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.