How Does Efflorescence Occur? Mechanism & Key Concepts Explained
The migration of a salt to the surface of a porous material, where it forms a coating, is known as efflorescence (which means "to flower out" in French). The most important step entails dissolving an internally held salt in water or, on rare occasions, another solvent. The salt is now kept in solution in the bath, which migrates to the surface and evaporates, leaving a salt coating.
Water is the invader in "primary efflorescence," and the salt was already present internally; "secondary efflorescence," on the other hand, is a phase in which the salt is initially present externally and then brought inside in solution.
Efflorescences can be present in both natural and man-made environments. It can often indicate internal structural weakness (migration/degradation of component materials) on porous construction materials, but it can also present a cosmetic outer issue (primary efflorescence causing staining). As seen in the spalling of brick, efflorescence can clog the pores of porous materials, causing internal water pressure to destroy the materials.
In this article, we will study mineral efflorescence, masonry efflorescence, brick efflorescence, white efflorescence, anti efflorescence and efflorescence treatment in detail.
Efflorescence Treatment
Cleaning the wall with high-pressure water has long been a popular method for removing efflorescence. However, since the salts are water-soluble, there's a chance they'll seep back into the wall and reappear as crystals later. As a result, the first line of protection should be to clean the walls with a stiff brush to eliminate the rest of the white. Water pressure washing can then be used to remove any remaining salt from the walls.
When calcium carbonate or calcium sulphate is the source of efflorescence, it is more difficult to remove. It sticks to the wall very firmly and is difficult to scrape with a brush. In this scenario, an acid-based treatment is recommended.
Masonry Efflorescence
Brick Efflorescence
In efflorescence, magnesium sulphate, calcium sulphate, sodium sulphate and carbonate (and sometimes chloride and nitrates) are commonly found. These salts have been connected to the brick itself, construction sand, base soil, groundwater, construction water, and loose earth left in contact with brickwork. Bricks containing more than 0.05 per cent magnesium sulphate should not be used in building. Sand should have a soluble salt content of less than 0.1 per cent (chloride and sulphate combined).
Efflorescence in Cement
As water percolates into poorly compacted concrete, cracks, or poorly constructed joints, the lime compounds in the concrete leach out, resulting in efflorescence (the formation of salt deposits on the concrete surface). Calcium hydroxide Ca(OH)₂, one of the hydration products that is slightly soluble in water, migrates to the concrete surface through the capillary mechanism, causing this.
The solid Ca (OH)₂ reacts with atmospheric carbon dioxide CO₂ to form calcium carbonate CaCO₃, a white layer on the concrete surface, after evaporation.
Efflorescence on Walls
White marks on internal walls can be caused by efflorescence on plaster, which can occur behind paint and wallpaper.
These white, fluffy salts are "crunchy" to the touch, and the crystals that form underneath wallpaper or paint are strong enough to force these coatings off the plaster, or "pop" the plaster. This type of sulphate crystal formation can occur in any building, regardless of age, where water enters the structure. Evaporation would cause the water to leave the wall, leaving the salts behind. While the salts can be rubbed off, they often reappear and cause more harm to the décor.
Efflorescence Plaster
Efflorescence on plaster surfaces is caused by the presence of salts in lime, cement, sand, bricks, and often even water used in building. The soluble salts dissolved by moisture are drawn to the surface through pores after the plasterwork is completed and fully dry. These soluble salts absorb moisture from the air and store it in patches as a white crystalline material when it dries. The surface is marred by unsightly efflorescence patches. This weakness allows the structure to deteriorate over time. It is usually of a transient type, as it vanishes in rainy weather and reappears in dry weather.
Efflorescence on Stone
Efflorescence will cease in all but the most severe cases as crystallised salts block capillaries in the stone. The white deposits will slowly wear off with use and exposure until the process has stopped. Water and a stiff-bristled (non-metal) brush will always suffice for clients who are in a hurry to get rid of it. The haze will resume if the efflorescing process is not stopped or the factors that cause it are still present, and it will need to be cleaned again.
Examples of Efflorescent Salt
By the process of homogeneous nucleation, a 5 molar concentration aqueous droplet of NaCl can spontaneously crystallise at 45 per cent relative humidity (298 K) to form a NaCl cube. The initial water is freed into the gaseous state.
Gypsum (CaSO₄.2H₂O) is a hydrate solid that will give up its water to the gas phase and form anhydrite in a sufficiently dry environment (CaSO₄).
When exposed to sunlight, copper(II) sulphate (bluestone) (CaSO₄.5H₂O) is a blue crystalline solid that steadily loses water of crystallisation from its surface, resulting in a white layer of anhydrous copper(II) sulphate.
When exposed to sunlight, sodium carbonate decahydrate (Na₂CO₃.10H₂O) can lose water.
