zlacker

At least those samples to be mixed just right to survive over 1,000 years had a mixture that got stronger with age. There's a big element of survivorship bias.

Also, due to a lack of reinforcement, Roman concrete structures, at least those that survived, avoided putting concrete in tension. Roman concrete won't last very long in areas of buildings that are under tension. Edit: the implication being that using Roman concrete would make many modern building designs unworkable, particularly taller thinner designs that sway a bit in the wind.