Which accelerating admixture is harmful for reinforced concrete and pre-stressed concrete?
Correct Answer: B. Calcium chloride
📚 Detailed Explanation: Calcium Chloride Harmful to RC and PSC
Why B (Calcium chloride) is correct: While calcium chloride (CaCl2) is an effective accelerator, chloride ions (Cl¹¯) are aggressive to steel reinforcement. They depassivate the protective iron oxide film on the steel surface, initiating corrosion. For reinforced concrete, a maximum of 0.3% Cl¹¯ by mass of cement is permitted (IS 456). For pre-stressed concrete, no calcium chloride is permitted at all due to the risk of stress-corrosion cracking of high-tensile steel wires.
Comparison of Common Accelerating Admixtures
| Accelerator | Effectiveness | Harmful to Steel? | Permitted in RC? | Permitted in PSC? |
|---|---|---|---|---|
| Calcium chloride (CaCl2) | High (most effective) | Yes — aggressive Cl¹¯ ions corrode steel | Limited (<2% by wt. cement) | NOT permitted |
| Soluble carbonates (Na2CO3) | Moderate | Less harmful | Yes (limited quantities) | Restricted |
| Silicates (Na2SiO3) | Moderate | Minimal | Yes | Limited |
| Fluorosilicates | Low-moderate | Minimal | Yes | Limited |
Mechanism of CaClâ‚‚ Attack on Steel
| Step | What Happens |
|---|---|
| 1. Normal state | Steel in concrete has a passive oxide film (Fe2O3) formed by high alkalinity (pH >11.5) |
| 2. CaCl2 added | Cl¹¯ ions migrate through concrete pore water to the steel surface |
| 3. Depassivation | Cl¹¯ disrupts and dissolves the passive oxide film at local anodic sites |
| 4. Corrosion begins | Fe → Fe²+ → rust (Fe2O3·H2O); volume expands 3×–6× |
| 5. Concrete cracks | Expansive rust pressure exceeds tensile strength; concrete spalls and splits longitudinally |
- CaCl2 is the most widely used accelerator but is prohibited in pre-stressed concrete and limited in reinforced concrete.
- IS 456:2000: maximum chloride content in concrete (from all sources) = 0.3% by mass of cement for RC.
- Stress-corrosion cracking of high-tensile PSC wires can occur at even lower Cl¹¯ concentrations.