A. Dave Gobis, executive director of the Ceramic Tile Education Foundation, responds: Surface-sealing a slab before applying tile is not required or recommended by the tile industry. Tile can readily tolerate basement slab moisture, as long as you choose the appropriate tile and tile setting material. In fact, applying sealer to concrete is likely to cause problems for tile by closing the pores that are required for cement-based compounds to bond to the concrete slab.
In any case, you definitely cannot use mastic, premixed thinset, or any latex-modified product in this application. If the slab is perfectly sealed, the material will not adhere because the pores will be closed; on the other hand, if any moisture does rise out of the slab, the adhesive will probably deteriorate and lose the bond that way.
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