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This study investigated the interfacial bond strength between a calcium sulfoaluminate cement (CSA) mortar as a repair material and an old substrate. The surface of the old substrate was treated to have a different roughness (smooth, slightly rough and seriously rough) and moisture. In addition, the effect of a polycarboxylate superplasticiser on the bond properties of the CSA mortar was also studied. The results show that the interface between the CSA mortar and the old substrate is a weak area, and all the test specimens break at the interface. As the interface roughness increases, the bond strength at 4h increases from 1.0MPa to 3.1MPa, indicating that the higher interface roughness is beneficial in improving the bond strength. The wet interface has a lower bond strength than the dry interface. Adding an appropriate amount of polycarboxylate superplasticiser is beneficial to improving the bond strength of the mortar. When the polycarboxylate superplasticiser dosage is 0.9%, the bond strength is the highest.