Rahul Malhotra


Autoclaved Aerated Concrete


Autoclaved Aerated concrete is a lightweight building material that is used for many kinds of purposes in the building industry. The material is considered to be more environmentally friendlier than other building products. AAC offers great properties that many building materials need to possess and that is why it is becoming more popular in construction. Autoclaved Aerated Concrete is now commonly known as 'Hebel' concrete as the company that manufactures the concrete is known as Hebel.1




Autoclaved Aerated Concrete has a history dating back over 90 years. The material was invented in the 1920’s by an architect in Sweden, Johan Axel Eriksson. In those times there was a big shortage in the supply of wood and new building materials were needed urgently. Johan Eriksson used simple raw materials that were available to manufacture this type of lightweight aerated concrete. Johan also put the concrete into an autoclave to speed up the production process and that is how the material got its name. AAC was officially patented in 1924 under the name; porous concrete.2

Development in History


Since its invention, AAC has had many improvements made to its design and structure. In 1929, the material was manufactured for the first time commercially in Sweden. The concrete was then called Ytong and its popularity increased rapidly due to its useful properties. In 1945, an improvement was made to the manufacturing process of AAC by Josef Hebel. He developed a method to include steel reinforcement in the manufacturing of this concrete. He patented this method and opened his own company called Hebel which also manufactured autoclaved aerated concrete. In 1975, Ytong stopped using a type of slate, which is considered unsafe for people, in its concrete products. This led to Autoclaved Aerated Concrete being even safer for people’s health. For the rest of the 20th century, Hebel and Ytong were the main suppliers of AAC but have since merged to form one company. Hebel Concrete is now sold in nearly 30 countries over the world. Its manufacturing process is still the same today as it was when it was invented except for a few improvements. Hebel can now be made into many different products such as blocks, wall panels and roof panels to let it be used for different purposes in a building.34


Autoclaved Aerated Concrete is manufactured from three main materials; sand, lime and Portland cement. Steel reinforcement is also added during the manufacturing process of wall and roof panels to make them even stronger. Aluminium powder, a rising agent, and water are also used during manufacture.5

Setting and Shaping


Firstly, all the raw materials are mixed into a mixture of thick liquid and poured into moulds ready to be shaped into different products. While the mixture is in the mould, the aluminium powder causes many hydrogen air holes to be formed in the mixture. These holes lower the density of the material and cause it to double its original size and this is why all AAC products are so lightweight. The material is then left to set for as much as 4 hours and if steel reinforcement is needed it is added in the moulds. As soon as it becomes hard, the material is cut by wires into the desired shape by using many automatic machines.67


The hydrogen is replaced by air and the concrete is sent to an autoclave so it can be cured. In the autoclave, 180 degree steam is applied at high pressures onto the concrete so a second chemical reaction can occur and make the material stronger and rigid. This process takes about 8-14 hours which is much faster than the conventional method of curing. The whole manufacturing process is usually done by automatic machines and doesn’t cause any pollution or toxic substances to be released into the atmosphere.89


Chemical Equations

There are two main chemical reactions that are used in the manufacture of AAC:
Firstly, the reaction of aluminium powder and sulphuric acid in the concrete causes millions of hydrogen bubbles to be formed in the mixture, the equation of this reaction is:

  • Aluminium + Sulphuric Acid —> Aluminium Sulphate + Hydrogen
  • 2Al (s) + 3H2SO4 (aq) —> Al2(SO4)3 (aq) + 3H2 (g).10

The other reaction is the Calcium Aluminium Silicate reacting with lime to form cement which is made up of Calcium Silicate and Calcium Aluminate:

  • Calcium Aluminium Silicate + Limestone —> Calcium Silicate + Calcium Aluminate + Carbon Dioxide
  • CaAl2Si2O8 + 6CaCO3 —> 2Ca2SiO4 + Ca3(AlO3)2 + 6CO2

This reaction is what makes Portland cement; a component of AAC.11

The manufacturing process is shown in the video below:


Environmental Impact

Autoclaved Aerated Concrete has both positive and negative environmental impacts. However, if it is compared to traditional concrete, it is much better for the environment.


