Efflorescence can wreak havoc on brick walls if not treated, leaving salt deposits on the brick which can then begin to damage and break apart the surface. Talking with Sarah Gray, an engineer with RDH Building Science Labs, lets discuss efflorescence: what it is, how it works, why it matters, and how to fix or prevent it from occurring.

What is Efflorescence?

Efflorescence is basically salt that has dried on the surface of brick or stone masonry. The salt originates from within the masonry (the mortar or brick) and is pulled to the surface of the brick, marking the stones with visible white deposits on the surface with a salty color and texture. The salt is carried by water and then deposited onto the brick when the water evaporates.

The salt can come from the mortar, which carries natural water soluble salts, or the minor amounts of salt contained within the clay that the bricks are made from. When water gets into the brick wall, the salt contained can dissolve into the water. Once the evaporation of the water begins (such as the hot sun beating down on the brick wall), the salty moisture gets pulled onto the exterior surface of the wall. While the water evaporates, the salt deposits are left behind.

Why Does It Matter?

 If trapped just beneath the surface of the brick, it can lead to problems and cause further damage. The salt crystals start to shrink and swell as the brick goes through drying cycles (a process known as subflorescence), which can cause the surface of the brick to flake off. If the cycle isn’t complete, the salt stays within the brick, where repeat cycles will cause the salt to grow and damage the crystalline structure of the mortar.

While rainwater wetting brick is manageable, water coming from the ground is more powerful due to it having a constant moisture source. As salt is deposited, and salty water is trapped within the masonry, physics takes over in the form of osmosis and capillarity. The pores in the masonry are quite small and have a capillary suction, which pulls the water up from the ground. The narrowness of the pores allows the water to travel far up into the brick. Capillarity sucks water into the brick, and then osmosis sucks up the salt.

This process can be the most damaging for wall structures that are in contact with the ground, especially in colder climates where lots of salt is being laid down on the pavement.

Efflorescence and subflorescence can attack the structure of a wall and can affect the durability of the brick. While a few millimeters of brick sloughing off the wall may not impact the strength of the wall, it can impact the durability of the brick itself.

Bricks have a strong outer layer known as the “fire skin” that forms when the brick is in the kiln. If damaged or destroyed by efflorescence and subflorescence, the brick is exposed. The inner part of the brick, while still strong, is very soft and porous, making it easier to wick up more water and transport more salt.

Preventative Measures and Solutions

Mortar plays an important role in solving this problem. Since it contains so much natural salt in its chemical composition, an excessive amount of moisture can cause a dissolution of the salt. Mortar is also typically weaker than the brick around it, making it more prone to destruction by the salt crystals.

Using mortar (specifically mortar that is weaker than the brick, block, or rock) is a great solution as it is much cheaper and easier to replace or repair. Choosing brick and mortar with a low salt (alkalinity) content is another great method of preventing efflorescence altogether. This can be done through factory tests to provide the correct formulation of the brick. As the mortar gets mixed with water, you want to ensure that the water is clean and potable.

Design is another factor to consider. Detailing wall assemblies such as ledges, roof caps, and windowsills in buildings helps to minimize the brick’s exposure to rainwater that can work its way through and cause efflorescence. Flashing is incredibly valuable, as it helps to protect and deflect water away from the wall.

A break between the bottom part of the wall and the rest of the assembly can also help. Flashing within the wall helps keep rising damp from the capillary action from getting pulled into higher levels of the wall.

If the wall needs to be retrofitted to accommodate this, it is difficult to install through flashing at the base. In that case, dig some soil away from the wall and put in a gravel fill about a foot deep and a foot wide. Gravel doesn’t hold moisture well, so if there’s no moisture in the soil, it can’t get wicked up into the brick and cause efflorescence.

This content was taken from the podcast “7 Minutes of BS (Building Science)” from Protradecraft.com.