My only question is -- was it better because the Roman Kingdom lasted long enough (~600BCE to 467CE => ~1100years) so that they had time to learn it or did they get lucky with the materials (volcanic ash + lime + seawater). Maybe both.

I also vaguely remember they were remarkably sophisticated with it, too. Like the walls of the Pantheon get thinner as they get higher so they have the strength to stand and last. Pretty amazing stuff.

Why 2,000 Year-Old Roman Concrete Is So Much Better Than What We Produce Today [sciencealert.com]