This paper reports the test results on the bonding strength between magnesium ammonium phosphate cement and Portland cement. The bonded specimens were prepared by immersion in Na2SO4 solution. Following immersion, the surfaces and positions of fractures on the specimens were examined. SEM and an optical microscope were used to analyse changes of the interfacial microzone following different immersion periods. Microstructure changes of the magnesium ammonium phosphate cement-based interfacial microzone within the bonded mortar specimens were studied and compared with those immersed in normal conditions. The bonded mortar specimens were found to have weaker bonding strength than that of the mortar specimens in normal conditions. Under normal conditions, slight crystal morphology changes of the interfacial microzonewere observed and the crystal structure appeared compact, while more noticeable changes occurred within mortar specimens immersed in Na2SO4 solution, with the crystal structures within them being more loose. Thebonding behaviour of the magnesium ammonium phosphate cement includes mechanical bonding, mutual diffusion, and chemical bonding. Structural diagrams of the interfacial transition zone (ITZ) in different environments are also presented. Further investigations are needed to determine the performance of the bonding interface microzone of Portland cements and magnesium ammonium phosphate cements within repair materials in order to enhance the performance of cement-based composite materials for concrete repair.