The fact of the matter is that, despite possessing technologies the Romans could hardly have imagined, their concrete lasts longer than ours. Why that should be the case comes down, in large part, to water: we put a great deal more of it into our concrete than the Romans did, in order to pour it more cheaply and easily. But this makes it more fragile and subject to deterioration over time (as seen in the early dilapidation of certain Brutalist buildings), even despite our use of chemical additives and steel reinforcement. Romans concrete was also mixed with seawater, which caused the formation of crystals within the material that actually strengthened it as it aged — thus cementing, as one wag in the comments puts it, the Romans’ place in history. Source: How to Make Roman Concrete, One of Human Civilization’s Longest-Lasting Building Materials | Open Culture