Why is there white powder on my new paving?

What the white bloom actually is, why it appears on new paving and then stops, and why the cure is usually patience rather than chemicals.

Difficulty: beginner
Time: 20 min
Read: 5 min
Safety: caution

Short answer

It is salt carried out of the bedding mortar or the slab itself by water evaporating at the surface. It is cosmetic, it is normal on new paving, and it usually weathers off over one to two winters as the free salts are used up. Brush it off dry, rinse, and resist the urge to attack it with acid, which often makes it worse.

Efflorescence appears when water moving through a cement-based material dissolves free salts, carries them to the surface, and evaporates. The salt is left behind as a white bloom. New paving has plenty of free salt — from the bedding mortar, from the jointing, and from the slabs if they are concrete — and plenty of water, so a first winter of white patches is extremely common and says nothing bad about the workmanship.

Safety

Acid-based efflorescence removers are hazardous — they burn skin and eyes, give off fumes, and runoff harms plants, ponds and watercourses. They also etch some stone and dull some finishes. Treat acid as a last resort, test on a spare slab, wear gloves and eye protection, and never let the runoff reach a drain that goes to a watercourse.

Step by step

Confirm it is efflorescence and not something else.Efflorescence is a fine white powder or crust that appears in patches, is worse after wet weather followed by drying, and brushes off dry at least partly. Lime bloom is a harder, more tenacious white film that does not brush off. Limescale from tap water splashes is localised. Paint or cement residue is uniform and dates from the laying.
Establish where the water is coming from.Efflorescence needs a water route. Rain landing on the surface produces mild, general bloom that fades. Persistent heavy bloom in the same place means water is moving through the bed — a downpipe discharging on the paving, a leaking gully, ground water at an edge, or an area that ponds.
Brush it off dry, first.A stiff dry brush removes most surface efflorescence. Do it dry: wetting it redissolves the salt and carries it back into the slab, where it will come out again on the next dry spell.
Then rinse and let it dry.After brushing, a rinse with clean water and a good drying period removes what is left on the surface. Repeat over a few dry spells.
Wait a season before doing anything more.The supply of free salt is finite. Most new paving stops efflorescing within one to two years as it is washed out. Aggressive treatment during that period is treating a process that is ending on its own.
Only then consider a proprietary remover.If it persists beyond a couple of years, a purpose-made efflorescence remover — usually a mild acid — will remove it. Test on a spare slab or a hidden corner: acids etch limestone and some sandstones, dull polished finishes and can bleach coloured concrete.
Fix the water route if you want it to stop.Chemical removal without dealing with a persistent water path simply resets the clock. Divert the downpipe, correct the fall, repair the joints, or drain the edge.
Do not seal until it has finished.Sealing paving that is still efflorescing traps the salt under the sealer, where it crystallises and can lift the sealer off. Wait until it has run its course, then seal if you want to.

Tips

It looks worst in winter and best in summer, because it needs the wet-then-dry cycle. Judge it in June, not January.
Photograph it each season. People remember the worst day rather than the trend, and the trend on new paving is almost always improvement.
If the paving is brand new and heavily bloomed, ask the supplier — some concrete products are known for it and manufacturers usually have specific advice and sometimes a remover they recommend.
A pressure washer removes the surface bloom quickly but drives water into the paving, which brings more salt out next time. Brushing dry is genuinely better.

Common mistakes

Washing it off with a hose as the first move — Water redissolves the salt and takes it back into the slab. It disappears for a day and comes back worse.
Going straight to acid — Acid removes the bloom and can etch the surface, dull the finish and, on some stones, cause a worse and permanent discolouration. It is a last resort after a season of patience.
Sealing new paving in its first autumn — The salts are still coming out. Sealing traps them under the film and the sealer blisters or goes white.

If it doesn't work

White bloom returns every time the paving dries

Cause: Water is still moving through the bed and there is still free salt to carry. — Fix: Find the water route. A downpipe or a gully discharging near or under the paving is the commonest cause and the easiest to fix.

It will not brush off at all

Cause: This is more likely lime bloom — calcium carbonate formed from lime leached out and reacting with air — which is chemically different and much harder. — Fix: Lime bloom needs a proprietary lime remover rather than brushing, applied with the usual test-first caution. It does not weather off as readily as efflorescence.

The joints are white but the slabs are clean

Cause: The salt is coming from the jointing mortar, which is normal in the first year. — Fix: Brush dry and wait. Cement-based joints commonly bloom and then settle down.

It appeared after sealing

Cause: Salt has crystallised under the sealer film, or the sealer was applied to paving that was not fully dry. — Fix: The sealer usually has to be stripped, the paving allowed to dry out and finish efflorescing, and then re-sealed. This is why sealing new paving too early is a mistake.

Questions people ask

Does efflorescence damage the paving?

On a patio, no — it is a cosmetic surface deposit. What can cause damage is the underlying water movement, if it is persistent enough to saturate the bed and cause frost damage. So the bloom is worth reading as information about water rather than treating as damage in itself.

Will it definitely go away?

Almost always, over one to two years, as the free salts are exhausted. What does not go away on its own is bloom driven by a permanent water source — a leaking gully, a spring, or paving in permanent contact with wet ground.

Is it the installer's fault?

Usually not. Efflorescence on new cement-based paving is a normal material behaviour, and most suppliers say so explicitly in their literature. What is a workmanship issue is a fall that ponds, a joint that lets water into the bed, or a downpipe left discharging onto the paving.