Efflorescence on Stone and Concrete: Causes and Removal

Efflorescence on a facade is a white or greyish deposit of salts carried by moisture from inside stone, concrete, masonry or mortar joints to the surface. The water evaporates, the salts crystallise, and stains, streaks and crusts appear on the cladding.

The key point is that efflorescence is a symptom of water movement, not a defect of the stone itself. As long as moisture keeps entering and evaporating through the cladding, the deposit will return after any cleaning. So the source of moisture and salts is found first, then the deposit is removed by a method suitable for the stone, and only then is the surface protected.

The article will be useful to clients and facility management teams of buildings clad in natural stone, concrete, GFRC (glass fibre reinforced concrete) and brick, and to technical supervisors at handover. A general overview of stones is in the section Natural stone for facades and interiors.

Efflorescence needs three conditions at once: soluble salts, water to dissolve and carry them, and evaporation from the surface. Remove any one and no deposit forms. The main sources:

• Cement mortars and adhesives. As cement hardens, calcium hydroxide forms; water carries it to the surface, where it reacts with carbon dioxide and turns into calcium carbonate, the most persistent type of efflorescence.

• New concrete and masonry. Primary efflorescence in the first year after construction is typical: construction moisture leaves the structure.

• Ground moisture. Without a damp-proof course, water rises up the plinth by capillary action and carries salts from the soil.

• De-icing salts. Splashes from pavements saturate plinths and steps with chlorides.

• Leaks. Poor sills, unsealed parapets, missing drip edges: water regularly runs down the cladding along the same lines.

• The stone itself. Some porous stones contain soluble compounds, but this is less common than external sources.

To remove efflorescence for good, first answer the question: where is the water coming from? The shape of the stains is a clue: a band along the plinth means rising damp, streaks under a window mean the sill, outlines around slabs mean mortar joints.

Prevention is built in at the design and installation stage; it is cheaper than washing the facade for years.

1. A ventilated rainscreen system instead of wet fixing. When stone, GFRC or porcelain stoneware hangs on a substructure with an air gap, there is no mortar between the cladding and the wall from which lime can leach. This is the main design measure for reducing the risk of efflorescence on cladding.

2. A damp-proof course in the plinth and drainage away from the building: apron, falls, downpipes.

3. Facade drainage. Sills projecting beyond the cladding with a drip, capped parapets, sealed junctions.

4. Low-salt materials. For wet-laid stone, adhesives and grouts designed for natural stone; for masonry, mortars with reduced free lime.

5. Site storage. Slabs on pallets, under cover, off the ground: stone that has stood in a puddle for a month arrives on the facade already saturated with salts.

6. Water-repellent treatment of all faces of porous stone before installation.

The removal method depends on the type of salt and the stone.

The rule for any method is a trial area. After cleaning, the surface is rinsed and allowed to dry completely, and only then is it decided whether water-repellent treatment is needed. Uncontrolled high-pressure washing can damage soft stones and joints.

Primary efflorescence on a new building often disappears by itself: construction moisture leaves, the stock of free salts in the mortar is exhausted, and after one or two cleanings the deposit no longer appears. If it does return, there is a constant source of water. Typical cases:

• On the plinth after every winter: rising damp from the ground or de-icing salts from the pavement.

• Under reveals and sills: water gets behind the cladding through unsealed junctions.

• Around slab edges: mortar is washed out of joints or the adhesive bed in wet-fixed cladding.

• On parapets and cornices: there is no protective flashing and horizontal surfaces are constantly wet.

• After incorrect cleaning: the deposit was washed off with water, the salts went deeper and came out again.

In such cases, repeated cleaning without repairing the drainage is useless. The source of moisture is eliminated first, then the surface is cleaned and protected.

Protection against efflorescence means reducing the amount of water passing through the cladding. After the causes have been eliminated and the surface cleaned, porous stone, concrete and joints are treated with a vapour-permeable silane or siloxane water repellent. It prevents rainwater from soaking in but lets moisture from the structure escape as vapour, without carrying salts to the face.

What not to do: coat the facade with film-forming lacquers and paints. The film traps moisture, salts crystallise beneath it, and the coating peels off together with the surface layer. Treat a dry, clean surface; if treating before installation, treat all faces of the slab including the edges. More on products and intervals in the article on natural stone protection.

TECHNOFORMA installs GFRC, natural stone and porcelain stoneware cladding on ventilated rainscreen systems: the company's own installation teams fit the substructure, insulation and panels. This method involves no mortar layer behind the cladding, which is the main source of efflorescence in wet fixing. An example with stone is the Dinastiya club house in Kislovodsk (GFRC 6,000 m², granite 700 m², porcelain stoneware 700 m²); an example of a large residential GFRC facade is the Dominanta residential complex in Makhachkala.

Other projects are in the TECHNOFORMA projects section. Where the stone comes from and how the warehouse works is covered in Stone imports: India, Turkey, China, Iran.