Marine concrete structures are exposed to one of the most hostile of natural environments. Many physical and chemical phenomena are usually interdependent and mutually reinforcing in the deterioration of marine exposed concrete: expansion and microcracking due to physical effects increases concrete permeability paving the way for harmful chemical interactions between seawater, concrete and embedded steel reinforcement. Early research in self-healing concrete has focused on the autogenous ability of hydrates to heal cracks over time, this form of healing is however restricted to early and small cross sectional crack width reductions, while limited research is available on the autogenous healing of concrete incorporating GBFS (Ground blast furnace slag). A novel approach to self-heal concrete is a bioinspired technique, where bacteria immobilized in the concrete are activated through crack induced water ingress, forming a mineral healing precipitate [1]. The current study characterises the autogenous healing of blast furnace slag cement (CEM III/B 42.5 N) mortar cubes submerged in both fresh- and synthetic sea- water, as the first step towards developing a bacteria-based self-healing concrete for application in the marine environment. Compression tests of the mortar cubes showed their strength to be in good agreement with the Norm EN 196-1 [2]. ESEM analysis of the specimens after 54 days submersion revealed two distinctive surface crystal morphologies. Specimens submerged in fresh water displayed rhomboidal surface crystals 10 ?m thick, while those specimens submerged in synthetic seawater were covered in a 50 ?m carpet of spicules. EDS analysis showed the mineral to be calcium based, suggests the presence of two calcium carbonate polymorphs. FT-IR analysis of the surface precipitates supported observations made by ESEM, as those specimens submerged in fresh- and synthetic sea- water displayed spectra indicative of two calcium carbonate polymorphs, calcite and aragonite respectively. This research provides a valuable reference from where an improved bacteria-based system can later be developed for combating crack-induced deterioration of concrete in the marine environment.