Which of the following is NOT a retarding admixture?
Correct Answer: B. Calcium chloride
📚 Detailed Explanation: CaCl2 is an Accelerator, NOT a Retarder
Why B (Calcium chloride) is the odd one out: All the other options — starch, sugar, calcium sulphate (gypsum) — are retarders that slow down cement hydration. Calcium chloride (CaCl2) does the opposite: it accelerates hydration, reducing the initial setting time and causing concrete to harden rapidly. It is therefore NOT a retarding admixture.
Classification: Retarders vs. Accelerators
| Admixture | Type | Effect on Setting | Typical Dosage |
|---|---|---|---|
| Starch | Retarder | Delays hydration; extends setting time | 0.1–0.5% by wt. cement |
| Sugar (sucrose) | Retarder | Extends setting by ≈4 hours (at 0.2%) | 0.05–0.2% by wt. cement |
| Calcium sulphate (Gypsum) | Retarder | Reacts with C3A to form ettringite; delays flash set | 2–3% added during cement grinding |
| Calcium lignosulphonate | Retarder (also plasticiser) | Extends setting; also reduces water demand | 0.1–0.4% by wt. cement |
| Sodium bicarbonate | Retarder | Delays hydration of C3S and C3A | Small doses |
| Calcium chloride (CaCl2) | Accelerator | Reduces initial setting time; speeds hardening | 1–2% by wt. cement (max) |
How Retarders Work
| Retarder Type | Mechanism of Retardation |
|---|---|
| Sugars (saccharose, glucose) | Adsorb onto cement particle surfaces; form a coating that prevents water from reaching unhydrated cement; delay C3S hydration |
| Gypsum (CaSO4·2H2O) | Reacts with C3A rapidly to form ettringite, a protective coating on C3A particles, slowing flash set; allows time for mixing and placing |
| Lignosulphonates | Adsorb on C3A and C3S surfaces; reduce hydration rate; also deflocculate cement particles (plasticiser effect) |
- Calcium chloride is an accelerator — it is NOT a retarding admixture.
- Common retarders: starch, sugar, gypsum, calcium lignosulphonate, sodium bicarbonate.
- Safe limit for CaCl2: 2% by weight of cement; >4% is detrimental to strength and corrosion protection.