Compositions for well cementing comprise a pumpable slurry of cement, water and at least one additive that swells in contact with an underground fluid (hydrocarbon, water or brines or mixtures thereof). In case of failure of the cement matrix after setting, the additive swells when contacted by the underground fluid and restores zonal isolation. The swelling additive is an elastomer compounded with an aqueous inverse emulsion of particles of a polymer comprising a betaine group.