The hydration kinetics of a compacted low-density bentonite is investigated from both experimental and numerical points of view. A water retention curve and water permeability under saturated conditions were determined to characterize the hydraulic behaviour of the compacted material. The evolution of swelling strains developed during a soaking test performed under oedometer conditions is interpreted as a measure of the hydration kinetics. While soaking was immediate, the vertical strains appeared to develop very slowly during the first two hours of testing. Numerical simulations of the soaking test were performed to achieve a better understanding of the hydration mechanisms. It is shown that the numerical results overestimate the hydration rate of the sample. Two explanations for the very slow hydration are proposed and discussed in the paper, namely the formation of clay gel and an initial drying of the sample.