How can I completely remove all traces of mastic from old concrete?
I had an entry way floor some 30 years ago that had old hard vinyl tile glued down in it with the blackish-brown type of adhesive. I wanted to replace that tile with a ceramic floor tile so I peeled out all of the old tile. I then used a wide razor scraper to get most of the old adhesive scraped off.

 

After that I purchased a few gallons of aggressive paint stripper, opened every window in the house, shut off all gas appliances and pilot lights and poured out a layer of the stripper across the whole floor. 



I let it sit for a suitable time (may have been about a half hour or so). Then working some with a wire brush and metal dust pan as a scoop I took up all the goop off the floor. 

 

Finally using a bucket and a large sponge I washed up the whole area with a copious amount of hot water to remove all the stripper residue. 



After the floor fully dried the surface was the nicest clean concrete surface you could wish for. I was able to put down the tile setting cement and had no problems with it adhering to the concrete.