Why does the drive have 'patches'?

Your problem is undoubtedly efflorescence; a naturally occurring phenomenon that sometimes appears on the surface on cement based building materials and clay products. The best means of treating an affected area remains open to debate with natural weathering, time and patience being the recommendation of many. Efflorescence on concrete is a transient phenomenon of Ordinary Portland Cement. As the cement and water chemically react together, calcium hydroxide is produced. As the concrete dries, the calcium hydroxide reacts with the carbon dioxide in the atmosphere to produce calcium carbonate, which manifests as a white solid, leading to a temporary bleaching of the paving. Efflorescence on clay products results when soluble salts go into solution in a saturated brick or block. As the clay dries out, the water containing the salts, migrates to the surface of the clay face and evaporates leaving a light grey or white powdery deposit. Since efflorescence is in fact simply the crystalline deposits of salts, it is water soluble and therefore, if left untreated, will weather away naturally.

its proberly salt stains coming from the sand used you can get a chemical cleaner or brick cleaner solution from any main building outlet. about fifteen pounds the cost in 5 ltr container and to clear water and brush the area over ,that should do the job...