Autoclaved aerated concrete (AAC) blocks were developed in the 1920s in Sweden to address timber shortages and have gained widespread acceptance globally, particularly in the Middle East. They are made from a mixture of cement, fly ash, and water, which results in lightweight, insulating blocks that contribute to reduced construction loads and better energy efficiency. With superior acoustic and fire-resistant properties, AAC blocks also support environmental sustainability by utilizing industrial waste and reducing the need for traditional clay bricks.

1. Autoclaved Aerated Concrete
Blocks
Thaju Zaman E K
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2. History
• Developed in Sweden in the 1920s in response to
increasing demands on timber supplies.
• Industries like Durox, Siporex etc. were established in
the early 1940s.
• Soon started spreading to other parts of the world
though its main hub remained at Western Europe
• Huge acceptance in Asia especially in the middle-east
countries because of the strong demand in housing
and commercial space.
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3. Introduction
• 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
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4. A multi-storey building made with AAC blocks
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5. 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.
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6. 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.
Environmental impact
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7. 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
Excellent acoustics
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8. 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.
Fire resistance
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10. Autoclaved Aerated Concrete is very airy thus
allowing more diffusion of water, reducing
humidity of the building.
Ventilation
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11. 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.
Energy saving
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13. 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.
Accuracy
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14. 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.
Rapid on - site assembly
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15. 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.
Long life
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16. Composition
1. Fly-ash - 59%
2. Cement (usually OPC grade 53) -33%
3. Lime - 8%
4. Aluminum powder - 0.07%
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18. AAC blocks coming out of the Autoclave vessel
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19. Close up view of air bubble formation during the production of
AAC blocks
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20. PROPERTIES
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21. 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
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22. Porosity
• 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.
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23. Permeability
• 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.
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24. 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
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25. Mechanical Properties
Compressive Strength
• Can carry loads of up to 8 MPa 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.
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27. 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.
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29. Durability
• 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℃
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30. Typical Properties of Autoclaved Aerated Concrete
Dry density
(kg/m3)
Compressive
Strength
(MPa)
Flexural
Strength
(MPa)
Modulus of
Elasticity (GPa)
Thermal
Conductivity
(3% moisture)
(W/mK)
450 3.2 0.65 1.6 0.12
525 4.0 0.75 2.0 0.14
600 4.5 0.85 2.4 0.16
675 6.3 1.00 2.5 0.18
750 7.5 1.25 2.7 0.20
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31. COMPARISON BETWEEN AAC BLOCKS AND
CLAY BRICKS
• Fly ash is an industrial waste product and its use in
AAC block production process takes care of the issues
concerned with its disposal whereas brick industry
consumes our natural top soil
• The AAC block is dimensionally more accurate as it is
produced with wire cut technology in a certified
factory.
• AAC blocks have air voids and hence have better fire
resisting property compared to red clay bricks.
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32. • Using AAC Blocks reduces the load on the foundation
and other structural components in a structure due to
its lower self-weight.
• AAC Blocks do not have any “micro-pores” or
continuous ‟capillaries‟ through which exterior
surface water can be absorbed to interiors. It means
longer life to the paints and interiors free from
growth of any kind of fungus, providing healthier and
long lasting interiors to the occupants
• AAC Blocks can be easily cut, drilled, nailed, milled
and grooved to fit individual requirements. Available
in custom sizes.
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33. AAC Blocks in India
• Currently there are around 35 largescale AAC blocks
manufacturing plants across India with a heavy
concentration near Surat, Gujarat.
• Besides there are a number of small scale
manufacturers concentrated near the coal industries.
• More and more AAC blocks plants are set up across
the India, as awareness about AAC blocks is growing.
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34. Cost of AAC blocks
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35. Disadvantages
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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36. Conclusion
• Autoclaved Aerated Concrete block is different from
conventional concrete in the mix materials and
properties.
• Though the compressive strength of Autoclaved
Aerated concrete is less than that of concrete, AAC
blocks possess many beneficial factors such as lower
density, enhanced thermal and sound insulation and
reduced dead loads.
• Hence it is an ideal alternative for traditional wall
brick or stone materials owing to its environmental
impact and eco- friendly features.
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