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Per §2.2.12, the chief reactive component in aggregates that drives the alkali-aggregate reaction is:
A{'text': 'Alumina silicate phase Al2SiO5 in feldspars', 'label': 'A'}
B{'text': 'Reactive silica reacting with cement alkalies', 'label': 'B'}
C{'text': 'Iron oxide Fe2O3 acting as a hydration trap', 'label': 'C'}
D{'text': 'Calcium carbonate CaCO3 in limestone fillers', 'label': 'D'}
Answer & Solution
Correct answer: B. {'text': 'Reactive silica reacting with cement alkalies', 'label': 'B'}
1. §2.2.12 reads: 'some of the aggregates contain reactive silica, which reacts with alkalies (Na2O and K2O… present in the cement).'
2. The reaction with cement alkalies produces an alkali-silica gel of unlimited swelling that disrupts the concrete.
_Source: IGNOU ET-522 Block-1 Unit-2 Aggregates, §2.2.12, p. 24_
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