[2III7]

The concrete the Romans made actually hardened over time because they used volcanic ash and quicklime which fills micro cracks by itself. Modern concrete lasts structurally only a couple of hundred years. There will be no detectable modern concrete in millions of years. It'll be decomposed to minerals in a couple of thousand years.

[andrewflnr]

Decomposed to... what, though? Minerals indistinguishable from natural accumulations? Doubt it. Even if it turns to powder with rust stains where the reinforcement was, that'll be sufficient clue. (At least if it gets buried, which I'll admit is a key assumption that won't be true for all cities.)