Steel-reinforced concrete does not perform well in salt water. You can use modern concrete without steel and build things about as good as Roman concrete. You'

Steel-reinforced concrete does not perform well in salt water. You can use modern concrete without steel and build things about as good as Roman concrete. You'll pay a whole lot more than reinforced because you'll use at least 3x more concrete and limit the shape -- only arches. Compare Prague's Charles bridge[0] to Seattle's West Seattle bridge. Or compare the windows of the Roman Pantheon (uh, none) to those in any skyscraper. You won't have as much self-healing in salt water as some Roman concretes. But modern concretes also have far more compressive strength. You can still buy lime mortars and build brick masonry houses, if you want to pay for the labor; the materials themselves have never been cheaper. [0] Yes I know it isn't Roman nor especially made of concrete but there's a lot of good information on its construction and it's basically the perfection of Roman-style bridge building, and without reinforcement you need to build just like that.