---
title: "Aerated autoclave concrete blocks"
subject: "Civil Engineering Materials"
chapter: 10
key: "semester_3/civil_engineering_materials:aac-blocks"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/aac-blocks
site: BCT Engineering
---

# Aerated autoclave concrete blocks

Civil Engineering Materials · Chapter 10: Asphalt, Bitumen, Tar and Miscellaneous Materials

- Asked in: 2081 Baisakh (new course), 2 marks; 2081 Chaitra (new course), 2 marks (asked 2 times)
- Video: [Difference Between AAC Blocks & CLC Blocks](https://www.youtube.com/watch?v=EW3lzFHYBCE) — Engineering Motive
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/aac-blocks
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-10

## Question

Write a short note on Aerated Autoclave Concrete (AAC) blocks.

## Short answer

AAC blocks are lightweight (500–800 kg/m³) precast cellular concrete made with fly ash, cement, lime, gypsum and aluminium powder. Steam curing in an autoclave forms tiny air cells, giving 3–4.5 N/mm² strength and good thermal insulation. Used for partitions and infill walls.

## Answer

**Autoclaved Aerated Concrete (AAC) blocks** are lightweight, precast, cellular blocks made from fly ash (or sand), cement, lime, gypsum and water, with a small amount of **aluminium powder** as the foaming agent. The aluminium reacts with lime to give hydrogen gas, which forms millions of tiny air cells. The mix is cut into blocks and cured in an **autoclave** (steam at about 180 °C and 12 bar pressure) for 8–12 hours.

#### Properties

- Density 500–800 kg/m³ (about one-third of clay brick), so dead load is reduced.
- Compressive strength about 3–4.5 N/mm² (IS 2185 Part 3).
- Good thermal insulation, sound absorption and fire resistance (2–4 hours).
- Accurate sizes (e.g. 600 × 200 × 100–300 mm), so thin joints and less plaster are needed.
- Easily cut, nailed, drilled and chased; quick construction.

#### Uses and limits

Used for non-load-bearing partitions and infill walls in framed buildings, and in load-bearing low rise buildings. Water absorption is higher than brick, so plastering and waterproofing are needed on outer walls, and special mortar (adhesive) is used.

## Check yourself

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

1. What creates the air cells in AAC?
   A. Aluminium powder reacting with lime to give hydrogen
   B. Steam
   C. Sand
   D. Gypsum
   Answer: A. Aluminium reacts with lime releasing hydrogen gas that forms tiny cells. (Key point: Foaming agent)

2. How are AAC blocks cured?
   A. Sun-dried
   B. Water curing 28 days
   C. In an autoclave with steam at about 180 degrees C and 12 bar
   D. Fired in kiln
   Answer: C. Autoclaving for 8-12 hours. (Key point: Curing)

3. AAC density is about what?
   A. 500-800 kg/m3, about one-third of clay brick
   B. 2400 kg/m3
   C. 3000 kg/m3
   D. 100 kg/m3
   Answer: A. Light weight reduces dead load. (Key point: Density)

4. Why do outer AAC walls need plaster and waterproofing?
   A. Weak sizes
   B. Water absorption is higher than brick
   C. Too heavy
   D. Low fire resistance
   Answer: B. Higher water absorption than brick. (Key point: Limit)

## Related in other subjects

- [Force on blocks after sluice gate jump](https://www.bctengineer.com/question.md?key=semester_4%2Fhydraulics%3Asluice-gate-jump-blocks-force-6m-channel) — Hydraulics, chapter 9
- [Hollow blocks and compressed earth blocks](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Ahollow-block-compressed-earth-block) — Concrete Technology and Masonry Structure, chapter 7