Quoting the EP Henry web site: "Efflorescence is a whitish powder-like deposit, common on concrete and masonry products, which normally will disappear over time with weathering. Although it may present an aesthetic concern, efflorescence will not affect the structural performance of pavers or wall systems. Efflorescence is a natural occurrence for which EP Henry accepts neither responsibility nor liability. Contact your EP Henry Hardscaping Center for additional information and a copy of the Interlocking Concrete Pavement Institute (ICPI) publication“Managing Efflorescence”." This is the link to the ICPI publication in PDF.
The Chemistry of Efflorescence:
All concrete products contain cement which produces lime or water soluble calcium oxide. Lime can also be in the bedding sand, aggregate base materials, or soil. Although concrete pavers are solid, strong, and very dense, they contain millions of microscopic capillaries that run from the interior to the surface. Moisture from rain, sprinkler systems, underground sources, poor site drainage, or dew enters these microscopic capillaries. Calcium oxide inside the paver Efflorescence emerged from pores within a magnified area of a concrete paver. The calcium has been carried to the surface by water.