Characterization of the interfacial transition zone between wet carbonated fine recycled aggregate and fresh cement paste

Extensive and complex interfacial transition zone (ITZ) was regarded as the primary weak regions when utilizing recycled concrete aggregate (RCA) in concrete. In order to reuse fine recycled concrete aggregate (FRCA) enhanced by a wet carbonation approach developed previously by the authors, this paper investigated the effect of wet carbonation on the ITZ between the adhered old mortar and the fresh cement paste. A modelled recycled aggregate concrete (MRAC) with FRCA ranging from 0.15 to 5 mm was prepared. The physical properties of FRCA and carbonated fine recycled concrete aggregate (C-FRCA) and the mechanical performance of corresponding mortar specimens were compared. The microstructural and micromechanical characteristics of the ITZ were analyzed including the particle size, porosity, width, microhardness, micromorphology, and hydration profiles by a series of experiments. It was found that there was a reduced water absorption and an enhanced density of FRCA after wet carbonation due to a significant increase of carbonation products; The 0.6-1.18 mm FRCA was regarded as a transition group for the effect of wet carbonation; The formation of additional Mc and CS-H in the ITZ with C-FRCA helped to form a denser mix in the ITZ; The wet carbonation increased the microhardness values of the ITZ and reduced its porosity, especially in the innermost 10 mu m from the old mortar; Carbonation of FRCA compensated the compressive strength loss when replacing river sand in mortar specimens while indicated limited effect on the flexural strength. Replacing 75 % river sand by C-FRCA in the mortar specimens would not result in remarkable strength degradation.