What is efflorescence?

Have you ever noticed a white-grey staining that seems to appear from nowhere on walls, floors, or building materials such as brick, masonry, stone, or concrete? That’s efflorescence; crystalline salt deposits which are left behind when water evaporates from the material’s surface.

Example of efflorescence on brickwall

Causes of efflorescence

Although efflorescence may not be a risk to the structure of your property, it is an eyesore. Therefore, understanding what causes it may help you to prevent it occuring in the first place or stop it from recurring once it’s been cleaned away.

The crystalline salty deposits can appear on both internal and external walls and are caused by water-soluble salts and other water-dispersible materials that come to the surface of building materials.

Low temperatures, moist conditions, condensation, rain, dew, groundwater, and added water to aid in cement trowelling can all contribute towards efflorescence. With water, snow, and rain having the biggest impact, it should come as no surprise that depending on where you are in the country, the likelihood, frequency, and intensity of efflorescence can change.

During winter months, or in areas prone to more rainfall, you may notice a greater degree of efflorescence. This is due to the fact that a lower humidity can result in the water evaporating before it reaches the material’s surface, meaning that the salt deposits remain unseen. High humidity, on the other hand, can result in slower evaporation, allowing the water to reach the surface and leaving visible salt deposits once evaporated. This is why efflorescence is more prominent after periods of consistent and/or heavy rainfall.

However, the above environments do not guarantee the appearance of efflorescence. For it to occur, three conditions must be met:

  • Water-soluble salts must be present.
  • Moisture must be available to transform salts into a soluble solution.
  • Salts must be able to move through a material to its surface; the moisture will then evaporate and cause the salts to crystallise, resulting in efflorescence.

If you’re noticing efflorescence on a new build, it’s important to note that it can even occur during construction of a property. If bricks or masonry units are left on the ground unpalletted overnight, or left without water proof covering, then they can absorb moisture from the ground (ie. damp soil) or rain.

Various building surface installation problems can also lead to efflorescence including:

  • Incorrect use of through-wall flashing
  • Use of masonry without sufficient ventilation
  • Use of masonry in areas that lack a proper moisture barrier
  • Joint material failure
  • Improper ground storage

Whatever the age or construction method of your property, wall, or flooring, it’s important to note that it could be at risk of efflorescence.

Water can carry salt to the surface of concrete, masonry, brick, and stone by way of migration through the capillaries in the materials. The fewer capillaries there are—meaning the higher the density of the material—the harder it is for water to migrate to the surface. As such, the higher the porosity of the material, the greater the likelihood of efflorescence.

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Identifying efflorescence

Learning how to tell the difference between efflorescence—and understanding that difference—is vital in removing it effectively and helping to prevent its return. Efflorescence is a powdery white-grey substance whereas staining from other sources can be all sorts of different colours.

While other stains may look very similar to efflorescence, it’s important that you can spot the difference to ensure effective removal without risk of damage to the surface.

Knowing what building materials are affected by efflorescence, how, and what causes it, you can more accurately diagnose any white staining and take the appropriate action to help prevent it and to also clean it.

Efflorescence salts are associated with a number of building materials, including:

Brick

Brick is a porous material so therefore it may be more susceptible to efflorescence as water will be able to reach the surface before evaporating. To find out how your bricks may react to efflorescence, you can do a simple test if you have any spare bricks. Take a handful of test bricks and immerse them in water for a week, then remove from the water and allow to dry. Once dry, compare them to the non-test bricks. If there’s a white powdery substance on the test bricks, it’s highly likely to be efflorescence and this is how you can assume the rest of the bricks will react.

Cement

Cement usually contains some water-soluble alkalis, meaning that it’s more susceptible to efflorescence than other building material. Portland cement, in particular, is cited as the key contributor to efflorescence in mortar and grout due to its high alkali content.

Sand

While used in mortar and grout, sand itself is not water-soluble. However, it is possible for sand to be contaminated with other materials that can contribute to efflorescence. Therefore, when used for construction purposes, using clean, washed sand from sources free of contamination will help minimise the risk of efflorescence.

Lime

Lime has been shown to improve the bond between mortar and brick and also increase the water resistance of masonry materials. However, lime is water-soluble and can react with unbuffered hydrochloric acid to produce calcium chloride which can migrate to a building material’s surface.

Clay

Both building bricks and face bricks (bricks used for aesthetics, not structure) contain clay and clay contains highly soluble salts. Clay can also react with calcium sulfate (a common salt source in brick) which will result in efflorescence.

Admixture

An admixture is a substance which can be added to a building material to achieve or modify its properties. For example, admixtures are commonly added to concrete in addition to cement, water and aggregate. An admixture’s strength and bond may increase the potential of efflorescence and generally, it is better to avoid their use entirely if you do not know exactly what’s in them.

Backings

Concrete is a common backing material and is likely to contain soluble salts, as are other backing materials. If enough water is present within these backings, the salt can dissolve, leaving efflorescence when the water evaporates.

Porous building materials can absorb or wick water over a six mile span. Water is carried through the materials like a tree moves water from its roots to its leaves. And once it evaporates, salt deposits can be left. If not controlled properly, efflorescence can spread.

Another note to take into account is that while efflorescence isn’t a risk structurally, it can be indicative of a moisture problem, especially if it’s happening without heavy or constant rainfall. Therefore, if you notice efflorescence it’s important that you take action.

Removing efflorescence

For fresh efflorescence, sometimes it is possible to simply wash the area by either pressure washing or wet scrubbing the surface (depending on the particular substrate affected). Fresh water is then applied by air jetting or a wet vacuum so that the property isn’t left with residue on the surface.

Once efflorescence becomes insoluble, it is far harder to remove. In this instance, a mild, diluted acid solution can be applied. Once the acid wash is complete, the surface will need thoroughly flushing with fresh water and neutralised with a form of sodium bicarbonate. However, careful consideration should be given to the implications of using an acid wash; it can erode mortar and discolour brickwork.

If efflorescence has been present for several months, light sandblasting can be used to clear it. Then once the surface is dry, a stiff brush applied to the affected area can help prevent the salt from penetrating again.

However, the safest method for removing efflorescence is with the use of TORC, by competent and fully trained operatives. The TORC system creates a swirling vortex and uses a fine granulate to gently clean the surface. It is highly effective at removing efflorescence while leaving the substrate in tact. The TORC system doesn’t saturate the surface and instead utilises low volumes of water, along with a fine inert granulate. In the hands of a fully trained operative, it is an extremely sensitive, efficient and gentle method of cleaning masonry, concrete, and brick.

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