Influence of the Microstructure on the Flexural Behaviors of Coral Sand Mortar with Different Supplementary Cementitious Materials

Coral sand mortar is widely used for airport runways, buildings, and foundations on islands and reefs. However, the effect of the microstructure of coral sand and its interaction with different supplementary cementitious materials (SCMs) on the flexural behaviors of coral sand mortar remains unclear. This study systematically analyzed the flexural behaviors and underlying mechanism of coral sand mortar containing silica fume (SF), metakaolin (MK), blast furnace slag (BFS), and fly ash (FA). The flexural strength of the coral sand mortar with four different SCMs ranged from 3.18 to 3.44 MPa. This study innovatively measured the rough fracture area of specimens using digital microscopy after flexural testing. It was 21.25% to 26.25% larger than the cross-sectional area. The proposed method provided a more accurate result of actual fracture energy by accounting for external loading and self-weight work relative to the rough fracture surface area. The results showed that actual fracture energy was significantly lower than theoretical fracture energy (by 20.71% to 27.18%) and differed substantially between the four groups (63.52 to 118.30 J/m2). SF reduced the tortuosity and complexity of cracks. The coral sand mortar containing 20% (mass fraction) SF had the highest fracture energy, with actual and theoretical fracture energies of 118.30 and 143.70 J/m2, respectively. The induction period of hydration lasted 6 hours and was followed by rapid hydration, with the SF-FA group exhibiting the highest reaction rate. The addition of MK resulted in a high aluminum content and the development of needle-like calcium silicate hydrate (C─ S─ H) during hydration, resulting in lower flexural strength. The results offer valuable insights into sustainable applications of coral sand mortar in marine engineering.