Where this starts
Concrete contains soluble compounds and it contains free lime. Water moving through it dissolves both, carries them to the surface, and evaporates — leaving what it carried behind. So a white deposit is not contamination arriving; it is the slab exporting part of itself, which means the interesting question is where the water is coming from.
THE SOLUBILITY TEST IS THE DIAGNOSIS and it takes a minute. Wet a small area with clean water. Soluble salts go into solution and the patch clears, at least until it dries again; that is efflorescence, and a soft brush or a dry cloth will lift it. Lime bloom — calcium carbonate formed when free lime reaches the surface and reacts with carbon dioxide in the air — will not dissolve in water, and rubbing does very little; it needs a dilute acidic cleaner and it tends to come back once before it stops.
WHETHER IT RETURNS IS THE SECOND READING, and it is the one that matters for what to do. A slab that is still drying will produce deposit repeatedly for months, and each recurrence is normal. A slab that has been dry for years and has suddenly started producing deposit is being wetted by something — a failed damp-proof membrane, a leak, ground water, or a change in how the space is used — and cleaning it is chasing a symptom.
WHERE IT APPEARS narrows it further. Deposit concentrated at joints, cracks and edges follows the paths water finds easiest. Deposit spread evenly over a whole slab is the slab drying as a whole. Deposit in one patch on an otherwise clean floor usually means water arriving in that patch.
The reason this matters beyond appearance: a floor still exporting salts is still passing moisture, and floor coverings, adhesives and coatings applied over it will be sitting on a wet substrate. Many adhesive and coating failures on slabs are not product failures — they are installations that went down before the slab had finished.
What it could be, and what tells them apart
These are candidates to check against your own building, in the order they most often turn out to be the answer. None of them is a diagnosis of your situation.
A new slab still drying out
Most commonDeposit spread broadly over a recent slab or screed, recurring after each cleaning, and diminishing over months without anything being done.
How to check: The age of the slab. Concrete takes far longer to dry than to cure, and deposits during that time are the drying itself.
Lime bloom rather than soluble salt
Most commonA deposit that does NOT dissolve when wetted, often a harder, more adherent bloom than a powder, and resistant to brushing.
How to check: The solubility test. If clean water does not shift it, it is calcium carbonate and a brush will not win.
No effective damp-proof membrane under the slab
CommonPersistent deposit on an OLD slab, often with a cold, damp feel, worse in one area, and worse after wet weather.
How to check: Tape a sheet of polythene down and leave it a few days. Moisture under the sheet means the slab is passing water continuously.
Water arriving from a leak or from outside
CommonDeposit in one patch or in a line rather than generally, sometimes near a wall, a pipe run or an external door, and appearing long after the slab was new.
How to check: Whether the pattern is local. A patch on an otherwise clean slab is a water source, not a drying slab.
Water being trapped by a coating or covering
CommonDeposit appearing at the edges of a coated area, under a vinyl or resin that is lifting, or around the perimeter of a rug or mat.
How to check: Whether deposits sit where the covering ends. Moisture that cannot leave through the middle leaves at the edge.
De-icing salt or chemical contamination carried onto the slab
Less commonSeasonal deposit on a garage or entrance slab, following winter and vehicle traffic rather than the slab's own age.
How to check: Whether it follows winter and traffic. Salt brought in on tyres and boots behaves like efflorescence and did not come from the concrete.
What to check, in order
The order matters — each step is here because of what it rules out for the ones after it.
- 1
It separates soluble efflorescence from insoluble lime bloom in one minute, and those need different removal and carry different meanings.
- 2
A drying slab is expected to do this and needs time rather than treatment; an old slab doing it has a water source that has not been found.
- 3
It is a free, non-destructive indication of whether the slab is still passing moisture, and it is the test that decides whether a floor covering can go down.
- 4
General means the slab as a whole; local means water arriving at that point, and only the second one has something to find and fix.
- 5
Water uses the easiest route out, so the pattern of the deposit draws the map of where moisture is moving through the slab.
The calculations behind this
Several of the causes above are a quantity that was specified or assumed. These compute them, with the formula and the standard behind each.
When to stop and get somebody in
- Where a polythene test shows moisture under the sheet on a slab that is meant to be dry, because that is a damp-proof membrane question and it governs every covering that follows.
- Where deposit keeps returning in one area on an old slab, which indicates water arriving rather than the slab drying.
- Where a floor covering, adhesive or resin coating has failed over the affected area, since re-laying it onto a slab still passing moisture repeats the failure.
- Where the slab is in a basement or below external ground level, which brings hydrostatic pressure into it rather than simple drying.
This page lists candidates to check, not a diagnosis — nobody has seen your building. If the symptom involves anything structural, read this first.