<p>The synergistic degradation of reinforced concrete (RC) structures under calcium leaching and chloride attack poses a critical challenge for sustainable coastal infrastructures. This study develops a multiscale, multiphysics framework that integrates ion transport, reaction kinetics, and microstructural evolution to simulate the real-time deterioration process of concrete incorporating supplementary cementitious materials (SCMs), including fly ash, slag, and silica fume. Physical adsorption and chemical binding of chloride are modeled separately using ion exchange theory and kinetic formulations. A quantitative relationship is developed between the critical parameters of calcium leaching solid–liquid equilibrium curve, the dosage of SCMs, and the concentration of the aggressive solution. Utilizing the Nernst-Planck-Poisson multi-ion transport model, a coupled framework is constructed, integrating diffusion, electromigration, chemical reactions, pore structure evolution, and mechanical strength degradation. The model's validity and reliability are validated by five sets of experimental data from third-party publications, featuring various mix proportions and SCM dosages, and assessed across multiple dimensions, including free/ total chloride concentration profiles, and the kinetics of bound chloride release. Using the environmental conditions of the Guangdong-Hong Kong-Macao Greater Bay Area as a case, the model analyzes the spatiotemporal evolution of phase composition and structural integrity during coupled degradation. Furthermore, the model quantifies the impact of different SCMs (plain cement, binary, and ternary mixes) on key indicators such as free chloride levels, service life, leaching depth, and strength loss. This framework supports durability design and lifecycle maintenance of marine RC structures and informs sustainable material strategies in line with the GCCA (Global Cement and Concrete Association) 2050 roadmap.</p>