This study systematically evaluates the influence of the recycled coarse aggregate (RCA) replacement rate and the number of freeze–thaw cycles (FTCs) on the frost damage of recycled aggregate concrete (RAC) through rapid freeze–thaw tests, and delves into the underlying damage mechanisms. The findings demonstrate that the incorporation of recycled aggregates deteriorates the frost resistance of concrete to a certain extent, primarily manifested by increased apparent damage with rising FTC numbers and RCA content. Specimens with an RCA replacement rate exceeding 50% exhibited extensive mortar spalling and aggregate exposure after 50 FTCs. The mass loss rate initially decreased in the early freezing-thawing stage, then began to increase after approximately 20 cycles, reaching a maximum of 5.09%. The relative dynamic elasticity modulus (RDEM) decreased with an increase in both FTCs and RCA content, dropping to a minimum of 71.99%. Furthermore, based on the relative dynamic elastic modulus, this study developed a GM(1, 1) freeze–thaw damage prediction model applicable to a full replacement range of 0%–100% RCA with a precision level of Grade I. Microstructural analysis revealed that microcracks and pores within the interfacial transition zones (ITZs) and the surrounding matrix of both NCA and RCA are critical for the initiation and propagation of freeze–thaw damage, thereby elucidating the damage mechanism in RAC.

Grey cast iron releases energy in the form of stress waves when damaged. To analyse the evolution of the physical and mechanical properties and acoustic emission characteristics of grey cast iron under uniaxial compression, acoustic emission signals were collected at different rates (0.5, 1, and 2 mm/s). Combined with load-time curves, damage modes were identified and classified using the parametric RA-AF correlation analysis method. The results indicate the loading rate effects on the strength, deformation, acoustic emission (AE), and energy evolution of grey cast iron specimens. The acoustic emission counts align with the engineering stress–strain response. To better illustrate the entire failure process of grey cast iron, from its internal microstructure to its macroscopic appearance, X-ray diffraction (XRD) and optical microscopy (OM) were employed for qualitative and quantitative analyses of the material’s internal microstructural characteristics. The equivalent crystal model of grey cast iron was constructed using a Particle Flow Software PFC2D 6.00.30 grain-based model (GBM) to simulate uniaxial compression acoustic emission tests. The calibration of fine parameters with indoor test results ensured good agreement with numerical simulation results. Acoustic emission dynamically monitors the compression process, while discrete element particle flow software further analyses the entire damage process from the inside to the outside. It provides a new research method and idea for the study of crack extension in some metal materials such as grey cast iron.