How Did the Romans Make Concrete That Lasts Longer Than Modern Concrete?

Are we talking about concrete structures in general, or specifically marine concrete structures? This topic (or variants thereof) have been floating around the net for months. They're mostly bullshit. All modern-day PCC (portland cement concrete) structures continue to harden ('cure') for an indefinite period after pouring. A 30 year old building is stronger than a 3 month old building.

There's nothing magic or secret about this phenomenon. Adding fly ash (very similar to volcanic ash) to PCC is a well known ingredient for making high strength concrete.

Related:

http://99percentinvisible.org/article/cracking-case-secret-i...