airtight conditions to ensure a minimum decrease in iodide concentration. The diffusion behavior of both chloride and iodide in concrete is in accordance with Fick Ⅱ pattern, and both bonding and free ions show an obvious linear relationship. Besides, with the increase of w/c, the content of free chloride and iodide in concrete gradually decreased, while the content of bonding ions gradually increased. However, at the same w/c, the concentration of iodide was relatively higher than that of chloride at the same concrete depth due to the higher molar mass of iodide. As both ions diffuse and migrate in the concrete, this also leads to the generation of two complex salts inside the concrete, 3CaO·Al2O3·CaCl2·10H2O and 3CaO•Al2O3•CaI2•8H2O, and with the w/c decreasing, there is a smaller porosity in matrix and the proportion of macropores gradually increases. Besides, the concept of D(chloride) and D(iodide) conversion coefficient was introduced, and a linear relationship between D(chloride) and D(iodide) was obtained using DRA, and the conversion coefficient also was obtained, and a better quadratic parabolic function relationship was found between the w/c adjustment coefficients. Overall, this study proposes a new natural iodide diffusion test method for the performance assessment of RC structures in some remote islands or marine areas to minimize the test error of ion penetration, and provides a new reference index for the structural design and reinforcement or repair of coastal RC structures.