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: When sulphate ions (from calcium sulphate, sodium sulphate, or magnesium sulphate in soil or groundwater) attack the calcium aluminate hydrate (C3A hydration products) in hardened concrete, they produce ettringite (trisulphoaluminate) and gypsum. Ettringite formation is highly expansive — its crystal growth generates internal pressure that cracks and disintegrates the concrete.
Chemistry of Sulphate Attack
Step 1: Calcium sulphate attacks calcium aluminate hydrate
CaSO4 + C3A hydrate + H2O → Ettringite (3CaO.Al2O3.3CaSO4.32H2O)
CaSO4 + C3A hydrate + H2O → Ettringite (3CaO.Al2O3.3CaSO4.32H2O)
Step 2: Calcium sulphate also reacts with Ca(OH)2 to form gypsum
CaSO4 + Ca(OH)2 + 2H2O → CaSO4.2H2O (gypsum) + Ca(OH)2 consumed
Step 3: Ettringite formation is expansive
Volume of ettringite >> volume of original reactants
Result: internal pressure, cracking, spalling, loss of strength
Ettringite — Properties and Effects
| Property | Detail |
|---|---|
| Chemical name | Calcium trisulphoaluminate hydrate: 3CaO·Al2O3·3CaSO4·32H2O |
| Appearance | Needle-like (acicular) crystals; white to colourless |
| Volume change | Expansive: occupies ≈2.5× volume of reacted C3A |
| Effect on concrete | Internal expansion → tensile cracking → loss of cohesion → disintegration |
| Typical appearance of sulphate attack | White deposits; surface softening; map cracking; eventual structural failure |
Prevention of Sulphate Attack
| Method | How It Prevents Attack |
|---|---|
| Use sulphate-resisting Portland cement (SRPC, IS 12330) | Low C3A content (<5%); less material available for ettringite formation |
| Low w/c ratio (≤0.45) | Dense paste; fewer capillary channels for sulphate ingress |
| Fly ash or GGBS blended cement | Consumes Ca(OH)2 pozzolonically; reduces alkali available for sulphate reaction |
| Waterproof surface coating | Physical barrier prevents sulphate-bearing water from reaching concrete |
- Calcium sulphate + calcium aluminate hydrate → ettringite (expansive; causes cracking).
- Ettringite = 3CaO·Al2O3·3CaSO4·32H2O; highly expansive crystal structure.
- Prevention: sulphate-resisting Portland cement (SRPC) with <5% C3A content; low w/c; dense, impermeable concrete.