I was told that I could just go over the remaining adhesive, that's black but not sticky, if I put down porcelain tile's with mortar type adhesive.
In my opinion that may lead to a failure in the adhesion of the tile. I would scrape any glue off the concrete prior to thinset of the tile.
Agree with redrover, my motto is if I don't know what is is (especially chemical makeup), it's best to remove it since it could have unfavorable reaction with anything put on top of it.
I also agree, it's always better to get rid of old adhesive as you don't know what problems it could cause later on
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