Water is the worst enemy of a concrete subfloor, and especially a concrete subfloor that is below grade. These floors are very vulnerable so you should never install floor tiles of any kind to a concrete tile underlayment until you fix any cracks, even out the high spots or repair any other problems you may find.
Check your concrete floor for level. Divide the floor into imaginary sections of six feet and then use a four-foot bubble level and a carpenter's pencil to mark any areas of the floor that have cracks, high spots, or any other defects you may find.
Keep in mind that cracks could be a sign of a structure defect. Some of which you may be able to repair while others may require more professional help.
Use a good quality hammer and a cold chisel to open any
small cracks, so you can later fill them. Try to angle your chisel
inside the crack so you can create a recess that is wider at the bottom
than on top. This method helps to hold the patching cement more securely.
Wash and Fill in the Cracks
First, wash out the crack with water, and then fill the crack with hydraulic cement or thinset tile adhesive.
Use either a margin trowel or mason's trowel to feather out the edges. You'll want the patch to be level with the surrounding floor.
Fill Up any Depressions
To fill depressions in your concrete floor pour a small amount of either thinset tile adhesive or self-leveling compound into the depression. Then trowel the thinset or self-leveling compound until level.
Grind Down The High Spots
Using a grinding tool equipped with a masonry grit abrasive wheel, grind down any of the high spots in your concrete floor.
A right angle grinder will make the job go quicker. For removing the dust simply hold a vacuum hose near the grinder as you work.
Once you're finished grinding down the high spots, thoroughly vacuum and damp mop your concrete floor.
Check To See If Your Floor Surface Has Had a finish Applied
Thinset or any other tile adhesives will not stick to a concrete tile underlayments that have been waxed, sealed, painted, or finished to a gloss.
Check for any of these finishes by dripping a few drops of water on the cement surface.
If the water bead the surface has a finish on it. If the water soaks in but the surface is otherwise slick or smooth, it most likely was done with a steel trowel.
Roughen Up A Slick Concrete Floor Before You Install Floor Tiles
You can roughen up slick or painted finishes with a sanding block or a floor sander. You can even make your sanding block by attaching a sheet of course abrasive on an eight inch 2x4.
Scuff up the entire floor. No need for a lot of elbow grease. Just a light pressure will do. When you finish vacuum the floor.
Remove Any Wax Or Sealers From Your Concrete SubFloor
remove wax and sealers use a solution of four parts water to one part
Muriatic acid. Muriatic acid is very caustic so be sure to follow the
Wear eye protection and rubber gloves and be sure to ventilate the room. Scour the floor with the Muriatic acid wash and a scrub brush.
Rinse your concrete subfloor with clear water and let the floor dry.