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Last edited 23 Jun 2023
Autoclaved aerated concrete
 What is Aerated concrete ?
Aerated concrete (AC) also called cellular concrete is often used to describe any concrete product that contains significant pockets of air in a controlled manner, usually it is prepared with water, cement, a foaming agent and perhaps a fine light aggregate such as sand, but rarely with aggregates. The foaming agent introduces air bubbles into the finished product, making it lighter and having insulative properties.
When the air content within concrete is about 1.5% - 3% and bubbles are relatively large in size, over around 1mm and randomly located throughout the mix it is referred to as entrapped air, mostly an unintended consequence of mixing concrete, with insufficient agitation. When the air is uniformly distributed in microscopic bubbles of less than 1mm it is referred to as air entrainment. When it makes up between 3%-8% by volume of the finished product it is often done so to improve the performance under freeze thaw conditions. Aerated concrete or cellular concrete however normally refers to concrete products that have a higher content around 25-25% of air bubbles, which may be higher is no structural performance is required.
 A background to aerated concrete (AC)
As early as 1889 Czech Hoffman tested and patented a method of aerating concrete with the use of carbon dioxide, whilst in 1914 Aylsworth and Dyer used aluminium powder and calcium hydroxide to create and patent a porous cementitious mixture. This might be considered as a very early form of aerated concrete, though much of the credit is associated with Johan Axel Eriksson who developed both aerated and autoclaved aerated concrete as described below..
The Architect and inventor Johan Axel Eriksson is generally accepted to be credited with perfecting the technique of producing a limestone and ground slate, lime formula in the early 1920's with Professor Henrik Kreüger at the Royal Institute of Technology in Sweden. They found that the foamed concrete product could withstand the moisture and pressure of autoclaving and that this sped up the curing process without causing shrinkage, but improving performance. The technology was patented using ground slate or alum shale and started to be mass produced from the late 30's.
Other AAC manufacturers in Sweden and other countries started to produce variations on the product from around the 1940's up to war time. These products also included reinforced autoclaved aerated concrete (RAAC) using Eriksson's initial formula to develop panel, roofing and flooring systems, in fact with the goal of developing a whole house from the same base product.
After the war and effectively between 1945 and 1975 mass building programs across Europe searched for modern methods to construct buildings quickly and cost effectively, and as such innovative new products were employed extensively, with autoclaved aerated concrete being a key one. This alongside insulation products, glazing and window frame systems, and a variety of prefabricated and precast systems that often included lightweight elements from AAC or RAAC, collectively today sometime referred to as post-war building materials.
Sometime later in the 1970's it was discovered that natural uranium within the alum shale found in Sweden, caused some radioactive radon gas exposure, which led to a new recipe being developed. This formula contained quartz sand, calcined gypsum, lime(mineral), cement, water and aluminium powder, but no contaminated alum shale and is the base recipe of many aerated concrete blocks that are produced today.
The aluminium powder is a air entraining agent and reacts with the calcium hydroxide formed on hydration of cement to produce hydrogen gas bubbles. The agent is mixed with the a fine aggregate (usually sand or fly ash), cement, lime, gypsum, and water and reacts on hydration creating pockets. Autoclaved aerated concrete (AAC) is essentially the same but once poured and setting, is placed in an autoclave which applies steam and pressure, to speeds up the curing process and create a stronger product.
- Aircrete blocks.
- Alkali-activated binder.
- Alkali-aggregate reaction (AAR).
- Applications, performance characteristics and environmental benefits of alkali-activated binder concretes.
- Cellular concrete.
- Fly ash.
- Precast concrete.
- Self-compacting concrete.
- Smart concrete.
- Types of concrete.
- Types of concrete specification.
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