As Kd from radwaste elements in concrete systems show a wide range of values, a modelling of cesium and lead immobilization in Calcium Silicate Hydrate (CSH, xCaO.SiO2.H2O, with 0.7 (aged cements)<Ca/Si<1.8 (fresh cements)) matrix has been developed. This model accounts for radwaste surface binding mechanisms on silanol sites (>SiOH) and precipitation equilibria. Values of Kd from cesium and lead in CSH matrix can thus be calculated with Ca/Si evolution and ionic strength effect. Predictive calculations have been carried out with success with different Ca/Si ratios, ionic strengths and liquid/solid ratios, and results are well superimposed with experimental isotherm data. If the CSH really allow accounting for the radwaste behavior in hydrated cement matrices, this model can be used in safety assessment calculations, with varying pH and [Ca2+] as cement degradation state parameters.