when and where was the concrete first being used ?

Answer 1

When the Roman emperor Caligula built what was called the "golden house", concrete was used in it's construction. Parts of that house still exist today.

Answer 2

The Romans first used a volcanic ash that was
chemically similar to portland cement as concrete.
Concrete is NOT the same as lime mortar.

Answer 3

"In Serbia, remains of a hut dating from 5600 BC have been found, with a floor made of red lime, sand, and gravel."