Hi, I have an internal concrete block wall that was tiled when originally built. A couple of the tiles 'lifted', off the wall, and when i took them off, the tiles next to them also came off!!!
The original tile adhesive remains on the concrete block, and is tough to knock off, even with a bolster chisel.
Can i leave this original tile adhesive on the wall, and tile over it?
Also, do i need any special kind of tile adhesive; it's only 6" plain wall tiles.
Many thanks, and have a very happy Christmas.

It would be better to knock off the old adhesive really, but if, and it's a big if, it's solid all the way, you could skim over it with some rapid set adhesive, but it would be better to take it off. As for re-fixing the new tiles, a powdered ( bagged ) adhesive would be the way to go, if your not sure of yourself, get the standard setting adhesive, it will give you plenty of working time.

is the old addy a dispersion (tubbed) or cement based addy?

As said above, it would be best to take it off. The problem with tiling over it is that you cant be sure how secure the adhesive is bonded to the wall, even if it is hard to get off, in other places it may well be loose. Its not a fun job but Id take it off.

Guys, thanks for the information, really appreciated.

Doug, I don't know what type of adhesive was originally used, I didn't do it, i'm afraid.

Guys, am I quicker (God...I hope the answer is "no") with a bolster, or a chisel attachment on my hilti?

try both and see what works best for you mate.

If you have a grinder you could buy a grinding cup and plane down to the brick with this.

I have used a hilt grinder with planing cup and when used with hilti dust extractor this thing left a perfect finnish on the substrate and there was almost no dust as the extractor was used.

give it a good wetting then try scraping

steamer my work