By integrating circular economy principles into its by-products management, ArcelorMittal is developing cost-effective solutions to reuse its by-products when the technical characteristics match with market requirements. Converter slag is a by-product generated by the steel making process. As dense material (~2 t/m3), the slag might be valued as ballast for gravity foundation bases for offshore windmill replacing natural stones and preserve resources. Due to its content of free lime and free magnesia, converter slag in contact with water will be hydrated and carbonated, causing its volumetric expansion. The current work investigated converter slag behaviour in contact with sea water inside closed structures. Two main mechanisms have been considered regarding slag volume expansion: internal pressure on the walls of the concrete structure and slag particles sticking. Through modeling and experimentation, it has been shown that slag expansion generates only a level increase inside the foundation with a very low contact pressure on the walls: 0.2 MPa, far below ordinary concrete strength (~35 MPa). Inside an experimental prototype (1/5 size), the slag level increased by 5 cm, after one year of contact with sea water, without cohesion between particles, while expansion decreases by 6%. Those results prove that slag hydration takes place by increasing its level without any danger for concrete gravity structures. The assessment of the ecosystem impact of slag used in maritime environment has shown a neutral effect in comparison with natural stone.