Hebel Concrete has been tested by Good Environmental Choice Australia (GECA) for how environmentally friendly it is and has proven to be 60% more environmentally-friendly than other building materials in aspects such as global warming effects, carcinogens, waste produced and energy used in manufacturing. Overall, AAC has 73% less embodied energy and 61% less greenhouse gas emissions than other building materials.13


During its production, AAC uses only a quarter of the raw materials that are used in traditional building products and therefore has a much lower carbon footprint. They even use raw materials which are abundant and can easily be sourced close to the manufacturing plant so the environmental effects of transport are minimised. Also, all the waste concrete and other materials that are produced during manufacture are reused so no permanent waste is created. The production process also saves energy as the steam curing in the autoclave is done at low temperatures and doesn’t require re-heating. No toxic gases are released in the process and water is also saved by being collected and reused many times on the concrete. Overall, AAC requires about 0.4kg of raw material per m3 (Refer to Graph).1415



During its use, the thermal properties of AAC reduce the heating and cooling costs in buildings and therefore their carbon footprint. The accuracy of the cutting process in production reduces the amount of waste from the cutting of products during building. If you build with AAC, you do not need to use insulation in walls and roofs therefore lowering a building’s impact on the environment.16

Disposal and Recycling

There are programs in place by the manufacturers of AAC which sort and recover waste AAC materials that can be recycled before they are sent to a landfill, therefore reducing its environmental impact. Also, the material contains no toxic substances so it is not harmful even if in landfill.17


Although Autoclaved Aerated Concrete seems like a really environmentally friendly material, it does have its negative effects. The automatic machines used in the manufacturing of AAC all use and burn fossil fuels and produce a large amount of carbon emissions. The carbon emissions from the transportation are present and leave a carbon footprint. The lime that is within the concrete also contains a large amount of carbon dioxide and is therefore less environmentally friendly. It contains about 1000kg of CO2 per tonne.18

Chemical Composition

Autoclaved Aerated Concrete is made up of the following materials:

  • Sand which contains more than 85% Silica (SiO3) and Clay (Aluminium Silicates- AlSi)
  • Aluminium Powder/Paste (Al)
  • Lime which contains 75% Calcium Oxide (CaO) as well as other compounds.
  • Silicon Dioxide (SiO2)
  • Gypsum; dehydrate or phosphogypsum which contains Sulfur Trioxide (SO3)19


  • Portland Cement, which is made up of Calcium Silicate- Ca2SiO4 and Calcium Aluminate-Ca3 (AlO3)220

The chemical structure of calcium silicate is:


Macroscopic Properties

The macroscopic properties concerning Autoclaved Aerated Concrete are in the table below:

Property Definition Uses
Density Density is the ratio of the mass of a substance inside a specific volume. Low density is having a small amount of mass in a certain volume. The low density of AAC makes it really lightweight and still durable. This allows for it to be used in any type of building which requires a material which uses less labour to install.
Hardness Hardness is a measure of how much a substance can resist denting or scratching on its surface. The hardness of AAC lets it avoid any damage to its surface and therefore is very useful if this material is used as the outside layer of walls in commercial and residential buildings.
Strength Strength is a measure of the power a material has to resist Stress and Strain that is applied on the material. The high compressive strength of AAC allows it to hold a lot of pressure before it deforms. Therefore, it can be easily used in large commercial building walls and roofs in the form of panels and blocks.
Durability Durability is how long a substance can last for before becoming worn out and defective. As AAC is a very durable material, it can be used in large commercial buildings and still last for a long time even with the effects of nature acting on it. In residential homes, the durability will mean that there is no need for regular repairs or other forms of maintenance.
Malleability The ability of an object to be shaped in any way, e.g. by hammering or rolling, is called its Malleability. The high malleability of AAC allows it to be used in building of all different shapes and sizes. It can be shaped to let plumbing and electrical wires through in residential building or shaped in moulds for special structures in commercial buildings.
Fire Resistance The more Fire Resistant a substance is, the longer the time for which it can hold out fire and not be easily burnt. It is a measure of how difficult a substance is to burn. As AAC is very fire resistant, it gives a higher level of security during a fire emergency in commercial or residential buildings and can prove to be a barrier as a roof panel or in walls.
Thermal Insulation (Conductivity of Heat) Thermal Insulation is a measure of how much a substance can retain or reflect heat so it doesn’t pass through it. Heat Conductivity is how easily a substance will let heat pass through it. The Thermal Insulating properties of AAC enable it to reduce the heating and cooling costs of building and therefore be used in all kinds of buildings from residential homes to commercial buildings. This also allows AAC to be used as roof panels, wall panels and blocks so it can easily reduce the heating and cooling costs of a building.
Acoustic Insulation Acoustic Insulation is how much a substance can prevent soundwaves passing through it to keep noise out. In residential homes, the acoustic properties of AAC increase the comfort of people by reducing the noise that gets into the home. These acoustic properties also let AAC be used in the wall of commercial offices and halls.
Water Resistant Water Resistance is how much a material can stop water going through its surface and soak the material. Being very Water resistant, AAC can be used on the outside of houses and on roofs as it will not let water damage its surface, just like any other concrete.

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