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Per §8.10, the principal carbonation reaction of hardened concrete with atmospheric carbon dioxide is:
A{'text': 'Ca(OH)2 + CO2 + H2O → CaCO3 + 2 H2O', 'label': 'A'}
B{'text': 'Ca(OH)2 + 2 CO2 → Ca(HCO3)2 only', 'label': 'B'}
C{'text': 'Mg(OH)2 + CO2 → MgCO3 + H2O', 'label': 'C'}
D{'text': 'NaOH + CO2 → NaHCO3 + H2O', 'label': 'D'}
Answer & Solution
Correct answer: A. {'text': 'Ca(OH)2 + CO2 + H2O → CaCO3 + 2 H2O', 'label': 'A'}
1. §8.10 diagram and text give the carbonation equation as Ca(OH)2 + CO2 + H2O → CaCO3 + 2 H2O.
2. Carbonation drops the alkaline pH (>12.6) to about 8, destroying the passivating layer on steel reinforcement and triggering corrosion.
_Source: IGNOU ET-522 Block-2 Unit-8 Durability of Concrete, §8.10 carbonation diagram, p. 79_
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