Mitigating sulfate ions migration in concrete: A targeted approach to address recycled concrete aggregate's impact

The attached old mortar on the surface of recycled concrete aggregate (RCA) can result in the introduction of sulfate-corrosive substances into the matrix when the parent concrete is exposed to an external sulfate attack. As a result, the presence of corroded RCA (i.e., CRCA) in new recycled aggregate concrete (RAC) could adversely affect its mechanical and durability properties. The interface transition zone (ITZ), which is recognized as the weakest region in concrete, especially in RAC where multiple weak ITZs exist, can act as a pathway for the migration of corrosive substances (in the form of sulfate ions). To address this issue, this study proposes the use of an interfacial coating comprising a cement@silica fume slurry on the CRCA surface (i.e., Slurry@CRCA) to mitigate the migration of sulfate ions into the cement matrix and optimize the ITZ in concrete. Experimental findings demonstrate that the Slurry@CRCA can effectively enhance the ITZ, enabling the concrete containing slurry-coated CRCA (i.e., C-Slurry@CRCA) to exhibit the lowest sulfate ions migration distance of approximately 10 mm and a total sulfate ions migration content of 0.25% among all RAC samples. Furthermore, the incorporation of Slurry@CRCA in concrete leads to significant improvements in mechanical strengths and water sorptivity. These improvements bring the performance of the C-Slurry@CRCA samples to a level comparable to that of C-RCA specimens, ensuring that they meet the necessary criteria for practical applications. The findings of this study highlight the effectiveness of using Slurry@CRCA in reducing the migration of sulfate ions into the cement matrix and optimizing the ITZ. As a result, the utilization efficiency of RCA is increased, offering a novel and sustainable approach for the development of RAC in sustainable and resilient infrastructure.