What is efflorescence?
Efflorescence is a deposit, usually white, that forms on the surface of brickwork, mortar joints, concrete or render when water carries dissolved substances to the surface and evaporates, leaving them behind. In Dutch it is called ‘uitbloeiing’. It is mainly a cosmetic issue, but it always tells you that water is moving through the wall.
How does it form?
Three things are needed: soluble material in or behind the masonry, water to dissolve it, and a surface where that water can evaporate. The salts come from the bricks, mortar, concrete, the ground or a previous use of the building. Efflorescence is common in the first wet seasons after construction, while the wall is drying out.
Salts or lime?
- Salt efflorescence: fluffy or powdery white crystals, often sulfates from the bricks or mortar. They are water-soluble and usually disappear as the wall dries out. Persistent salt deposits on damp internal walls are often called saltpetre.
- Lime staining (lime bloom): a hard white film or streaks running from joints or concrete. Calcium hydroxide from cement or lime is washed out and reacts with carbon dioxide in the air to form calcium carbonate. This is no longer water-soluble, so it will not weather away.
- Green or yellow stains on light bricks: often vanadium salts from the brick itself. Never treat these with hydrochloric acid, as it makes them worse.
Application
The term comes up at handover of new brickwork, in facade refurbishment and in damp surveys.
Tips and points of attention
- On new work, wait first: light salt efflorescence often clears after one or a few seasons. Dry-brush it off; do not wash it.
- Lime staining needs a suitable masonry cleaner. Wet the wall thoroughly beforehand, test on a hidden area and rinse well afterwards.
- Prevent it on site: store bricks off the ground and covered, and protect fresh brickwork from rain.
- Find the water source if efflorescence keeps coming back: leaking gutters, failed pointing, missing cavity trays or a failed damp-proof course.
- Do not rush to apply a water repellent: on salt-laden masonry, salts can crystallise behind the treated zone and spall the brick face.
Related terms
- Saltpetre on walls
- Lime
- Facing brickwork
- Construction moisture
- Hydrophobizing
- Damp-proof course
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