For the adhesion to the drywall, any good construction adhesive should work fine. As to sealing the backsplash to the countertop, I would think getting a good match to the concrete color (I am assuming your are coloring the concrete mix) could be tough, I would suggest clear silicone, in fact, you could use this to adhere it to the wall also. If your interface between the countertop and the backsplash is pretty accurate, there will be an almost invisible bead of the clear silicone remaining. Actually, use more than necessary when setting it, then use a plastic scraper to remove the excess at the joint. Once dry, any residiual silicone can be removed with a razor blade and then final clean/polish with fine steel wool.