Frost resistance durability and damage evolution law of recycled concrete incorporating ultra-fine fly ash under rapid freeze-thaw cycling conditions

Pressing environmental and resource challenges in the construction industry have accelerated the use of recycled aggregates and fly ash (FA) to support the low-carbon transformation of construction materials. This study addresses key challenges in applying recycled concrete with composite recycled aggregates and FA, including developing application guidelines, clarifying freeze–thaw failure mechanisms, and evaluating freeze–thaw damage evolution. The freeze–thaw performance of recycled concrete with ultra-fine fly ash (UFA) was investigated through rapid freeze–thaw tests, complemented by X-ray diffraction and scanning electron microscopy. Ultra-fine fly ash enhances the compactness of the interface transition zone of recycled aggregates and can significantly optimize the microstructure of recycled concrete. Frost resistance in recycled concrete containing both recycled aggregate and UFA can be enhanced with adequate curing time. Microscopic observations elucidated the coupling mechanism by which recycled aggregate and UFA influence freeze–thaw damage in concrete. It also confirmed that recycled concrete with UFA is well-suited for urban river and lake projects in humid environments, and the recommended dosages of recycled aggregate and UFA are not exceeded 100% and 40%, respectively. The damage evolution of recycled concrete with UFA is quantitatively characterized through the development of a control parameter ( ξ n ) for the freeze–thaw damage. Comprehensive analyses establish a novel set of accurate and robust equations for assessing damage evolution in recycled concrete with UFA under freeze–thaw cycles. These findings and the proposed freeze–thaw models provide practical tools for designing durable, low-carbon recycled concrete for challenging environments. • Freeze–thaw damage mechanism of recycled concrete incorporating UFA is elucidated. • Freeze–thaw damage evolution in recycled concrete incorporating UFA is robustly characterized. • A series of equations are derived to assess freeze–thaw damage in recycled concrete incorporating UFA.