Cobblestone Driveway - White Residue

Hi Brains Trust,

We are considering laying some dark coloured cobblestones over a proposed concrete driveway.

I have however, noticed too often, a white residue that comes out of dark coloured cobblestones that looks quiet unsightly - refer to photo. We have noticed this happens on new driveways too...

Do you know what this is? How does it happen and why?

Is there a way to prevent it?

Thanks!
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