From: Johnny Drozdek <jdrozdek(--nospam--at)keymark.com>
Date: Tue, 08 Feb 2000 12:28:37 -0700
To Nels Roselund:
I forwarded your message to a grad student at the University of Wisconsin -
Madison, who has spent several years researching various concrete mixtures.
His reply is pasted below. I hope it's useful.
Johnny Drozdek, E.I.T.
Keymark Engineering, Inc.
>From Richard A. Walls <walls(--nospam--at)cae.wisc.edu>
>I believe the reaction is similar to the reaction with NaOH and water. The
>dissociation of water drives the reaction. The aluminum sulfate (aluminum
>powder) or rather the hydroxides suck CO2 and H2O from the air and produces
>H2 gas a by-product. Thus the air bubbles are present in the concrete.
>There have been a few papers about this subject. The official reaction is
> 2 Al+ 6 H2O 2 Al(OH)3+ 3 H2
>It was discovered in 1914 in Sweden that adding aluminum powder to cement,
>lime, water, and finely ground sand caused the mixture to expand
>dramatically. The Swedes allowed this "foamed" concrete to harden in a mold,
>and then they cured it in a pressurized steam chamber--an autoclave.
>Autoclaved aerated concrete (AAC) was finally developed in Europe in 1923.
>Commercial production of the material began in 1930. In 1995, more than 31
>million cubic meters were produced by over 50 factories worldwide.
>Autoclaved aerated concrete (AAC, also called autoclaved cellular
>concrete--ACC) is produced by about 200 plants in 35 countries and is used
>extensively in residential, commercial, and industrial buildings. At a
>density of roughly one-fifth that of conventional concrete and a compressive
>strength of about one-tenth, AAC is used in load-bearing walls only in
>low-rise buildings. In high-rises, AAC is used in partition and curtain
>walls. The material is also fairly friable and must be protected from
>weather with stucco or siding. On the positive side, it insulates much
>better than concrete and has very good sound absorbing characteristics.
> In other words,the material (ACC) is inert but requires a coating to remain
>durable. It's extremely workable and can be shaped and "cut" with a regular
>saw. The aluminum will not be detrimental to anything after the product has
>cured. There are currently two patents on this material and an ASTM
> C1386-98 Standard Specification for Precast Autoclaved Aerated Concrete
At 10:47 AM 2/8/2000 EST, you wrote:
>Years ago, on my first existing building project, the bottom of an existing
>concrete footing needed to be exposed so that a deeper footing could be
>placed under it. The contractor placed the new footing to within a
>convenient few inches of the old footing and then made a batch of concrete
>fill the space between the new and old footing -- it was a conventional
>site-made mix with a handful of aluminum powder added. The fill was formed
>and packed into place, but not with the care we expect today for a drypack
>fill. The fill material was expansive in the plastic state and resulted
>tight fill. My understanding was that the aluminum reacted with "something"
>in the portland cement with hydrogen gas being one of the byproducts; tiny
>bubbles of hydrogen gas kept the concrete expansive as long as it was in the
>Sika Grout Aid and PMP Grout Additive seem to have similar action in the
>grout used in concrete block construction today. Apparently they consist of
>aluminum powder and a fine aggregate. My understanding is that these
>admixtures act on portland cement and on flyash (which is principally
>amorphous silicon dioxide).
>I have a project for conservation of the walls of an old stone building in
>which I would like to use an injected grout that includes Sika Grout Aid or
>PMP Grout Additive to control shrinkage. I've developed a mix that has the
>mechanical properties of appropriate strength, pumpability and shrinkage
>control that I want. A question from the materials conservator has arisen,
>"How do I know that the aluminum admixture is not detrimental to the
>surrounding materials in the long term?"
>I don't know, but I think that an understanding of the specific chemical
>reaction would help us evaluate the long term effects. Does anyone know the
>specific chemical reactions? My assumption has been that the reaction
>includes a break down of water to produce hydrogen, leaving oxygen to
>with the aluminum to produce alumina (Al2O3), which is a usual component of
>portland cement. The alumina (by my understanding) gets incorporated into
>the binder as a calcium aluminate or a calcium aluminaferrite so that there
>is nothing particularly unusual about the resulting concrete except perhaps
>there is some un-reacted metallic aluminum remaining in the hardened
>Can anyone help me with the specifics of the chemical reaction?