Sulphate Attack on Concrete Explained

Sulphate attack is a type of chemical attack that causes deterioration in concrete. It happens when concrete is exposed to water or soil containing sulphate ions ($\text{SO}_4^{2-}$). These sulphate ions react with certain components of the hardened cement paste, particularly calcium hydroxide ($\text{Ca(OH)}_2$) and calcium aluminates (like $\text{C}_3\text{A}$), to form expansive products.

Mechanism of Sulphate Attack

The primary reactions involved in sulphate attack are:

• Reaction with calcium hydroxide: $\text{Ca(OH)}_2 + \text{SO}_4^{2-} + 2\text{H}_2\text{O} \rightarrow \text{CaSO}_4 \cdot 2\text{H}_2\text{O}$ (Gypsum)
• Reaction with calcium aluminates (in presence of calcium sulphate from previous reaction): $\text{3CaO} \cdot \text{Al}_2\text{O}_3 (\text{C}_3\text{A}) + 3(\text{CaSO}_4 \cdot 2\text{H}_2\text{O}) + 26\text{H}_2\text{O} \rightarrow \text{3CaO} \cdot \text{Al}_2\text{O}_3 \cdot \text{3CaSO}_4 \cdot \text{32H}_2\text{O}$ (Ettringite)

Both gypsum and ettringite are expansive compounds. Their formation within the pore structure of concrete creates internal stresses, leading to cracking, expansion, and eventual disintegration of the concrete.

Severity of Different Sulphates: Magnesium Sulphate

Different types of sulphate salts can cause attack, including sodium sulphate ($\text{Na}_2\text{SO}_4$), potassium sulphate ($\text{K}_2\text{SO}_4$), calcium sulphate ($\text{CaSO}_4$), and magnesium sulphate ($\text{MgSO}_4$). While all are damaging, their severity can differ.

Magnesium sulphate ($\text{MgSO}_4$) is often considered particularly aggressive. Its attack mechanism is more complex than that of alkali sulphates ($\text{Na}_2\text{SO}_4$, $\text{K}_2\text{SO}_4$). In addition to reacting with $\text{Ca(OH)}_2$ and $\text{C}_3\text{A}$ to form gypsum and ettringite, magnesium sulphate also directly attacks the calcium silicate hydrate (C-S-H) phase, which is the primary binder in concrete. This reaction leads to the decomposition of C-S-H and the formation of non-cementitious magnesium hydroxide ($\text{Mg(OH)}_2$, brucite) and magnesium silicate hydrate (M-S-H).

• Reaction with C-S-H: $\text{C-S-H} + \text{MgSO}_4 \rightarrow \text{M-S-H} + \text{Mg(OH)}_2 + \text{CaSO}_4$

The decomposition of C-S-H weakens the concrete structure significantly by reducing the amount of binding material. The combined effects of expansion from ettringite/gypsum formation and the loss of binding capacity due to C-S-H decomposition make magnesium sulphate attack very damaging.

Therefore, the statement "Magnesium Sulphate has less damaging effect than other sulphates present" is incorrect. Magnesium Sulphate is often more damaging due to its attack on C-S-H.

Other Options Analysis

• The vulnerability of concrete to Sulphate attack can be reduced by the use of cement having low percentage of Tricalcium aluminate. This is generally correct. $\text{C}_3\text{A}$ is the most reactive phase with sulphates, leading to the formation of expansive ettringite. Cements with low $\text{C}_3\text{A}$ content (like Sulphate Resisting Portland Cement, SRPC) offer better resistance.
• Improved resistance to sulphate attack can be achieved by the use of Portland Pozzolana cement. This is generally correct. Pozzolanic materials in PPC react with calcium hydroxide ($\text{Ca(OH)}_2$) produced during cement hydration, converting it into stable C-S-H. This reduces the amount of $\text{Ca(OH)}_2$ available for reaction with sulphates and also refines the pore structure, making the concrete less permeable to sulphate solutions.
• Concrete attacked by Sulphates has a whitish appearance, with start of damage at the edges and corners, followed by cracking and spalling of concrete. This is a correct description of the visual signs of sulphate attack. The whitish appearance is often due to deposits of gypsum or ettringite. Damage starts at exposed surfaces like edges and corners where ingress of sulphate ions is easier. The internal expansion leads to cracking and eventually spalling (breaking away) of the concrete cover.

Conclusion

Based on the mechanisms of sulphate attack, particularly the aggressive nature of Magnesium Sulphate due to its attack on the C-S-H phase, the statement that Magnesium Sulphate has less damaging effect is incorrect.