Durability of Concrete MCQ with answer and solution

Concept

Volume expansion during sulfate attack is primarily caused by the formation of ettringite (calcium sulfoaluminate) due to the reaction between sulfates and hydrated aluminates in hardened cement paste.

Step-by-step solution

1. Identify the aggressive agent: Sulfates (SO4²-) present in soil or groundwater enter the hardened concrete matrix. 2. Identify the reactive components: Sulfates react with calcium hydroxide (Ca(OH)2) to form gypsum, and further react with hydrated calcium aluminate compounds (like C3A). 3. Ettringite formation: The reaction produces ettringite (tricalcium sulfoaluminate-32-hydrate), which has a significantly higher volume than the reactants. 4. Resulting damage: The substantial increase in solid volume creates internal expansive pressures, causing cracking, spalling, and loss of strength.

Explanation

Sulfate attack occurs when sulfates from soil or groundwater react with hydrated cement paste, notably calcium hydroxide and calcium sulfoaluminate. This reaction forms expansive crystals of ettringite, leading to internal stresses, volume expansion, cracking, and eventual disintegration of the concrete.

Why the other options are wrong

A: Expansion of ASR gel is responsible for alkali-silica reaction (alkali-aggregate reaction), not sulfate attack. B: Expansion of aggregates is typically related to thermal or alkali-aggregate reactions, not the primary mechanism of chemical sulfate attack. C: While gypsum is formed during sulfate attack and can cause some expansion, the primary and most damaging expansive product is ettringite.

Shortcut

Remember: Sulfate attack = Ettringite = Expansion and cracking. ASR = Alkali-Silica Reaction = Gel expansion.

Common mistake

Confusing sulfate attack with Alkali-Silica Reaction (ASR); ASR involves alkali-silica gel expansion, whereas sulfate attack involves ettringite and gypsum formation.