of these two main phases in cement, portlandite is rather compact while the gel phase has a very irregular morphology, probably fractal. This work essentially deals with sorption that takes place as absorption in the gel phase, but the same theory is applicable for any of the phases present in cement. It is also able to handle sorption in any combination of phases in the cement. A model for multi component solid mixtures has been developed, essentially following the Guggenheim excess free enthalpy approach. Measurement of Kd for one gel composition makes it possible to calculate Kd values during the entire process of cement degradation. The main limitation is that the concentration of the radionuclide has to be low enough for all higher terms in the series expansion to be negligible. The model is mostly valid for elements forming cations and it also requires the sorbed element to form a compound of reasonably low solubility with one of the anions in cement (hydroxide or silicate). Kd values for Ni in cement has been taken from the literature and used to illustrate the model.