Which of the below is an example of plasticizer?
Correct Answer: A. Hydroxylated carboxylic acid
📚 Detailed Explanation: Plasticizers in Concrete
Why A (Hydroxylated carboxylic acid) is correct: Plasticizers (normal water-reducing admixtures per IS 9103 Type A) include:
• Hydroxylated carboxylic acids (gluconic acid, citric acid, tartaric acid) and their salts
• Lignosulphonates (by-product of paper pulping)
• Hydroxycarboxylic acid salts
These adsorb onto cement particle surfaces and impart a negative charge, causing electrostatic repulsion between particles — dispersing them and releasing trapped water, effectively reducing water demand by 5–15% while maintaining the same workability.
• Hydroxylated carboxylic acids (gluconic acid, citric acid, tartaric acid) and their salts
• Lignosulphonates (by-product of paper pulping)
• Hydroxycarboxylic acid salts
These adsorb onto cement particle surfaces and impart a negative charge, causing electrostatic repulsion between particles — dispersing them and releasing trapped water, effectively reducing water demand by 5–15% while maintaining the same workability.
Why others are wrong: Option B (Fluoro-silicate) is used as a surface hardener, not a plasticiser. Option C (Gypsum) is a retarder added to cement clinker to control flash setting — not a workability admixture. Option D (Surkhi) is a pozzolana made from burnt brick powder.
Classification of Common Admixtures
| Admixture | Type | Purpose |
|---|---|---|
| Hydroxylated carboxylic acid | Plasticiser (Type A) | Reduce water demand 5–15% |
| Lignosulphonate | Plasticiser (Type A/D) | Water reduction + retardation |
| Polycarboxylate ether | Superplasticiser (Type F) | Water reduction 15–30% |
| Gypsum | Retarder (in cement) | Controls flash set of C₃A |
| Fluoro-silicate | Hardener | Surface densification |
- Plasticiser example: Hydroxylated carboxylic acid (e.g., gluconic acid).
- Mechanism: adsorption onto cement → electrostatic repulsion → dispersion → free water released → workability gain or water reduction.