The impact of original aggregate and attached mortar types of recycled aggregates on the sulfuric acid resistance of geopolymer recycled concrete

Abstract

Recycled concrete aggregate (RA) is composed of original aggregate and attached mortar, both of which are essential in determining the durability and mechanical characteristics of recycled concrete. In this investigation, the sulfuric acid resistance of geopolymer concrete (GPC) was assessed using various RA types, incorporating limestone and quartz as original aggregates, and geopolymer and cement as the attached mortars. The results revealed that although the presence of RA accelerated the degradation of GPC, the concretes maintained a compressive strength of over 20 MPa and exhibited a mass loss ratio of less than 13 % after withstanding 91 days of sulfuric acid exposure. These results suggested that RA was viable to be utilized in environments subjected to acid attack. Notably, when quartz was used as the original aggregate, the GPC demonstrated remarkable acid resistance; the compressive strength and mass loss ratio were minimally affected, at just 1.7 % and 5.8 % respectively. Moreover, the geopolymer, when serving as the attached mortar, exhibited excellent compatibility with the new mortar within GPC, fostering the development of denser interfacial transition zones (ITZs). These robust ITZs played a pivotal role in mitigating the detachment of RA under acidic conditions. Conclusively, for concrete structures in acidic environment, the optimal selection of RA entailed pairing quartz as the original aggregate with geopolymer as the attached mortar. This combination provided superior acid resistance and extended the durability of the concrete.