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What is AAC Blocks?

by Devendra Kumar on November 20, 2021

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Autoclaved Aerated Concrete(AAC) is a type of eco-friendly precast concrete that is certified as a green building material and is composed of natural materials such as fly ash, limestone, aluminium powder, etc. It is highly durable, has high load-bearing capacity, high insulating capacity and is quite lightweight, approximately three times lighter and 8-9 times bigger than traditional baked red bricks.

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AAC is also known as Autoclaved cellular concrete, Autoclaved lightweight concrete, lightweight concrete and Aircrete. It was first developed in Sweden in the 1920s by Dr Johan Axel Eriksson who was trying to create an alternate building material that could replicate the properties of wood, i.e., easy to work with, solid structure and high thermal insulation, but without the usual drawbacks of wood like highly combustible, easily decaying and prone to termite damage. The Swedish architect along with Professor Henry Kreuger was the first to combine a conventional cement concrete mixture of cement, sand, lime and water with a small percentage of aluminium powder that causes the mixture to expand like a dough resulting in a type of concrete that is almost 80 % air.

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This resulting product is known as Autoclaved Aerated Concrete(AAC) and is typically made into blocks to be used as bricks in erecting mortared walls. AAC blocks can be used as an eco-friendly alternative to traditional blocks. These blocks are porous, non-toxic, renewable, reusable and recyclable. Even the manufacturing process adopted for the production of AAC blocks is energy efficient and eco-friendly. AAC blocks are typically used in the construction of apartments and other residential buildings owing to their excellent heat insulation capacity and are generally ideal for high rise structures.

Materials Used

Materials Used in the Manufacture of AAC blocks are:

  • Cement-OPC 53 is most suitable for the preparation of these blocks
  • Fly Ash-Being an industrial waste product, it reduces the overall construction cost of the AAC blocks. It provides properties such as thermal insulation, sound absorption and fire insulation to AAC blocks.
  • Limestone-Limestone is used in powder form, either crushed to powder at the AAC factory or purchased in powder form itself.
  • Aluminium Powder-Al powder is an expansion agent that releases hydrogen gas in reaction with calcium hydroxide and water. The aerated concrete is a result of this reaction.

Manufacturing Process

The manufacture of AAC blocks roughly consists of 4 major steps that have been given below:

  1. Preparation of Raw Materials-Except for fly ash all other constituent materials are taken in the required proportion as powder and fly ash is mixed with water to form a slurry so that it can be mixed with other constituents.
  2. Mixing in Proportions-The quality of the blocks being produced is greatly influenced by this step as the appropriate proportion of constituent materials will result in superior final products. The proportion of these materials is decided in this step as per the requirement of the final product. The ratio of the materials are- sand/fly ash : lime : cement : gypsum = 69 : 20 : 8 : 3. The ratio of water used is 0.6-0.65 and the percentage of Aluminium used in the mix is 0.08% of the total dry materials.

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The steps involved in the mixing process are as follows:

  • Fly Ash is pumped into the container until the desired weight is poured in.
  • Likewise lime powder, cement and gypsum are poured into individual containers using conveyor belts.
  • Once the required amount of all the constituent materials are poured into the individual containers, the control system pours all these materials together in a mixing drum.
  • The mixture is churned for a specific time after which it is ready for the next step of moulding the mix.
  • Moulding, Rising and Pre-curing- Moulds can be of any size depending upon the requirement and quantity of the mix. The standard size generally prepared is 600mm x 200mm x 200mm. The moulds are coated with oil to prevent the sticking of the mix to the moulds. Aluminium causes the slurry mix to expand by releasing hydrogen that forms tiny cells. The expansion can be up to three times the original volume of the mix and is thus responsible for the insulating and lightweight properties of the AAC block. The mix is green in colour and is allowed to settle and cure after being placed in the mould.

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The expanding and curing process takes approximately 60-240 minutes. The curing process takes place in an autoclave, a large pressure vessel that reaches a pressure of about 800-1200kPa and temperature up to 180’C.

  1. Demoulding and Wire Cutting-The moulds are carried to the cutting area where they are cut using cutting wires as per required sizes in two stages. In the first stage, the cutting is performed vertically and in the second stage, it is cut horizontally.

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Demoulding and cutting off the mould gives a dimensional finish to the blocks and must be performed meticulously to avoid rejection.

  1. Autoclaving the AAC Blocks-The final step in manufacturing an AAC Block is autoclaving of the wire cut blocks in an autoclaved chamber. The blocks are autoclaved for 8-12 hours at around 180’C.

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Curing inside an autoclave reduces drying shrinkage and the amount of heat and time for which the blocks are heated decides the grade of the blocks. Also, even if autoclaving may be an expensive process that requires more energy, the blocks take much less time to be manufactured.

Advantages of AAC Blocks over Red Clay Bricks

Many advantages are related to the use of blocks and some of them are:

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  • Lightweight-AAC blocks are approximately 3-4 times lighter than traditional red bricks, thus they are easier and cheaper to transport, and due to this property taller buildings can be easily constructed with AAC blocks.
  • Eco-friendly and Sustainable-Since no toxic materials are used, and industrial waste product in the form of fly ash is used, no toxic gases are emitted in its manufacture process, low energy consumption, etc making AAC blocks eco-friendly and sustainable, whereas the manufacturing process of red bricks results in the emission of polluting gases.
  • Better Workability and Design Flexibility-AAC blocks can be easily cut, drilled, nailed, milled, grooved as compared to red bricks according to individual requirements. Any type of design changes can be easily made to a structure after the blocks have already been laid.
  • Faster Construction-The construction done with AAC blocks is 25% faster as compared to traditional bricks due to its lightweight property and better workability.
  • Fire Resistant-AAC blocks are non-combustible and are fire resistant up to 1600’C and withstand fire up to 6 hours.
  • Better Insulation and Energy Efficiency-The high density of air pores in AAC blocks provide better thermal insulation and makes structures made with it more energy efficient.
  • Less Wastage-AAC blocks have negligible breaks, i.e., less than 5% that reduces its wastage and increases utilization.
  • Better Acoustic Performance-Since the AAC blocks are highly porous, it offers super sound-absorbing properties that can offer sound attenuation up to 42 decibels. Therefore it blocks out all significant sounds and disturbances, which makes it suitable for schools, hospitals, offices, etc.
  • Better Seismic Resistance-AAC blocks reduces the mass of a structure than usual that making the structure resistant to earthquakes.
  • Cost-Efficient-The mortar required while placing AAC blocks is much less due to its surface accuracy than the amount used for traditional bricks. The overall cost of the construction is thus reduced by about 2.5%. Also as it needs less jointing the requirement of cement and steel is reduced, which in turn helps in saving energy costs by about another 30%.

Comparison of AAC blocks with Traditional Bricks

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