Influence of coating recycled aggregate surface with different pozzolanic slurries on mechanical performance, durability, and micro-structure properties of recycled aggregate concrete

Surface treatment of recycled aggregate concrete (RAC) using a pozzolanic slurry to cover these aggregates' porous and weak surfaces has been recently considered. This study used pozzolanic slurries of micro-silica, nano-silica, micro-zeolite, and nano-montmorillonite to improve recycled aggregate surface strength. In the first phase of this study, the effect of pozzolanic slurries on the density and water absorption of recycled aggregates was evaluated. Due to the dispersion quality, non-ideal conditions of covering the recycled aggregates surface, and settling a portion of cement replacement materials in the slurry, a correction factor obtained through atomic force microscopy (AFM) and different amounts of cement replacement materials were considered. In the second phase, 15 concrete mixtures were made regarding the variables of varying amounts of pozzolans in slurries (100, 200, and 400 times of the aggregate surface), different percentages of replacing recycled aggregates (50 and 100 %), and the duration of treatment (12 and 24 h). Characteristics of treated aggregates, compressive strength, electrical resistance, chloride ion penetration, the appropriate gravel-to-sand ratio in the mixture design, and the microstructure of the new interfacial transition zones (ITZ) were investigated. The results showed that the most significant improvement of almost 18 % in compressive strength was achieved by replacing 100 % of recycled aggregate, treatment for 24 h, and the amount of pozzolan 400 times the surface of the aggregate with micro-zeolite slurry. Also, this slurry had the lowest penetration amount, with a 35 % decrease in chloride ion penetration. In addition, the optimal time for soaking recycled aggregates and the amount of pozzolan in the slurry was 12 h and 200 times, respectively.