Ettringite is a needle-shaped calcium sulfoaluminate mineral found in cement and concrete. Under normal conditions ettringite forms during the first hours after mixing — that is harmless. The problem arises when ettringite forms later, or recrystallises: then it causes expansion, cracking and degradation of the concrete.
What is ettringite chemically?
The chemical formula: Ca₆Al₂(SO₄)₃(OH)₁₂·26H₂O
Ettringite crystallises as fine, needle-like to columnar crystals that take up significant volume in concrete. The mineral is named after Ettringen in Germany, where it was first identified.
Primary and secondary ettringite
| Type | When | Effect |
|---|---|---|
| Primary ettringite | During cement hydration, shortly after mixing | Desired, aids set |
| Secondary ettringite | Later, after hardening, due to external sulfate | Harmful, expansion |
| Delayed ettringite formation (DEF) | Only after months or years | Very harmful |
How does secondary ettringite form?
For damaging ettringite formation, the following are usually present:
- Sulfate — from soil, groundwater, seawater or industrial emissions
- Moisture — water as the transport medium
- Aluminate-rich cement — certain cement types
- Elevated temperature — typical of steam-cured precast
Without moisture no ettringite forms. Dry construction is therefore relatively safe.
Consequences
Secondary ettringite formation leads to:
- Volume expansion — needles push the concrete apart
- Microcracking — at first invisible, later open cracks
- Surface spalling — under freeze-thaw cycles
- Strength loss — concrete becomes friable
- Irreversible damage — repair usually impossible
Sulfate attack is a major form of concrete deterioration in aggressive environments.
Prevention
Ways to prevent ettringite damage:
- Correct cement choice — sulfate-resisting cement (SR/HS) in sulfate-rich environments
- Low water-cement ratio — denser concrete, less permeable
- Pozzolans — fly ash, GGBS (CEM III) reduce risk
- Limited curing temperature — prevents DEF
- Protective coatings — against direct sulfate contact
- Good drainage design — prevents trapped moisture
Assessment in practice
In concrete used in:
- Sewers — H₂S oxidises to H₂SO₄ → ettringite risk
- Seawater — sulfate is present
- Industrial settings — fertilisers, chemicals
- Groundwater containing gypsum — in certain regions
…sulfate-resisting or low-aluminate cement is always specified.
Related terms
- Concrete
- Portland cement
- Blast furnace cement
- Concrete deterioration
- Sulfate attack
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