Efflorescence in Colored Concrete: Causes, Prevention, and Where the Pigment Supplier Fits

A white bloom on a red paving tile is usually reported as a colour problem. It is a salt-transport problem. Efflorescence in colored concrete is the deposit left behind when water carrying dissolved salts reaches the concrete surface and evaporates. The pigment is not the salt, and in most cases it is not the cause — but its water-soluble content is a parameter that can be specified on a number and tested, which makes it one of the few parts of this problem a supplier can be held to.

What efflorescence in colored concrete actually is

Efflorescence forms in three steps, and all three have to happen: soluble salts present in the pore water; water migrating to an evaporation surface by capillary flow and diffusion; and evaporation that leaves the salts to crystallise. Remove any one of the three and no bloom appears — which is why this is a moisture-and-salt problem, not a colouring problem.

The salt most commonly reported in concrete efflorescence is calcium carbonate. Free lime — calcium hydroxide, Ca(OH)2 — is released as cement hydrates, dissolves in the pore water, travels to the surface and reacts there with atmospheric carbon dioxide to form calcium carbonate (CaCO3), which has low solubility and precipitates as the white deposit. Sulfates and chlorides appear too, where the materials or the mix water bring them in.

Three white surface conditions get confused with each other, and they have different causes:

Only the first is a salt-transport problem. Efflorescence is also described in two phases: primary, during manufacture and curing where units are cured wet or stored with poor air movement, and secondary, later in service, driven by water entering from outside — rain, irrigation, rising damp or wash-down.

Why the same bloom is worse on coloured concrete

On grey concrete a thin white deposit reads as a slightly lighter patch. On a red, black or brown pigmented surface the same deposit is a high-contrast mark, so the tolerance in the buyer's eye is much tighter. The deposit has not changed; its visibility has.

The pigment itself does not travel. Iron oxide pigments are water-insoluble inorganic oxide particles, so they stay where they were placed in the matrix; bloom, by contrast, is built from material that dissolves, moves with water and re-crystallises at the surface.

Where pigment does enter the discussion is the soluble salt load it carries in. Iron oxide grades are supplied against a water-soluble matter limit, commonly reported on published technical data sheets in the region of 0.3% max, with sulfate content as a second parameter in the same family.

The four inputs that decide whether bloom appears

Water. The pore network is the pathway, and water-cement ratio is the main control on it. A mix placed stiffer and finished without adding water for workability has a denser surface layer, so less water reaches the evaporation surface and less salt is carried with it. Curing water counts as an input: ponding or spraying with water that itself carries dissolved salts adds to the salt load, so appearance-critical work is normally cured with clean potable water.

Cement. Free lime is the raw material for the most common deposit, so the cement's alkali content — commonly expressed on a mill certificate as sodium oxide equivalent — and its free-lime behaviour set how much soluble material is available. Where a project has a history of bloom and the mix design allows it, specifying a cement with lower alkali release is one of the few upstream levers available.

Sand and aggregate. Unwashed or marine-derived sand can carry chlorides and a high fines content. Chlorides add to the soluble salt load, and fines raise water demand for the same workability. A washed, well-graded sand is a cheaper fix than any change to the pigment.

Admixtures. Calcium chloride accelerators are a commonly reported contributor to both efflorescence and chloride load; where acceleration is needed, non-chloride types are normally specified instead. Water-reducing admixtures can also change bleeding behaviour, and so change how much water migrates to the surface while the unit is still plastic.

External water. A correctly made unit can still effloresce in service if it stays wet, which is why fall, drainage and jointing set how far the pigment decision is ever tested.

What the pigment supplier owes you — and what it does not

The pigment is one line item in a salt balance. It is a real line item, and it is testable:

What the supplier does not control: the water-cement ratio, whether the sand was washed, how the units were cured and how they were stored. A pigment at its water-soluble limit can be blameless in a mix placed wet and cured under ponded water.

