Efflorescence

(A deposit of bases or salts, usually white, on a concrete surface)

CAUSES

Efflorescence results when chemicals in hardened concrete are carried to the surface by moisture. Calcium hydroxide, a product of portland-cement hydration, is the most common source of efflorescence. Calcium hydroxide is a water-soluble base, but after appearing on the surface it reacts with carbon dioxide in the air and forms calcium carbonate, which is water insoluble and thus more difficult to remove.

Less common sources of efflorescence are sea salts or sulfates that are brought into the concrete on the aggregates or in the mixing water. Depending on their composition, these salts may or may not form deposits on the concrete surface. In some cases, rain simply washes them away.

For efflorescence to occur, water must move through the concrete. This movement may result from hydraulic pressure on the backside of the surface or moisture evaporation from the visible surface of the structure. Movement of water resulting in efflorescence usually occurs at cracks, joints, or other openings in the concrete. In cases of extremely permeable concrete, the moisture movement may be directly through the concrete.

Efflorescence is usually considered harmless, except for the damage it does to the appearance of an affected structure.

PREVENTION

The first step in preventing efflorescence is to reduce water movement through the concrete, primarily by improving drainage of water away from the structure or by decreasing the permeability of the concrete. You can improve drainage by sloping the finish grade away from the structure and using in-plane drains and granular fill that direct water to a drain-tile system. Moisture barriers and water-stops may need to be installed during construction if the structure is intended to contain water.

The second step in preventing efflorescence is to reduce or eliminate the chemicals within the concrete that are brought to the surface. It's relatively easy to eliminate sea salts and sulfates: Use only clean mixing water, and wash aggregates to remove any contaminating salts that may cause efflorescence. However, eliminating calcium hydroxide from the concrete is impossible because a cubic yard of concrete contains about 100 to 150 pounds of this hydration by-product.

If you add a suitable pozzolan, such as fly ash, slag, or silica fume to the concrete, it will react with some of the calcium hydroxide and water to form an insoluble calcium-silicate hydrate. However, ACI Committee 232, cautions that not all fly ashes are equally effective in this role. Also, the reaction between some pozzolans and calcium hydroxide is slow, meaning that efflorescence may occur before the calcium hydroxide combines with the pozzolan.

Besides reducing the amount of soluble calcium hydroxide, pozzolans also reduce concrete permeability. However, they won't prevent water movement through cracks or joints.

How Can You Remove Efflorescence?

The most common treatments are to remove the deposits at very early ages by wire brushing or at later ages by acid washing. Either of these approaches can change the appearance of the concrete, so they must be done carefully.

References:
ACI 232.2R-96 - "Use of Fly Ash in Concrete.
ACI 332R-84 - "Guide to Residential Cast-in Place Concrete Construction.