---
title: "Why pozzolanic materials are used"
subject: "Civil Engineering Materials"
chapter: 4
key: "semester_3/civil_engineering_materials:why-pozzolanic-materials-used"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/why-pozzolanic-materials-used
site: BCT Engineering
---

# Why pozzolanic materials are used

Civil Engineering Materials · Chapter 4: Lime

- Asked in: 2080 Bhadra, 3 marks; 2078 Bhadra, 1 marks (asked 2 times)
- Video: [What's the Matter With... Ancient Roman Concrete? With Admir Masic, MIT CEE](https://www.youtube.com/watch?v=Jtjeyy_zvVY) — CSHubMIT
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/why-pozzolanic-materials-used
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-4

## Question

Why are pozzolanic materials used in the field of civil construction (added to lime)?

## Short answer

Pozzolanic materials are added to lime to impart hydraulic properties, enabling it to set under water, and to increase strength, durability, and chemical resistance. They also reduce heat of hydration, permeability, and cost, while improving workability and utilizing waste materials.

## Answer

Pozzolanic materials are added to lime (and cement) for the following reasons.

1. They react with free lime to form **insoluble calcium silicates and aluminates**, giving hydraulic properties to fat lime so that it can set under water and in damp places.
2. They increase the **strength and durability** of mortar and concrete, and improve resistance to sulphate and chemical attack.
3. They give a **low heat of hydration**, which is useful in mass concrete such as dams.
4. They reduce permeability and lower the cost, since fly ash, surkhi and ash are cheap by-products.
5. They improve workability and reduce bleeding.
6. They save cement and help the environment by using waste material.

## Check yourself

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

1. Pozzolana added to fat lime:
   A. Makes it slake faster
   B. Makes it dissolve
   C. Gives it hydraulic properties by forming insoluble silicates
   D. Reduces strength
   Answer: C. It can then set in damp places. (Key point: Hydraulic property)

2. Pozzolanic materials give:
   A. High heat of hydration
   B. No benefit
   C. Higher bleeding
   D. Low heat of hydration, useful in mass concrete
   Answer: D. Useful in dams. (Key point: Heat of hydration)

3. Fly ash and surkhi are:
   A. Cheap by-products that save cement
   B. Expensive imports
   C. Harmful to strength
   D. Not usable
   Answer: A. They reduce cost. (Key point: Cost)

## Related in Civil Engineering Materials

- [Define pozzolanic materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Adefine-pozzolanic-materials) — Civil Engineering Materials, chapter 4, asked 7×
- [Properties of pozzolanic materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aproperties-pozzolanic-materials) — Civil Engineering Materials, chapter 4
- [Important properties of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aimportant-properties-civil-materials) — Civil Engineering Materials, chapter 1, asked 8×
- [Importance and scope of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aimportance-scope-subject) — Civil Engineering Materials, chapter 1, asked 8×
- [Classification of civil engineering materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aclassification-civil-materials) — Civil Engineering Materials, chapter 1, asked 2×
- [Factors for selecting construction materials](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Afactors-selection-construction-material) — Civil Engineering Materials, chapter 1, asked 9×

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