Your coin is more than likely a Roman Bronze coin...perhaps a Follis, there are quite a lot of Roman Bronze coins that have the letter K on the reverse, but with out any Legend to read, a ID will very difficult, or impossible
The crud on it may not be cement, but rather 1500 years of corrosion mixed with the dirt it was sitting in. I have tried to clean some Roman coins, and some of that stuff does take a jackhammer, but then there is nothing left of the coin.