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