I have a lot of old parquet flooring blocks the base is covered with bitumen which was used to lay them in the first place, quite a lot have some concrete / cement stuck to the butumen and the bitumen is quite thick, what i do is use a blow lamp the type a plummer may use,
this melts the bitumen very quick and all you have to do is scrape the molten bitumen off i use the back of a strong knife but you need to be quick as the bitumen will harden again.
you may need to go over them a second time, also you will need to have some sort of surface to work on i use a old paving slab because you will have to scrape the bitumen off the knife as you work use the edge of the slab.
This is a outside task the hotter the bitumen the easer to remove, this method does not effect the surface of the block.
