Efflorescence – what is it and how to stop it?

Efflorescence is a crystalline deposit of salts that forms on a surfaces. It forms when types of soluble salts within a water solution are drawn to the surface of a structure. As the moisture evaporates it leaves the salt staining commonly seen on concrete, masonry and tiled surfaces.

Efflorescence will occur in any material containing Portland cement. It occurs when calcium hydroxide formed in the hydration reaction of Portland cement is transported by water to the surface through capillaries in the concrete. There it combines with carbon dioxide from the air to produce calcium carbonate. It is almost impossible to stop the formation of efflorescence on cement type products because calcium hydroxide forms as part of the cement setting process. This type of efflorescence that is attributed to the soluble salts within the cement based mix is called “primary” efflorescence.

Primary efflorescence is generally not an issue and can often be removed by a pressure wash or light acid wash. Secondary efflorescence however, is typically more challenging as it caused by a continual supply of moisture.

Other than salts found within cement, efflorescence can also be caused by hydroxides and sulphates of either sodium or potassium. These salts typically come from rain and run off, and from groundwater. With large amounts of dissolvable sulphates being stored in soil, building foundations create a capillary action or 'wicking' upwards into masonry and concrete, leaving behind efflorescence. In the case where soil conditions exhibit water soluble sulphates, precautions should be taken to preclude the passage of this sulphate-bearing water to the structure.

How to stop efflorescence.

Screed beds and open slab faces are two of the more common sources of efflorescence in the construction industry today. Here are a few simple ways of preventing it

Screed beds are notorious for promoting the formation of efflorescence as they are typically highly porous and contain cement. Contaminated sands with soluble sulfates will cause efflorescence so using a clean, washed variety is advisable.

Methods to reduce the incidents of efflorescence associated with screeds include;

  1. If the screed is porous, ensure the substrate below has built in falls to drain this moisture away and to prevent water ponding. Admittedly this is not always feasible and sometimes why screeds are employed
  2. Minimise the absorption of water into the screed. Admixtures, including the Gripset 11Y, can be added to cement based compounds to enhance water resistant properties to reduce permeability.
  3. Applying a membrane over the screed. Dual waterproof coatings over and under screed beds is becoming more popular as a way to eliminate screeds as a source of efflorescence. Care should still be taken in having adequate falls and using quality tile adhesives and grout boosters to further improve water resistance.

Slab edge dampness refers to the absorption of water into slab faces typically from groundwater from surrounding soils or water run off. This water can have high concentrations of sulphates. As the water absorbs into the slab face it migrates through the concrete capillaries to evaporate on the internal side, leaving the soluble salts behind.

Sealing the face of the slab using a quality Gripset membrane such as Gripset C-1P or C-1R is a simple method of preventing moisture penetration.

In summary, efflorescence cannot occur without the presence of two components, water and salt. By ensuring proper falls and drainage, or by negating the entry of water into a substrate, and by definition the dissolvable salts contained therein, the chances of efflorescence are effectively eliminated.