Investigation of the Combined Chloride-Sulphate Attack and its Impact on Reinforced Concrete Structures in Coastal-Industrial Environments

Abstract

Chloride ingress and sulphate attack are the primary drivers of Reinforced Concrete (RC) deterioration in coastal-industrial environments. While chlorides trigger reinforcement depassivation, sulphates degrade the concrete matrix through the formation of expansive products. Coastal structures face a simultaneous coupled attack that is significantly more aggressive than single-ion benchmarks. Literature establishes that, for single-ion diffusion, chloride coefficients (Dc) average 10-12 m2/s, while sulphate ingress is markedly slower, often near 10-14 m2/s. In both cases, ionic concentrations decrease with depth, and diffusion coefficients attenuate over time as continuous hydration densifies the pore structure. Combined ion ingress is not a linear process. Initially, the concrete exhibits competitive antagonism, where chloride and sulphate ions share restricted diffusion paths, effectively retarding each other’s penetration by 30–50%. During this stage, reaction products like Friedel’s salt and early ettringite fill the capillary pores, densifying the matrix and improving resistance. However, once these expansive products exceed the concrete's internal tensile capacity, the mechanism shifts. Micro-cracking transforms the matrix into a network of "preferential paths," allowing aggressive agents to bypass diffusion-control and reach the steel rapidly. Furthermore, ingressing sulphates chemically displace bound chlorides from Friedel’s salt due to their higher reactive priority, releasing them as free ions. Simultaneously, sulphate attack consumes portlandite, lowering the pore solution pH. This dual action significantly increases the Cl−/OH− ratio at the reinforcement level, accelerating reinforcement depassivation and electrochemical corrosion in marine structures. Experimental data collected from an industrial RC structure complex located in coastal environment confirms this severity; concrete in high-ingress zones suffered a 25–35% strength reduction, far exceeding the 10–15% loss typical of single sulphate exposure. Half-Cell Potential (HCP) mappings in these zones shifted aggressively to values more negative than -350 mV, signifying a 90% probability of active corrosion. This coupled aggression pushes coastal RC structures into the "Very Severe" exposure category (ACI 201.2R), necessitating advanced multi-ionic diffusion models for accurate service-life prediction.