---
title: "Short note: setting action of lime"
subject: "Civil Engineering Materials"
chapter: 4
key: "semester_3/civil_engineering_materials:setting-action-of-lime"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/setting-action-of-lime
site: BCT Engineering
---

# Short note: setting action of lime

Civil Engineering Materials · Chapter 4: Lime

- Asked in: 2070 Chaitra (old course), 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/setting-action-of-lime
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-4

## Question

Write a short note on setting action of lime.

## Short answer

Fat lime sets by carbonation: Ca(OH)₂ absorbs CO₂ to reform CaCO₃, slow and only in air. Hydraulic lime sets by hydration of silicates/aluminates forming interlocking crystals within days, even underwater; later carbonation adds strength. Lime sets slower and weaker than cement.

## Answer

**Setting** is the stiffening of lime mortar or paste into a hard mass. It happens in two ways.

#### Fat lime (non-hydraulic): setting by carbonation
Slaked lime paste first loses water by evaporation and then absorbs CO₂ from the air to turn back into calcium carbonate, which binds the sand particles.

$$
\text{Ca(OH)}_2 + \text{CO}_2 \rightarrow \text{CaCO}_3 + \text{H}_2\text{O}
$$

The action is slow and starts from the surface inwards, so thick walls take years to harden fully. It does not happen under water or in damp inside.

#### Hydraulic lime: setting by hydration
The calcium silicates and aluminates formed during burning react with water to form insoluble hydrates which crystallise and interlock, giving strength within a few days, even under water. Later, carbonation of the free lime adds strength.

Lime sets slower and with less strength than cement.

## Check yourself

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

1. Fat lime sets by:
   A. Carbonation, absorbing CO2
   B. Hydration
   C. Freezing
   D. Cooling
   Answer: A. Forms CaCO3. (Key point: Fat lime setting)

2. The reaction is:
   A. CaO + CO2 gives Ca(OH)2
   B. Ca(OH)2 + CO2 gives CaCO3 + H2O
   C. CaCO3 gives CaO
   D. Ca(OH)2 gives CaO
   Answer: B. It binds sand. (Key point: Reaction)

3. Carbonation proceeds:
   A. Instantly
   B. From inside out
   C. Slowly from surface inwards
   D. Only underwater
   Answer: C. Thick walls take years. (Key point: Direction)

4. Hydraulic lime sets by:
   A. Carbonation only
   B. Drying
   C. Cooling
   D. Hydration of silicates and aluminates
   Answer: D. Strength within days. (Key point: Hydraulic setting)