Scientists replaced 40 percent of cement with rice husk cinder, limestone crushing waste, and silica sand, giving concrete a rubber-like quality, six to nine times more crack-resistant than regular concrete. It self-seals, replaces cement with plentiful waste products, and should be cheaper to use.

[u/mvea]

https://newatlas.com/materials/rubbery-crack-resistant-cement/

[u/ImFamousOnImgur]

Yup. It’s quite amazing the amount of knowledge they had. A lot of that knowledge was lost when the empire fell.

They think the secret to the quality was the volcanic rock used, and if I recall, it was especially good at setting underwater even.

[u/Opeewan → u/ImFamousOnImgur]

There's a bit more to it than that, salt plays a big part in it:

https://www.nature.com/news/seawater-is-the-secret-to-long-lasting-roman-concrete-1.22231

[u/TheLamerGamer → u/[deleted]]

It was with trial and error. Early Roman structures where built typical of the time period. Not the marbled white grandeur we often associate with Rome. That was wayyyy later. They also weren't white. Romans had a hard on for deco colors, gaudy oranges and pinks. Rome would have look more like a pride parade, rather than the bleached out marble we depict it as. Anyways, wood frames and mud and clay bricks where far more common. Over time, they developed better and better bricks and mortars. Which eventually formed the recipe for their cement. Which wasn't even really exclusive to Rome. Other peoples ALSO developed the same recipe. However, Rome's social and economical power allowed them to mass produce the stuff in ways other regional powers could not. So we simply associate it with them. It's not something they just sorta came up with in a few years, it was developed over a period of 200+ years. Which absent modern scientific methods is more than enough time.