Yes, it would be clinker or furnace slag much used as temporary paving and on hardstandings and to create temporary footpaths.
It was the curse of our Bomb Disposal Flights when carrying out airfield and bomb store clearances because the mine detectors of the day (Mk 3A and later 4C) would
'ring' continuously masking any real signals. There was a setting on the control box marked 'PAVE' which was used supposedly to filter out the slag noise. It worked up to a point but if there was any suggestion that the ground being searched had buried objects we would be forced to dig trial holes to get clear of the noise.