zlacker

I'm not sure, but this probably fits the facts better: the Romans figured out how to make cement with volcanic ash and salty sea water, and it turns out that it gets stronger with age.

It's not clear to me that the Romans knew that, or that it informed their choice of that building material. I suspect it's more that they built with what they had, and got lucky that it turns out to be incredibly durable stuff.

>>samatm+(OP)
The Roman scholar Pliny the Elder described underwater concrete structures that become "a single stone mass, impregnable to the waves and every day stronger."

https://www.nature.com/news/seawater-is-the-secret-to-long-l...

Sounds like they knew.

>>samatm+(OP)
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.