From Bob Formisano,
Your Guide to Home Repair.
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Symptoms: Efflorescence
A problem of painted masonry construction, efflorescence is identifiable by crusty white salt deposits that bubble through the paint film from a masonry structure. Salts in the brick or concrete become dissolved with water and then leach to the surface as the water evaporates.

Potential Causes:

• Poor paint surface preparation where prior efflorescence was not entirely removed and washed before the surface was repainted.
• Heavy moisture migrating through exterior masonry walls from inside the home.
• Inadequately waterproofed basement walls allowing ground water penetration.
• Painting masonry construction before the concrete or mortar had adequately cured and dried out.
• Cracks in masonry wall or poor tuckpointing is allowing water to get behind masonry wall.

Possible Repairs:

• If moisture is getting into the masonry wall eliminate source of moisture by properly tuckpointing any cracks or missing mortar in the wall or patching concrete with a latex concrete patch, clean out gutters and downspouts, caulk joints around windows and doors with a butyl rubber caulk.
• If moisture is migrating through the wall from the outside (e.g., basement wall), apply waterproofing to outside of wall.
• Remove all efflorescence and loose flaking, chalking paint with a wire brush, scraping or power washing before repainting.
• Clean area with a trisodium phosphate cleaning solution and rinse with clean water.
• Let completely dry and paint with a high quality latex house paint.