Using Rockite with a Ready-Mix concrete
Once upon a time I worked for
an industrial fence company, and sometimes we had to set a fence post
in sub-freezing conditions. The trick to making this work is to mix in
just the right amount of Rockite with the Ready-Mix concrete. The Rockite helped to
speed the process along, preventing the mixture from freezing.
I decided to apply this
technique to the creek
stepping stones where I
filled in the holes of each cinder block with 1 part Rockite to 8 parts Ready-Mix. A few of the holes had a fence
post driven through them to act as an anchor.
It seems like it worked, but
I won't know for sure until I see how well they stand up to the
constant pressure of our creek.
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It is probably a mixture of gypsum plaster and portland cement.
Both harden by hydration, the reaction with gypsum plaster being very fast and exothermic (releases heat).
According to the MSDS (PDF), it's 98% gypsum and 2% portland cement. I'm guessing the portland cement is added to give it a gray colour? Or maybe to improve the adhesion to surrounding concrete.
If you want to know the composition of something, looking for an MSDS is always a good idea.
There is a couple of caveats, which can vary depending on the jurisdiction the MSDS was made for.
So it is pretty unusual to find a 100% composition on an MSDS. In this case, both portland cement and plaster of paris are labelled as irritants, so that is why the numbers add up.
At a guess, I'd say plain plaster of paris is probably way cheaper... Especially if you buy it in bulk.
Selling plain stuff under a brand name for an inflated price is an age-old trick after all.