Hi, I have a concrete bathroom floor that has had the tile removed, 9 in. tiles and am pretty sure that the black tar glue contains asbestos. My question is, can I seal and then put durarock over the glue to lay ceramic tile and if so what type of sealer? Do I just need to have the glue removed? Thanks

You cannot use cement boards on a slab. The adhesive residue may contain asbestos but as part of the matrix of the adhesive, it is considered non friable, so unless you grind, pulverize or sand it, it will not become loose and airborn. While keeping it wet, scrape away any flakey sections of adhesive with a 4" razor scraper, rinse, vac the slurry with a shop vac and then set your tile with a thinset that lists "cutback adhesive" as an allowable subsurface. That's all there is to it. Most highly modified thinsets in the $25/bag range will say go for it over cutback, just double check on the bag.