Why is Roman concrete considered stronger than modern concrete?

Why was ancient Roman concrete superior to modern versions of concrete?

Back in Roman times, it was vastly superior, and in fact more practical. That is because “modern” ordinary Portland cement (OPC) had not been invented.

“Roman concrete” is a blend of whatever they could use as aggregate, with a blend of lime and “pozzolana” as the binder. The pozzolana is a volcanic ash that combines with the lime to give a calcium silicate hydrate (CSH). That CSH is also present in Portland cement.

The Roman material is said to be more durable than OPC in a marine environment. However, there are downsides. The availability of the pozzolana is limited. The recipe used for the binder and concrete was poorly controlled, due to the variability on ingredients and their mix ratio.

Keep in mind that the relatively small number of “Roman concrete” elements we see today is most likely a tiny fraction of those made 2000 years ago. We should not be fooled into stating that “Roman concrete lasts 2000 years”. They did not use steel reinforcement as a matter of course.

That limited the scope for what they could use the material for. On the other hand, if their material were used with steel reinforcement in a marine environment these days, it would most likely suffer the same fate as OPC bound concrete does. Spalling. Iff it is not a well designed and executed mix, preferably using fly ash, blast furnace slag, silica fume etc, as well as a repertoire of admixtures.