Ion transport mechanism and diffusion model establishment in recycled aggregate concrete subjected to magnesium, sulfate, and chloride attack

Abstract

Recycled aggregate concrete (RAC), which is used in Northwest China, is inevitably subjected to various aggressive environments. However, the aggressive ions of chloride, sulfate, and magnesium and the original ions of calcium and hydroxide in RAC under the combined action of magnesium-sulfate-chloride attack and dry–wet (D&W) cycles have received little attention in previous studies. Therefore, in this study, the content of aggressive and original ions in exposed RAC was quantified, and the influence of the replacement ratio of supplementary cementitious materials (SCMs) on diffusion was investigated. Results showed that the contents of chloride, sulfate, and magnesium in exposed RAC increased with an increase in the replacement ratio of silica fume (SF) and metakaolin (MK). In contrast, the contents decreased with the increase in the replacement ratio of granulated blast furnace slag (GBFS). At the same exposure time and depth, the content of RAC with ternary cementitious materials from the highest to the lowest was in the order of cement–fly ash –SF, cement–fly ash–MK, and cement–fly ash–GBFS. Visual appearance, relative compressive strength, and the diffusion coefficient exhibited a similar change law. An exponential relationship was observed between the diffusion and time delay coefficients and the replacement ratio of SCMs, and the content of aggressive ions on the exposed surface decreased linearly with increasing exposure time. Diffusion models of aggressive ions were established to consider the multiple effects. Aggressive ion–original ion interaction influenced the mineral phases’ composition in the corrosive zone and led to a difference in the diffusion process and depth.