The Process for Autoclaved Aerated Concrete

AAC works much like regular concrete but provides better insulation.

Autoclaved aerated concrete, also called AAC, is a special type of very light concrete that contains around 80 percent air. This material provides better thermal and sound insulation than ordinary concrete while retaining concrete blocks' strength and fire resistance. AAC must be made in a factory and requires some special care and materials to produce workable blocks.

Materials

AAC materials very somewhat by location and manufacturer. However, most autoclaved aerated concrete blocks include silica sand, calcined lime, gypsum, class F fly ash and hydraulic cement. AAC also contains an expansive agent such as aluminum paste or powder, which encourages a high air content. Like all concrete, AAC is mixed with water and cured in molds to produce blocks, beams and other building units.

Mixing

The AAC production process starts by grinding sand to the appropriate fineness. Workers then measure out the materials by weight and place them in an automatic mixer. Expansive agents and water are added last to create a cement slurry. As soon as the expansive agents, water and concrete come into contact, a chemical reaction occurs, creating microscopic bubbles of hydrogen.

Molding

Immediately after mixing, workers pour fresh concrete slurry into steel molds. If they are producing reinforced AAC, these molds contain steel reinforcing materials. Within about three hours, the concrete expands to around twice its original volume, filling the mold. It takes a few more hours for the AAC to cure enough to support its own weight. The material can then be cut and shaped.

Curing

While AAC can support its own weight within a few hours, it is not yet fully cured. The concrete must be placed inside an autoclave, a device that subjects it to high pressure steam at temperatures of around 360 degrees Fahrenheit. This cures the concrete completely, giving it the necessary structural properties and stability to be used in buildings.

Construction

Autoclaved aerated concrete can be used in a variety of structural and nonstructural applications, just like regular concrete. Unlike ordinary concrete, however, AAC requires a stucco-type finish to prevent weather damage. Special polymer-modified stuccos make AAC more resistant to water penetration while still allowing the material to breathe. Some builders also finish AAC with heavy acrylic paints.

References

• National Association of Home Builders: Autoclaved Aerated Concrete
• Masonry Magazine: Autoclaved Aerated Concrete
• Portland Cement Association: Autoclaved Aerated Concrete