What is the cause of my grout staining, and how can I remove the stain?

QUESTION

About a year ago we had a light colored porcelain tile installed on our pool deck. We had a rolled chattahoochee rock deck which we had sealed multiple times with a marine epoxy sealer. This deck was removed with a chipper in preparation for the next layer which was a mortar mix used to pitch the deck along with the tile to move water away from the house. The pitch seems to be perfect because we do not have standing water at all after a rain.

Shortly after the install we noticed some of the grout lines had turned dark. I emphasize that the stains are only on some of the tiles and only on some of the sides. Most, but not all, of the staining seems to be in the same general area.

Since we here in Miami get a lot of rain and construction on our home had been continuing with dust, etc. I thought maybe we had some mold and mildew. I tried a few products to remove mold and mildew but had no luck, so we had the tile acid washed and the grout re-sealed. I know they advise not to use acid wash but we had successfully done that inside the house with what was supposed to be the same tile in a different color. We needed to do that because that tile inside our home had a haze left from the grout which I could not remove because the tile has a small textured surface. Anyway, the stains on our deck are still there and seem to grow during the time when it is dry and has not rained for a couple of days.

I do not know if the grout is sanded or unsanded but it is an off-white. We are now in the rainy season so the rain is keeping the staining down, but it occurred to me today that it might be rust. I tried using hydrogen peroxide on a small area, but I got the same result I always get. A good deal of brown stain comes off on the cloth I use but the grout is still stained and the tile next to the grout also has surface staining. I have notice that the surface stains can be removed with Barkeepers Friend.

One of my other thoughts was that the grout is deteriorating for some reason. It does seem to be cracking in some tiles that do not have staining. Again I point out that although most of the staining is in one general area, there are some tiles in other places that are isolated that also have this staining. At this point I am thinking that our only recourse is to cut out as much of the grout as possible and re-grout and re-seal. I would appreciate any suggestions or advice you might give us.
Thank you so much in advance.

ANSWER

ANSWER - You didn't mention what kind of tile you had.  Natural stone of various types are known to contain ferrous oxide that can lead to staining in the tile or the adjacent grout.

Since the staining is isolated to certain areas, it could be that there is some underlying metal or other contaminate that is causing the staining.  Maybe there are some residuals from the original epoxy chattahooochee rock deck that is causing the staining.

Sometimes the staining could be due to an area being subjected to excessive moisture.  Although the slope of the tile surface may be adequate, the underlying slope under the tile could have low spots where water collects and causes or contributes to the staining.

If it were me I would remove the stained grout and look for underlying evidence of what caused the staining.  I would leave the removed grout joints open to dry and to observe what may or may not occur over several days.  I would regrout and see if the condition reoccurs.  If it does reoccur then it practical, I would remove all of the tiles in that area to look for evidence that caused the staining.

Keep in mind that when you remove the grout and regrout with the same grout, at first the grout may not match well and will stand out.   Over time it normally will blend in.

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