Understanding concrete efflorescence

Concrete efflorescence is the appearance of white mineral salt deposits on or near the surface of concrete causing a change in its appearance.

Efflorescence is the phenomenon that occurs when mineral salt in concrete or groundwater is deposited as an unattractive white powder or stain on the surface of a wall, floor, driveway or path. Stains can be white, grey, pale yellow or even green depending on the type of soluble salts in the concrete, clay brick, paver or in the groundwater.

These salts form naturally and are generally concentrated depending on geographical location and the source of the raw materials in the concrete/clay.

There is two main types of efflorescence that affect concrete

Primary efflorescence – salt in concrete or clay products is dissolved or carried by capillary action to the surface by water and left when the water evaporates. This type of efflorescence generally occurs for about 2 to 3 years and reduces naturally as the available salts are depleted.

Secondary efflorescence – salts in ground water or another source are carried to the surface of concrete or brickwork by hydrostatic pressure or osmosis/evaporation (also referred to as evapotranspiration) and left when the water evaporates. This type of efflorescence continues, providing the source of the salty groundwater remains available.

Water carrying soluble and non-soluble salts leached from the earth flow naturally in the ground. As this water travels under a building, the downward pressure of the building’s weight results in hydrostatic pressure. The waterproof membrane under a house slab and its footings prevent rising damp in the building, however most concrete and paved driveways, patios and paths do not have a waterproof membrane underneath. When groundwater under pressure reaches a concrete drive or gap between the drive and a footing, it will make its way to the surface.

When water containing salt evaporates, salt crystals are formed on the surface which appear as a fluffy white powder or white stain deposit. These crystals continually grow as long as the salt and water source are available. The type or types of salt will determine the nature and appearance of the efflorescence.

Typical appearances of efflorescence

Mineral salts react with Carbon Dioxide in the air and form non soluble Calcium Carbonate. This is the hard white or yellow efflorescence seen on concrete and bricks.

Because it is not soluble, Calcium Carbonate cannot be washed off the surface. A mild solution of muriatic acid (Hydrochloric Acid) is normally required to remove Calcium Carbonate.

(Photo Credits - Sakkie Meyer, Eben - Buildcon Solutions (Pty) Ltd

Normally fluffy white salts are water soluble like Sodium salt crystals. This white powder is very unsightly on coloured paving or brickwork, as seen in the photographs below. Water soluble salts can be easily washed off the surface using water and a brisk scrub.

Efflorescence can occur on both walls and floors and is primarily caused by hydrostatic pressure building as a result of an interruption in the natural ground water.

Example 1: efflorescence on driveways and brickwork

 A house or building has footings which penetrate the ground. The building generates downward pressure which builds hydrostatic pressure. This pressure manifests as salts in porous substrates such as stone, brick and concrete.

Example 2: efflorescence on internal garage floor

Most houses built on a concrete slab have a vapour barrier preventing moisture from entering the slab. However, in the past, this has not been mandatory on garage slabs and is often excluded or forgotten. Where the building’s concrete slab is lower than the outside ground level, moisture can rise through the slab causing efflorescence to occur.

Example 3: efflorescence on external concrete pavements and pathways

Concrete paths and driveways rarely have a vapour barrier underneath them. Therefore, moisture in the surrounding subsoil can easily penetrate the concrete which can result on salts manifesting on the surface.

Control joints, expansion joints and cracks make it easy for water to penetrate the concrete which can affect any coatings applied to the concrete’s surface.

Delamination of coating caused by efflorescence:

Photo Credit - Alcon

Efflorescent salts forming under clear sealer:

Photo Credit - Alcon

Coating failure on slate concrete as a result of efflorescence:

Photo Credit - Alcon

How does efflorescence affect concrete sealers?

When efflorescence occurs under concrete coatings, the salts will grow and begin lifting and damaging the concrete coating. Most coatings, whether single pack, two pack, polyurethane, acrylic or epoxy can be affected by efflorescence.

Where efflorescence damage has started, the process will continue providing the source of the moisture and the salt remains.

Rectifying efflorescence problems

Re-coating efflorescence damaged concrete will not generally solve the problem, the most effective way is to remove the source of moisture or salts which will prevent efflorescence from occurring.

To remove the water or salt, the following options are available:

1. Wash off the surface salts using a weak acidic solution (1-part Hydrochloric acid, 100 parts clean water) or household vinegar.
2. Install interceptor drainage to prevent ground water pressure building at the perimeter of the building.
3. Lower the exterior ground level and re-grade surface away from the building or concrete to reduce ground water movement.
4. Where coatings have failed due to efflorescence, the efflorescence problem will need to be treated before applying new coatings. To apply new coatings, first strip off the old sealer, then clean the surface correctly including acid etching the surface and finally apply 2-3 coats of sealer according to manufacturers instructions.

Need more expert advice about dealing with and preventing concrete efflorescence ? Talk to the team at SAPAC.

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