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Re: Expansive Grout

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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)>
>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
>as follows:
>                                            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
in a 
>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 
>plastic state.
>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?
>Nels Roselund
>Structural Engineer