ACCIONA Infrastructure, a Spanish contractor, uses the caisson method for the construction ofbreakwater structures. One of its floating docks, Kugira, is one of the largest of its kind in the world and itcan produce concrete caissons measuring 70 m long by 36 m wide and 35 m high. Concrete caissonsmade in floating docks are gradually immersed in sea water as they are built; thus, the set concrete isexposed to sea water at a very early stage, within 18 to 48 hours after casting in sliding formwork. In orderto study the durability of slag concrete using this construction method, the properties of concrete exposedto sea water at a very young age have been tested. Specimens were exposed to artificial sea water by theponding method at different ages (16h, 1, 2, 3, 7 and 28 days) for a period of 6, 18 and 36 (analysispending) months. The following properties were measured and compared with unexposed specimens:compressive strength, water and oxygen permeability, pore size distribution, migration coefficient DNT492and chloride profiles. After 6 and 18 months ponding, lower chloride contents were measured for thespecimens 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 themigration coefficient DNT492 decreases significantly with the increase of the age of the concrete. Withregards to water and oxygen permeability, no significant differences were found. In this paper, a summaryof all these works is presented.Apart from this laboratory analysis, real scale tests were performed at site with the aim to increasedurability by the addition of nanosilica particles. The characterization of these real scale tests are alsodescribed and discussed in this paper.