Correct Answer - Option 1 : Atmospheric corrosion
Carbonation occur when carbon dioxide from the air penetrates inside the concrete through pores and reacts with calcium hydroxide (formed during hydration of cement) to form calcium carbonates.
Ca (OH)2 + CO2 → CaCo3 + H2O
This reaction reduces the pH of the pore solution as low as 8.4 and leads to corrosion of steel reinforcement due to carbon dioxide received from atmosphere, hence called Atmospheric corrosion.
Important points about Carbonation:
1. The phenolphthalein indicator solution is applied to a fresh fracture surface of concrete. If the indicator turns purple, the pH is above 8.6. Where the solution remains colourless, the pH of the concrete is below 8.6, suggesting carbonation.
2. The carbonation depth is proportional to the square root of time.