Realkalisation is an electrochemical technique for repairing concrete structures damaged by rebar corrosion due to carbonation. The treatment aims at restoring alkalinity of the concrete by application of a continuous current between the rebar, acting as a cathode, and an external auxiliary electrode placed in a carbonate solution and connected to a positive pole of a power supply. Here we report the application of neutron diffraction in the in situ monitoring of a realkalisation treatment, analysing at the same time the development of the electro-osmotic flux and the microstructural variations in the surroundings of the rebar.