ACCIONA Infrastructure, a Spanish contractor, uses the caisson method for the construction of breakwater structures. One of its floating docks, Kugira, is one of the largest of its kind in the world and it can produce concrete caissons measuring 70 m long by 36 m wide and 35 m high. Concrete caissons made in floating docks are gradually immersed in sea water as they are built; thus, the set concrete is exposed to sea water at a very early stage, within 18 to 48 hours after casting in sliding formwork. In order to study the durability of slag concrete using this construction method, the properties of concrete exposed to sea water at a very young age have been tested. Specimens were exposed to artificial sea water by the ponding method at different ages (16h, 1, 2, 3, 7 and 28 days) for a period of 6, 18 and 36 (analysis pending) months. The following properties were measured and compared with unexposed specimens: compressive strength, water and oxygen permeability, pore size distribution, migration coefficient DNT492 and chloride profiles. After 6 and 18 months ponding, lower chloride contents were measured for the specimens exposed to sea water after 28 days curing compared to those exposed at early ages. However, this difference significantly decreases within a small depth from the exposed surface. Also the migration coefficient DNT492 decreases significantly with the increase of the age of the concrete. With regards to water and oxygen permeability, no significant differences were found. In this paper, a summary of all these works is presented.