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Expansive Grout

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Nels,

I can't give you the specifics of the chemical reaction, but aluminum, either 
in powder form (including aluminum abraded from tools, pumping, etc.) or by 
embedding aluminium in concrete, does cause concrete to expand in an 
unpredictable manner and lowers the strength of the concrete mix.  Expansive 
grout used to be made with aluminum powder and caused so much trouble that it 
was prohibited about 30 or so years ago.  (Petrosky's 30 - 35 year 
syndrome.)  This is why "non-metallic, non-shrink" grout is now specified.

My specs prohibit concrete from being mixed, transported, conveyed or 
finished using any equipment made of aluminum.  The UBC masonry chapter 
prohibits using aluminum tools.

Some years ago, an ad appeared in EN-R for aluminum concrete chutes.  That ad 
quickly disappeared.  A local ready-mix company was using aluminum liners in 
their chutes, but when they learned that concrete and aluminum don't mix, the 
aluminum liners quickly disappeared.  Aluminum electrical conduits in 
concrete slabs have caused the concrete slabs to crack below the conduit (See 
Construction Failures, by Jacob Feld).  This latter effect is exacerbated if 
the concrete contains calcium chloride or deleterious amounts of the chloride 
ion (Phoenix Plaza underground parking garage, early 1970's).

A. Roger Turk, P.E.(Structural)
Tucson, Arizona

Nels Roselund 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 to 
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 combine 
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 
concrete. 

Can anyone help me with the specifics of the chemical reaction?

Nels Roselund
Structural Engineer<<