The use of which of the following chemicals as adulterants in water used in concrete affects the setting time of cement?
Correct Answer: C. Sodium carbonates
📚 Detailed Explanation: Sodium Carbonates and Cement Setting Time
Various dissolved chemicals in mixing water can either accelerate or retard the setting of cement. Understanding which chemicals cause which effect is important for controlling workability and avoiding premature stiffening in the field.
Why C (Sodium carbonates) is correct: Sodium carbonate (Na2CO3) and sodium bicarbonate (NaHCO3) are well-known accelerators of cement setting. They react with the gypsum retarder in cement, effectively neutralising its retarding action and allowing C3A to hydrate rapidly — causing flash set or quick set. In small concentrations (<1000 ppm), the effect is moderate; at higher concentrations, the concrete can stiffen before placement. Comparison: Calcium chloride (B) is also an accelerator but mainly shortens the setting time without flash set; Sodium sulphate (A) can cause expansive sulphate attack on hardened concrete; Lime (D) has negligible effect on setting in normal concentrations.
Effect of Common Dissolved Salts on Setting Time
| Chemical | Effect on Setting | Concern |
|---|---|---|
| Sodium carbonate / bicarbonate | Rapid acceleration (flash set) | Loss of workability before placing |
| Calcium chloride (<2%) | Mild acceleration | Corrosion of steel |
| Sodium sulphate | May retard (high conc.) | Sulphate attack on hardened concrete |
| Lime | Negligible | Excess can affect alkali content |
Key Concepts for Students
- IS 456 limits sulphates in mixing water to 400 mg/l (as SO3) to prevent ettringite expansion.
- Sodium carbonates cause flash set — rapid stiffening that cannot be reversed by further mixing.
- Any water that is not potable must be tested for pH, sulphates, chlorides, and organic matter before use in concrete.