QUESTIONHi, I have limestone laid on new concrete last year and this year some edge around the grouting area turn brown or yellow. I have someone professional using efflorescence cleaner to clean and it’s still there. Can someone tell me what is it?
ANSWERANSWER - Normally if you have a yellow/brown rust-like color stain it is due to a combination of the stone being subjected to excessive moisture and due to the presence of metal, or that the limestone contains the ferrous oxide mineral. It is not uncommon for natural stone to contain ferrous oxide or other metal based minerals that under a wet environment could lead to staining.
There are stone poultices that can be used to remove these stains depending on the source of the stain and how long it has been present. You can normally purchase these poultices through companies who produce stone sealers and cleaners or you can hire a professional stone restoration company to attempt to clean it for you.