An innovative electrochemical treatment for the sealing of hardened mortar by nanosilica migration is proposed. The composition of the colloidal suspension and the electric field power were assessed as significant parameters of the transport efficiency. The interaction between the nano-SiO2 and the cement matrix was characterized by different techniques: electrical resistivity measurements, DTA–TGA, BS-SEM with EDX microanalysis coupled and MIP. Results showed that the transport of colloidal nano-SiO2 under the action of the electric field can be achieved. An active interaction with the solid phases of the mortar samples was confirmed from the decrease in the portlandite content of treated mortar and from the smaller C/S ratio found in the solid hydrated phases of the treated mortars. A significant decrease of capillary pores was also identified at the age of 28 days after finishing the treatment. The sealing ability of migrated silica nanoparticles was enhanced with the ageing of the treated sample; smaller amount of micropores was measured with the treated mortar ageing.