Binder-dependent anti-washout performance and microstructural evolution of ductility underwater engineered cementitious composites

Marine underwater construction requires cementitious composites with adequate anti-washout stability, mechanical performance, and crack-control capacity. This study prepared underwater engineered cementitious composites (ECC) using artificial seawater, coral sand, polyethylene fibers, and a fixed dosage of non-ionic polyacrylamide (NPAM). The binder composition was varied using fly ash (FA), ground granulated blast-furnace slag (GGBS), and coral powder (CP). The effects of binder composition on fresh-state stability, mechanical performance, multiple cracking, and microstructure were evaluated under quiescent immersion and seawater curing. The FA-dominated mixture showed the most favorable fresh-state response, with a 12-h apparent mass gain of 1.59% and the lowest final turbidity of 23.1 NTU. The GGBS-dominated mixture achieved the highest seawater-cured compressive strength of 63.92 MPa and tensile strain capacity of 9.76%, together with an ultimate tensile strength of 8.84 MPa. It also developed 62 cracks with a nominal average spacing of 1.29 mm. In contrast, high-volume CP substitution reduced fresh-state stability and tensile performance and promoted crack localization. It was also associated with lower chemically bound water, a coarser pore structure, and a looser matrix. Overall, FA- and GGBS-based binders provided better fresh and hardened performance, whereas high-volume CP substitution was unfavorable under the investigated conditions. These findings provide a macro–micro basis for designing ductile, anti-washout cementitious composites for underwater construction and marine engineering applications.