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Eco-friendly concretes: Autoclaved Aerated Concrete (AAC)

Rana Hamid Shabbar
Sustainability is a method which is used to develop the human life using natural resources. The essential aspects of the sustainability are environmental, social and economic. Consequently many researchers tried to use suitable materials with new technics to expand the sustainability concept. In addition, decrease the carbonation effect and energy consumption. AAC is classified as ultra-lightweight concrete which consists of cement, water, fine aggregates (sand, fly ash) and aluminium powder (Al) with a distinct cellular structure. It has one-fifth the weight of conventional concrete with density range from 400 to 800 kg/m3 and compressive strength from 2 to 7 MPa. The entrained air bubbles are the main reason for enhance its physical properties. AAC is a brittle in tension, therefore, it should be reinforced. In addition, AAC is considered as eco-friendly building material and manufactured by steam curing of raw materials. It is the term applied to a homogeneous fine-grained matrix enclosing small, non-communicating voids. It is produced by introducing gas bubbles into a matrix composed of cement with siliceous filler such as sand and fly ash. At the mixing stage, aluminium powder (Al) is added to react with the caustic solution that evolves during the hydration reaction to form hydrogen gas bubbles. On hydration, cement produces gels of calcium silicate hydrate and calcium aluminate hydrate together with calcium hydroxide.
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After initial setting, the specimen should be placed in the autoclave and cured under high-pressure steam range from 4 to 16 N/mm2 at 180 – 210 ˚C for 8 -16 hours’ duration. High-pressure steam curing affects the cement when finely ground silica is added, owing to the chemical reactions between silica and Ca (OH) 2 released on hydration of C3S. The sufficient curing in an autoclave causes a neutral of the tobermorite-11Å and crystallised C-S- H, which are the main structural minerals of AAC. As a result, voids may be more than 80% of the total volume [6-8]. An optimum curing temperature has been reported by Neville to be about 177°C with a steam pressure of 8 bars. AAC is generally used for structural and non-structural purpose which reduces the total dead weight of structures. Subsequently, reinforcement can be reducing. AAC has zero-pollution manufacturing process (zero emission of gases) and only emission of steam. Energy consumption for producing AAC is lower than other building materials. Further advantages it has higher strength-weight ratio, better tensile strain carrying capacity, lower coefficient of thermal expansion and superior heat and sound insulation characteristics due to air voids present in the concrete. It is also typically used for reduction of carbon emission, easy transportation and fast construction; therefore reduce the costs of artificial maintenance of building’s temperature.
On the other hand, crack after installation is considered as disadvantage of the AAC, which can be eliminate by decreasing its strength and be a sure that the blocks are dry during installation. Also, it needs much careful handling than clay bricks to avoid any breakage because of their brittleness. In addition, it has an initial cost per unit higher than conventional concrete. According to the Portland Cement Association, AAC has shear stress range from 0.05 to 0.15 MPa with compressive strength from 2.06 to 6.20 MPa. However, conventional concrete has shear stress closer to 0.28 MPa with compressive strength around 10.34 MPa.

    An autoclave machine

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