Ettringite
Definition
A calcium sulfoaluminate hydrate that forms early in cement hydration and can re-form later from delayed sulfate attack; its presence or alteration products help distinguish fresh from old concrete and flag aggressive groundwater chemistry.
- Mineral class
- Calcium sulfoaluminate hydrate
- Crystal form
- Acicular (needle-shaped)
- Forms when
- First hours of cement hydration
- Forensic use
- Distinguishes fresh from aged concrete; signals sulfate attack or aggressive groundwater
Common questions
What is ettringite and why does it matter in cement forensics?+
Ettringite is a needle-shaped mineral that forms early when cement hydrates. It inhibits the flash set of C3A, preventing the cement from setting too quickly. In older concrete, ettringite can re-form from sulfate attack, which makes it useful for distinguishing fresh from aged samples and detecting aggressive groundwater.
How do you spot ettringite under a microscope?+
Under SEM (scanning electron microscope), ettringite appears as thin needle-shaped crystals. These acicular crystals form within the first few hours of cement hydration as calcium sulfoaluminate hydrate.
What is the chemical formula for ettringite?+
Ettringite has the formula 6CaO.Al2O3.3SO3.32H2O (also written as 3CaO.Al2O3.3CaSO4.32H2O). It contains 32 water molecules per formula unit.
Related terms
- Portlandite
- Calcium hydroxide (Ca(OH)2) crystallised during cement hydration; a bright birefringent phase visible in thin section that dissolves progressively as concrete carbonates, providing...
- Clinker
- The nodular intermediate product of Portland-cement manufacture, consisting of C3S, C2S, C3A and C4AF mineral phases before grinding.
- Portland Cement
- A hydraulic binder produced by firing calcium and silica sources at 1450°C; the four clinker phases (C3S, C2S, C3A, C4AF) hydrate on...
- Aggregate
- The granular filler (sand, gravel, crushed rock) that occupies 60-80% of a concrete volume; its mineralogy, grain shape, and size distribution are...
- Alite (C3S)
- Tricalcium silicate, 3CaO.SiO2. Dominant clinker phase (50 to 70 percent). Hydrates fast, gives early strength up to 28 days.
- Aluminate (C3A)
- Tricalcium aluminate, 5 to 10 percent. Reacts violently with water, attacked by sulphate. SRC cement is low in C3A.
- ASTM C150
- US standard specifying Portland-cement composition limits and performance requirements by type (I through V); referenced in forensic mineral-phase characterisation.
- Belite (C2S)
- Dicalcium silicate, 2CaO.SiO2. 15 to 30 percent. Hydrates slow, gives long-term strength beyond 28 days.
- C-S-H Gel
- Calcium silicate hydrate, the main hydration binder. Amorphous, broad hump on XRD.
- Calcium Silicate Hydrate (C-S-H)
- The principal binding phase produced when Portland cement reacts with water; a gel-like material that gives concrete most of its strength and...
- Calcium Sulphate Hemihydrate
- CaSO4 . 1/2 H2O; produced by heating gypsum, rehydrates to gypsum on setting; the basis of plaster of Paris and most interior...
- Carbonation Front
- The boundary in concrete where atmospheric CO2 has converted portlandite to calcite; the depth of this front (measured in millimetres) is used...
Explained in these topics
- Cement, Mortar and Concrete: Composition and Forensic AnalysisCalcium trisulphoaluminate hydrate 3CaO.Al2O3.3CaSO4.32H2O. Needle-shaped under SEM, formed in the first hours from C3A and gypsum.
- Cement, Plaster and Building-Material EvidenceA calcium sulphoaluminate hydrate (6CaO . Al2O3 . 3SO3 . 32H2O) forming acicular crystals in early cement hydration; inhibits the flash set of C3A.
- Concrete, Mortar, and Cement as EvidenceA calcium sulfoaluminate hydrate that forms early in cement hydration and can re-form later from delayed sulfate attack; its presence or alteration products he...