The effect of curing age on mechanical properties and mesoscopic damage mechanism of recycled aggregate concrete modified with zeolite powder

To alleviate the environmental problems caused by building demolition and to reduce the exploitation of natural resources, the use of recycled coarse aggregate (RCA) instead of natural coarse aggregate (NCA) to prepare recycled aggregate concrete (RAC) is an effective solution. The effects of curing age and zeolite powder (ZP) contents on the uniaxial compressive mechanical properties and mesoscopic damage evolution mechanism of RAC were investigated, where the cement substitution levels of ZP were 0 % and 10 %. Nuclear magnetic resonance (NMR) and scanning electron microscopy (SEM) were used to investigate the internal microstructural changes of RAC and the micromorphological characteristics of the hydration products; and the micro-crack development process of RAC was monitored by acoustic emission (AE). The results showed that with the increase of age, the peak stress and modulus of elasticity of the specimens gradually increased, the peak strain gradually decreased, and the internal microstructure gradually became dense. It is worth noting that the use of ZP deteriorated the mechanical properties of RAC from 7 to 56 days of age, but enhanced the mechanical properties from 90 to 120 days of age. Due to the volcanic ash activity of ZP, more secondary hydration products were generated at the later stage, which improved the densification of the specimen microstructure. This demonstrates the potential of ZP to promote the development of concrete properties under long age. Based on the statistical damage theory and considering the existence of two damage modes, yielding and fracture, the mesoscopic damage evolution mechanism of RAC was analyzed. The relationship between the macroscopic mechanical properties of RAC and the mesoscopic damage mechanism was established by exploring the evolution patterns of the four characteristic parameters εa, εh, εb and H. In addition, the phenomenon that the peak AE signal lags the peak stress was explained from the perspective of effective stress.