Prevention, in the order it works

1. Control the water first. Place at the lowest water-cement ratio the mix design allows, and do not add water on site for workability. This is the largest single lever on the pore network that carries salts to the surface.
2. Fix the solids. Use washed sand within a graded envelope, keep chlorides out of the aggregate and mix water, and specify a non-chloride accelerator where acceleration is required.
3. Finish for density, not for shine. Full compaction and a consistent finish close the surface layer. Over-trowelling draws paste and fines upward and can leave a weak layer that later dusts or chalks — a different defect, often reported as efflorescence.
4. Cure uniformly, then let it dry. Curing supports strength development but also keeps a moisture path open, so method and duration are managed rather than maximised. Ponding with hard water adds salts while it cures.
5. Dry before you wrap. Palletising and wrapping units while still wet traps moisture against the faces and is commonly reported as a driver of primary efflorescence on pavers.
6. Choose a penetrating repellent over a film. Silane and siloxane repellents reduce water ingress while leaving the surface able to release vapour. Film-forming coatings can hold moisture beneath the film, commonly reported to worsen bloom on a damp substrate.
7. Remove existing bloom dry. Dry brushing, then a clean-water wash and a full drying period, removes fresh deposit without driving it into the surface. Acid washing is used on grey concrete, but on integrally coloured surfaces it can change the surface colour, so trial it on a spare unit first.

How to verify before you commit to volume

Efflorescence is a system effect, so it must be verified on the system: a trial that uses the supplier's pigment but not the buyer's cement, sand, admixture, curing cycle and storage conditions tests almost nothing.

1. Cast trial units with the real materials — own cement, sand, aggregate and admixture, the intended addition rate and finish. Include one pallet cured and stored exactly the way production is cured and stored, because that pallet is where primary efflorescence shows up if it is going to.
2. Evaluate after curing, not at demoulding. Check at 7 days and again at 28 days, in the same light, against a reference sample held under agreed conditions. Most disputes about "wrong colour" are disputes about the reference.
3. Work from documents, not assurances. For integrally coloured concrete the reference point is ASTM C979 / C979M-16, Standard Specification for Pigments for Integrally Colored Concrete — water wettability, alkali resistance, sulfates, water solubility, atmospheric curing stability, light resistance and colour match of shipment. For pigments used in cement- and lime-based building materials, EN 12878 covers requirements and test methods. ASTM C1400-23, Standard Guide for Reduction of Efflorescence Potential in New Masonry Walls, is written for masonry walls rather than concrete units, but its principles on limiting soluble salts and controlling moisture transfer apply to paving and tile production. ASTM C936 / C936M covers product requirements for solid concrete interlocking paving units.
4. Ask the supplier for the lot file — water-soluble matter, sulfate content and alkali resistance for the delivered lot; ΔE against the supplier's standard; and a retained reference sample.

Where the pigment is genuinely not the variable

Pigment selection changes hue, depth, tinting strength and dispersion behaviour. It does not change the pore structure of the concrete, the water in the mix, the alkali content of the cement, or how the units were dried. So when a bloom appears, the questions that locate the cause come first: the water-cement ratio the mix was placed at, what the sand carries, and how the units were cured and stored.

About the supplier

Hebei Tianhuibao Technology Co., Ltd. has produced iron oxide pigments for 26 years, with an annual production capacity of 50,000 tons, and supplies customers in 150+ countries and regions. The iron oxide red series covers grades 110, 130 and 190, plus nano, ultra-fine, easy-dispersing, acid- and alkali-resistant and food-contact grades, and factory-direct colour matching. Certifications held include REACH, RoHS, CE and ISO, with IATA DGR for air transport and IMDG Code for sea transport. The minimum order quantity is 1 metric ton in 25 kg woven bags available ex-stock; small samples are supplied separately for testing and do not count toward the MOQ. Stock items ship in 3–5 working days, customised items in 7–15 working days. Quality control runs from raw material testing through to finished product inspection, and multi-batch colour consistency data (ΔE) is available on request.

Our article Iron Oxide Pigment Dosage for Concrete: kg per m³ in the News section covers addition rates and how to verify them against your own materials.

To evaluate a pigment against your own mix and curing cycle before efflorescence in colored concrete becomes a dispute, send us your cement and sand details, your addition rate and your curing method, and request a specification sheet and a trial sample. Contact: [email protected] / +86-13400211655.