Mechanical and microscopic properties of low alkali sulfoaluminate and magnesium phosphate cement slurries modified recycled aggregate concrete

The use of recycled coarse aggregates (RCAs) in concrete often results in a decrease in mechanical properties due to the poor bonding performance between new and old interfaces. This study impregnated RCA with low-alkali sulfoaluminate (LASAC) and magnesium phosphate cement (MPC) to improve the mechanical properties of RCA, and the axial compressive strength and stress–strain characteristics of the recycled aggregate concrete (RAC) were performed. The experimental results showed that LASAC impregnation modification produced thick encapsulation layers and decreased the compressive strength. MPC modified RAC showed good mechanical properties. The thick encapsulation layer caused by LASAC weakened the bonding performance of the interfacial transition zones (ITZs), which led to mechanical properties decreased. MPC modification can effectively improve the ITZs between RCAs and new mortar. Scanning electron microscopy with energy dispersive X-ray spectrometry (SEM–EDS) results showed that LASAC slurries encapsulation layer loosened the ITZs of RCAs and new mortar. The hydration products of MPC acted on RCAs and the new mortar, mainly being clustered on the new mortar to densify the interface between RCAs and the new mortar, which increased the mechanical properties of RAC. Nanoindentation method was used to verify the microscopic mechanical properties between phases in RAC and support the experimental conclusion.