Behavior of the interfacial transition zone in aafs-based green recycled aggregate concrete at elevated temperatures

Alkali-activated fly ash-slag-based green recycled aggregate concrete (AAFS-RAC) is an eco-friendly alternative for Portland cement concrete (PCC). Nonetheless, the current understanding of its microstructural performance at elevated temperatures, especially concerning the complex interfacial transition zone (ITZ) with recycled aggregates (RA), remains insufficiently explored. This study systematically investigated the physical and mechanical properties of AAFS-RAC containing recycled coarse and fine aggregates from crushed PCC at temperatures ranging from 20°C to 800°C. Microstructural analysis was conducted using SEM, BSEM, and EDS to examine two ITZ types: the A-N interface (unbound aggregate and new paste) and the O-N interface (old paste and new paste). The results revealed that while the compressive strength of each mixture initially rose with temperature, peaking at 200°C before sharply declining above 600°C , the mix with 25% slag demonstrated the highest residual strength. Below 400°C, the O-N ITZ had a higher volume fraction of reaction products, particularly C-A-S-H and high-calcium C-(N)-A-S-H gels, as well as a lower porosity in contrast to the A-N ITZ, enhancing the microstructure. Above 400°C, porosity increased in both interfaces, with matrix damage exceeding ITZ degradation. The expansion of interface cracks between the aggregates and the ITZ, along with the progressive formation of pores and the decomposition of gels in the matrix, contributed to the decline in high-temperature performance. These findings provide a theoretical basis for optimizing AAFS-RAC for high-temperature applications and suggest potential directions for ITZ modification to enhance performance.