[danmaz74]

I read many times that Roman concrete used volcanic ash, which made it much more durable (but I guess it would be much more expensive at scale today).

[dredmorbius]

I'm fairly certain Roman concrete also relied on chemistry as an alternative to thermal energy in its manufacturing process, as thermal energy simply wasn't as abundant to the Romans.

The longevity may simply be a fortuitous consequence, though a highly significant one.

(A topic I should add to my research list.)

[lostlogin]

Roman engineering was also rather more overengineered. You compare their columns to modern or say gothic ones, and they clearly have a lot of excess strength.

[dredmorbius]

Overengineering doesn't buy you much if your substrate is disintegrating.