Hi Dr Chemical. Please see the attached photos. I’ve been battling with this mould for the last few years. The stuff on the paving is green now and then goes black. It comes off with the high pressure cleaner in the warmer months without too much trouble. The mould on the blocks clean off as well but its gradually getting worse. Is it OK to just continue with high pressure cleaning the paving and the blocks or is there a long term or better solution that will last. The blocks are diamond cut limestone. I use a Box Jet Turbo pressure cleaner. 1350PSI. The area is on the NW edge of our block.
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Caustic Soda will help. Sprinkle flakes on, wet with a misting spray and rub in with a stiff broom. This will loosen it up and the HP cleaning will work better