The uptake kinetics of ³²Si and ⁴⁵Ca on cement minerals including C-S-H phases, portlandite, AFm phases, ettringite, and aged hardened cement paste were determined through batch sorption experiments. A two-step uptake kinetics was observed, with a fast initial step during ∼1 day followed by a much slower second step, not yet completed after one year. The working hypothesis that the fast uptake is caused by exchange of the radioisotopes with stable isotopes adsorbed on the mineral surface, whereas the slow uptake step is due to uptake in the crystal lattice during recrystallisation, was tested with the help of phenomenological models that combine surface adsorption and homogeneous recrystallisation. The experimental data could be reproduced satisfactorily using a refined version of the formerly published continuous homogeneous recrystallisation (CHOR) model, supporting the working hypothesis and allowing equilibrium sorption coefficients (R_d; L kg⁻¹) for these radionuclides to be calculated on a mechanistic basis. This provides insight into the intrinsic rates and mechanisms of interaction between cementitious materials and their pore fluids which contain dissolved calcium and silicon.