80 litres of Xylene and a team working floor scrapers, buckets and rags is worth a try but that's a hell of a large area, it'll take more than a little while and a decent PPE budget!
All rags used will have to be disposed of once done as they'll be no good for anything else.
I've had success with this method on Terrazzo which was way trickier than concrete so I'm sure it'll do the trick for you.
I only did about 16 metres square and that took over 4 hours, although I had to take all sorts of care as the area was surrounded by newly laid carpet tiles and edging strip (by newly I mean the fitters were there at the same time as me!).
One question: why?