Efflorescence is a white, powdery deposit of salt that forms on the surface of brick, concrete, stone, stucco, and other porous building materials. It appears when water dissolves soluble salts inside the material, carries them to the surface through tiny pores, and leaves the salt crystals behind as the water evaporates. While it looks alarming, surface efflorescence is cosmetic, not structural. The real concern is when salt crystallization happens inside the material, where it can cause lasting damage.
How Efflorescence Forms
Three conditions must be present for efflorescence to appear: soluble salts within the material, water to dissolve those salts, and a force that moves the dissolved solution toward the surface. That force can be capillary action (water wicking upward through pores), hydrostatic pressure from groundwater, gravity, or simple evaporation pulling moisture outward.
As water moves through the porous material, it picks up soluble salts along the way. When that moisture reaches the surface and evaporates, the dissolved salts are left behind as a visible crystalline deposit. In cement-based materials like concrete and mortar, the most common culprit is calcium hydroxide from the cement paste. It reacts with carbon dioxide in the air to form calcium carbonate, the white residue you see on the surface.
The process involves a constant tug-of-war between two forces: advection (the flow of water carrying salts toward the drying surface) and diffusion (which tries to redistribute the salt concentration evenly throughout the material). When advection wins, salts concentrate at the surface and crystallize as visible efflorescence.
What the White Deposit Is Made Of
The most common salts found in efflorescence are sodium sulfate and potassium sulfate. Analysis of efflorescence samples across different building types consistently shows some combination of sodium and potassium salts as the primary component, according to research published by the National Research Council of Canada. Mortar joint efflorescence is a notable exception, where sodium carbonate tends to dominate instead.
Beyond those, efflorescence deposits can contain calcium sulfate, calcium carbonate, magnesium sulfate, sodium carbonate, sodium bicarbonate, and trace amounts of sodium chloride. Less commonly, salts of vanadium, chromium, and molybdenum have been identified. The specific composition depends on what’s in the building material and what the water picks up as it travels through.
Which Materials Are Affected
Any porous building material can develop efflorescence, but some are more susceptible than others. Brick, concrete, concrete pavers, stone, and stucco are the most commonly affected because they readily absorb or wick water. The cement in concrete and mortar is typically the primary source of soluble salts, making these materials particularly prone.
Efflorescence appears as a white or grayish tint on vertical surfaces like walls, or as a powdery layer on floors and horizontal surfaces. On brick, it often shows up in patches or streaks. On concrete slabs, it can cover large areas uniformly, especially when moisture is wicking up from below.
Surface Deposits vs. Internal Crystallization
When salts crystallize on the surface, the result is unsightly but harmless. You can brush or wash it off without any lasting effect on the material. The real problem occurs when salts crystallize inside the pores of the material, a process sometimes called cryptoflorescence or subflorescence.
Salt crystals growing inside pores generate pressure against the walls of those pores. Repeated cycles of wetting and drying dissolve and recrystallize salts over and over, building up stress with each cycle. Over time, this leads to spalling (surface flaking), crumbling, and structural deterioration. Sodium sulfate is particularly destructive because it can change between crystal phases with different sizes, amplifying the internal pressure. This is why controlling moisture matters even when the visible efflorescence seems minor: the surface deposits signal that salt migration is happening, and some of that crystallization may be occurring inside the material as well.
How to Remove It
Start with the simplest approach: dry brushing. A stiff-bristled brush can often remove the powdery salt deposits without any chemicals. If the efflorescence doesn’t come off with brushing alone, you have two main chemical options.
Vinegar is the gentler choice. Dilution ratios range from 1 part vinegar to 1 part water (stronger) up to 10 parts vinegar to 1 part water (weaker). Start with a weaker solution and work up if needed. For more stubborn deposits, muriatic acid (hydrochloric acid) is effective at a ratio of 1 part acid to 10 or 12 parts water. Apply either solution with a pump sprayer and scrub with a stiff bristle brush if needed. Products specifically formulated for efflorescence removal are also available at most hardware stores.
Removal only solves the visible problem. If the moisture source isn’t addressed, the efflorescence will return.
Preventing Efflorescence
Since efflorescence requires soluble salts, water, and a path to the surface, prevention targets at least one of those three conditions.
- Control moisture from below. For slab foundations, install a capillary break: 4 inches of aggregate stone or sand beneath the slab, topped with a vapor barrier of at least 6-mil polyethylene sheeting. Lap seams 6 to 12 inches and seal them. Seal the sheeting at joints with foundation walls and around any pipes or posts penetrating the floor.
- Choose low-alkali cement. Portland cement meeting ASTM C 150’s low-alkali specification reduces the amount of soluble salts available to migrate. This is one of the most straightforward prevention steps during construction.
- Manage water exposure. Proper flashing, drainage, and grading keep rainwater and groundwater from saturating masonry walls. Crawlspaces should have plastic sheeting over bare dirt floors to limit moisture vapor from the ground.
- Test materials before building. ASTM C 67 describes a test for evaluating whether brick units are likely to cause efflorescence. You can do a simplified version yourself: immerse a single brick in distilled water for about seven days, let it dry, and compare it to an untreated brick. Visible salt deposits on the dried brick indicate efflorescence potential.
The limitation of material testing is that efflorescence often results from the interaction between multiple components. A brick that tests clean on its own may still develop efflorescence when paired with a particular mortar or exposed to groundwater carrying dissolved minerals. ASTM C 1400 provides broader guidance for reducing efflorescence potential in new masonry walls by addressing the system as a whole, not just individual components.
Why It Often Appears on New Construction
Efflorescence is most common in the first year or two after construction. New concrete and mortar contain the highest concentration of soluble salts, and construction moisture (water used in mixing and curing) provides the vehicle to move those salts to the surface. As the materials dry out over time and the readily available salts are depleted, efflorescence typically diminishes on its own. This early-stage efflorescence is sometimes called primary efflorescence.
When efflorescence appears on older structures, it usually signals a new or worsening moisture problem: a failed flashing, rising groundwater, a plumbing leak, or inadequate drainage. This secondary efflorescence is more concerning because it points to an ongoing issue that can lead to the internal salt crystallization cycles that damage masonry over time.