Was very skeptical about such a detergent, already have many tried my cobblestones to clean, not a success. So I bought Quart concentrate to try it again. Have it as described diluted with water and sprayed onto the surface. Could for days and even weeks nor detect any effect of the product. But now after about half a year, the stones are clean again. But have to say that my patch was very dirty, black algae had (black spots on the surface), I was told. I think it depends on the pollution, how long it takes until you noticed a change. I can only recommend this agent. If quart concentrate effect would have shown faster, I would have awarded 5 stars, only 4. My conclusion give Quart concentrate some time to look around and everything will be fine.
