Whitish deposits of calcium carbonate, CaCO3, frequently develop on Portland cement concrete and on masonry units, including brick and tile, which have been bonded with Portland cement. These surface deposits are termed efflorescence and are most frequently encountered in new or recent construction. While efflorescence is not normally damaging, except possibly to decorative coatings, it is aesthetically undesirable. The origin of efflorescence is explained and a physicochemical model is developed to explain and quantify the key features of its formation. Calculations and experiments highlight the important role of soluble alkalis in the formation of efflorescence. Mechanistic interpretations and calculations suggest ways in which efflorescence can be mitigated by interrupting one or more steps of the process in conjunction with improved materials selection.