The Belgian reactor 1 fuel cladding is made of aluminium. If disposed of in a geological disposal repository, aluminium could come into contact with ordinary Portland cement and is going to corrode to form hydrogen gas and aluminium hydroxide. In this study, the long‐term corrosion was evaluated by electrochemical impedance spectroscopy on metal embedded in cement paste immersed in saturated calcium hydroxide solution under anaerobic conditions. Mass transfer effects were investigated by embedding aluminium in cement paste with two different porosities. LiNO3 was also added to the systems to study its corrosion inhibiting effect. After high initial values, the corrosion rate rapidly declined to reach a steady state after a few days. After ~100 days, the corrosion rate was the lowest when cement paste possessing a low porosity with the addition of LiNO3 was used and the highest when cement paste with high porosity without LiNO3 was used. Finally, scanning electron microscopy–energy X‐ray dispersive spectroscopy revealed that the corrosion product layer was thicker than expected by electrochemical impedance spectroscopy measurements, while it is only composed of aluminium and oxygen. Aluminium was found to diffuse into the cement paste close to this corrosion product layer.