Autoclaved Aerated Concrete (AAC) Research at Penn State

Autoclaved Aerated Concrete (AAC) was developed in Sweden in the early 1900s. It was found that a mixture of lime (CaO) and finely divided quartz (SiO2) could be foamed with aluminum powder and then autoclaved at 180 centigrade to produce a lightweight, insulating, sound absorbing building material. The Germans refined the technonogy and have been producing lime-slica AAC for over 50 years under licensed trademarks such as Hebel and Ytong. Others, most notably the English and Eastern Europeans, substituted Class F fly ash for quartz and have been marketing fly ash derived AAC for almost as long (H＆H Celcon, Spirox). Although AAC is used for construction on every continent (>200 plants in 35 countries including Mexico), its use in the United States and Canada has been rather limited. Seemingly, potential homeowners in the U.S. and Canada make choices based on factors other than longevity and durability. North America has always had an abundant supply of wood, an innovative wood products industry, and a very strong lobbying arm in Washington. Although AAC has an impressive use history in Europe and Japan, it is still a relatively new construction material for those who live in North America. At this point in time, a house built with AAC would cost more than a similarly sized wood framed house, but in its favor an AAC house would be allergen free, maintenance free, water proof and last considerably longer. AAC block can be assembled with thin set mortar and window and door openings as well as electrical and plumbing recesses fashioned after the fact with hand tools. It is conceivable that lower labor costs could eventually close the affordability gap as AAC block and panel become more widely available, i.e. more plants are opened and shipping costs are reduced.

Current research continues to explore the feasibility of using use alkali activation (NaOH) to promote concurrent tobermorite and tectosilicate (zeolite) growth in a traditional AAC block. Adding small amounts of alkali to an AAC mix suggests that in situ growth of a zeolite in the tobermorite matrix will work, but only with considerable effort. However it was found that toughness could be improved slightly by including fly-ash based zeolites (pretreated with NaOH and converted to zeolites) in the starting materials used to make the AAC. The results of the study can be viewed using the publications link at the bottom of the page. This "next-generation" AAC was based upon the premise that minor changes in the chemistry and processing of AAC could provide a way to make major changes in the phases present in the AAC and its microstructure. Although it was found that zeolites would not significantly increase toughness, the study suggested that the presence of zeolites in an AAC masonry product might have the potential to absorb toxins and thus help "clean the air."

For example, when zeolite microfiber were mixed with an AAC formulation they tended to inter grow with the developing tobermorite matrix. Thus one could produce a composite in a single step. The zeolites will act to toughen the AAC (by a few percent) and at the same time adsorb various air born contaminants from the surrounding air. If zeolite AAC walls/panels were used inside office buildings and schools the AAC composites could adsorb water vapor, VOCs and perhaps even bacteria/viruses. Currently AAC is used to bear weight and enclose space. As its zeolite content is increased, AAC will be able to adsorb contaminants from the air. This allows the AAC to perform additional tasks. Although formulations are studied in the laboratory, they must eventually be scaled up to full sized block for mechanical property testing.

Pictured is a research grade software controlled steam autoclave (3 foot ID and 41/2 feet long0. It provides a convenient way to carry out formulation experiments. The autoclave is currently operational and is housed in Research Unit A on the Penn State Campus.



The exciting aspect of the work is the idea that masonry building materials might be made to serve two functions: traditional and environmental. Zeolites are commonly made in an autoclave at elevated temperatures much like AAC. Thus making a zeolite containing AAC is not so much a chemical or equipment limited problem, but one related to processing. In order to make a zeolite block Class F fly ash can be mixed with NaOH solution. It is critical to contral water content in order for foaming to occur. Unfortunately mixing the material with Al powder has been a major problem; Al foams spontaneously when added to the mix. Foaming agent research is underway.

Making a zeolite AAC block is currently in its infancy. With a little effort it should be possible to develop a suitable block with the added properties of insulation, sound abatement and purification that can replace traditional masonry. Field-testing will necessarily be longer term in nature and might be carried out as part of instructional programs at the university. Should efforts to promote this "green" technology become a reality, Penn State could become the major player in unibersity-based research.

Eventually AAC will begin to gain market share and manufacturing costs will come down. Finally, as governments adopt more stringent energy and environmental requirements, and affordable energy efficient housing becomes increasingly important, AAC building materials could well become the building material of choice for the 21st Century.