Mastic adhesive is the most common adhesive used to lay flooring like tile, linoleum and carpet over concrete. Professionals prefer it because it gets incredibly hard when dried, making it very strong. Unfortunately, these very qualities make it extremely difficult to remove with regular cleaning solvents. If you are trying to remove mastic adhesive, you need to buy a strong adhesive remover, which is available at home improvement stores, hardware stores and some department stores.
Scrape up the glue with a razor blade or a paint scraper. Place the blade as flush with the floor as you can and use slow, gentle strokes to separate the glue from the floor. This method of scraping will help prevent gouging the floor with the blade. Discard the glue scrapings.
Pour an adhesive remover over the remaining mastic adhesive. Adhesive removers come in many different formulas, but the safest ones to use are made from soy and citrus. Citrus-based adhesive removers generally take longer to work. Read the instructions on the packaging for information on how long to wait before removing the adhesive. Most products take between 15 and 30 minutes to work.
Wipe up as much of the softened glue as you can with a damp, roughly textured cloth. Scrape up the remainder of the softened glue with the razor or paint scraper. Discard all of the glue scrapings.
Buff away any glue that you could not remove with the scraping tools gently with sandpaper. Work slowly and check the glue often to make sure you are not sanding the concrete below.
Mop the entire area with warm, soapy water and allow the floor to dry. Place fans near the floor and open any windows in the room to help it dry faster.