Mechanical properties, microstructure, and non-destructive strength prediction of alkali-activated fly ash-slag recycled coarse aggregate concrete after exposure to elevated temperatures

Alkali-activated recycled concrete (AARC), formed by combining recycled coarse aggregate (RCA) with an alkali-activated binder, exhibits superior mechanical performance and offers a promising sustainable, low-carbon alternative to conventional concrete. However, under fire exposure, elevated temperatures readily cause strength degradation in concrete structures, which poses serious risks to practical applications. To investigate the macro and micro degradation of AARC after high-temperature exposure and to enable rapid strength prediction and damage assessment, this study tested AARC samples with RCA replacement rates of 0 %, 25 %, 50 %, 75 %, and 100 % under conditions of 20, 200, 400, 600, and 800 °C. Macro behavior was assessed through visual inspection, mass loss, compressive tests, flexural tests, splitting-tensile tests, and failure-mode analysis. Microstructure was examined by BSEM and TG. Finally, ultrasonic testing supported a non-destructive strength-prediction model, and WPT-derived WPES provided an index for thermal-damage evaluation. Experimental results showed that compared to the reference state at 20 °C, AARC with 0 % and 25 % RCA exhibited compressive strength increase of 4.4 % and 1.4 % at 200 °C, respectively. At 600 °C, AARC with 100 % RCA shows a 41.4 % reduction in compressive strength, whereas ordinary concrete decreases by 55 %. Accordingly, the 100 % RCA AARC exhibits better thermal stability. At 400 °C, the flexural and splitting tensile strength of AARC decreased by 58.3 % and 48 %, respectively, indicating higher sensitivity to thermal damage than compressive strength. BSEM/TG observations revealed that the ITZ between the original aggregate and the old mortar primarily exhibited non-propagating microcracks in high-replacement mixtures, resulting in a relatively mild degree of microstructural degradation. This implies that increasing the RCA replacement level can enhance the high-temperature stability of AARC. In addition, ultrasonic pulse velocity (UPV) showed a strong correlation with mechanical indices (R² > 0.94), and a non-destructive strength prediction model was established based on the strong correlation. WPT results indicated that the WPES shifted from higher to lower frequency bands as temperature increased. The derived D s,c can serve as a quantitative indicator of thermal damage and demonstrated a strong correlation with the damage index (DI) (R² = 0.92). When D s,c ≤ 60.4， DI ≤ 54.53 % and the compressive strength exceeds 35 MPa. This study verifies the reliability of AARC under fire conditions and provides engineering guidance for post-fire rapid assessment, non-destructive grading, and repair decision-making. • Clarifies post-heating degradation of FA–slag AARC, supporting waste-reuse and safety assessment frameworks. • BSEM/TG indicate low ITZ cracking and good thermal stability in high-RCA AARC. • Use UPV and WPT to predict strength and quantify damage non-destructively.