Autoclaved aerated concrete, also called Aircrete or just AAC, is a lightweight and durable building material with excellent acoustic and thermal insulation properties. It is made using an expansion agent that creates gas bubbles so that the final concrete mixture contains between 65 and 85 percent air by volume. The mixture is hardened in an autoclave by steam curing.
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AAC is made from a mixture of quartz sand or pulverised fly ash, Portland cement, calcined lime and water together with small amounts of gypsum or anhydrite. Aluminium powder is used as the expansion agent. Unlike standard cement, AAC does not contain any aggregate and all the components react chemically together. Calcined lime is the chemical compound calcium oxide that is produced by burning limestone to remove carbon dioxide. Fly ash is the powdered residue from burned coal. Like sand, it is a pozzolan. This means that fly ash and sand are inert on their own but react with calcium oxide and water to produce a durable concrete. Gypsum and anhydrite are the chemical compound calcium sulphate and act as strengthening agents in the concrete.
The AAC process starts by grinding the sand or fly ash with water in a ball mill. This produces an extremely fine powder that creates slurry when mixed with water. The cement, lime and anhydrite (or gypsum) are stored in separate silos on the factory site. Aluminium powder is produced by an atomisation process where molten aluminium passes through a vacuum, gas or centrifuge and solidifies into a fine powder. This powder may also be supplied by an aluminium manufacturer.
The precise composition of AAC depends on the density, strength and insulation properties required. According to Brickwall Construction Machinery, when sand alone is used in the concrete mixture, Portland cement comprises between 10 and 20 percent of the solid materials, lime between 20 and 30 percent, the sand between 55 and 65 percent, gypsum or anhydrite about 3 percent and aluminium powder 0.8 percent. The density of the block falls if the proportion of aluminium powder – the expansion agent -- increases because more gas bubbles are formed.
Once prepared, the sand slurry is pumped to a slurry tank. Weighed amounts of lime and cement are moved from their silos to the slurry tank. Powdered aluminium is mixed with water to form a paste, added to the slurry and mixed. The resulting mixture is poured into moulds. These are detachable, rectangular containers mounted on rails. Their inner sides are covered in oil to prevent the concrete from sticking. The containers are transported to a pre-curing area for up to several hours at a temperature of 40 degrees Celsius (104 degrees Fahrenheit). The mixture reacts to produce “greencake,” a mixture of calcium and aluminium silicates called ettringite and calcium silicate hydrates, together with hydrogen gas. The gas bubbles cause the mixture to expand and create a cellular structure rather like bread dough. Air moves into the cells as the hydrogen gas escapes.
Cutting and autoclaving
The mixture stiffens as it rises. Once it has reached a correct consistency, each block of concrete passes through a series of cutting wires to make smaller blocks of required sizes. These blocks are loaded into an autoclave for between 8 to 10 hours where they cure in steam at 200 degrees Celsius (392 degrees Fahrenheit). Once the curing process is complete, the concrete blocks are unloaded and packed for dispatch.
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