Chloride diffusion behavior in partially saturated fly ash-cement blends: A microstructural simulation approach

Chloride transport in cement-based materials is significantly influenced by fly ash content and the partially saturated conditions typically encountered by concrete structures in service. However, predicting chloride diffusivity while simultaneously accounting for both factors remains challenging. To elucidate the chloride transport mechanism in these unsaturated blended systems, this study develops a comprehensive numerical and experimental framework. From a numerical perspective, a hydration model incorporating solid-phase solubility equilibrium and pore-solution electroneutrality is used to analyze the evolution of pore-solution pH and pore size distribution under different water saturation levels. In addition, an opening-operation-based analysis method is proposed to characterize the pore structure and construct an unsaturated model based on an idealized water-vapor distribution. This approach enables a quantitative analysis of critical pore size and pore connectivity and culminates in a first-passage cube theory for predicting chloride diffusivity. Experimentally, a bespoke apparatus incorporating X-ray computed tomography is designed to accelerate the preparation of unsaturated specimens, and diffusivity is measured at various saturation levels using an electromigration method. The experimental results are then used to validate the proposed numerical model. By analyzing the changes in diffusivity as connected capillary pores, gel pores in the outer hydration layer, and gel pores in the inner hydration layer successively lose water, this study clarifies the fundamental chloride transport mechanisms in partially saturated fly ash–cement systems.