> You may wonder why we don’t use Roman concrete today if that is the case; well, one of the reasons as to why is because, although it gets stronger over time and withstands erosion from water, when this cement is still young and has not had time to develop its strength from seawater, it likely does not have the compressive strength to handle modern use.

https://www.forthvalleyconcrete.co.uk/the-differences-betwee...

Also, Roman concrete didn't contain rebar which is necessary in many modern applications and rebar rusts which reduces the overall lifespan of the structure.

That said, apparently Roman concrete has some potential issues in structures adjacent to bodies of saltwater since the saltwater would accelerate the strengthening process.