Efflorescence occurs with all concrete and is the most frequent problem that concrete contractors face with colored concrete. Owners don't care about “plain” concrete, but colored concrete is another matter. They complain that their contractor didn't give them the color they ordered, and sometimes they withhold payment.
Efflorescence is caused when soluble salts and other water dispersible materials come to the surface of concrete and mortars. It's induced by low temperatures, moist conditions, condensation, rain, dew, and water added to the surface of fresh concrete to assist troweling. It can occur very soon after exposure to moist or cool conditions or gradually, especially when it comes from within the concrete or from the subgrade.
Any material containing portland cement results in efflorescence. The most usual reaction occurs when calcium hydroxide (lime) formed in the hydration reaction of portland cement (approximately 140 pounds per cubic yard of concrete) is transported by water to the surface through capillaries in the concrete. There it combines with carbon dioxide from the air to produce calcium carbonate (an insoluble material) and water. But efflorescence can also be caused by hydroxides and sulfates of either sodium or potassium, which are much more soluble in water than calcium. And they form efflorescence more rapidly than calcium hydroxide. These salts can come from cement, aggregates, water, or admixtures.
Efflorescence is normally white and shows up more on darker colors than white or light gray because of the contrast. Only 0.2 ounce of calcium carbonate per square yard of surface is needed to cause a significant shift in color. Some forms are very difficult (if not impossible) to remove, while others are easy—especially if they are removed right after they form.
The easiest time to remove calcium hydroxide efflorescence is before it combines with carbon dioxide. Up to this time it will dissolve in water, so pressure washing or wet scrubbing will put it in solution with water so it can be rinsed away. You must be careful to rinse the surface with fresh water so that no residue is left to dry on the concrete. Use an air jet or a wet vacuum to remove any standing water. Any remaining solution will cause new efflorescence to appear
When efflorescence proceeds to the calcium carbonate phase, it becomes insoluble and is much more difficult to remove—perhaps impossible. The application of a mild acid solution becomes the first course of action. These acids include vinegar (5% acetic acid), muriatic acid, or citric acid. Muriatic acid is purchased in full strength and must be diluted. So it's the most dangerous, requiring proper safety gear.
After acid washing, slabs should be rinsed thoroughly and neutralized with baking soda (sodium bicarbonate) or an equivalent. Acid residues can harm plants. The reaction products of acid on concrete are all soluble calcium and iron salts, which can cause more efflorescence.
When efflorescence can't be removed with acid washes, other commercial products are available. One is ethylenediamine tetraacetic acid (EDTA), which rapidly dissolves calcium salts. EDTA will also damage concrete, so it's best to test it on an inconspicuous sample area first.
There are a few things you can do to reduce the possibility of efflorescence. Including either Class-F fly ash or metakaolin can lock up significant amounts of calcium hydroxide in the concrete. And as stated earlier, the efflorescence reaction is driven by water, either water from above or below a slab. Only vapor barriers can prevent the movement of moisture from the subgrade to the surface of a slab. And the application of sealers and coatings can prevent surface water from penetrating slabs. Apply them as soon as surfaces are clean and dry.
A final thought
Efflorescence naturally occurs on all concrete. Part of the problem—and the solution—may have to do with the way we sell colored concrete. Customers often have the idea that their concrete will have uniform color. But they should be sold on the idea that concrete has variable color, providing interest and the impression of color depth.
— Joe Nasvik, based upon an interview with Stan Stratton, Director of Technology Development, L.M. Scofield Company