When calcium sulphate attacks calcium aluminate hydrate present in concrete, it produces:
Correct Answer: C. Ettringite
📚 Detailed Explanation: Sulphate Attack Produces Ettringite
Why C (Ettringite) is correct: Sulphate attack occurs when sulphate ions from external sources react with two key components of hydrated cement paste: (1) calcium aluminate hydrate (from C3A) and (2) calcium hydroxide [Ca(OH)2]. The primary product is ettringite (calcium trisulphoaluminate), a highly expansive crystalline compound. Its volume is much greater than the reactants, so its formation generates internal expansion forces that crack and disintegrate the concrete.
Chemistry of Sulphate Attack
Reaction 1 (main): Sulphate + Calcium Aluminate Hydrate → Ettringite
CaSO4 + C3A.H6 + H2O → 3CaO.Al2O3.3CaSO4.32H2O (ettringite)
CaSO4 + C3A.H6 + H2O → 3CaO.Al2O3.3CaSO4.32H2O (ettringite)
Reaction 2 (secondary): Sulphate + Ca(OH)2 → Gypsum
Na2SO4 + Ca(OH)2 + 2H2O → CaSO4.2H2O (gypsum) + 2NaOH
Net effect: Ettringite + gypsum form = expansion → cracking → disintegration
Ettringite Properties
| Property | Detail |
|---|---|
| Chemical formula | 3CaO·Al2O3·3CaSO4·32H2O (trisulphoaluminate) |
| Crystal form | Needle-like (acicular); white to colourless |
| Volume change | Expansive: ≈2.5× volume of original C3A reactant |
| Effect on concrete | Internal expansive forces → tensile cracking → spalling → strength loss → disintegration |
UPV Quality vs. Sulphate Attack Susceptibility
| Prevention Method | How It Works |
|---|---|
| Sulphate-resisting Portland cement (SRPC, IS 12330) | Low C3A content (<5%); less material available to form ettringite |
| Low w/c ratio (≤0.45) | Dense, less permeable paste; sulphate ions cannot penetrate as easily |
| Pozzolanic additions (fly ash, GGBS) | Consumes Ca(OH)2; less secondary gypsum formation; denser matrix |
| Waterproof coating | Physical barrier to sulphate-bearing water ingress |
- CaSO4 attacks calcium aluminate hydrate → produces ettringite (highly expansive).
- Expansion from ettringite formation causes internal cracking and progressive disintegration.
- Prevention: SRPC (low C3A), low w/c ratio, pozzolanic blends, dense concrete.