cleaning old tiles for new tiling

[zahmed]

i am going to be tiling ontop of some old floor tiles which are directly next to me new concrete floor
i have read you should properly clean the surface of the old tiles to get a good bond/grip
being next to the new floor the old tiles have been left with a very thin layer of concrete i.e. spillage from the concrete mixer & wher builders walking around spreading it
should i scrape/remove this concrete off & clean with sugar soap or just leave & tile on top?

[royaloakcarpentry]

I would advise to take them up.

[jabroni88]

yes i would also take them up be4 laying new 1's