The Effect of Metakaolin and Limestone Powder on the Mechanical Performance and Microstructural Characteristics of Ultrahigh-Performance Engineered Cementitious Composites with Seawater and Sea Sand

Ultra-high-performance seawater and sea sand engineered cementitious composites (UHPSSECC) have gained increasing attention in recent years due to their suitability for marine engineering applications. Compared with ultra-high-performance engineering cementitious composites (UHPECC), UHPSSECC offers distinct advantages, particularly in offshore projects for which the availability of construction materials is limited and transportation costs are high. However, the high cement content in UHPSSECC production contributes to substantial carbon emissions, highlighting the need for more sustainable alternatives. This study investigates the development of eco-friendly UHPSSECC by partially replacing cement with metakaolin (MK) and limestone powder (LP). The mechanical and microstructural properties of UHPSSECC incorporating MK and LP were comprehensively analyzed. Experimental results revealed that the addition of MK and LP slightly reduced early-age strength, and the pozzolanic activity of MK at later ages, coupled with the synergistic reactions between LP and MK, significantly enhanced the formation of hydration products such as monocarboaluminate and hemicarbonate. This led to improved long-term mechanical properties and a denser microstructure. At 90 days, compared to the reference group (without MK and LP), the tensile strain capacity of the group containing 30% MK and 15% LP increased by 27.61%. The combined use of MK and LP increased the frictional resistance at the fiber-matrix interface, enhancing tensile strength. Additionally, the accumulation of hydration products at the fiber-matrix interface densified the interfacial transition zone, enabling fibers to fully exert their bridging effect, thereby improving tensile strain capacity. Advanced characterization techniques, including mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM), thermogravimetric analysis (TGA), and X-ray diffusion (XRD), were employed to systematically evaluate the synergistic effects of MK and LP on hydration product formation and microstructural evolution. This study demonstrates that cement replacement levels with LP and MK can reach up to 45%, achieving optimal long-term strength and tensile strain capacity. These findings offer valuable insights for optimizing the mechanical performance of UHPSSECC and expanding its applications in marine engineering.