Experimental study on performance of cast-in-situ recycled aggregate concrete under different sulfate attack exposures

Sulfate attack is a type of widely distributed corrosion to cement-based materials, and the utilization of recycled aggregate concrete is beneficial for reducing construction solid waste and the consumption of natural resources. To study the performance of ordinary cast-in-situ recycled aggregate concrete subjected to sulfate attack under various service conditions, five different exposure conditions, full and partial immersion and drying-wetting cycles for 40 weeks were investigated. The sulfate concentration, compressive strength, mass and diameter variation, and visual appearance of the recycled aggregate concrete cylinders were measured. Furthermore, microscopic analysis of the scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and mercury intrusion porosimetry (MIP) was carried out to observe the microstructure and corrosion products. The micro observations of the interfacial transition zone (ITZ) were analyzed by SEM and EDS. The evolution of the pore structure under sulfate attack was measured by the MIP test. The results show that the specimens under the partially immersed condition endured the most severe damage, and significant mass loss and scaling were observed due to chemical attack and salt crystallization. The specimens under drying-wetting conditions deteriorated far more rapidly than those under fully immersed conditions. In addition, ettringite, gypsum and micro-cracks were observed regardless of the exposure conditions. However, no evident characteristic of ettringite or gypsum appeared in the ITZ. This paper suggests that the key role of ITZ could be to provide a diffusion path for sulfate due to its high permeability and porosity. (C) 2020 Elsevier Ltd. All rights reserved.