I tiled our entire house with ceramic tile about 25 years ago.  I used a good brand (can't recall the name right now) of latex modified thinset mortar to adhere the tile to the concrete floor.  I didn't use a substrate.  I just set the tiles to the floor with the thinset.

With the exception of the laundry room, the tile has adhered to the concrete floor with an annoying tenacity.  If we wanted to change to a different style, it would be cleaner and easier to tile over the 25 year old tile, instead of pulling it up.

It's different in the laundry room.  Every washing machine we have ever had leaks a little bit, from the hoses or from a little sloshing from the tub.  The new dryer has a steaming function, so it has a water connection.  Over 25 years the leaks and humidity have caused the thinset mortared tiles to lose their adhesion to the concrete floor.  Some have come off, others are abut to do so.

Can you recommend a good waterproof thinset?  

I started out looking at epoxy thinset mortars.  I found one that would probably work, Laticrete Latapoxy 300, but it costs about $180 for the amount that might be enough.  Laticrete 254R (R means rapid set) is designed for pool tiles and might do the job.  And it costs much less.  Do you have any experience with either?  Any suggestions?

Many thanks from the loose tile guy.

I would suggest you go to John Bridge.com and ask there. You will probably have a pro answer in less than an hour.

I just visited that website and looked around.  If I can't find my answer there, I'll just give up.  

Thanks!

We always used TEC products either Full Flex or Super Flex whichever is the better grade.
I've installed porcelain tile over porcelain tile with no bond loss.
You may want to abrade the surface of the old tile first.

What about ardex 8 plus 9 or laticrete hydro ban cementicious water proof membrane.

Redguard

Maybe a decoupling membrane like ditra

I use Ardex mortar for showers. Mapei is nice to work with as well.

I used the premixed latex fortified mortar in a bucket from HD. It is rated for showers and shower floors, so a little water from the washing machine won't affect it. I used it 4 years ago and not a single piece of tile or trim has loosened yet.

I have had very good results with Flexibond from home depot. About $35 a bag.

I have little tile experience so I'm just spitballing here. What if its not moisture but the vibration from the washer/dryer causing the bond to fail?

J-