During the last few years, the valorization of marine sediment has been marked by the incorporation of this material in cementitious matrices. Some studies have depicted, more particularly, the use of the sediment in alkaline activation of Ground Granulated Blast Furnace Slag (GGBFS) for Civil Engineering applications. The objective of this work is to understand the effect of clear sediment incorporation on the reactivity of GGBFS alkali-activation. Two alkaline activators are used, a sodium hydroxide solution (NaOH) and a mixture of sodium hydroxide and sodium silicate solution (NaOH + silicate). The influence of sediment incorporation on the reactivity of Alkali-Activated GGBFS (AAS) is determined through X-ray Diffraction XRD, thermogravimetry TG and Solid State Nuclear Magnetic Resonance (NMR) analyses. Results show that the presence of sediment does not modify the structure of the hydration products formed, neither the proportion of Aluminum atoms incorporated into hydrate chains. The substitution of slag by sediment causes a delay in the initiation of alkaline activation particularly in the short term, but this effect diminishes at 28 days.