cleaning

Quoted text begins.cleaned them using a high pressure water pump but there are some black
spots on some of the squares which cannot be cleaned that way
End of quote.

Bill, If the black spots can't be removed with a water jet, then they may be
iron stains from impurities in the gravel: try undiluted oxalic acid
(available from a hardware store as timber stain remover). Dampen the
surface and sprinkle the acid crystals over the stain: the acid acts as a
chelating agent, that is, it wraps around the iron molecules and removes
them from the concrete matrix. It also acts as a bleach as Walter has
suggested, and it won't etch the concrete surface.

Simon