What is efflorescence?

Efflorescence is the white powdery or fluffy crystalline deposit that appears on the surface of brick, concrete, or stone masonry. It is caused by soluble salts (typically sulphates, carbonates, or chlorides) present in the brick, mortar, or sand. When rainwater or groundwater penetrates the wall, it dissolves these salts. As the water migrates to the surface and evaporates, the dissolved salts crystallise on or near the surface as white deposits. Efflorescence is primarily a cosmetic problem - it rarely affects structural integrity - and commonly appears on new brickwork in its first year.

Is efflorescence harmful to brickwork?

Surface efflorescence (where salts crystallise at the wall face) is cosmetic rather than structural - it can be brushed off and will diminish over time as the soluble salt reservoir in the masonry is depleted. Crypto-efflorescence (or sub-florescence) is more damaging - here the salts crystallise just below the brick surface rather than at the face. The crystallisation pressure within the brick can cause the face to spall (pop off) in flakes, permanently damaging the brick surface. Crypto-efflorescence is more common where the surface is sealed or painted, trapping moisture that would otherwise evaporate through the face.

How do you remove efflorescence?

Fresh, dry efflorescence can simply be brushed off with a stiff dry brush - washing it with water will dissolve the salts back into the wall and may make the problem worse. For persistent staining, a proprietary efflorescence remover (a dilute acid solution - typically hydrochloric acid, used with great care and correct PPE) can be applied and rinsed off. The brickwork must be thoroughly wetted before and after acid treatment to prevent the acid from attacking the mortar joints. Never use undiluted muriatic (hydrochloric) acid on brickwork - it will damage the mortar and etch the brick faces.

How do you prevent efflorescence?

Prevention focuses on limiting moisture entry and using low-salt materials. Specify bricks with low soluble salt content (Category S1 in BS EN 771-1 - active soluble salts below specified limits). Use sand with low sulphate and chloride content in the mortar. Protect new brickwork from rain during construction - cover the top of walls overnight and in wet weather. Ensure copings, DPCs, and flashings correctly shed water away from wall faces. Applying a water repellent (silane or siloxane) to new brickwork can reduce water uptake, though this may also reduce breathability and is generally not recommended for most traditional brickwork.

Masonry noun

Efflorescence

/ ˌɛflɔːˈrɛs(ə)ns /

Also known as: white staining, salt staining, lime bloom, crypto-florescence (sub-surface)

Definition

Efflorescence is the white powdery or crystalline deposit that forms on the surface of brick, concrete block, stone, or mortar when soluble salts dissolved in water migrate to the wall surface and crystallise as the water evaporates. It is caused by sulphates, carbonates, or chlorides naturally present in brick, mortar, or aggregate. Surface efflorescence is a cosmetic defect rather than a structural problem. Sub-surface crypto-efflorescence - where salts crystallise just below the brick face - can cause spalling damage.

In practice

Efflorescence most commonly appears on new brickwork in its first year, when construction moisture is drying out through the wall and carrying dissolved salts to the surface. It often disappears naturally as the salt reservoir in the masonry is depleted over one or two winters of wetting and drying. It can be brushed off dry with a stiff brush - the worst thing to do is wash it off with water, which simply dissolves the salts back into the wall and delays their removal.

Persistent or recurring efflorescence indicates a continuing moisture source - a failed DPC, damaged coping, blocked gutter, or missing flashing allowing water to enter the wall. Simply removing the salt deposits without addressing the moisture source will result in recurring staining. For stubborn deposits, dilute proprietary efflorescence removers (acid-based) are applied and thoroughly rinsed. Correct PPE including eye protection and chemical-resistant gloves must be worn when using acid cleaning solutions on masonry.

Building Regulations

Salt categoryBS EN 771-1 Category S1 - active salts below specified limits

Mortar sulphatesUse sulphate-resistant cement where ground sulphates are present

Crypto-efflorescenceCan cause brick spalling - structural concern if widespread

Efflorescence itself is not directly addressed in Approved Documents, but it is a symptom of moisture movement through walls - which relates to Approved Document C (resistance to weather and ground moisture). Brick soluble salt content is classified in BS EN 771-1 (Category S1 for low active soluble salts). Widespread crypto-efflorescence causing brick spalling in a structural wall could constitute a defect under Part A - Structure if sufficient brick faces are lost to affect the integrity of the masonry.

Full Building Regulations guidance

Efflorescence

Just so you know...