Thats right. I remember the engineering dept. of my university would hold "concrete boat" competitions to highlight this point exactly. Concrete is a mixture, and different mistures are used for different purposes. Im not a civil engineer, but I wonder how well would roman concrete perform in 100+ floor buildings or expansive multi-floor complexes. Even if it performs "well", is it cost effective? I doubt it will be better than a min-maxed mix determined by project/area/budget/etc