Cement-based disposal systems for radioactive will undergo chemical alterations due to interaction with the environment. One of the most relevant geochemical alteration processes is decalciﬁcation or leaching of the cement phases. Consequently, the cementitious components will evolve through different chemical degradation states, altering physical and chemical parameters. This paper presents an approach in which geochemical modelling serves as a basis for assessing the evolution of geochemical conditions within a cement-based near-surface disposal facility. Geochemical modelling is used to quantify uncertainties related to C–S–H leaching. On the other hand, the concept of mixed tank reactor is used to represent cement degradation within the entire disposal system and linked it with the retardation factor. The usefulness of the approach is demonstrated via a number of case studies concerning leaching of radionuclides from a cementitious disposal facility. The studies reveal that there is a large effect of the conceptualisation on calculated ﬂuxes from the disposal facility. A crucial factor is the amount of radionuclide mass present in the disposal system when large changes in the retardation factor occur, for instance, when different retardation factors exist in different chemical degradation states.