Improving the sulfate resistance of recycled aggregate concrete (RAC) by using surface-treated aggregate with sulfoaluminate cement (SAC)

Recycled aggregate (RA) was surface-treated in a sulfoaluminate cement (SAC) slurry in order to improve the resistance of recycled aggregate concrete (RAC) to external sulfate attacks. Three groups of concrete, were prepared with non-treated natural or recycled aggregate, and surface-treated RA (SRA), respectively. The results indicate that the mechanical performance of SRAC (recycled concrete prepared with SRA) was improved compared to the reference RAC. On the micro scale, a relatively dense SAC coating was observed on the surface of SRA, which led to a layered, sandwich-shaped structure of the new interfacial transition zone (ITZ) formed between SRA and the new cement mortar in SRAC. The sandwich shaped new ITZ presented an overall higher micro-hardness in comparisons with that in the reference RAC. When subjected to external sodium sulfate attacks coupled with drying-wetting cycles, the accumulation of sulfate ions was slower in SRAC, meanwhile the deterioration in mechanical performance of SRAC was less drastic than that of the reference RAC after a long-term sulfate exposure. This paper suggests that the enhanced sandwich-shaped new ITZ should help hinder the SO(4)(2-)accumulation in SRAC. Moreover, the dense SAC coating which well sustained the sulfate-induced tension can protect the old ITZ and old mortar from the attacks of sulfate ions. This explains why the resistance of SRAC to external sulfate attacks was improved by the SAC treatment on RA. (c) 2021 Published by Elsevier Ltd.