Why Are There Dark Stains in Surface of Limestone After Installed?


Dark Stains in surface of Limestone After Installed - we have just completed a new limestone installation over cmu .after a few days moisture started migrating to the surface of the tiles. white prolite thinset was used in the installation. it was spread with 1/2 x 1/2 notched trowel to cmu and also to the back of the tiles to prevent spotting. the moisture is still there after 3 weeks. tried to used forced air heater to dry the tiles (this was working)prior to grouting but was told by general contractor not to use the heaters indoors. is this normal on 1/2 " thick limestone? if so , how long will it take to dry on its own? maybe dehumidifiers? do you know another method we can use to remove the moisture. haven't grouted yet? wanted walls to be able to breath. i read the letter from the gentleman from new zealand. I cant wait 3 months, can you help me?


ANSWER - You indicated you installed a 1/2" thick limestone over CMU (concrete block) and have not grouted it yet.  Apparently there are dark stains on the surface of the limestone showing that have not gone away.  You think the dark stains is moisture trapped in the stone.

There are many different types of limestone in terms of their physical properties.  Some limestones are very dense, like a porcelain tile, and others are very porous.  It is more likely to see the moisture staining with more porous stones, but they should dry faster. Denser stones will take longer to dry.  You imply that you back-buttered the stones, but I wonder if you have full thin-set contact behind the stone with 95% contact or after you skim coated the backs and then spot bonded the tiles?

I have one project we recently investigated where all the limestone tiles were spot bonded and you could see the 5 spots on the face of the tile.  We thought we would have found mastic as the adhesive, but it was a white thin-set mortar.  This was over steel stud wall so the stains were not due to moisture, but due to something from the thin-set mortar staining the stones.

Sometimes we see where they spot bond stone with thin-set mortar over a concrete substrate that has moisture and the moisture will migrate through the spots to the stone surface leaving stains.

In theory, if the stone has damp spots on the surface then once the source of the moisture is gone the stains go away.  So I would not grout the stone until the stains go away.  I would check to see if there is excessive moisture in the CBU and if there is then the stone won't try until it is dissipated out of the CMU. You could have used a liquid applied waterproof membrane over the CMU to avoid this. If it isn't damp and you have waited over a week for the stone to try and it doesn't look like it is improving then I would say you have a staining problem.  The waterproof membrane wouldn't have helped in this case.

If you can't resolve your problem then you will have to investigate this yourself or hire a company like us to investigate it for you.  Good luck.

2 thoughts on “Why Are There Dark Stains in Surface of Limestone After Installed?

  1. Wes Myers says:

    My name is Wes Myers. I have an unsightly shifting brown stain issue on limestone. Last summer My company repaired an Indiana limestone patio (buff color) that I had installed 15 years earlier. The grading and subsequent vegetation growth around the patio did not allow for proper drainage, so the limestone had deteriorated over time due to the excessive moisture retention. I removed the affected pieces and replaced them with new 2” thick tiles. All pieces were completely backbuttered with thinset with an acrylic additive before installation. I set them in a Portland cement bed (installed dry like a tile setters mortar), and applied a fresh thinset coating on the back surface for adhesion. This is the same process I used 15 years ago without an issue.

    I installed the majority of the tiles during a dry weather period. However, before grouting there was a significant rain event. Afterwards, I noticed brown stains forming around all the edges of the newly installed stone tiles. The stain would wash away when wet but return when the surface dried. It grew worse and worse and spread throughout the whole surface of the stones with each successive rain.

    From my research, I believe that what I am experiencing is a stain from alkaline water reacting with some chemical residue in the stone from the quarrying process. That residue is leaching out to the surface. I was hoping that in time the stain would dissipate, however , the patio retains water at the edges, and though the staining pattern continually shifts, there are still substantial stains remaining to date. The owners and I agreed to wait until late spring 2020 to see if the problem gets resolved on its own.

    Is my assessment of the problem correct? Will the stains dissipate naturally with so much moisture being retained? I have recommended a french drain system being installed around the patio to alleviate the moisture at the beginning, however, nothing has been done to date. Please give me any recommendations you may have.

    Thanks so much for your thoughts,

    Wes Myers

    • Donato Pompo says:

      Without knowing the chemical make up of the stain, or knowing the physical properties of the stone or knowing the underlying conditions it is difficult to make a conclusion of what is causing the staining.

      Could be a high water table or some other source of water migrating up through the stone. Natural stone is just that so the physical properties of the stone will vary not only from piece to piece but from portion of the stone versus another portion.

      I would check out the grout to see if it is contaminated since the staining is along the edge of the stone. Maybe cut out the grout in an area and see if it could be source of the stain. It is possible to test the stain stone to determine the chemical make up of the stain to determine its source, but sometimes it isn’t practical to spend the money to do the testing.

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