---
title: "Common admixtures; accelerating admixture"
subject: "Concrete Technology and Masonry Structure"
chapter: 1
key: "semester_5/concrete_technology_and_masonry_structure:accelerating-admixture"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/accelerating-admixture
site: BCT Engineering
---

# Common admixtures; accelerating admixture

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

- Asked in: 2072 Chaitra, 6 marks
- Video: [Accelerators, Retarders or Retarding Admixture  and Water reducers or Water reducing Admixtures](https://www.youtube.com/watch?v=EPKHiwXIAJg) — CementPDM by CemNex
- 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/accelerating-admixture
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1

## Question

List out common admixtures available in the market. Elaborate in brief the accelerating admixture.

## Short answer

Common admixtures include water reducers, superplasticizers, retarders, accelerators, and air-entraining agents. Accelerators like calcium chloride speed up setting and early strength gain by enhancing cement hydration; they are used in cold-weather concreting and repairs but chloride-based ones are avoided in reinforced concrete due to corrosion risk.

## Answer

#### Common admixtures available in the market
- **Chemical**: plasticisers (water reducers), superplasticisers (SNF, PCE), accelerators, retarders, air-entraining agents, water-proofing compounds, corrosion inhibitors, shrinkage reducers, viscosity modifiers, bonding agents.
- **Mineral**: fly ash, silica fume, ground granulated blast furnace slag (GGBS), rice husk ash, metakaolin.
- Others: pigments, fibres, expansive agents, pumping aids.

#### Accelerating admixture (ASTM C494 Type C, IS 9103)
An accelerator is an admixture that **speeds up the setting and early strength gain** of concrete by speeding up the hydration of C$_3$S and C$_3$A.

**Types**
- Soluble inorganic salts: calcium chloride (most common and cheap), calcium nitrate, calcium nitrite, sodium thiocyanate, sodium carbonate.
- Organic: triethanolamine, calcium formate.
- Rapid-setting shotcrete accelerators: aluminates, silicates.

**Dosage**: calcium chloride up to 2% of cement mass (IS 456: chlorides in the mix limited, so it is not used in RCC and prestressed concrete).

**Effects**
- Initial and final setting times are shortened; 1-3 day strengths rise, with lower or similar 28-day strength.
- Heat of hydration is liberated earlier.
- Chlorides cause corrosion of steel; also shrinkage, creep and sulphate attack may increase.

**Uses**: cold weather concreting, early removal of formwork, emergency repair, precast production, shotcrete, concreting under water or against flowing water, plugging leaks.

**Precautions**: avoid in RCC when chloride-based; trial mixes and accurate dosage; quick placing; sufficient curing.

## Check yourself

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

1. An accelerator:
   A. Delays hydration
   B. Speeds up setting and early strength by hastening C3S and C3A hydration
   C. Entrains air
   D. Reduces cement
   Answer: B. Accelerators speed hydration of C3S and C3A. (Key point: Accelerator function)

2. Why is calcium chloride not used in RCC?
   A. It delays setting
   B. Chlorides cause corrosion of steel
   C. It reduces workability
   D. It is too costly
   Answer: B. Chlorides corrode reinforcement, so chloride-based accelerators are avoided in RCC and prestressed concrete. (Key point: Calcium chloride)

3. Accelerators typically give:
   A. Higher 1-3 day strength with lower or similar 28-day strength
   B. Lower early strength
   C. Higher 28-day strength always
   D. No change in setting
   Answer: A. Early strength rises; 28-day strength is similar or lower. (Key point: Strength effect)

4. A typical use of accelerators is:
   A. Hot weather concreting
   B. Long transport
   C. Cold weather concreting and early formwork removal
   D. Mass concrete
   Answer: C. They suit cold weather, early form removal, repair and shotcrete. (Key point: Uses)

## Related in Concrete Technology and Masonry Structure

- [Admixture vs additive; chemical admixtures](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aadmixture-vs-additive-chemical-types) — Concrete Technology and Masonry Structure, chapter 1
- [Chemical admixtures per ASTM; recommendations](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aadmixture-recommendation-weather) — Concrete Technology and Masonry Structure, chapter 1
- [Admixture vs additive; apparent vs absolute specific gravity](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aspecific-gravity-differences) — Concrete Technology and Masonry Structure, chapter 1

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