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Using Rockite with a Ready-Mix concrete

Experimenting with Rockite in a stepping stone project


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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What is Rockite, Mark?
Comment by John Sat Oct 22 19:31:15 2011
That's an excellent question. It's easy to find out what Rockite is used for --- making cement set faster and stronger. But as for the actual chemical composition...I'm not sure.
Comment by anna Sat Oct 22 19:55:36 2011

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).

Comment by Roland_Smith Sun Oct 23 05:30:23 2011
I wouldn't be terribly surprised if it was straight gypsum plaster. On their website, the company that manufactures rockite says that it won't react with aluminum the way portland cement will, which seems to suggest it's not a mixture of the two.
Comment by anna Sun Oct 23 11:44:10 2011

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.

Comment by Roland_Smith Sun Oct 23 16:24:36 2011
I knew there was something obvious I wasn't googling --- should have added "msds" to my search terms. Thanks for clearing that up!!
Comment by anna Mon Oct 24 10:50:27 2011

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.

  • Usually only ingredients recognized as in some way harmful have to be listed.
  • And there has to be a minimum fraction of it.

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.

Comment by Roland_Smith Mon Oct 24 11:47:23 2011
Excellent data. I can see why the companies wouldn't want to report all of their non-toxic components so no one could steal their product. I wonder how the price of Rockite compares to the same size container of plain gypsum plaster....
Comment by anna Mon Oct 24 14:53:37 2011

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.

Comment by Roland_Smith Mon Oct 24 15:55:35 2011

One very unique homestead, $1,500 per acre, the opportunity of a lifetime