Autoclaved Aerated Concrete (AAC), also known as autoclaved cellular concrete (ACC), autoclaved lightweight concrete (ALC), autoclaved concrete, cellular concrete, porous concrete, Aircrete, Hebel Block, and Ytong is a lightweight, precast, concrete building material invented in the mid-1920s that simultaneously provides structure, insulation, and fire &mold-resistance. AAC products include blocks, wall panels, floor and roof panels, cladding (facade) panels and lintels.
AAC was perfected in the mid-1920s by the Swedish architect and inventor Dr. Johan Axel Eriksson, working with Professor Henrik Kreüger at the Royal Institute of Technology. It went into production in Sweden in 1929 in a factory in Hällabrottet and quickly became very popular.
Approximate 80% of the volume in AAC blocks is air, making its weight just a third of a clay brick. The AAC block is formed on the reaction of aluminium with blended proportion of cement, gypsum, fly ash and lime. When hydrogen gas emanates, it imparts a robust honey-comb structure to AAC blocks.
Eriksson’s success immediately attracted a much-needed commercial interest and in 1929 the first large-scale manufacturing facility of these artificially-made crystalized stone blocks was launched in a factory “YxhultsStenhuggeriAktibolag“, Sweden under the name Yxhult. In 1940 the “Yxhult” name was changed to YTONG as this name was easier to pronounce. In 1932 the factory CarlsroKalkbrukSkovde started with AAC block production and the product acquired the brand name Durox. Important competitor arose in 1934 which started to manufacture AAC blocks under the brand name Siporit (as of 1937 a well-known Siporex). Siporex was also the first to introduce AAC reinforced elements in 1935, namely roof/floor panels and lintels. Good structural properties of the newly created AAC material soon spread all over Western Europe, with more than 6 plants in Sweden alone.
AAC manufacturing went international in 1937 with introduction of technology licensing and know-how transfer. After WWII, there existed only few leading AAC technology suppliers: Siporex and Ytong (both belonging to the Swedes), Durox (bought by the Dutch) and Hebel (German). Throughout the 20th century all of them successfully sold AAC technology licenses around the world while at the same time annual conventions contributed to further developments in AAC production, product quality and its applications. Given different manufacturing technologies, production of AAC blocks became associated with Ytong (tilt-cake system) while production of both AAC blocks and reinforced elements was led by Durox, Siporex and later Hebel (flat-cake systems).
Germany, United Kingdom, Sweden, Denmark and the Netherlands established themselves as the main AAC hubs after WWII, albeit the fact that countries used different technologies to produce similar products. Following the triumph of AAC material on an international arena, competition grew stronger between the parties in that relatively small market, often ending up in a battle of patents. Slowly in the 1980s, the influence of Swedes was diminished due to the suffering domestic market and as a result Siporex activities were reduced to a minimum and no new plants were built since the 1990s. Additionally, during the 1980s Germans took over and improved on the know-how of Ytong from the Swedes. Despite fierce competition, multiple plants were realized in Asia, Middle East and Eastern Europe, based on all four different technologies. In the beginning of the 1990s the first AAC plant based on a tilt-cake technology (Ytong) was supplied to China. From that point onwards technology outflow became widespread and, as of 2014, there are more than 3,000 AAC production facilities with an estimatedproduction capacity of 450 million m3 per year of non-reinforced blocks. Mass production of blocks is also popular in Central and Eastern Europe and India while Japanese, Korean, Australian and Western European markets are focusing more and more on reinforced panels and high precision blocks.
The technology of aircrete production has developed significantly over the last decennia. Production of ordinary non-reinforced AAC blocks is not linked to any exclusive know-how anymore and as a result AAC blocks became a commodity in many markets. Manufacturing light and heavy reinforced AAC elements is still a big challenge for majority of world producers, primarily with tilt-cake technologies. Nevertheless, with time physical properties of AAC material improved and application became more universal, from a construction point of view. Today, AAC is a structural solid building material, excellent thermal insulator, good sound absorber and also an attractive decoration material.
AAC has growing market shares in several countries. More than 40% of all construction in UK and 60% of all construction in Germany accounts for AAC. In India, the demand of AAC block has grown 10 fold in the past few years. Today, AAC is used as building material in all types of buildings. Capable of withstanding all weather conditions, it can be used external or internal walls. These can be efficient for basement walls, framed structures and loadbearing applications.
AAC market development went through a major revolution since the 1990s. With large increase in absolute number of AAC manufacturing facilities, producers worldwide are striving to improve the balance between manufacturing cost and physical material properties, with a focus on thermally efficient building. International ‘green’ policies and strict building regulations are putting pressure on AAC producers demanding more energy-efficient materials (low density blocks and panels), better quality products (high product accuracy, surface quality) and wider range of product application (residential, commercial and industrial). Beyond the existing AAC commodity market of blocks there is a growing worldwide demand for integrated building solutions. It is known that building with AAC panels makes it possible to reduce the total cost of ownership for the final consumer. Offering buildings made of solely prefab AAC elements results in a fast, easy construction and no on-site waste.