Hi I am trying to improve our bathroom. I pulled up the carpet..Gross! Underneath was lanolium..than old Asbestos tiles!! With tons of mold under the partical kick board that goes around the bottom of the wall. The problem is now I find there is a crack in the cement of the floor! What do I use to fix this before installing tile flooring??

sounds like you need to do a full gut job on the whole bathroom with that amount of crud. at the least, cut a hole in the wall and look around with a mirror and flashlight. if there is any contamination, figure on a gut job.

minor cracks in the floor slab mean nothing unless you flunk a radon gas test. something that would catch and hold a dump truck, now that needs fixing. I'd say anything under a quarter inch should benefit from urethane caulk/sealer labelled as a concrete sealing material. gets costly up to about a half inch, but it could still be done.

over a half inch, like maybe an inch crack, you have serious issues. assuming the foundation walls are OK, you might get OK using a chisel to dovetail the bottom of the crack wider, then fill it in with a 5000 psi concrete mix. if there is water entering the crack, use hydraulic cement instead.

larger than that, I'd call a contractor to assess the stability of the slab and foundation.

Thank you for responding! (: Ummmmm.. Looks like I have a lot of work to be done! One other question..if you don't mind? After the repair in concrete..do I lay down some kind of ..something.. before putting in tile? Thank you again!!

There's almost always small cracks in slabs, not that big a deal unless there really big. Post a picture.
That mold could very well have came from the fact some dumb as* installed carpet in a bathroom, a huge no no.

IKaryn:

That crack won't be a problem unless it's opening and closing, in which case that crack will cause problems with your tiling unless you do something to accomodate that movement.

Schluter makes an orange plastic matt that you thin set down to a floor, and then tile directly over that matt. The matt is formed in such a way to allow movement of the concrete under the matt without causing tension or compression on the tiling above the matt.

If I recall correctly, Mapei makes a green matt that allows for movement of cracks under the matt without causing tension or compression in the tiling above the matt. If you go to Mapei's web site, you should be able to find a 1-800 phone number for technical support. If you describe the problem, they can tell you how to solve it.

Once again, that crack won't be a problem unless it's opening and closing. So, you can't go wrong to assume it is active and put down a movement isolation matt to prevent any such movement affecting your tiling.

Just mix up a small amount of hydraulic cement and fill it in. Pay close attention to the directions since it hardens very quickly.
Make sure the Hcement is level with rest of the floor because it's hard to get off. Since this is a concrete slab just clean it well add the adhesive and tile, then grout and seal.