Performance evolution of the interfacial transition zone (ITZ) in recycled aggregate concrete under external sulfate attacks and dry-wet cycling

Performance evolution of different types of interfacial transition zones (ITZs) contained in recycled aggregate concrete (RAC) under external sulfate attacks coupled with dry-wet cycling is experimentally studied in the labotrary. Comparisons are made between different ITZs in the erosion products, the morphology changes, and the microhardness evolution. Changes in the pore structure and microhardness of the old and new mortars are also discussed. Test results show that sulfate ions accumulate faster in RAC than in natural aggregate concrete (NAC), while the erosion products are the same. Erosion occurs in all ITZs but the predominant erosion prodcuts are not identical: AFt and gypsum are found in new ITZs (i.e., ITZ(1) between the unwrapped old virgin aggregate and the new mortar, and ITZ(3) between the old and the new mortars), while in the old ITZ(2) (embedded inside RA between the old virgin aggregate and the adhering old mortar), fewer gypsum crystals can be seen; due to the dry-wet cycling, thenardite crystals are generated but mainly in ITZ(1) near the exposure surface. It is also found that the size of AFt in ITZ(1) and ITZ(3) is larger than that in ITZ(2). The microhardness of ITZs and cement mortars is found to first increase and then decrease with the growth of the erosion age. The old ITZ(2) presents the most stabilized performance thereby manifesting itself better to resist the sodium sulfate attacks compared to the new ITZ(1) and ITZ(3). The old mortar is inferior to both the new mortar and the ITZs in resisting sulfate attacks at a long erosion age. The new ITZ(1) and ITZ(3), as well as the old cement mortar, should be thereby identified to be the predominant weak points in RAC under external sulfate attacks, on the micro scale. This paper also suggests enhancing these weak points for the purpose of improving RAC's resistance to sulfate attacks in dry-wet cyclic environments. (C) 2019 Elsevier Ltd. All rights reserved.