Concrete problems
Concrete efflorescence
There is a white powdery deposit on my concrete. Is it mould, or damage?
Neither. Efflorescence is mineral salt that was dissolved in water inside the concrete, carried to the surface, and left behind when the water evaporated. The deposit is the residue of a journey.
That makes it genuinely useful evidence — it marks where water has moved through — while being close to harmless in itself. It is one of the few conditions where the right response may be to read it rather than remove it.
What it is
Concrete contains soluble minerals. Water moving through dissolves some, and when that water reaches a surface and evaporates, the mineral stays as a white or greyish crystalline deposit.
Its location maps the water path. Efflorescence along a crack shows water has travelled through it. In a band at a consistent height on a wall it can indicate the level water reaches. Spread over a floor it suggests moisture rising through the slab.
It is distinguishable from mould by texture and behaviour: efflorescence is crystalline and mineral, generally brushes off dry, and does not have the fuzzy appearance or musty odour of biological growth.
What seeing this cannot establish
- Efflorescence does not tell you when the water moved. A deposit can persist long after the source has been resolved.
- It does not indicate volume. A significant deposit can come from a modest amount of water evaporating repeatedly at the same spot.
- It says nothing about structural condition.
- Its absence does not establish that no water is moving — evaporation conditions and mineral content both affect whether it forms.
What this can be associated with
Candidate explanations, not findings. Nobody can establish which applies to your concrete without seeing it.
- Water moving through the concrete from wet ground or a saturated backfill zone
- A crack or joint acting as a path, with evaporation at its exit
- No capillary break or vapour retarder beneath a slab in contact with damp ground
- Repeated wetting and drying cycles at the same location, which concentrate the deposit
- Poor exterior drainage keeping the surrounding ground wet
What you can safely check yourself
- Brush a small area dry and see whether it lifts as powder. Efflorescence generally does; biological growth generally does not.
- Photograph the pattern before disturbing it — the pattern is the evidence and cleaning destroys it.
- Note whether it follows a crack, sits in a band at a height, or spreads generally. Each indicates a different path.
- Mark the extent and check whether it returns after cleaning, and how quickly. Recurrence means the path is active.
- Note whether it correlates with rain or season.
- Check the exterior directly behind or above the deposit for a water source.
When this is frequently cosmetic
- A light deposit that does not return after cleaning, which suggests the water event was historical
- Deposit on an exterior surface where water evaporating is expected and nothing else is wrong
When professional evaluation matters more
- It returns quickly after cleaning, indicating an active path
- It appears in a band on a foundation wall, which can indicate the level water reaches
- It accompanies damp, mould, or damage to finishes and stored items
- It is spreading across a floor slab, which can be associated with moisture rising through it
- It appears alongside a horizontal crack or a bowed wall, in which case the structural question comes first
Evidence worth having before you ask for estimates
Every item here is something you can produce without tools or risk, and every one of them makes an estimate more accurate.
- Photographs of the pattern before any cleaning
- A note of whether it brushes off dry
- A marked extent and a dated follow-up after cleaning
- Correlation with rain or season
- Exterior photographs of the corresponding area
Questions to ask whoever comes out
- What does the pattern of this deposit tell you about where water is moving?
- Is the path still active, and how are you judging that?
- What outside would you change first?
- If the deposit is removed, will it return, and what does that depend on?
- Does anything need doing about the concrete itself, or is this purely a water question?
Where estimates for this commonly differ in scope
These are the places two prices for apparently the same work stop being comparable.
- Cleaning efflorescence and addressing the water path are entirely different scopes, and the first without the second is cosmetic.
- Sealing a surface showing efflorescence can trap moisture behind the sealer, which is a consequence rarely raised in an estimate.
- The exterior work that addresses the source is usually a different trade.
Often reasonable to monitor
Conditions in this band are frequently stable and frequently cosmetic. Monitoring means dating a photograph and re-photographing from the same spot, not ignoring it.
Who tends to do this work
- A waterproofing contractor
- A drainage contractor or landscaper who does grading
- A concrete repair specialist
ConcreteOwner does not inspect, certify or approve contractors. This describes which trade usually holds this work, not a recommendation of anybody in particular.
Common questions
Is efflorescence dangerous?
The deposit itself is mineral salt and is not a health hazard in the way mould is. Its value is as evidence: it marks where water has moved through concrete. So the deposit is not the problem, but it is pointing at something worth understanding.
How do I tell efflorescence from mould?
Efflorescence is crystalline, generally white or grey, brushes off dry as powder, and has no smell. Mould tends to be fuzzy or blotchy, can be various colours, smells musty, and smears rather than powdering when dry. Brushing a small test area dry usually settles it quickly.
Should I clean it off?
Photograph the pattern first, because the pattern is the most useful thing about it and cleaning destroys it. After that, cleaning is reasonable — and whether it returns, and how fast, is itself a valuable test of whether the water path is still active.
Will sealing stop efflorescence?
It can stop the deposit appearing on the surface, which is not the same as stopping the water. Sealing a surface that moisture is moving through can trap that moisture behind the sealer, which sometimes causes the sealer to fail and occasionally makes things worse. Addressing where the water comes from is the more durable order of operations.
Where to go next
Methods that may apply
Related problems
Worth comparing
What drives the cost
Want a straight answer about efflorescence?
Both decision tools are free and need no contact details. If you would rather have somebody local look at it, you can ask us to review whether we can match you — and we will say plainly if we cannot.