Which of the following acts as a retarder for concrete?
Correct Answer: B. Calcium lignosulphonate
📚 Detailed Explanation: Calcium Lignosulphonate — Retarder and Plasticiser
Why B (Calcium lignosulphonate) is correct: Calcium lignosulphonate (also written as calcium ligno-sulphonate) is derived as a by-product of the wood-pulp (sulphite) paper-making process. It functions as a retarder by adsorbing onto C3S and C3A surfaces, delaying their hydration, and extending the setting time. It also acts as a plasticiser (water reducer), reducing water demand by 5–10% at the same workability.
Classification of the Four Options
| Admixture | Type | Primary Function | Use Case |
|---|---|---|---|
| A. Calcium chloride (CaCl2) | Accelerator | Speeds hydration; reduces setting time; early strength gain | Cold weather; emergency repairs; precast |
| B. Calcium lignosulphonate | Retarder + plasticiser | Delays hydration; extends setting time; reduces water demand 5–10% | Hot weather; long transport; deep oil wells; ready-mix |
| C. Calcium stearate | Waterproofing/water-repelling agent | Fills capillary pores; reduces water absorption; waterproofs concrete | Water-retaining structures; roof slabs |
| D. Aluminium powder | Gas-forming agent | Reacts with Ca(OH)2; releases H2 gas; creates air bubbles; makes aerated/cellular concrete | Autoclaved Aerated Concrete (AAC) blocks; thermal insulation |
How Calcium Lignosulphonate Works as a Retarder
| Step | Mechanism |
|---|---|
| 1. Dissolution | Calcium lignosulphonate dissolves in mix water; anionic surfactant in solution |
| 2. Adsorption | Lignosulphonate molecules adsorb onto positively charged C3S and C3A particle surfaces |
| 3. Barrier layer | Adsorbed layer prevents water from accessing reactive cement surfaces |
| 4. Retardation | Hydration rate slowed; setting time extended by 1–3 hours depending on dosage |
| 5. Plasticising effect | Adsorbed layer also deflocculates cement clusters; frees water; improves workability or allows water reduction |
- Calcium lignosulphonate = retarder (also a plasticiser); extends setting time by 1–3 hours.
- CaCl2 = accelerator (opposite effect); calcium stearate = waterproofing; aluminium powder = gas-forming.
- IS 9103 classifies admixtures; lignosulphonate-based products are among the most widely used concrete admixtures worldwide.