In this study, fly ash and ground granulated blast-furnace slag (GGBFS), by-products from the industry, are used as binder materials to manufacture high-performance mortars that will be used to repair marine concrete structures. Four mortar mixtures were designed with a constant water/binder ratio of 0.49 and the GGBFS was used to substitute for 0, 10, 20, and 30 wt.% of cement. Changes in the engineering properties of the mortars corresponding to the GGBFS replacement levels were evaluated. The findings proved that the presence of GGBFS significantly increased the mortar’s strength rate at later ages and increased the early-stage water absorption level. At 56 days, the mortar specimens containing 30% GGBFS had the highest water absorption capacity of 8.2% and respective values of compressive and flexural strength of roughly 49 and 11 MPa. Additionally, it was discovered that adding more GGBFS to the mixtures reduced the drying shrinkage and delayed the setting time of the mortars.