The influence of blast furnace slag on the mechanical properties and mesoscopic damage mechanisms of recycled aggregate concrete compared to natural aggregate concrete at different ages

To improve the mechanical property defects caused by recycled aggregate (RCA) in recycled aggregate concrete (RAC) as well as the effect of blast furnace slag (BFS) addition on the mechanical properties and microscopic damage mechanisms of natural aggregate concrete (NAC) and RAC at long ages. In this paper, the effects of different curing ages (T = 7d, 14d, 28d, 56d, 90d, and 150d) and blast furnace slag contents on the mechanical properties and microscopic damage mechanisms under uniaxial compression of NAC and RAC were investigated, where the cement substitution rate of BFS was 0 % and 35 % (water-cement ratio of 0.46, water-reducing agent dosage of 0.25 %), respectively. Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were used to study the internal pore changes and the generation of hydration products in the specimens, and acoustic emission (AE) showed the microcrack development process. The results showed that the peak stress and modulus of elasticity of the specimens gradually increased with increasing age, the peak strain gradually decreased, and the internal porosity gradually decreased. It is worth noting that the addition of BFS caused the mechanical properties of RAC to decrease at 28 days but enhanced the mechanical properties of RAC at 56–120 days. This was attributed to the fact that the properties of BFS were not yet activated in the early stage, whereas it was activated in the later stage under an alkaline environment and reacted with CH to generate more C–S–H hydration products, which enhanced the interfacial transition zone and reduced the porosity. This highlights the potential of BFS to improve the performance of RCA over long periods of time. In contrast, BFS improved the mechanical properties of NAC throughout the age period and more significantly after 56 days. AE results showed that the peak ring counts lagged behind the peak stresses. Finally, combined with the statistical damage principal model, the fitting was performed by Matlab to analyze the microscopic damage mechanism of the specimen during uniaxial compression. It was found that the changes in the four characteristic parameters (εa, εb, εh and H) followed a certain regularity, reflecting the regular changes in fracture damage and yield damage. This revealed the intrinsic linkage between mesoscopic damage mechanisms and macroscopic mechanical properties. It was demonstrated that it was feasible to speculate on the law of the stress-strain curve at the macroscopic level by using the law of change of the four parameters (εa, εb, εh and H) at the microscopic level.