In the last decade, the use of the Deep Soil Mixing (DSM) technique to improve low bearing soils has been growing consistently. Usually, the construction of the DSM columns involves large volumes of cement, whose production increases the exploitation of natural resources and CO2 emissions, justifying the development of alternative binders. In the present work, an industrial slag, activated by sodium hydroxide and sodium silicate, was used to stabilize a marine soil, dredged from the Portuguese coast. The mechanical behaviour was assessed, using flexural and compression tests, and the results were compared with the same soil stabilized with high-performance Portland cement. The specimens were cured under saltwater, at a temperature of 18ºC, for 7, 14 and 28 days. The marine soil presented a pH of 3 and it was contaminated with heavy metals like chromium and lead and oxides like sulphur trioxide which with water can become sulphuric acid. This, together with the submerged curing conditions, formed a problematic, although realistic scenario for the application of the alkaline activation technique. The results showed that the alkali-activated slag developed better performances than the Portland cement.