Last September, we had a Pavestone Sahara Sandstone laid as a new patio. There are now various stains (or algae) that have developed on the stone. Is this because they were laid on "blobs" of cement, and the porous nature of the stone has absorbed something? Is there any way we can treat this and remove the stains?
He's laid it on blobs alright, but it only needs to be cleaned. Soapy water or mild bleach and scrubbing brush, alternatively a pressure washer, should do the job.
Cleaning will provide only a temporary fix. Spot bedding - always a sure sign of incompetence - will cause the markings to re-appear in a few months time.
And two instances of "four corners meeting" only serve to reinforce the sense that this is not a professional flagger.
Because it's on blobs there is water under the slabs. This keeps the stone moist where the big voids are. The retained moisture allows perfect conditions for algae and lichens to grow. The moisture isn't transferred as freely where the blobs are bonded to the slabs and therefore those bits stay cleaner.
