In order to determine the effect of the use of limestone sand on chloride ion ingress in mortar, specimens were cast with two different sands: siliceous sand (used as reference) and limestone crushed sand (used for this study). To compare and assess the resistance of this mortar to chloride penetration, two different diffusions tests were employed: slow migration and rapid migration (AASHTO test). In this study, calculation of the effective diffusion coefficient is proposed using a model based on Nernst. Planck equation. The diffusion coefficients from each sample were compared. The results for all tests show that the diffusion coefficients for siliceous sand mortar are larger than those obtained with limestone sand. It appears also that the diffusion coefficient varies as a function of the W/C ratio.