Did You Know?
Three key factors contribute to the formation of efflorescence on concrete and brick masonry walls. The following are the conditions:
Soluble salts could be present in concrete and brick masonry walls, and the salts could be found in masonry brick, mortar, adjacent dirt, and backing material.
Water must be present in the concrete and brick masonry walls, and it must come into contact with soluble salt in order to dissolve it.
The pore structure of concrete and brick masonry walls must allow soluble salt to migrate to the surface, where water can evaporate and leave the salt.
FAQs on Efflorescence in Chemistry: Definition, Causes & Examples
1. What is efflorescence in chemistry, with examples?
Efflorescence is the phenomenon where a crystalline hydrated salt spontaneously loses its water of crystallisation when exposed to the air, often becoming an amorphous powder. This process occurs when the aqueous vapour pressure of the hydrate is higher than the partial pressure of water vapour in the surrounding atmosphere. Common examples of efflorescent substances include:
Washing Soda (Sodium Carbonate Decahydrate): Na₂CO₃·10H₂O
Glauber's Salt (Sodium Sulphate Decahydrate): Na₂SO₄·10H₂O
Epsom Salt (Magnesium Sulphate Heptahydrate): MgSO₄·7H₂O
Blue Vitriol (Copper(II) Sulphate Pentahydrate): CuSO₄·5H₂O
2. What are the primary causes of efflorescence?
The primary cause of efflorescence is a pressure difference related to humidity. The process is driven by three essential conditions:
Presence of a Hydrated Salt: The substance must contain loosely bound water molecules within its crystal structure (water of crystallisation).
Vapour Pressure Gradient: The aqueous vapour pressure exerted by the hydrated salt must be greater than the partial pressure of water vapour in the surrounding air.
Exposure to Air: The salt must be in contact with the atmosphere, particularly one with low humidity, for the water molecules to escape.
This pressure difference forces water molecules to leave the crystal lattice, causing the crystal to lose its structure and crumble.
3. How is efflorescence different from deliquescence and hygroscopy?
These three terms describe how substances interact with atmospheric moisture, but they represent opposite or different phenomena:
Efflorescence: The loss of water of crystallisation from a hydrated salt to the atmosphere, causing it to become powdery. It happens in dry conditions.
Deliquescence: The process where a substance absorbs so much moisture from the air that it dissolves in it, forming an aqueous solution. This occurs in humid conditions.
Hygroscopy: The general ability of a substance to attract and hold water molecules from the surrounding environment. All deliquescent substances are hygroscopic, but not all hygroscopic substances become so wet that they dissolve.
4. Why does efflorescence happen more quickly in dry weather?
Efflorescence is more pronounced in dry weather because the driving force for water loss is significantly greater. Dry weather implies that the air has a very low partial pressure of water vapour (low humidity). According to the principle of efflorescence, water molecules naturally move from an area of higher vapour pressure (the hydrated crystal's surface) to an area of lower vapour pressure (the surrounding air). The drier the air, the larger this pressure difference, which accelerates the rate at which the salt loses its water of crystallisation.
5. Is efflorescence a chemical reaction?
No, efflorescence is a physical process, not a chemical reaction. During efflorescence, the hydrated salt loses its water of crystallisation, which results in a change of its physical state from crystalline to amorphous (powdery). However, the fundamental chemical composition of the salt (e.g., the sodium carbonate in Na₂CO₃) remains unchanged. A chemical reaction involves the formation of entirely new substances with different chemical bonds and properties, which does not occur in this case.
6. Can any hydrated salt show efflorescence?
No, not every hydrated salt is efflorescent under normal conditions. A hydrated salt will only exhibit efflorescence if its internal aqueous vapour pressure at a given temperature is higher than the partial pressure of water vapour in the surrounding atmosphere. If a hydrated salt's vapour pressure is lower than that of the surrounding air, it will not lose its water of crystallisation and will remain stable. Therefore, efflorescence is a property specific to certain hydrated salts under specific atmospheric conditions.
7. In construction, how is efflorescence on bricks or concrete prevented?
In construction, preventing efflorescence involves managing water, soluble salts, and their movement. The key chemical and physical strategies include:
Using Low-Alkali Cement: Choosing cement with a lower content of soluble sodium and potassium salts reduces the primary source of efflorescence.
Applying Sealants: A high-quality hydrophobic or waterproof sealant creates an impermeable barrier on the surface, preventing internal moisture from evaporating and leaving salt deposits behind.
Controlling Water: Implementing proper drainage and using water-repellent admixtures within the concrete or mortar mix minimises the amount of water available to dissolve and transport salts to the surface.
Using Pure Materials: Ensuring that the sand, gravel, and water used for the mix are clean and free from contaminating salts (like sea salt) is a crucial preventive step.
