Therefore, the purpose of this study is to analyse the accuracy of the determination of the chloride diffusion coefficient in the RCM test for various blended cement mortars. The diffusion coefficients are obtained by two approaches: the basic RCM test model (Eqs. (1)–(3)) [5,6] and the extended model presented by Spiesz et al. (Eqs. (6)–(8)) [16,23].