[AGA Member] Re: Portyland Cement and aquatic structures

["S. Hieber" <shieber@yahoo.com>]

So this happens this strongly even with plastic cement?

I supposed one couldn't drastically impede the effect by
reducing the exposure -- by coating the cement after it has
achieved it's first cure and dried.  For a pond, a
waterproof liner wood do the trick.  For in-aquarium forms
I would guess that a thin coat of epoxy would work and
still allow other material to adhere.

If you're using the cement like a plaster over a
substructure, wire mesh, metal lath, plastic lath,
styrofoam, etc., and it's basically the last layer of
material, then I guess coating is out of the question. 
Other materials might work better, but portland cement is
so doggone cheap.

The pH problems are due to what portland cement is made of.
 Basically, in water it is the opposite of acid.  In
industry, where pH is an issue, cement not used or another
material covers it.  In a plant canning tomatoes or catsup,
the floor must be cover with clay tiles and instead of
cement grout, special epoxy or other materials are used. 
The acidity of tomatoes simply eats into cement.

RE cure characteristics.  There are lots of kinds of
portland cememt (named for the visual similarity to the
stone from the famous Portland Quarry).  All cements
eventually stop curing and begin spalling.  It might be
twenty years, it might be a hundred, depending on
composition and environment.  But the process is continual
and the cement is always changing.  If you think tht kind
of change is slow, then you haven't travelled by glacier.

Here's a good description of cements, some (Type III)
better for ponds (under liners) and some (Type II or IIA,
which contains more air) better for in aquarium use:

http://www.calportland.com/colton/coltprods.htm

You can make very lightweight cement structures by using a
lot of perlite mixed with the sand.  I mean the expanded
type of perlite that's used as an extender in potting
soils.  As an aggreagate, it prevents cracking during first
cure as does the sand, but it entrains a lot of air -- no,
your a cement aquarium background still won't float ;-)

We used to make kitchen counters this way as a substrate
for tiling -- it's amazing how light a bucket of cement can
be.

Scott H.

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