I recently removed 9"x9" tiles from my basement concrete slab which have tested positive for asbestos. I am now left with a black adhesive on the concrete which is not thick but rather patchy and resembles a tar-like stain. I've tried an adhesive remover with a metal scraper and it had little effect. We are looking to lay a porcelain tile and I am fearful I will not be able to properly bond to the concrete with this layer. My question is, does this have to be removed and if so, is there a safe way of doing so besides grinding or futile liquid removers?