Developed in Sweden in the late 1920s, autoclaved cellular concrete (ACC) is a lightweight precast concrete building material that is cured under elevated pressure inside special kilns called autoclaves. Though ACC has been used successfully throughout most of the world since the end of World War II, ACC made a mark in the United States only recently.

ACC, sometimes known as autoclaved aerated concrete, is made with all fine materials-nothing coarser than finely ground sand. What makes ACC different from lightweight aggregate concrete is that ACC contains millions of microscopic cells that are generated during the manufacturing process. In addition, ACC is unlike many other concrete products because it may be drilled, sawed, chiseled, nailed, or screwed using conventional carpentry tools.

Several Formulas

Although several formulas are used for manufacturing ACC, the basic raw materials are portland cement, limestone, aluminum powder, water, and a large proportion of a silica-rich material-usually sand or fly ash. Once raw materials are mixed into a slurry and poured into greased molds, the aluminum powder reacts chemically to create millions of tiny hydrogen gas bubbles. These microscopic, unconnected cells cause the material to expand to nearly twice its original volume—similar to the rising of bread dough—imparting the lightweight cellular quality to ACC. After a setting time ranging from 30 minutes to 4 hours, the foam-like material is hard enough to be wire cut into the desired shapes and moved into an autoclave for curing.

The autoclave uses high-pressure steam at temperatures of about 356° F (180°C) to accelerate the hydration of the concrete and spur a second chemical reaction that gives ACC its strength, rigidity, and dimensional stability. Autoclaving can produce in 8 to 14 hours concrete strengths equal to strengths obtained in a concrete moist-cured for 28 days at 70° F (21°C). The final products are usually shrink wrapped in plastic and transported directly to the construction site.

ACC, which is about one-fourth of the weight of conventional concrete, is available in blocks, wall and roof panels, lintels, and floor slabs. Each of these products can be manufactured in a range of sizes depending on specific applications, allowing for maximum efficiency and flexibility in construction. ACC can be used for all types of structures ranging from single-family housing to large industrial complexes.

ACC is an inert, nontoxic substance that has an energy-efficient and pollution-free manufacturing process. Perhaps the most significant environmental benefit of using ACC is that fly ash can be used as the silica-rich component. The electric utility industry generates more than 50 million tons of fly ash each year—only a fraction of which can be recycled.

ACC is reasonably frost and sulfate resistant, allowing it to be used around the world in all climatic zones and for a wide range of applications. When it is used on the exterior, ACC is normally protected by stucco or other protective coatings. ACC also is an inorganic material, making it 100 percent termite and vermin proof and resistant to rotting and mold.

 
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