A model is proposed for the hydration rate of slag in slag-blended cement at long term, from 3 days up to 3 years. The model accounts for the main parameters influencing on the slag hydration kinetics: water-to-binder ratio, slag substitution level, temperature, slag Blaine fineness, proportion of reactive slag. Additionally, the chemical composition of both slag and clinker is accounted for through a new composite indicator which is the expected mass ratio of C-A-S-H produced by slag to all hydration products of slag. The model is expressed as an analytical formula based on Knudsen equation. Model parameters are first calibrated on a large experimental database of 239 points from the literature. The model is then validated on 42 independent data points. The reaction rate is shown to increase with temperature, Blaine fineness, reactive slag proportion, and water-to-binder ratio, and decrease with slag substitution level. The proposed composite chemical mass ratio is the most influential parameter. The interpretation is that the higher the amount of C-A-S-H, the slower the diffusion of ions through the forming hydrate layers, which inhibits slag hydration.