What is Aircrete or Autoclaved Aerated Concrete (AAC)?

Aircrete is lightweight concrete that contains stable air cells uniformly distributed throughout the concrete mixture. It is simply concrete with bubbles that harden to get a highly dense material with impressive compressive strength.

Aircrete or Autoclaved Aerated Concrete (AAC) 

An aircrete is designed to be light and airy compared to traditional concrete, where it is made heavy and durable. Both have certain strength criteria that work best for specific construction applications.

In this article, we will discuss in detail the making of aircrete and its applications in the construction industry. 

Autoclaved Aerated Concrete (AAC) or Aircrete

Aircrete is also mentioned and described as Autoclaved Aerated Concrete (AAC) or Aerated Cellular Concrete. Axel Erikson in 1923 discovered that in the masonry industry, this moist foam mass of concrete has the ability to handle a pressurized steam curing process, called autoclaving. 

During the autoclave process, the wet aircrete mix hardens fast with no issues of shrinkage even after steam curing, compared to the normal curing process. This later led to improving the aircrete concrete mix with pulverized ash instead of lime or cement to economize the product. 

Hence, aircrete cured in an autoclave results in autoclaved aerated concrete. This technology has undergone significant improvement in terms of mix design, and product design that has resulted in AAC blocks, panels, and buildings with increased insulation and sound absorption properties. 

How to Make Aircrete Concrete?

Aircrete is made from a mixture of:

  1. Cement
  2. Lime
  3. Gypsum (anhydrite)
  4. Finely ground sand
  5. Aluminum powder
The above mixtures when combined together undergo few chemical reaction, in which the final products are Tobermorite or Hydrates Calcium Silicate C5S6H5.
The reactions are:
  • CaO+H2O →Ca(OH)2 + 65,2 KJ/mol 2 
  • 3Ca(OH)2 + 2Al + 6H2O → Ca3(Al(OH)6)2 + 3H2 3
  • 6SiO2 + 5Ca(OH)2 → 5CaO · 6SiO2 · 5H2O
The chemical reaction between aluminum and calcium hydroxide creates an alkaline element, which contributes to the foam in concrete. 

Production of Aircrete or AAC

An autoclave is a large pressure vessel in which an aerated concrete mix is cured. It is normally a steel tube of 3m in diameter and 45 m in length. Steam curing at high pressure of 800 kPa and 180 degrees celsius is fed during the process.

Here, the production and hydration reaction of AAC blocks are explained for better understanding. The hydration reaction of AAC concrete completes with the completion of the autoclave process. Hence, after the curing process, the AAC concrete block are ready to use.

Production of AAC Block
Image Courtesy: MASA

The production process varies with the production plants, but a general production overview is explained here. 
  • The aircrete mix containing cement, lime, sand, and fine aluminum powder is mixed to form a slurry and added to the cellular mold structure.
  • The density of the final block is varied by varying the amount of aluminum powder.
  • The cement mixture in the mold starts hydration within a period and stiffens to form a "green cake".
  • Green cake formation is due to the evolution of hydrogen gas during the reaction between aluminum articles and the alkaline liquid. These bubbles give cellular structure to the product.
  • The temperature of the green cake increase, as the hydration reaction is exothermic and proceeds at a faster rate.
  • Once, the green cake rise to the required height, it is moved to cut into the required block size using cutting wires.
  • The cut blocks are still soft and they are loaded into the autoclave. It is held inside for 8 to 10 hours for further hardening to achieve high density and high strength aircrete.

Hydration Products in Aircrete

During the green stage of aircrete mix, the hydration products formed are similar to the hydration products of normal concrete, i.e. C-S-H, Ettringite, and Monosulfate. After the autoclaving process, the principal final reaction product is Tobermorite. 

During the autoclaving process, the 1.1 nm Tobermorite starts to crystalline from the C-S-H already formed. Hence, the total proportion of C-S-H decreases, and 1.1 nm tobermorite increases. Hence, C-S-H is an intermediate hydration product in AAC manufacture.

Hence, the hydration products in Aircrete are:
  • 1.1nm Tobermorite
  • Residual C-S-H
  • Hydrogarnet
1.1 nm Tobermorite (C5S6H5) is the normal hydration product in aircrete/AAC. When the concrete is subjected to longer and higher autoclaving time, then 1.4 nm Tobermorite is formed. This is also called Xonotlite (C5S6H).

Advantages of Aircrete or AAC in Construction

  • Aircrete is lightweight making it easy to handle and transport.
  • Employ economic raw materials, which make it cost-effective
  • Reduce overall construction costs
  • Aircrete products have low thermal conductivity.
  • They are good acoustic properties due to their porous structure.
  • They do not combust and are fire resistance
  • Have high permeability to water vapor
  • Resistant from insects, rodents, and water
  • Available in different colors to match the aesthetics.
  • Employs eco-friendly materials

Applications of Aircrete

  • Lightweight filling over underground structures
  • Landfilling 
  • Acoustic buildings
  • Shock absorbent floors
  • Floor slab construction
  • Insulated roof construction
  • Pre-cast blocks and panels
Aircrete production demands expensive equipment and high-power consumption. In addition to these, aircrete loses strength when subjected to continuous water. 

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