The numerical modelling of chloride diffusion in concrete structures requires an appropriate description of input parameters. The main inputs for the model are the diffusion coefficient of concrete and derived aging factor. The model itself can be susceptible to the values of these parameters because of the size of the finite elements and size of the time step. Due to the potential use in probabilistic calculations, which requires millions of simulations, it is desirable to create a highly optimized model. It is important to pay attention to the accuracy of the calculation, but also to its calculation time. The presented results show the possible limits of the finite element model of the diffusion in a concrete structure. This is demonstrated on the reference ordinary Portland cement mixture and 32 various binary and ternary concrete mixtures, which show a significant effect of different diffusion coefficients and aging factors on the overall convergence. This study provides guidance on which extreme material parameters, out of the potential range, may adversely affect model results.