Posterosion Performance of Seawater–Sea Sand Concrete: Synergistic Reinforcement by Ultrafine Metakaolin and Nano-TiO2 under Tidal Action
The direct use of untreated seawater and sea sand for concrete production may adversely affect the mechanical properties and durability of concrete, thereby limiting their widespread application in civil construction. To address this issue, this study has investigated the performance evolution of seawater–sea sand concrete (SWSSC) under long-term exposure to a natural marine tidal environment. The effects of adding ultrafine metakaolin (UMK) and nano - TiO 2 (NT) were also explored in their enhancement on the SWSSC performance. Seven mix designs, including control, binary (UMK), and ternary (UMK and NT) mixtures, were exposed to real-world tidal wetting–drying cycles for up to 360 days. Mechanical properties, including cubic and axial compressive strengths, elastic modulus, and flexural strength, were periodically evaluated. The results showed continuous strength development in all mixtures, with the ternary group exhibiting the most balanced and cost-effective improvement, where SWSSC-UMK-NT10 reached a 28-day compressive strength of 65.4 MPa. The synergistic action of the UMK and the NT also markedly enhanced the matrix compactness and cracking resistance. Scanning electron microscopy (SEM) revealed the refined pore structures and denser interfacial transition zones in the modified SWSSC. X-ray diffraction (XRD) confirmed a reduction in the harmful crystalline phases and an increase in the formation of dense hydration products. Furthermore, mercury intrusion porosimetry (MIP) demonstrated the reduced total porosity, along with a shift in the pore size distribution toward the finer gel pores, thereby restricting the fluid transport pathways. In conclusion, the combined addition of UMK and NT is an effective strategy to improve the microstructure and durability of SWSSC in marine environments.