Roman concrete holds up against the erosive properties of seawater better than the materials used today. Scientists discovered it's because they used volcanic ash to make their concrete.
That's part of it, but mainly roman concrete holds up much better because they never used reinforcing rebar. Volcanic ash improves longevity further, but if they had included rebar (volcanic ash or no) all those structures would have rotted away a long time ago.
http://newscenter.lbl.gov/2013/06/04/roman-concrete/