Autoclaved Aerated Concrete

AAC block is made of Portland cement, fine aggregates (fly ash or sand), water and an expansion agent. The autoclaving process results in the production of air voids in the material, making it less dense, easy to cut/ mould and better insulating. Autoclave is a strong, pressurized, steam-heated vessel.

Reduced Dead Loads

  • Using AAC Blocks reduces the load on the foundation and other structural components in a structure due to its lower self-weight.
  • About 55% reduction in weight of walls can be obtained when compared to that of walls made with clay bricks.

Environmental impact

  • AAC blocks uses fly ash (70% of its weight), thus provides the most constructive solution to the nation’s fly-ash utilization problem.
  • Fly ash is an industrial waste product and use of fly ash in the AAC block production process takes care of the issues concerned with its disposal.

Excellent acoustics

  • Autoclaved Aerated Concrete has excellent acoustic performance and can be used as an effective sound barrier, e.g. (AAC wall panel).
  • The small air pockets that are generated during AAC production prevents the sound from travelling from one side of a wall to the other.

Fire resistance

  • An important reason for the AAC good fire resistance is it’s is relatively homogeneous structure.
  • Unlike the normal concrete where the presence of coarse aggregate leads to deferential rates of expansion, cracking and disintegration.


  • Autoclaved Aerated Concrete is very airy thus allowing more diffusion of water, reducing humidity of the building.

Energy saving

  • The remarkably good insulation properties of Autoclaved Aerated Concrete mean a pleasant interior environment is achieved.
  • In most cases the need for supplementary insulation can be avoided.
  • It has very low specific conductivity. This makes heating or cooling (Air Conditioning) very efficient.


  • The accurate manufacturing process ensures that Autoclaved Aerated Concrete panels and blocks are always produced to size as they leave the factory.
  • This results in less on-site trimming and reduced quantities of mortar and finishing materials used.

Rapid on – site assembly

  • The low weight of AAC blocks helps in the easy loading and unloading of these items when transportation to work sites are required.
  • Besides it also helps the mason as he can easily lift and place the blocks.

Long life

  • AAC does not deteriorate over time and they retain good finishes even after many years.
  • They have better resistance against alternate cycles of heating and cooling.


  1. Fly-ash – 59%
  2. Cement (usually OPC grade 53) -33%
  3. Lime – 8%
  4. Aluminium powder – 0.07%

Physical properties Density

  • Ranges between 300- 1800 kg/m3 according to a source (RILEM).
  • AAC is thus a unique industrial product that covers such a range in apparent density.
  • AAC products with density up to 350 kg/m3 can be used as load bearing construction material.
  • AAC with lower density are generally used for thermal insulation purposes.


  • Classified as capillary pores, gel pores, macro-pores due to deliberately entrained air, and micro-pores due to inadequate compaction.
  • Fly ash is used as for more uniform distribution of air- voids by providing uniform coating on each bubble and thereby prevents merging of bubbles.
  • It is observed that the strength, permeability, diffusivity, shrinkage and creep properties of AAC blocks are considerably related to its porosity and pore size distribution.


  • Permeability of aerated concrete is greatly influenced by the type, size and distribution of the pores, and not the pore volume.
  • Pores are generally classified into two types – open pores and closed pores.
  • Permeability of aerated concrete is contributed by the open pores and not the closed pores.

Compositional Properties

  • Scanning Electron Microscopy (SEM) showed that the micro capillaries in AAC are plate shaped crystals of 11.3 Å tobermorite with a double-chain silicate structure.
  • The growth rate and the degree of orientation of this structure cause differential pore distribution.
  • Tobermorite is a calcium silicate hydrate mineral with chemical formula: Ca5Si6O16(OH)2·4H2O or Ca5Si6(O,OH)18·5H2O 24.

Mechanical Properties Compressive Strength

  • Can carry loads of up to 8MPa approximately 50% of the compressive strength of regular concrete.
  • Influenced by factors such as density, age, curing method, component and mix proportion.
  • The rate of strength development is initially very high and it gets decreased with age.

Thermal conductivity

  • Thermal conductivity increases as the moisture content of AAC increases.
  • Also increases with the increasing density.
  • The amount of pores and their distribution are also critical for thermal insulation.
  • Finer the pores, better the insulation.


  • Carbonation is one of the main factors of the ageing deterioration of AAC.
  • In cold countries frost deteriorations are observed in outer walls made from AAC.
  • Surface scaling caused by freezing and thawing.
  • The other is the wide cracks formed due to the inner part AAC temperature being reduced to 0℃.


  • Familiarisation of the Product
  • Few contractors are currently familiar with the product, and masons must adjust to using thin-set mortar as opposed to traditional cement-based mortar, which requires less precision in its application.
  • Scarcity of Manufacturing Plants.
  • Projects far from manufacturing facilities will suffer from higher initial costs.


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