coloured paving slabs

[claybury]

WHAT CAUSES COLOURED PAVING SLABS TO GO MOTTLED, IT LOOKS LIKE GROUND IN DIRT, " BLACK MARKS" HAVE CLEANED WITH A POWER WASHER BUT HAS NOT REMOVED THE MARKS, ANY IDEA WHAT HAS CAUSED THIS PROBLEM, MANY THANKS,

[johnk]

The surface is slightly porous and under a microscope you will see lots of tiny holes which fill with dirt and are inpossible to clean. This also happens with recycled plastic.