The leaching action in concrete is an example of:
Correct Answer: C. Chemical reaction
📚 Detailed Explanation: Leaching in Concrete is a Chemical Reaction Process
Why C (Chemical reaction) is correct: Leaching (also called dissolution attack or soft water attack) is classified as a chemical deterioration process. Water — especially soft water, rainwater, or slightly acidic water — percolates through the concrete, chemically dissolves soluble compounds from the cement paste (primarily calcium hydroxide, Ca(OH)2), and carries them out through the concrete. The dissolution reaction and the resulting change in the microchemistry of the paste classify leaching as a chemical reaction type of deterioration.
Leaching Process in Detail
| Stage | What Happens |
|---|---|
| 1. Water ingress | Soft or slightly acidic water permeates concrete through capillary pores and cracks |
| 2. Chemical dissolution | Water dissolves Ca(OH)2 (calcium hydroxide / portlandite) from cement paste: Ca(OH)2 → Ca2+ + 2OH− in solution; reaction is thermodynamically favoured in soft water |
| 3. Transport and deposition | Ca2+ ions migrate to concrete surface; react with CO2 in air: Ca2+ + CO2 → CaCO3 (white efflorescence) |
| 4. Microstructure degradation | As Ca(OH)2 is leached out, the cement paste becomes more porous; eventually C-S-H gel destabilises (requires very advanced leaching) |
| 5. Loss of alkalinity | Reduces concrete pH; compromises passive oxide layer on reinforcement; secondary risk: corrosion initiation |
Classification of Concrete Deterioration Mechanisms
| Category | Examples |
|---|---|
| Chemical reactions | Leaching (dissolution), sulphate attack (ettringite formation), alkali-silica reaction (ASR), acid attack, carbonation |
| Crystallisation | Sulphate crystallisation in pores; salt attack in tidal/splash zones; physical rather than chemical degradation |
| Creep | Long-term deformation under sustained load; a mechanical/rheological phenomenon; not a chemical reaction |
| Decomposition | Thermal decomposition (high temperature >300°C: Ca(OH)2 dehydrates); fire damage; not leaching |
Note: The white deposits (efflorescence) visible on concrete surfaces are often the product of leaching — Ca(OH)2 brought to the surface and reacting with CO2 to form CaCO3. While the white deposit looks like crystallisation, the underlying process that causes it is a chemical reaction (dissolution of Ca(OH)2).
- Leaching is a chemical reaction: water dissolves Ca(OH)2 from cement paste, increasing porosity and causing white efflorescence.
- Soft water, rainwater, and slightly acidic water are the most aggressive leaching agents.
- Prevention: dense, low-permeability concrete (low w/c); pozzolanic additions (reduce Ca(OH)2 content); surface coatings.