Efflorescence results when chemicals in hardened concrete are carried to the surface by moisture. Calcium hydroxide, a product of portland-cement hydration, is the most common source of efflorescence. Calcium hydroxide is a water soluble base, but after appearing on the surface it reacts with carbon dioxide in the air and forms calcium carbonate, which is water insoluble and thus more difficult to remove. Less common sources of efflorescence are sea salts or sulfates that are brought into the concrete on the aggregates or in the mixing water. For efflorescence to occur, water must move through the concrete. This movement may result from hydraulic pressure on the backside of the surface or moisture evaporation from the visible surface of the structure and usually occurs at cracks, joints, or other openings in the concrete.