Unfortunately, this is a natural phenomenon called "Efflorescence" and can appear when using cement-based products that are constantly exposed to water or wet environments. Efflorescence is a process when the soluble salts dissolved by rain and groundwater in the cement, soil, water, sand, gravel or other sources crystallize and are carried to the surface of the stone or joint. Water moves from areas with lower temperatures to places with higher temperatures e.g. when the sun warms the surface of the paving it pulls the water through the paving and joints leaving behind a white residue. We recommend using hot water, fairy liquid and a stiff brush to wash the slabs with. A few repetitions of this will cause it to fade and mostly disappear. Another method is to use Lithofin outdoor cleaner. If it is not that bad then leaving it may be the best course of action as it will gradually disappear. (This photo was taken from our display area 5 days after laying.) Although this is unattractive and rather annoying if you have just had your new patio laid, it is completely natural and in no way damages the stone.