Investigating the mechanism of multi-walled carbon nanotube (MWCNT) in the early cement hydration enables better utilization of the potential of MWCNT as reinforcing agents in cement composites. The interaction of MWCNT with ions and its effect on cement hydration were investigated. Zeta potential measurement was intended to discuss the interaction of the MWCNT surface with Ca2+. Then, the formation processes of C–S–H were observed with a scanning electron microscope (SEM). The influence of MWCNT on the early cement hydration was investigated using isothermal calorimetry. The results showed that MWCNT significantly accelerates early cement hydration. It is likely attributed to its high surface area and strong adsorption for Ca2+, which greatly promotes the migration of ions, especially Ca2+, and thus the precipitation of ions on the cement surface. This facilitates the C–S–H nucleation and growth process, thus the cement hydration. These results also indicated that the MWCNT would not provide a stable nucleation site for C–S–H, since the size of MWCNT is less than that of C–S–H.@en