Based on the alkali activated slag-limestone powder （AAS-LS） low carbon cementitious composite system with different limestone powder contents， X-ray diffraction（XRD）， Fourier transform infrared spectrometer（FTIR）， thermogravimetric analysis（TG-DTG） and scanning electron microscope（SEM） were used to study the phase， functional groups and microstructure of the hydration products in the atmospheric and underwater areas of marine environment. The results show that during the natural carbonization of AAS-LS pastes in the atmospheric area of marine environment， the main new carbonization products are the metastable vaterite and aragonite， which are the polycrystalline phases of calcium carbonate. Mg（OH）₂ and a trace amount of Ca（OH）₂ are formed in the continuous seawater immersion environment， and the hydrotalcite-like phase appears remarkably after 12 months immersion. Limestone powder have a potential chemical effect in alkali-activated system.