Yorkstone driveway setts - Black after sealing

[kenny155]

Looking for some advice from the experts.

Around 5 years ago I professionally laid my driveway with genuine yorkstone cobles. Done a nice job without any sinking.

Variation of colour was nice from yellows and greys. Decided to seal it with a professional sealer which appears to have done a good job for 99% of the setts.

After around 2 years a couple went totally black on the surface and now there are around a dozen.
My guess is that black mould is under the surface of the sealant. Neither Thick bleach or heavy duty powerwash removes the problem.
I can attach a photo if necessary.

[RAPressureWashing]

Please post some photos, also what sealer did you use? IE brand & product

[kenny155]

Adseal Heavy Duty was the sealer used
I contacted them and they advised me to buy one of their products to remove the coating, treat the black ones and then reseal.
Strange how only a few are badly affected,
I couldn't see how to attach a photo.

[RAPressureWashing]

Adseal Heavy Duty was the sealer used
I contacted them and they advised me to buy one of their products to remove the coating, treat the black ones and then reseal.
Strange how only a few are badly affected,
I couldn't see how to attach a photo.

Shit product, the price is a give away TBH, you got what you paid for I'm afraid, but if you can post some photos I'll try to help, or you can go with their advice. Out of interest what advice did they give for "treating" the black pavers? Again photos will help before I can comment further.

[bespokemasonry]

Hi, I always find that the best cure for dirty flags/patios etc is a solution of Sodium Hypochlorite. It works great on green slime.

If your marks are black, it could be lichen. Which are a lot harder to get rid of. If you post a picture, maybe we can help.