<p>The long-term durability of concrete is essential for sustainable construction and reducing carbon emissions. A key factor affecting durability is the ingress of hazardous substances, making the understanding of ionic diffusion in concrete crucial. While ionic diffusion is significantly affected by the microstructure of cement paste, existing diffusion models often oversimplify this microstructure by neglecting particle internal pores (PIPs). This simplification can lead to inaccuracies in predicting ionic diffusivity and, consequently, the long-term performance of concrete structures. This study addresses this critical gap by developing a hydration-diffusion simulation method to investigate the effects of PIPs on ionic diffusion in hydrated tricalcium silicate (C₃S), a primary constituent of cement clinker. Our simulations revealed that: (1) the effects of PIP size distribution and particle porosity (PP) on ionic diffusion depended on the water content, water-to-solid ratio, solid C₃S content, and hydration age; (2) at early ages, in hydration systems with a fixed C₃S content or water-to-solid ratio, higher PP would result in lower ionic diffusivity in hydrated C₃S paste. However, contrasting conclusions emerged under conditions of fixed water content; (3) the effects of PIPs on the ionic diffusion in hydrated C₃S also varied with hydration age and water-to-solid ratio.</p>