I think it is a mistake to take the knowledge of the Roman engineers lightly and assume they used this special lightweight concrete by accident.
They built many specialized projects out of concrete using various techniques that could be used today. I remember one article on FR about the way they built a harbor in the eastern Med by setting down forms under the sea to build concrete piers. I think they were ‘internal cured’ concrete as well.
Then there are their acqueducts with concreted tunnels and pipes that had to have very precise ‘drops’ to harness gravity to move the water while still keeping the flow under control.
Once the Roman engineers were gone, the world lost the formula for concrete for a millenia.
I wholeheartedly agree.
They were also better than pretty much anyone at building enormous wooden ships.