This scenario is inspired by this article.
Long ago, it was thought that this super-cool, extra strong material called carbon nanotubes is a recent invention. After all, what ancient culture could have thought up of something as complex and complicated as a piece of metal as thin as an atom with a tensile strength of 4500 tons per square inch (100 times greater than steel)? Apparently, one such people did.
Turns out that whoever invented the forgotten art of Damascus Steel used strands of carbon nanotubes to make it both superplastic and hard at the same time. How did they make it? I have read nothing answering that question.
The real question here involves another material that is shown to be stronger than steel--graphene. In OTL, it was discovered as recently as 2004 and some articles boast it to be 200 times stronger than steel. The article linked above talks about how graphene could be used to make concrete more eco-friendly.
Now the question is--could the Romans, the culture who invented concrete, have the technology to invent graphene and use them in creating concrete?