Carbonation of concrete occurs when CO2 from the atmosphere reacts with:
Choose the correct answer
C3S in unhydrated cement
Ca(OH)2 (portlandite) in hardened cement paste to form CaCO3, reducing pH from ~12.5 to below 9 and destroying the passive layer on steel reinforcement
Water in concrete pores
Aggregate minerals
Correct Answer
B. Ca(OH)2 (portlandite) in hardened cement paste to form CaCO3, reducing pH from ~12.5 to below 9 and destroying the passive layer on steel reinforcement
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Carbonation: CO2 + Ca(OH)2 -> CaCO3 + H2O. CaCO3 is insoluble, fills pores (slightly beneficial for permeability) but reduces pH from ~12-13 to ~8-9. Below pH 9, the passive oxide layer on steel dissolves -> corrosion initiates. Carbonation depth d = k x sqrt(t). Low w/c, high cement content reduces carbonation rate.
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