This study reports chemical, mineralogical and petrographic characterization of ancient hydraulic concretes from the Roman piers at Santa Liberata Orbetello (Grosseto) Italy (~ 50 BC), and breakwaters at the harbour of Caesarea Palestinae, Israel (c. 25 BC), drilled by the ROMACONS (Roman Maritime Concrete Study) team in 2003-2005. Both sets of concrete contain a pozzolanic sanidine-and clinopiroxene-bearing tuff, identified as coming from the pyroclastic deposits of the Phlegrean Fields (Naples), the so-called pulvis Puteolanus of Vitruvius. However, the content of tuff changes, being predominant at Santa Liberata, whereas it is only a smaller fraction of the total aggregate at Caesarea that is mostly composed of local kurkar calcareous sandstone, with occasional ceramic fragments. The cementitious binding matrix presents amorphous gel-like, silica-rich CASH, with subordinated" sparry" calcite cement, and unusual dull white grains composed of calcite, tobermorite, and ettringite, apparently derived from reaction with hydrated lime in seawater. Saline encrustations, from the diffusion of chlorides and sulphates, and characteristic authigenic spherical zeolites with the" rosette" texture also occur within the mortar's porosity. These new data put further constraints on the various reactions occurring in roman concretes in over two thousand years