Concrete appears completely solid but has microscopic pores from which flow tiny droplets of moisture. The moisture evaporates, leaving behind tiny salt crystals called efflorescence. Efflorescence strongly resembles white fungus but is harmless and easily removed from concrete.
Concrete itself does not decay. Any fungus or mould appearing on concrete originates from bacteria in the presence of moisture or a material in proximity to concrete, such as wood. Efflorescence denotes the presence of moisture, either from the delayed curing of the concrete or a persistent source of moisture such as a leak or high water table.
Fungus, a living organism, is typically green or black and slimy. Efflorescence appears as a white, powdery crystalline growth on the surface of the concrete. Excessive efflorescence may lead to mould growth, especially in very wet areas and standing water.
Bleach kills fungus. Efflorescence may be removed with a mixture of white vinegar and water or a muriatic acid solution for extreme cases. Removing the source of the moisture will prevent efflorescence from developing.