A simplified model of cement attack by pure water or undersaturated lime solutions is proposed for long-term predictions of concrete degradation. The degradation is modeled by ionic diffusion in the cement pores coupled with mass exchange between the solid and aqueous phases. The local equilibrium hypothesis is discussed and justified for the present application. The only solid phases taken into account are portlandite [Ca(OH)2] and calcium-silicate-hydrate (C-S-H). The model predicts the existence of an undamaged core separated from the free surface being attacked by a sharp portlandite dissolution front with a progressive C-S-H decalcification zone located between this front and the surface. Good agreement has been found between the model and experimental results for the degradation of thin portland cement discs kept for three years in an undersaturated lime solution (1.6 mM/L). The model is completely determined with a few parameters measurable by simple tests (porosity, calcium and portlandite contents, diffusion coefficient). The model predicts a √t kinetics for the cement degradation (as long as an undamaged core remains).