How many major types of chemical admixtures are there?
Correct Answer: D. 5
📚 Detailed Explanation: Five Types of Chemical Admixtures
Why D (5) is correct: Chemical admixtures are classified into five major types based on their primary function in modifying the properties of fresh or hardened concrete. Each type works through a different chemical or physical mechanism.
Five Types of Chemical Admixtures
| # | Type | Function | Examples | Typical Dosage |
|---|---|---|---|---|
| 1 | Accelerators | Increase hydration rate; reduce setting time; speed early strength gain | CaCl2, soluble carbonates (Na2CO3), silicates | 1–4% by wt. cement; CaCl2 max 2% |
| 2 | Retarders | Slow hydration; extend setting time; maintain workability | Gypsum, calcium lignosulphonate, sugar, starch, sodium bicarbonate | 0.1–0.5% by wt. cement; gypsum 2–3% |
| 3 | Air-entraining agents | Introduce stable micro-air bubbles; improve freeze-thaw resistance, workability | Natural resins, animal fats (tallows), soaps, vinsol resin | 0.005–1% by wt. cement |
| 4 | Water-repelling agents (waterproofing) | Reduce water absorption; improve impermeability; reduce capillary suction | Sodium silicate, stearates, zinc sulphate, bitumen emulsion | 1–3% by wt. cement |
| 5 | Gas-forming agents | Generate gas bubbles to create lightweight aerated/cellular concrete | Aluminium powder, zinc powder, hydrogen peroxide | 0.02% of cement weight (Al powder) |
Note on Plasticisers / Superplasticisers
| Admixture | Primary Function | Standard Classification |
|---|---|---|
| Plasticisers (water reducers) | Reduce water demand by 5–15%; improve workability | Often included under a 6th category or as sub-type of retarders/chemical admixtures in IS 9103 |
| Superplasticisers (HRWR) | Reduce water by 20–30%; enable high-strength or self-compacting concrete | IS 9103: Type F (HRWR); separate category in IS classification |
- There are 5 major types of chemical admixtures: accelerators, retarders, air-entraining agents, water-repelling agents, gas-forming agents.
- IS 9103: 1999 governs the specification for admixtures for concrete.
- Plasticisers/superplasticisers are sometimes listed separately as additional categories beyond the classic 5.