There it is, on the floor or a wall: a whitish, crystalline substance staining the surface of the concrete or brick. It looks like it may be efflorescence… But what is that, exactly, and what causes it? Are these deposits worrisome? How can they be cleaned? Let’s find out.

Efflor… eh, what?

Efflorescence, sometimes called “whiskers,” is a powdery substance caused by mineral salt deposits on brick walls, the interior faces of foundation walls, or on concrete slabs.
The presence of efflorescence means that the affected material:

  • contains soluble mineral salts;
  • contains enough moisture to dissolve those salts; and
  • is porous enough for the salt solution to make its way to the surface.

Mineral salts are found in certain porous materials (stone, concrete, etc.) or the materials used to assemble them (e.g., Portland cement, masonry cement, lime). The salts dissolve when water gets into the material, and then show up on the surface as whitish deposits when the water evaporates.
How can you tell if the deposits aren’t mould, as opposed to efflorescence? This table sums up the differences between the two:

EfflorescenceMould
Dissolves in waterYesNo
Crumbles to a powder when pinched between the fingersYesNo
Forms on organic materialsNoYes

It is a serious problem?

Efflorescence on the surface of a material is mostly an esthetic issue: it stains the material. But its presence is a sign of excess moisture in the affected material, and this type of anomaly must be examined immediately to avoid any unpleasant surprises, such as unseen structural damage or mould formation.

There are three possible situations, with very differing degrees of severity.

  • Primary efflorescence is due to moisture captured by the material when it is first installed: for example, the water used for a cement or mortar mix evaporates. In such cases, the problem will not recur once evaporation is complete and the surface has been cleaned.
  • Secondary efflorescence is caused by moisture being absorbed into a dry material, long after its installation. Common causes are water infiltration from behind brick cladding, or water pooling in the ground beside or beneath a building. In these cases, the efflorescence can last for as long as the problem is not addressed. The necessary corrective action varies depending on the situation: repairs to masonry, waterproofing or drainage of the foundation, etc.
  • Another problem involving efflorescence can occur in very dry conditions: water within a material can evaporate before reaching the surface, which means the salt deposits occur underneath the surface. The efflorescence can degrade the material by scaling or spalling.
  • In the last two instances, simple cleaning will not solve anything.

Proper cleaning

If you are tired of seeing those whitish stains all over the place, here is how to clean efflorescence deposits:

  • Buy a cleanser specially designed for the purpose.
  • This may be a water-based (non-acid) product low in volatile organic compounds (VOCs), which you can find in a hardware store.

You can also opt for more powerful, acid-based cleansers, available from retailers specialized in masonry products (where industry professionals go for their supplies).

  • Start with dry brushing using a stiff brush. It’s best to wear a respirator, safety glasses and gloves.
  • Continue the operation using the chosen product, making sure to follow the manufacturer’s instructions, which may include wetting the surface beforehand and rinsing it afterward.

Finally, a warning about one product traditionally used for removal of efflorescence deposits: muriatic acid (or diluted hydrochloric acid). It is so dangerous to people as well as to the surfaces to be cleaned that it is preferable to entrust the job to a specialized contractor. Better safe than sorry!