The most commonly used admixture to accelerate the initial set of concrete is:
Correct Answer: B. Calcium chloride
📚 Detailed Explanation: Calcium Chloride — The Most Common Accelerator
Why B (Calcium chloride) is correct: Calcium chloride (CaCl2) is universally recognised as the most commonly used and most effective accelerating admixture for Portland cement concrete. It accelerates the initial set by speeding up the hydration of C3S and C3A — the compounds responsible for early strength and setting. Among all accelerators, CaCl2 is the most widely studied and the standard reference accelerator in most codes and textbooks.
Why CaCl2 Is the Common Choice
| Factor | CaCl2 Advantage |
|---|---|
| Effectiveness | Most effective commercially available accelerator; can halve setting time at 2% dose |
| Cost | Relatively cheap; widely available as industrial by-product |
| Solubility | Very soluble in water; easy to introduce into mix as solution |
| Predictability | Well-characterised dosage-response curves; reliable performance |
Why Other Options Are Wrong
| Option | What It Is | Why Wrong |
|---|---|---|
| A. Gypsum | CaSO4·2H2O; retarder added during cement grinding | A retarder, not an accelerator; delays initial set |
| B. Calcium chloride | CaCl2; the standard accelerator | Correct |
| C. Bitumen and inert material mixture | A waterproofing/damp-proof course material | Not an admixture; not related to setting time |
| D. By-product of bitumen | Vague description; not a standard concrete admixture category | Not a recognised accelerating admixture |
- Calcium chloride is the most common admixture to accelerate the initial set of concrete.
- Used in cold weather, emergency repairs, precast production; limited to 2% in RC, banned in PSC.
- Gypsum (option A) is the opposite: a retarder used to prevent flash set.