In order to limit the release of CO₂ emissions produced by cement manufacturing, clinker, the major cement component, is often partially replaced by mineral additions such as Ground Granulated Blast-Furnace Slag (GGBFS). Civil-engineering structures made with GGBFS cement can however present cracking at early age (< 28 days) due to re-strained shrinkage that significantly affects their durability and their material transport properties. Self-healing may limit these phenomena. In order to study and quantify self-healing kinetics, X-ray tomography tests for mortars with different GGBFS contents are performed. Results show that X-ray tomography provides valuable information inside the specimens: crack openings and healing products distribution. It is also shown that self-healing evolves rapidly during the first weeks of water curing and it is more important for cementitious materials containing GGBFS. This is due to their lower early age hydration degree allowing an ongoing hydration after cracking and the formation of supplementary C-S-H along the crack.