Recyclability potential of multi-generation carbonated recycled aggregate concrete under coupling action of high stress and freeze-thaw cycles

With the growing emphasis on sustainable construction, multi-generation recycling of waste concrete has gained increasing attention due to its dual environmental and economic advantages. However, existing studies have largely underestimated the detrimental effects of real-service environments on the recyclability of recycled concrete, particularly freeze-thaw cycles. This study systematically investigates the properties of repeatedly recycled aggregates (RRA) and carbonated RRA (CRRA) after freeze-thaw-loading exposure, along with the durability of repeatedly recycled aggregate concrete (RRC) and carbonated RRC (CRRC). The results demonstrate that successive multi-generation recycling processes lead to progressive deterioration in both RRA and CRRA. Pore structure characterization reveals that carbonation treatment unexpectedly exhibits high efficacy in enhancing the properties of freeze-thaw-damaged RRA, increasing apparent density by 5.4 %, while reducing water absorption, crushing value, and mass loss by 38.0 %, 20.6 %, and 24.9 %, respectively. In concrete, third-generation CRRC (CRRC3) achieved a compressive strength of 33.0 MPa, withstanding 62.6 % more than RRC3, attributable to the enhanced cement hydration of fresh mortar induced by carbonation products. Moreover, CRRC1 and CRRC2 could withstand 300 and 250 freeze-thaw cycles without failure, respectively, with an increase of 50 cycles than RRC1 and RRC2. Notably, CRRC3 exhibited a 43.3 % improvement in chloride penetration resistance compared to RRC3, satisfying the permeability index for a 50-year design service life of structural concrete in an E-type chloride environment as stipulated in relevant standards. In general, it is feasible to apply accelerated carbonation treatment as a modification technique to enhance the properties of RRC exposed to freeze-thaw and chloride-rich environments.