Dual Effects of Brown Algae Impurities on Coral Concrete: Sulfate Resistance Decay and Multiscale Predictive Modeling

Abstract In island construction, the application of coral aggregate concrete (CAC) is challenged by marine biological impurities, particularly brown algae, which impact material performance. This study systematically investigates the effects of brown algae content (0%–6%), water-to-cement ratio, and sulfate ion wet–dry cycles on CAC’s fluidity, air content, compressive/splitting/axial strengths, and sulfate ion diffusion behavior. Results showed that brown algae content ≤2% has negligible effect on fluidity, but higher contents significantly reduce workability. All strength parameters decreased with increasing impurity content and water-to-cement ratio. Early-stage seawater-immersion curing enhanced strength, but prolonged exposure led to significant degradation. A proposed strength prediction model demonstrated strong correlation with experimental data ( {R}²>0.94 ). Under sulfate cycles, CAC’s mass loss rate, relative dynamic elastic modulus, and compressive strength exhibit a nonmonotonic increase-then-decrease trend, and the damage layer thickness grows exponentially with cycle, impurity content, and water-to-cement ratio. Controlling brown algae ≤2% and water-to-cement ratio ≤0.3 optimizes sulfate erosion resistance. Numerical simulations reveal an exponential relationship between erosion depth and cycle, with uneven sulfate ion accumulation in CAC’s porous structure, highlighting diffusion heterogeneity. These findings provide critical technical guidelines for deploying CAC in marine environments, balancing ecological sustainability and engineering durability.