Autoclaved aerated concrete (AAC)

Autoclaved aerated concrete (AAC), also
known as autoclaved cellular concrete (ACC)
or autoclaved lightweight concrete (ALC), [1]
was invented in the mid-1920s by the Swedish
architect and inventor Johan Axel Eriksson. [2][3]
It is a lightweight, precast building material that
simultaneously provides structure, insulation,
and fire and mold resistance. AAC products
include blocks, wall panels, floor and roof
panels, and lintels.
It has been refined into a highly thermally
insulating concrete-based material used for both
internal and external construction. Besides
AAC's insulating capability, one of its
advantages in construction is its quick and easy
installation, for the material can be routed ,
sanded, and cut to size on site using standard
carbon steel bandsaws , hand saws , and drills .
Even though regular cement mortar can be
used, 98% of the buildings erected with AAC
materials use thin bed mortar , which comes to
deployment in a thickness of ⅛ inch. This varies
according to national building codes and creates
solid and compact building members. AAC
material can be coated with a stucco compound
or plaster against the elements. Siding materials
such as brick or vinyl siding can also be used to
cover the outside of AAC materials.
AAC has been produced for more than 70 years,
and it offers advantages over other cementitious
construction materials, one of the most
important being its lower environmental impact.
AAC’s improved thermal efficiency reduces the
heating and cooling load in buildings.
AAC’s workability allows accurate cutting,
which minimizes the generation of solid waste
during use.
AAC’s resource efficiency gives it lower
environmental impact in all phases of its life
cycle, from processing of raw materials to the
disposal of AAC waste.
AAC’s light weight also saves cost & energy in
transportation.
AAC's light weight saves labour
Raw materials
Unlike most other concrete applications, AAC is
produced using no aggregate larger than sand.
Quartz sand, lime , and/or cement and water are
used as a binding agent. Aluminum powder is
used at a rate of 0.05%–0.08% by volume
(depending on the pre-specified density). When
AAC is mixed and cast in forms, several
chemical reactions take place that give AAC its
light weight (20% of the weight of concrete) and
thermal properties. Aluminum powder reacts
with calcium hydroxide and water to form
hydrogen . The hydrogen gas foams and doubles
the volume of the raw mix (creating gas bubbles
up to 3mm (⅛ inch) in diameter). At the end of
the foaming process, the hydrogen escapes into
the atmosphere and is replaced by air.
When the forms are removed from the material,
it is solid but still soft. It is then cut into either
blocks or panels, and placed in an autoclave
chamber for 12 hours. During this steam
pressure hardening process, when the
temperature reaches 190° Celsius (374°
Fahrenheit) and the pressure reaches 8 to 12
bars , quartz sand reacts with calcium hydroxide
to form calcium silica hydrate , which accounts
for AAC's high strength and other unique
properties. After the autoclaving process, the
material is ready for immediate use on the
construction site. Depending on its density , up
to 80% of the volume of an AAC block is air.
AAC's low density also accounts for its low
structural compression strength. It can carry
loads of up to 8 MPa (1,160 PSI),
approximately 50% of the compressive strength
of regular concrete. [4]
Since 1980, there has been a worldwide
increase in the use of AAC materials. New
production plants are being built in the USA ,
Eastern Europe, Israel , China , Bahrain , India ,
and Australia. AAC is increasingly used by
developers, architects, and home builders.
History
The material was perfected in the mid-1920s by
Dr. Johan Axel Eriksson, an architect working
with Professor Henrik Kreüger at the Royal
Institute of Technology . [2][3] It went into
production in Sweden in 1929 in a factory in
Hällabrottet and became very popular. In the
1940s, the trade mark Ytong was introduced,
but it was often referred to as "blue concrete" in
Sweden due to its blueish tinge. This version of
Ytong was produced from alum slate , whose
combustible carbon content made it beneficial
to use in the production process. The competing
concrete brand Siporex used other raw
materials. However, the slate deposits used for
Ytong also contain uranium , which makes the
material give off small amounts of radioactive
radon gas to the surrounding air. In 1972, the
Swedish Radiation Safety Authority pointed out
the unsuitability of a radon-emitting
construction material, and the use of alum slate
in the production of Ytong ceased in 1975.
Ytong produced after 1975 has used raw
materials without the uranium content.