When calcium sulphate attacks calcium aluminate hydrate present in concrete, it produces:

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) 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.