Hi. I have a shower with tile on both the walls and floor. It is a natural stone type tile. The grout (at least I think it is grout) at the joint where the wall tile meets the floor tile has turned black with mold and/or mildew and I need to replace it. I think the cause is inadequate ventilation and the fact that the builder never sealed it when it was installed three plus years ago.

My question is what do I use on that joint once I have removed the existing stuff and thoroughly cleaned the joint/gap between the floor tile and the wall tile? I have always heard that you should use caulk between two different substances, like where the tub meets the surrounding tile or wood for example. However, in my case I have the same substance on both the floor and walls - tile is meeting tile. Should I use grout or caulk in that joint?

Thanks.

I'd suggest silicone sealant.

The tile may be the same on the walls and the floor. However, The walls and the floor will have slightly different movement. I know it sounds weird but your concrete floor does actually move. I would not only suggest silicone, as recommended by dclynch, but look for a selant that says it is mildrew proof or contains microban. Mildew is in the air in most homes and it is looking for an inviting place to put down roots. DAP makes a product called Kwik Seal Plus with Microban that claims to kill mildew. Periodic spritzing with 1:3 bleach and water will keep the problem in check.