There is no one chemical to clean all paving slabs, much depends on the mineral/colouring content of each slab.
Remember to test first on a small area
Try these: Lemon Juice - let it soak in for 5 mins then scrub with brush and then wash off with clean water, or you could use white vinegar (spirit vinegar) in the same way.
As a last resort - Oxalic Acid is good on concrete or stone, im not sure on using it paving slabs, but its very toxic, so use with caution!
