Clay substitute?

The more I read about clay, the more I'm confused.
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Would this work ok?
http://www.rutland.com/productinfo/dry-mix-211%C2%AE.html
Their Description:
Dry Mix 211®
A refractory mortar that is used for joining or repairing masonry flue tile or as a parge coating for building fireboxes. Non water soluble, this product is a hydraulic set mortar that does not require heat to cure. Resists washing out when exposed to rain or water. Develops a compressive strength above 2500 psi and withstands 2550° fahrenheit without softening. Acid resistant, meets or exceeds all the requirements of the National Fire Protection Association standard NFPA 211 and NYSCMA Tek Spec. Dark Gray color.

If so...
Would I want to use it as is, for the brick building part? I'm considering the riser being brick too.

.......
I'm basically doing something like Paul's portable, for a green/aquaponics house.
The barrel will be outside and the box(mass area) will come through the wall.
And guessing I could add some sand and straw, for the little bit of forming around the heater.

Thanks,
Bete

P.S.
Ok here's the kind of stuff I keep dealing with, just trying to figure out what I can use for "Clay"...
In the PDF of the above mentioned 211 stuff. Seen here... http://www.rutland.com/back/tds/document_20.pdf
It states...
"Limitations:
When cured it is water insoluble, but not water
impermeable. For outdoor applications, use Rutland
Castable Refractory Cement."

This stuff... http://www.rutland.com/productinfo/castable-refractory-cement.html
And the PDF... http://www.rutland.com/back/tds/document_22.pdf
Which mentions nothing about using as a mortar.
Plus it states..
"It sets up fast, so only
mix an amount that can be used within 20-30
minutes after adding water. Mix thoroughly taking
care not to lose any water which contains the binder."

And well, since this will be my first experience of setting brick. All I can do is lol at me mixing up a 1/4 cup at a time, to manage using it with-in that time frame.

P.P.S.
I've probably killed a few brain cells over the past couple of weeks, tweaking and re-tweaking. The re-tweaking the re-tweaks, trying to find a way to make this project come together with my ridiculously low budget for it.
And this "Clay" ordeal is killing the budget.
The clay seems to have went from... Dig some up in your back yard to... must be refractor fire clay powder that is white and cost 3 arms and 5 legs. Then sift that out... etc, etc.
Help? : /

In for results... I just asked at my local masonry supply house for firebrick mortar and they sold me a 15 lb tub of powder mix for $34. Was thinking of wetting the firebrick then mortaring a thin layer to join my base unit together. My riser is going to be triple wall stainless found at the junk yard for cheap insulated with perlite/clay slip and a stainless tank for an outer sleeve. Mudding the transition with cob, curious how it is going to hold since I have never made cob before. Good luck.

What goes outside around the barrel, feedtunnel is mostly about being capable of dealing with great amounts of heat that hit fast (thermal shock) what goes inside and forms the mass is mostly about insulation so it can hold the heat well. I think what you should depends very much on where you are. Some people can dig clay out of their backyard other can get it easier in bags. Do You have any usable sand close by? Straw or sawdust?

A good working rocket mass heater can reach temperatures up to 3000 f Refractory Cement only goes up to 2200 f I would choose clay!

Would a Clay silt work for sealing the fire bricks?