How can I remove a Carpet from concrete stuck down with black mastic/bitumen
I've had to do this in the past in a very similar situation. We used a 'diamond' cone angle grinder wheel, like this:



Image of one in use:



The first step was to pull off as much material by hand or with a scraper / flat shovel, so we were mainly grinding only through the adhesive. The grinder literally takes off a very thin layer of concrete so the strength of the adhesive bond was not a problem.

These cup wheels aren't cheap but you should be able to rent an equivalent tool locally (at least, that is true here in the US). There are also walk-behind grinders which work on the same principle, maybe that would be a good choice though I have not used one.

Angle grinders have some definite safety issues to be aware of which I would absolutely advise reading about if its an unfamiliar tool (even if it isn't). One of them is that this method produces a ton of fine dust. We rigged up a plastic shroud attached to a vacuum around the head of our grinder and that worked really well to contain the dust. (Again, this is where a rental tool might be well worth it if that feature is available).