Engineered cementitious composites with 100 % coral sand aggregate: Effect of water-to-binder and sand-to-binder ratios on mechanical properties and cracking behavior

: With growing demand for offshore and island infrastructure, developing high-performance, resource-efficient cementitious composites is increasingly vital. Engineered cementitious composites (ECC) using seawater and coral sand as sole mixing water and fine aggregate were developed. The effects of varying water-to-binder (W/B) and sand-to-binder (S/B) ratios on mechanical performance, cracking behavior, and microstructure were systematically investigated. Despite the lower strength of coral sand, its rough morphology enhanced interfacial bonding, enabling Seawater and Coral Sand Engineered Cementitious Composites (SCS-ECC) to achieve compressive strength up to 136.83 MPa (W/B = 0.14, S/B = 0.50), with less than 10% reduction compared to Quartz Sand Engineered Cementitious Composites (QS-ECC). Under tensile loading, SCS-ECC exhibited unique cracking behavior, with cracks penetrating coral particles, unlike QS-ECC where cracks followed the aggregate–matrix interface. SCS-ECC tensile strength increased with decreasing W/B and increasing S/B, peaking at 13.46 MPa. The ultimate tensile strain showed a non-monotonic trend due to competing effects of matrix toughness and aggregate-induced defects. These results suggest coral sand is a viable alternative to conventional aggregates in ECC, offering a sustainable solution for marine and resource-limited construction environments.