Admixtures which cause early setting and hardening of concrete are called:
Correct Answer: A. Accelerators
📚 Detailed Explanation: Accelerators Cause Early Setting and Hardening
Why A (Accelerators) is correct: Accelerators are chemical admixtures that increase the rate of cement hydration, resulting in earlier setting and faster early strength gain. This is their defining characteristic — they accelerate all the reactions that normally take place between cement and water, compressing the timeline of hardening.
Accelerators vs. Retarders — Direct Comparison
| Property | Accelerators | Retarders |
|---|---|---|
| Effect on setting time | Reduces; earlier initial and final set | Extends; later initial and final set |
| Effect on early strength | Higher 1, 3, 7-day strength | Lower 1, 3, 7-day strength (catch up by 28 days) |
| Effect on heat of hydration | More heat generated faster | Heat spread over longer period |
| Primary use condition | Cold weather; emergency repairs; precast | Hot weather; long transport; deep wells |
| Common examples | CaCl2, Na2CO3, silicates, triethanolamine | Gypsum, sugar, starch, calcium lignosulphonate |
Why Other Options Are Wrong
| Option | What It Actually Does |
|---|---|
| A. Accelerators | Correct: cause early setting and hardening |
| B. Retarders | Opposite effect: delay setting and slow hardening |
| C. Air-entraining agents | Introduce air bubbles; improve freeze-thaw resistance; do not primarily affect setting time |
| D. Workability admixtures | Plasticisers/superplasticisers improve flow; do not primarily cause early setting |
Use of Accelerators in Cold Weather
| Condition | Problem | Accelerator Solution |
|---|---|---|
| Temperature 0–5°C | Very slow hydration; concrete may freeze before gaining strength | CaCl2 or non-chloride accelerator; speeds hydration to outpace freezing |
| Temperature <0°C | Water in fresh concrete freezes; hydration stops; concrete destroyed | Heated mixing water + accelerator + insulated formwork |
- Accelerators = admixtures causing early setting and hardening of concrete.
- Examples: CaCl2, soluble carbonates (Na2CO3), silicates (Na2SiO3), triethanolamine.
- Used in cold weather, emergency repairs, precast production, tunnel linings.