Study on macroscopic mechanical properties and mesoscopic damage mechanism of recycled concrete with metakaolin under sodium sulfate erosion environment

The sodium sulfate resistance of recycled concrete (RC) under dry-wet (DW) cycle was studied, and the mesoscopic damage mechanism of RC under uniaxial compression was revealed by using statistical damage theory. The substitution rate of metakaolin (MK) and the number of DW cycles were considered in the test variables. The microstructure of RC was characterized by acoustic emission (AE), scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray fluorescence spectrometer (XRF). The results showed that with the increase of the number of DW cycles, the peak stress and elastic modulus of the specimen increased at first and then decreased, and the peak strain and mass change rate tended to increase. When the cycle times extended from 0 to 120, the peak stress of MK0 increased from -37.2 MPa to -50.7 MPa and then decreased to -29.1 MPa. The deterioration degree and mechanical properties of RC were improved by adding MK. The peak stress of MK15 increased by 26.4% and 16.4% respectively when it was cycled for 0 and 120 times compared with that of MK0. Based on the statistical damage theory, it was considered that there were 2 mesoscopic damage modes of fracture and yield, and the effective stress dominated the damage evolution of specimen during the failure process. The peak state and critical state were distinguished, and the mean values of & sigma;cr/& sigma;p and & epsilon;cr/& epsilon;p were 0.884 and 1.392 respectively. This paper provided experimental support for improving the sulfate resistance of RC, and provided theoretical basis for the use of in erosive environments.