---
title: "Define admixtures in brief"
subject: "Civil Engineering Materials"
chapter: 5
key: "semester_3/civil_engineering_materials:admixtures-define"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/admixtures-define
site: BCT Engineering
---

# Define admixtures in brief

Civil Engineering Materials · Chapter 5: Cement

- Asked in: 2075 Asoj, 1 marks; 2068 Baisakh, 3 marks (asked 2 times)
- Video: [Admixtures & Additives in Concrete | Types, Functions & Uses | Concrete Technology](https://www.youtube.com/watch?v=ddA8ZBpCP48) — Polytechnic Class (READBY) 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/admixtures-define
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-5

## Question

Define admixtures. Explain their engineering application.

## Short answer

Admixtures are materials (other than cement, water, aggregate) added before or during mixing to modify concrete properties. Examples: accelerators for early strength, retarders to delay setting, plasticizers for workability, air-entrainers for freeze-thaw resistance, mineral admixtures for durability.

## Answer

**Admixtures** are materials, other than cement, water and aggregate, added to concrete or mortar just before or during mixing to modify its properties in the fresh or hardened state. They are used in small doses (usually less than 5% of the cement weight).

#### Engineering applications
| Type | Purpose / application |
|---|---|
| Accelerators (calcium chloride, for plain concrete) | Faster setting and early strength; cold-weather concreting, urgent repairs |
| Retarders (sugar, gypsum) | Delay setting in hot weather, long hauls and large pours |
| Plasticizers / superplasticizers | Increase workability or cut water by 10-30% for high strength, pumping, congested reinforcement |
| Air-entraining agents (resins, fats) | Entrain tiny air bubbles for freeze-thaw resistance and workability |
| Water-proofing agents | Reduce permeability of basements, water tanks, roofs |
| Mineral admixtures (fly ash, silica fume, slag) | Higher durability, lower heat of hydration, economy |
| Pigments | Coloured concrete and plaster |
| Bonding agents | Join new concrete to old |

## Check yourself

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

1. What is an admixture?
   A. Extra cement added for strength
   B. Any aggregate over 20 mm
   C. Curing compound sprayed later
   D. A material other than cement, water and aggregate added during mixing to modify concrete properties
   Answer: D. Admixtures are small-dose additions (usually under 5% of cement weight) that modify fresh or hardened concrete. (Key point: Definition)

2. Where is a retarder applied?
   A. Hot weather, long haul and large pours
   B. Cold weather early formwork removal
   C. Freeze-thaw resistance
   D. Coloured plaster
   Answer: A. Retarders delay setting; accelerators are for cold weather and quick repairs. (Key point: Retarder use)

3. What is the purpose of air-entraining agents?
   A. Faster setting
   B. Freeze-thaw resistance and workability
   C. Reducing permeability by pore-filling only
   D. Colouring
   Answer: B. Tiny air bubbles improve frost resistance and workability. (Key point: Air entrainment)

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