Re: [ATM] portland cement; grind. etc.

[Dominic-Luc Webb <dlwebb@canit.se>]

Thanks Dune for offering the list an excellent treatment of
this subject. I could add a couple remarks. I have also used
concrete for a long time, including Portland cement.

I basically follow the rules about mixtures, etc, as much as I
can, although, I have ended up adding more water in order to ease
up the mixing process. This still has given excellent tools.

I think air pockets are a bit of a problem and I eliminate these
by vigorous shaking/aggitation after I pour. Bubbles can be seen
rising to the top. When no more bubbles come up, I leave the
concrete to dry for about a day, remove the cardboard and tape
I use to make the form, and let it dry a bit longer before
submerging.

So far no breaks other than a single case in which I tried pressing
the concrete against a thick piece of glass with a furniture clamp.
This was done in an effort to match the tool (having a wax coating)
to a mirror. I did a REALLY bad job of pouring the wax and the two
surfaces did not match well. I since learned my lesson - Remove
the wax and pour again in such situations. This is the only case of
a break I have had.

More problematic is that the concrete tools can be unstable and
can shrink, etc depending on water content. This can be particularly
problematic with embedded tiles. I manage, but have not found a
reliable way to maintain a long-term stable surface in this case.

Dominic-Luc Webb

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