A new look at the resistance of recycled aggregate concrete (RAC) to the external sulfate attacks: The influence of the multiple mesoscopic material phases

• Published in: Journal of Building Engineering 2023-04, Vol.64, p.105653, Article 105653
• Main Authors: Zhang, Hongru, Liu, Wenshen, Zhang, Jingbo, Liu, Fujiang, Lin, Xujian, Ji, Tao
• Format: Article
• Language: English
• Subjects: External sulfate attacks; Mesoscopic material phases; Recycled aggregate concrete; Volumetric fraction of aggregate; Water-to-binder ratio
• ISSN: 2352-7102; 2352-7102
• DOI: 10.1016/j.jobe.2022.105653

Recycled aggregate concrete (RAC) prepared with recycled coarse aggregate (RCA) has more complex material phases on the meso scale compared to natural aggregate concrete (NAC), including the old virgin aggregate, the new/old mortar, and the three types of interfacial transition zone (ITZ). These mesoscopic material phases would influence the RAC to external sulfate attacks in different ways. This paper provides a new look at the performance of RAC subjected to external sulfate attacks by considering such influence. The quality and quantity of the multiple mesoscopic phases were changed by taking varying water-to-binder ratios (i.e., w/b) and volumetric fractions of RCA (i.e., VRCA), respectively. The changes in properties of both the material phases on the meso scale and concrete on the macro scale subjected to external sulfate attacks were experimentally investigated. The old mortar and ITZ phases (regardless of the ITZ type) were identified as the weaker phases against external sulfate attacks as their microhardness deteriorated faster than that of the new mortar. Improving the quality of the new mortar/new ITZ phases by taking a lower w/b can greatly enhance RAC's resistance to external sulfate attacks: the ingress of sulfate ions was retarded, meanwhile the degradation in compressive strength and dynamic elastic modulus were postponed obviously. Increasing the volumetric fraction of old virgin aggregates by taking a higher VRCA can also help RAC sustain better under sulfate attacks, even though it was accompanied by the simultaneously increasing quantity of the old mortar and ITZ which were identified as the weaker phases against sulfate attacks. Basically, changing the quality of the multiple mesoscopic material phases had more remarkable effects on RAC's sulfate resistance than changing the relative quantity ratios. This paper thereby suggests that RAC's sulfate resistance should be better enhanced by the strengthening or removal of the old mortar phase, or by the improvement of the new mortar and ITZ phases, rather than adjusting their quantity ratios in concrete. •RAC's resistance to sulfate attacks was studied by considering the influence of the multiple mesoscopic material phases.•The quality and quantity of the material phases were set by taking varying w/b ratios and volumetric fractions of RCA.•The ITZ and the old mortar were identified as the weaker phases against sulfate attacks in RAC compared to the new mortar.•Enhancing the new mortar/new