In porous materials like concrete, the chemical composition of the pore solution varies spatially, particularly in the cover zone. This variation causes diffusion potentials due to differences in ionic mobility. The transport of charged species is not determined solely by their individual diffusion rates; the electroneutrality condition must also be met throughout the pore solution. This study aims to use numerical simulations to enhance our understanding of diffusion potential magnitudes in porous materials and under various configurations. The Poisson-Nernst-Planck (PNP) equations are applied to simulate ion transport, enabling diffusion potential determination based on the spatial distribution of positively and negatively charged ions. Simulations initially focus on porous materials without perm-selective behavior, which align well with values predicted by the Henderson equation; this equation, however, provides only a single diffusion potential value corresponding to the maximum from the simulation. Subsequently, the numerical model is adapted to cementitious materials. Comparison with literature data indicates the need to incorporate perm-selective behavior by adjusting the mobility ratio of negative and positive ions. This simulation study offers valuable insights into evaluating diffusion potentials in porous cementitious materials and correcting electrochemical measurements.