First off there's a wide variety of modern concrete. The portland cement part of concrete is the same but what you add to it and the water/cement ratio can vary quite a bit depending on what properties you want out of your concrete.
Roman concrete had volcanic ash added to it. That's similar to modern concrete that has fly ash added to it. Pozzolan (Roman volcanic ash) might wind up being superior to modern fly ash for a couple of very boring reasons involving cement chemistry that I will skip here.
They've known about that both from chemical analysis of Roman cement and from records that survived from Roman times.
hi! there's lots of room for more info here, but you may be interested in previous responses in these posts
Why was "roman concrete" not used for centuries after the fall of the empire?
related thread from yesterday "Or they used concrete. Medieval Europe thought the ancient Romans were magic for being able to create seamless floors and pillars, but that was because the secret of concrete was lost to the medieval Europeans." How accurate is this quote?
The last thread is still active, but if you have any followup questions on the first two (which are old and locked), just ask them here & include the relevant user's username so they'll be auto-notified
That's probably a better question for /r/askscience or engineering. Especially since there are many many many different types of concrete, high strength, corrosion resistant, sulfate resistant, asphalt concrete, etc.