QUESTIONWe have a glass-walled shower with 2 porcelain tile walls and small limestone tiles on the floor. The grout near the base has started to get mold and mildew. We had a professional give us an estimate to clean the shower and repair/replace the grouting. He would use heated high pressure extraction to clean it. I'm wondering if it is safe to use that method on limestone.
ANSWERANSWER - The reason you will get mildew on the tile in a shower is because the water is not draining or drying properly. This could be cause of the tile not being installed correctly with the correct slope on the tile floor.
The transition joint between the limestone floor tile and porcelain wall tile is not suppose to be filled with a cementitous grout. Many installers make this mistake It should be filled with a resilient mildew resistant ASTM C920 silicone sealant. This allows for movement so it doesn't crack and it waterproofs the joint, and it will help retard the growth of mildew.
There are many different types of limestone. Some are more durable and moisture resistant than others. Not sure why they want to use heat, but it is reasonable to clean it with a high pressure washer and the heat shouldn't affect it, if the tile is installed correctly and the tile is durable enough. They should test a small area first to make sure it works to your satisfaction.
After the tile is cleaned and thoroughly dried then they should seal the tile so it will be easier to maintain.