Coral Sand Dissolution in Fresh/Saline Groundwater of Reclaimed Reef Islands: Dominant Mechanisms, Key Factors, and Alteration Effects

Coral sand dissolution may weaken particle strength and compromise the foundation stability of reclaimed reef islands. However, its dissolution mechanisms and associated effects under saline–freshwater conditions remain poorly quantified. This study combined dissolution experiments, inverse hydrogeochemical modeling, statistical analysis, machine learning, and multiscale characterization to identify dominant mechanisms, quantify their contributions, determine key factors, and evaluate alterations in reef island groundwater. Results demonstrated that the dissolution capacity of coral sand (q) ranged from 0.04 to 0.24 mg, increasing with salinity but decreasing with pH and particle size. Coral sand dissolution was mainly controlled by carbonic-acid-mediated dissolution and Ca-Na cation exchange. The cation exchange contribution (p) reached 63–95% under alkaline conditions and increased with pH, salinity, and particle size. Random Forest accurately predicted q and p, with R2 values of 0.875 and 0.980, respectively. SHAP analysis identified salinity and pH as the dominant predictors of q and p, respectively. With increasing q, the relative aragonite content decreased, whereas calcite content increased; particle surfaces became rougher, BET specific surface area and porosity increased by 5–28% and 2–10.5%, respectively, and single-particle compressive strength decreased by 70–87%. These findings provide a theoretical basis for assessing stability and reinforcing coral sand foundations on artificial islands.