What is that white substance on your concrete?
Usually white in color, efflorescence is a discoloration caused by crystalline deposits of salts on masonary, stucco and concrete surfaces. These deposits often contain compounds such as calcium, sodium and potassium hydroxides or carbonates, bicarbonates, chlorides, and sulfates of calcium and magnesium. These substances typically originate as soluble compounds within the concrete that are transported and deposited on the surface by upward moisture migration and evaporation. Sometimes, they originate in the underlying soil where they are carried by moisture upward through the concrete and deposited on the surface.
Water is the solvent and vehicle for transporting the soluble salts to the surface. While not harmful, it is unsightly, especially on integral-colored concrete. Efflorescence can form a thick layer on the surface, which can completely hide the concrete’s color or create unsightly white patches.
Steps to removing efflorescence
A general rule for cleaning efflorescence is to try gentle methods first before moving on to harsher techniques. But really, the best approach to dealing with efflorescence is to keep it from forming in the first place.Try to wash and scrub off the white deposits using clean water as soon as they appear. This may work if the CH (calcium hydroxide) deposits have not fully reacted with carbon dioxide in the atmosphere and become insoluble. Be sure to remove any ponded water; otherwise, efflorescence may reoccur.
Simple washing can sometimes remove efflorescence. Scrub with a stiff brush and mild detergent or plain water. Efflorescence is most soluble when it first appears, so sooner is better than later if you want to try this approach. Just wetting efflorescence can make the film seem to go away (it actually becomes transparent), but you’ll need to apply some elbow grease to do a thorough job. Always be sure to rinse thoroughly. If you leave dissolved salts on the surface, they’ll return as new efflorescence.
Power washing also can be effective in removing surface deposits. Keep the pressure as low as you can to do the job. A spray that’s too intense may actually open pores in concrete or brick and encourage further efflorescence.
Sand blasting is effective, but should be used with care. The abrasion may damage surfaces, making bricks and mortar more porous. If you choose to try sand blasting, seal the surface you’re working on after you’re done.
Chemical cleaning might be needed for some cases of efflorescence. For your project, you will need a mild or diluted acidic solution that is stronger than vinegar water. For gray concrete, the following solutions are recommended:
- One part hydrochloric acid (muriatic) in 10 to 20 parts water (10 percent to 5 percent concentration)
- One part phosphoric acid in 10 parts water
- One part phosphoric acid, plus one part acetic acid in 20 parts water
- Prepackaged/proprietary efflorescence removers.
However, more diluted solutions are recommended for integral-colored concrete to avoid surface etching that may reveal aggregates and change the color and texture of the surface. Start with one part hydrochloric acid in 50 or 100 parts of water (2 percent to 1 percent concentration) and increase the concentration as needed.
Before using the acidic solution, flood the surface with clean water to prevent the acid from being absorbed into the concrete. Allow the acidic solution to set three to five minutes before scouring off the efflorescence with a stiff brush. Immediately and thoroughly flush the surface with clean water to remove all acid. Apply the solution uniformly in terms of concentration, amount, and duration. Protect surfaces that can be damaged by acid and treat the entire slab to achieve a uniform color and texture.
Always perform a trial treatment on an inconspicuous area to check for adverse effects and to perfect the technique. Read the Material Safety Data Sheets to be aware of the hazards associated with the acid. After removing the efflorescence, consider sealing the surface with an exterior concrete sealer.
Related Article: Efflorescence: What Causes it and How do You Remove it?