To overcome the accelerating effect of high temperature on initial setting time of concrete, which admixture is added to the fresh mix?
Correct Answer: B. Retarders
📚 Detailed Explanation: Retarders Counter High-Temperature Acceleration
Why B (Retarders) is correct: High temperature increases the rate of cement hydration. In hot weather (>30°C), concrete can reach initial set in as little as 15–20 minutes, leaving inadequate time for transport and placement. Retarders are added to the fresh mix to slow hydration, extend the setting time, and ensure concrete remains workable for the required concrete operations.
Temperature Effect on Concrete Setting Time
| Temperature (°C) | Approximate Initial Setting Time (OPC) | Problem |
|---|---|---|
| 10°C | 120–180 min | Too slow; cold weather requires accelerators |
| 27°C (reference) | 30–60 min | Normal; IS 269 minimum: 30 min IST |
| 38°C | 15–25 min | Too fast; retarders needed |
| 45°C+ | <15 min | Flash set possible; severe retardation required |
Applications Where Retarders Are Used
| Application | Reason Retarder Is Needed |
|---|---|
| Hot weather concreting (>38°C) | Prevent premature setting before placement is complete |
| Long-distance transport (ready-mix) | Maintain workability during extended transit time |
| Deep oil-well grouting | Grout must remain fluid for hours as it descends the borehole |
| Long tunnels / tall chimneys | Placement operations take extended time; cannot allow premature set |
| Mass concrete pours | Extended placing time; internal heat of hydration also elevates temperature |
Why Other Options Are Wrong
| Option | Why Incorrect |
|---|---|
| Superplasticisers | Increase workability/flowability; do not primarily slow setting time in hot conditions |
| Retarders | Correct: slow hydration; extend setting time; counteract heat effect |
| Air-entraining agents | Improve freeze-thaw resistance; do not address accelerated setting from heat |
| Accelerators | Would worsen the problem — they further speed up setting at high temperature |
- High temperature → faster hydration → shorter setting time. Solution: add retarders.
- Retarders extend initial setting time; used in hot weather, long hauls, deep wells, tunnels.
- Common retarders: gypsum, sugar, starch, calcium lignosulphonate, sodium bicarbonate.