White, blotchy concrete is one of the most common but least understood phenomenons with concrete.
As moisture migrates up to the surface of the concrete, it carries along with it calcium salts from within the concrete. When the salts reach the surface, they react with CO2 in the air and form insoluble calcium carbonate. This white, dusty, scaly salt can be minimal or dramatic, depending on the amount of free calcium salt present in the concrete. Exposure to rain, standing water, and sprinklers only make the situation worse, as water triggers the reaction and creates more efflorescence.
Efflorescence is not as noticeable when it occurs on gray concrete, but even a little efflorescence on colored concrete can be a contractor’s worst nightmare. Efflorescence makes red look pink, brown look tan, and black look gray or even white. The good news is that it will eventually go away on its own as the free calcium is depleted. The bad news is that this can take months or even years. And if this is the situation, you can’t wait.
To fix the problem at this point, clean the surface with a mild acid or efflorescence remover (some manufacturers make special efflorescence cleaners) followed by sealing.
Usually white in color, efflorescence is a discoloration caused by crystalline deposits of salts on 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.
Usually, efflorescence is primarily composed of calcium carbonate formed when leached calcium hydroxide (CH) from within the concrete reacts with carbon dioxide in the atmosphere. CH is one of the hydration products created by the chemical reaction between portland cement and water. It is readily soluble in water and easily leached.
During initial drying, excess mix water saturated with CH migrates to the surface of the concrete where it evaporates, leaving the CH. The deposited CH then reacts with carbon dioxide to form water-insoluble calcium carbonate. CH only remains water soluble for a brief period after exposure to the atmosphere. The efflorescence that forms during initial concrete drying or soon after concrete placement is called primary or new efflorescence.
Formation of primary efflorescence is strongly affected by weather conditions such as ambient temperatures, humidity, and wind. During high rates of surface evaporation, small amounts of soluble salts are brought to the surface, which reduces the risk of efflorescence. Occurrences are greater during cool and wet weather followed by a dry and warm period. Slow and extended evaporation periods allow more soluble salts to migrate to the surface, which increases the potential and severity of efflorescence.
After initial drying, secondary efflorescence can occur if the concrete is exposed to alternate wetting and drying, such as sprinklers or heavy rain, or to a moisture source beneath the concrete that allows or promotes soluble salts to be leached out of the concrete.
As stated previously, most efflorescing salts are water soluble and many will disappear with normal weathering unless there is some external source of salts.
Try to wash and scrub off the white deposits using clean water as soon as they appear. This may work if the CH 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.
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:
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.