Kinetic Electrochemical Reactive-Transport Modeling of Calcium Leaching and Porosity Evolution in Multiphase Concrete

Calcium leaching degrades concrete durability through progressive dissolution and decalcification of cement hydrate phases, yet it is commonly modelled by assuming local equilibrium between the pore solution and solid hydrates. While useful for estimating how much calcium is eventually depleted, such approaches do not fully resolve the kinetic coupling among hydrate dissolution, ionic transport within the bulk concrete and across the concrete-solution interface, and ultimately porosity evolution. Departing from local solid-liquid equilibrium, we couple dissolution-precipitation kinetics of calcium silicate hydrate, portlandite, ettringite, and monosulfate with multi-ionic diffusion and electrochemical migration in a unified concrete-solution domain, wherein interfacial ion concentrations emerge from flux continuity rather than prescribed fixed values. We reveal two coexisting Ca 2+ transport modes: (i) outward diffusion into the surrounding solution and (ii) accumulation at the active dissolution front, where Ca 2+ drives hydrate re-precipitation. The resulting near-surface porosity profile is non-monotonic, highest at the exposed surface due to dissolution, locally falling below the bulk value due to Ca 2+ -assisted re-precipitation, before recovering to a stable interior plateau. Lower w/c ratios reduce bulk porosity, whereas prolonged exposure extends the dissolution-dominated zone deeper into the concrete. Validated against long-term leaching profiles ( t = 3150 d; w/c = 0.30 and 0.55), the framework provides a mechanistic and experimentally grounded basis for predicting porosity evolution and strength degradation induced by calcium leaching in concrete.