Efflorescence

Fibre Reinforced Facade Encyclopedia: Finishes

Efflorescence is the white deposit that forms when soluble salts inside a cementitious material are dissolved by water, carried to the surface and left behind as the water evaporates. On concrete, glass fibre reinforced concrete, precast and masonry the deposit is mostly calcium carbonate, produced when calcium hydroxide from the cement reaches the face and reacts with carbon dioxide in the air. It is an appearance issue rather than a structural defect, but it is a real and recurring one and it should be planned for, not denied.

At a glance

Why it happens

Portland cement hydration produces calcium hydroxide as a by-product, roughly a fifth of the hydrated paste by weight, and it is slightly soluble. Wherever liquid water moves through the pore structure it carries dissolved calcium hydroxide with it, and where that water reaches a surface and evaporates the calcium hydroxide is left behind and carbonates to calcium carbonate, which is insoluble, white and firmly attached.

Water vapour alone does not do this. The salts move in liquid water, which is why the controls that work are the ones that stop liquid water travelling through or across the section, and why a panel that is kept dry after cure rarely effloresces at all.

Primary and secondary efflorescence

Primary efflorescence comes from the mix water itself, working its way out during the first weeks after casting. It is usually a uniform light bloom over a whole panel, it appears early, and in most cases it weathers away over the first year as rain dissolves the outer deposit and carbonation stabilises the surface.

Secondary efflorescence comes from water entering the hardened material later: rain on an exposed face, water tracking behind a panel, a leaking joint, or run-off from an adjacent element. It appears as streaks, patches or runs concentrated at a joint, a sill or a fixing, it recurs, and it will not weather away because the water source is still there. Lime run, where water passing through a crack leaves a hard deposit on the face, is the extreme version.

Reducing it in the mix and the cure

• Keep the water to cement ratio as low as workability allows; less capillary porosity means less transport
• Use pozzolans such as silica fume, metakaolin, fly ash or slag, which consume free calcium hydroxide in a secondary reaction
• Consider low alkali cement and, for GRC, polymer modification, which reduces permeability
• Cure properly and consistently; an undercured, porous surface is the most efflorescence-prone surface there is
• Use an integral hydrophobic admixture where the exposure justifies it
• Keep stacked panels covered and ventilated, and avoid wetting and drying cycles before installation

None of these eliminates efflorescence. They reduce its likelihood and severity and shorten how long primary bloom persists. Any manufacturer claiming a cementitious product cannot effloresce should be asked to put that in writing.

Designing water away from the face

Detailing does more than mix design. Drips under sills, copings and projecting elements stop water running back onto the face below. Throats at reveals and returns break the flow path. Falls on horizontal surfaces shed water in a controlled direction instead of ponding. Copings and parapet cappings need continuous cover and sealed joints, because water entering the top of a panel comes out somewhere on the face.

Behind the panel, a drained and ventilated cavity lets the back dry rather than holding it wet, which is one reason rainscreen construction efflorescences less than a face sealed wall. Run-off from an unlike material above, such as an uncoated concrete or limestone coping, deposits its own salts on the panel below and is a frequent cause of streaking blamed on the panel.

Removal and remediation

Start with the least aggressive method. Much primary efflorescence brushes off dry, or comes away with water and a stiff brush, particularly before it has fully carbonated. Where a wash is needed, a dilute acidic cleaner, commonly 5 to 10 percent acetic or phosphoric acid, dissolves calcium carbonate. Wet the surface first so the acid cannot be drawn into the pores, work in small areas, then neutralise and rinse thoroughly.

Avoid hydrochloric acid on architectural faces. It etches the paste, changes the texture and colour, and leaves chloride salts behind that cause further efflorescence. Avoid high pressure water, which drives water into the section and feeds the next cycle. Avoid sealing an efflorescing surface with a film forming coating, since the salts continue to arrive and will lift the film. After the face is clean and dry, a penetrating hydrophobic treatment reduces water uptake without forming a film and is the usual final step.

In the specification, set realistic acceptance criteria: require the cause of any recurring deposit to be identified rather than simply cleaned, allow uniform primary bloom to weather, and assess against an approved mock-up at the stated viewing distance.

Standards and references

• ASTM C1400 Standard Guide for Reduction of Efflorescence Potential in New Masonry Walls
• BS EN 1170 series, GRC test methods including water absorption and bulk density
• GRCA Specification for the Manufacture, Curing and Testing of GRC Products
• National Precast Concrete Association Australia, Precast Concrete Handbook
• Cement Concrete and Aggregates Australia guidance on efflorescence in concrete

Related terms

• Curing
• Water absorption
• Surface finish
• Glass fibre reinforced concrete (GRC / GFRC)
• Weathering
• Hydrophobic treatment
• Architectural precast

Questions specifiers ask

Reviewed by Richard Halvorsen, General Manager, ShapeShift Technologies. Last updated September 2026.