Concrete is a fundamental material for infrastructure in coastal and saline-alkali regions, which persistently faces a severe challenge of coupled chloride and sulphate attack. Chloride ingress induces steel corrosion, while sulphate ions lead to concrete cracking and mechanical degradation through the formation of expansive products. Under real service conditions, dynamic variations in temperature and humidity significantly affect the transport and reaction processes of these aggressive ions within concrete, resulting in complex competitive interactions between chloride and sulphate ions. Although previous research explored the transport behavior of single ions under constant environmental conditions, the synergistic migration mechanisms and competitive effects under combined salt, temperature, and humidity multi-field coupling remain inadequately understood. It is thus crucial for accurately assessing the durability and service life of concrete structures in complex environments to establish a comprehensive transport model that integrates temperature, humidity, ionic competition, and material damage.

A chloride-sulphate ion transport model was proposed based on multi-species transport theory, explicitly accounting for temperature and humidity variations, ionic competition, and the evolution of material microstructure. In the model, ionic diffusion coefficients were modified via introducing a temperature-dependent function and a humidity-dependent function. A competitive correction factor was also introduced to characterize the competition between ions for diffusion pathways. Within the reaction term, a kinetic parameter was incorporated to describe the decomposition of Friedel's salt (Fs) by sulphate ions, representing the dynamic two-way competitive reaction. Furthermore, the model could account for the volumetric expansion induced by the formation of ettringite (AFt) and Fs and its subsequent impact on the pore structure and transport properties, coupled with a damage function to quantify the acceleration of transport due to cracking. Besides, natural immersion tests were conducted on mortar specimens under various conditions. The concentrations of chloride and sulphate ions at different depths were measured. Numerical simulations were performed using a software named COMSOL Multiphysics, and the simulated results were compared with the experimental data.

The model validation demonstrates a good agreement between the simulated results and experimental data with relative errors for chloride and sulphate ions of below 0.32% and 0.55%, respectively. This indicates the model's applicability under complex salt-temperature-humidity conditions. The further numerical simulations also reveal some key findings. The concentrations of both free and total chloride ions increase significantly as the upper limit of the temperature increases from 283 K to 308 K. The peak values of the competitive interaction-induced nett free chloride concentration [Δc(Cl-F)] and nett total chloride concentration [Δc(Cl-T)] reach 266.55 mol/m³ and 290.91 mol/m³, respectively, indicating that elevated temperatures intensify the competitive interaction between ions. The response of sulphate ions to temperature is less pronounced, without an increase in free concentration, although the total sulphate concentration increases with temperature. High humidity conditions (i.e., RH = 1.0) markedly promote chloride ion diffusion and the formation of corrosion products. The peak values of Δc(Cl-F) and Δc(Cl-T) increase with rising humidity. In contrast, the effect of humidity on the nett free sulphate concentration [Δc(S-F)] is relatively minor. However, humidity significantly affects the distribution of total sulphate ions, resulting in a deeper and wider peak for nett total sulphate concentration [Δc(S-T)] under a high humidity. An increase in the environmental SO₄^2-/Cl^- ratio reduces the penetration depth of chloride ions, but significantly increases Δc(Cl-F), up to 259.39 mol/m³. Conversely, increasing the Cl^-/SO₄^2- ratio reduces the sulphate ion concentration and intensifies the competitive effect on Δc(S-F), although the magnitude of this increase is lower than that occurred in the chloride system. For a 40 mm cover thickness, increasing the temperature from 283 K to 308 K reduces a predicted service life due to sulphate attack and chloride-induced corrosion from 22 a and 43 a to approximately 6 a and 7 a, respectively, compared to the associated exclusive attack. Under a coupled attack, the service lives are 11 a and 16 a, respectively, indicating that coupling retards the degradation process. Increasing relative humidity also substantially shortens the service life, highlighting a critical influence of temperature and humidity control in practical engineering.

This study established a numerical model that could characterize the coupled competitive transport of chloride and sulphate ions in concrete under complex salt-temperature-humidity conditions. The modelling performance was validated with a good accuracy against experimental results. The key findings were summarized as follows: 1) Elevated temperature intensified ionic competition, significantly inhibiting chloride transport, while its effect on sulphate ions was weaker; 2) High humidity promoted chloride diffusion and competitive effects, affecting the distribution of total sulphate ions; 3) Variations in the environmental ionic concentration ratio could regulate the intensity of competitive interaction, particularly affecting chloride ingress behavior; 4) The influence of temperature variation on the transport of aggressive ions was more pronounced than that of humidity, and sulphate attack played a dominant role in controlling the durability of coastal concrete structures. The proposed model could provide a theoretical tool and a design basis for durability assessment and service life prediction of concrete structures exposed to complex environments.