How it forms
Concrete and masonry contain soluble compounds, mainly from cement, and can pick up others from soil, groundwater, de-icing salts or process spills. When water moves through the material, it dissolves these salts and carries them toward the surface. As the water evaporates, the salts crystallize. Calcium hydroxide reacting with carbon dioxide from the air forms calcium carbonate, which is hard to remove with water alone. Sodium and potassium sulfates and other salts may also appear.
What it tells you
| Observation | Likely meaning |
|---|---|
| Deposit after construction, then stops | Initial drying of new concrete |
| Recurring deposit in the same area | Continuing moisture source: slab on grade, leak, poor drainage |
| Deposit along cracks or joints | Water path through the defect |
| Deposit under an existing coating, with blisters | Moisture and salts trapped behind the film |
In every case, efflorescence means that water is moving through the substrate. Removing the deposit without addressing that water usually lets it return, under the new coating this time.
Risks for coatings
Salts at the interface interfere with adhesion and, being soluble, draw water through the film by osmosis. The result can be blistering, delamination or a white bloom pushing through. Moisture vapor pressure from below adds to the problem on slabs without an effective vapor retarder.
Treatment before coating
- Identify and correct the moisture source where possible: drainage, leaks, cracks, vapor retarder.
- Measure moisture conditions per ASTM F1869 (moisture vapor emission rate), ASTM F2170 (in situ relative humidity) or ASTM D4263 (plastic sheet), as the coating data sheet requires.
- Remove the deposit by dry brushing and mechanical preparation such as shot blasting or grinding, which also removes laitance.
- Where acid cleaning is allowed, rinse and neutralize thoroughly, then check the surface pH.
- Select a coating system tolerant of the measured moisture, or a moisture mitigation primer, according to the manufacturer.
Where efflorescence returns after coating, the cause is usually the moisture path, not the coating product.
SSPC-SP 13/NACE No. 6 covers the overall preparation of concrete. See our protective coatings service and our concrete protection page.