Microstructural Evolution of the NC-UHPC Near-Interface Composite Region Under Sequential Carbonation and Seawater Exposure.

The long-term durability of repair systems combining normal concrete (NC) and ultra-high-performance concrete (UHPC) in marine environments depends on the response of the near-interface composite region to sequential carbonation and seawater exposure. However, the effects of seawater immersion following pre-carbonation remain insufficiently understood. This study compared an unexposed reference (REF), specimens carbonated for 28 d (C28), and specimens carbonated for 28 d and then immersed in simplified artificial seawater for 60 d (C28-SW60) using X-ray diffraction, thermogravimetry, backscattered electron imaging with energy-dispersive X-ray spectroscopy, and mercury intrusion porosimetry. Pre-carbonation promoted portlandite consumption, carbonate formation, and pore refinement. Subsequent seawater immersion further enhanced calcite-related diffraction and carbonate decomposition signals, while no typical crystalline salt-attack product was detected as dominant. The initial Ca-rich-to-Si-rich gradient from the NC side through the overlay transition zone to the UHPC side was accompanied by marked Cl accumulation and further S and Mg enrichment and redistribution. After seawater immersion, the measured total intrusion volume increased from 0.026 to 0.043 mL/g, the volume-based median pore-entry diameter increased from 27.49 to 58.42 nm, and the >1000 nm pore-volume fraction reached 39.82%, a change consistent with a shift toward coarser mercury-accessible pore entries. Together, the results link the initial heterogeneity of the NC-Overlay transition zone (OTZ)-UHPC region to a sequence-dependent response in which carbonate enrichment coexisted with multi-ion redistribution and transport-relevant defects, distinguishing carbonate accumulation from sustained near-interface refinement.