Identifying Key Pore Structure Parameters for Predicting Apparent Chloride Diffusion Coefficient in Fly Ash Cement Pastes

Understanding chloride transport in fly ash cement pastes is essential for improving durability, yet the relationships between pore structure parameters and the apparent chloride diffusion coefficient remain insufficiently understood. This study investigates these relationships and assesses the relative associations of selected pore structure parameters with the apparent chloride diffusion coefficient within the present experimental dataset. Cement pastes with 0–70% fly ash were prepared at a water-to-binder ratio of 0.53 and cured for 90 days. The hydration phase assemblage was assessed by X-ray diffraction and thermogravimetric analysis, with particular attention to portlandite (CH), ettringite (AFt), and layered calcium aluminate hydrate (AFm) phases. Pore structure was characterized by nitrogen adsorption and mercury intrusion porosimetry. The apparent chloride diffusion coefficient was evaluated by fitting water-soluble chloride profiles obtained from bulk diffusion tests conducted for 30, 60, and 90 days. Fly ash significantly altered the CH content, AFm phase assemblage and pore structure. At fly ash replacement levels of 50–70%, capillary porosity increased from 20.77% at 50% fly ash to 30.31% at 70% fly ash, while the critical pore diameter increased from 47 nm to 75 nm, indicating substantial pore coarsening. At 90 days, the apparent chloride diffusion coefficient initially decreased from 6.3 × 10−12 m2/s for pure cement to 5.7 × 10−12 m2/s at 30% fly ash replacement and then increased to 10.1 × 10−12 m2/s at 70% fly ash replacement. Among the investigated pore structure parameters, critical pore diameter showed the strongest association with the apparent chloride diffusion coefficient after 90 days of immersion (R2 = 0.791).