---
title: "Chemical and mineral admixtures; plasticizers"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:plasticizer-water-reduction"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/plasticizer-water-reduction
site: BCT Engineering
---

# Chemical and mineral admixtures; plasticizers

Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials

- Asked in: 2064 Jestha (old course)
- Video: [Admixtures || Plasticizers, Superplasticizers || Concrete Technology || Ch 03 E01](https://www.youtube.com/watch?v=GRBV53XOdHA) — TRICKY DOSE
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/plasticizer-water-reduction
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

Mention the various types of chemical and mineral admixtures used in concrete. Explain how plasticizers can reduce the water content in concrete.

## Short answer

Chemical admixtures include water reducers, retarders, accelerators, superplasticizers, air-entraining agents. Mineral admixtures are fly ash, silica fume, slag. Plasticizers adsorb on cement particles, imparting like charges that break flocs and release trapped water, thereby lubricating the mix and allowing 5-15% water reduction for the same slump.

## Answer

#### Chemical admixtures (IS 9103 / ASTM C494)
1. **Water reducers / plasticisers** (Type A): lignosulphonates, hydroxylated carboxylic acids.
2. **Retarders** (Type B): sugar, gluconate, phosphates.
3. **Accelerators** (Type C): calcium chloride, calcium nitrate, triethanolamine.
4. **Water-reducing retarders / accelerators** (Type D and E).
5. **Superplasticisers** (Type F and G): SNF, SMF, PCE.
6. **Air-entraining agents**: Vinsol resin, fatty-acid soaps.
7. **Water-proofing, corrosion inhibitor, shrinkage-reducing, viscosity modifying** admixtures.

#### Mineral admixtures
Finely divided pozzolanic or cementitious materials added in large dosage: **fly ash**, **silica fume**, **ground granulated blast furnace slag**, **rice husk ash**, **metakaolin**, natural pozzolanas (volcanic ash), and inert fillers (limestone powder).

#### How plasticisers reduce the water content
Cement particles in water carry opposite electric charges at their surfaces and attract each other, forming **flocs** that trap part of the mixing water inside. This trapped water does not help workability.
1. The plasticiser molecule has a hydrophilic and hydrophobic end; it adsorbs on the cement surface.
2. It gives all the particles the same negative charge, so they repel each other (dispersion), and a lubricating water film forms around each grain.
3. The flocs break up and the **trapped water is released** to lubricate the mix, so the paste becomes more fluid.
4. Therefore, for the same slump, mixing water can be reduced by **5-15 per cent** (up to 30 per cent with superplasticisers), giving a lower w/c, higher strength, lower permeability, or for the same w/c a higher slump.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why does cement paste form flocs?
   A. Particles carry opposite charges and attract each other, trapping water
   B. Cement dissolves
   C. Aggregate attracts cement
   D. Air bubbles bind it
   Answer: A. Opposite surface charges cause flocs that trap mixing water. (Key point: Floc formation)

2. A plasticiser works by:
   A. Giving particles the same negative charge so they repel and release trapped water
   B. Adding more water
   C. Reacting with sand
   D. Removing air
   Answer: A. Adsorbed molecules cause repulsion; flocs break up and water is released. (Key point: Plasticiser action)

3. Plasticisers allow water reduction of about:
   A. 5-15%
   B. 40%
   C. 50-60%
   D. 0%
   Answer: A. 5-15% (up to 30% with superplasticisers). (Key point: Water reduction amount)

4. Which is a mineral admixture?
   A. Lignosulphonate
   B. Vinsol resin
   C. Silica fume
   D. Calcium chloride
   Answer: C. Silica fume, fly ash, GGBS, RHA and metakaolin are mineral admixtures. (Key point: Mineral admixtures)

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