Black mold underneath the caulk that is above the tile

[Philippe01]

Hi,
The tile of our pool was redone in June 2022. A few weeks after, black marks appeared underneath the caulk ( see pictures). I think it is black mold but I am not 100% sure. The owner of the company who has done the job made several promises to come before the closing but did not show up. He looks reluctant.
Is there a professional who can tell me why this black substance is there and what was not correctly done by the pool company?

 

[Texas Splash]

That looks to be mold trapped underneath the caulking. Either the area wasn't prepared well before the caulk application, or there are gaps allowing moisture to get trapped in there. Let's see what others think, but I have a bad suspicion the only way you will truly fix the problem is by removing the sealant and cleaning the area really well. In the meantime, you can try spraying the caulked areas with a diluted mixture of water & bleach (50/50) and gently brush with a nylon brush to see if you can get the chlorine into any nooks & crannies. You might get lucky, but I'm a bit pessimistic based on those pics.

 

[BMerrill]

Just a wild shot... Mold develops a film on the outside which is difficult to penetrate with bleach. Aww-some is one product known to break this film. Maybe the Aww-some guy will chime in for guidance.

 

[Philippe01]

Thank you both for your advice. I just read again the warranty and it says:
¨There is no warranty for the material and the labor in filling the gap between the coping and decking with caulk. This is a non-warranty item due to the uncontrollable effects of climate contraction and expansion and, the damage such causes, e.g., the caulk separates and/or cracks.¨
so no luck for me but the contractor said this morning that he will come to clean up. Like Texas Splash, I am not sure that a clean-up would be sufficient.

 

[ajw22]

That looks like the wrong material to use. What product is it? It looks like a silicone sealer.

Water runs under the coping from the ground and you cannot help the area dry if you block moisture in. That blocked moisture leads to mold.

Mortar or grout are cementious products that let moisture through and can breath. That can lead to efflorescence but to me that is preferable to mold.

I would scrape that product off, clean up the mold, and fill the area with grout or mortar.

 

[Philippe01]

Thank you ajw. I think it is silicone. I agree with you, it should have been grout or mortar.