Kaolin Clay or "FIRE CLAY"

I am planning the construction of a rocket mass heater and need some advice. I have read the books and a lot of forums regarding them, so I have a pretty good idea of the concept. It’s the riser that is my main concern. I think I have settled on a Perlite/Clay/Sodium silicate mix rammed into a form to make 2+ inches of insulated wall stock.

Here are my questions.

1. What clay is best? I have sourced a supplier that I can get most anything from. They carry Hawthorn Bond fire clay, as well as about 6 different Kaolin clays, and other materials like silicate sand and wollastonite. Kaolin clay is just a little more expensive so it is an option if people think it will hold up longer then the Hawthorn Bond.
2. Are there any other additives I should consider? I have seen the Wollastonite (calcium Metasilicate I think) as an ingredient in furnace cements.
3. How many years can I expect out of a well-made heat riser? I would not want to be tearing apart all my work all the time to replace it.

As for the core and burn chamber, I planned on casting most of it using the some material as mentioned above for insulation, but leaving the inside 2 ¼” bigger so I can line the burn chamber with fire brick so it can be replaced once worn out from cramming wood in and cleanings.

Thanks for your help in advance!

Fire clay is what you want. Hawthorn Bond is a known brand. Aside from heat resistance, fire clay has the property of not expanding or contracting much with temperature changes, which is important in riser construction.
Casting the core with space for firebrick liners is good, though you don't need to worry about the back half. Unless you are rough with things, only the feed tube and floor will be subject to abrasion. Wood does not get loaded into the burn tunnel except as coals spill out into it, and ash cleaning is a very minor and gentle procedure. I use a sardine can mounted on a short handle to scoop ash, takes a few seconds once or twice a week.

Glenn Herbert wrote:Fire clay is what you want. Hawthorn Bond is a known brand. Aside from heat resistance, fire clay has the property of not expanding or contracting much with temperature changes, which is important in riser construction.
Casting the core with space for firebrick liners is good, though you don't need to worry about the back half. Unless you are rough with things, only the feed tube and floor will be subject to abrasion. Wood does not get loaded into the burn tunnel except as coals spill out into it, and ash cleaning is a very minor and gentle procedure. I use a sardine can mounted on a short handle to scoop ash, takes a few seconds once or twice a week.

Great information, my first go around will be for a garage and a simple J-tube configuration, so I will take your advice on the fire brick being used towards the front only. The final build that I will put in my small home (building in the next couple years) will be a batch box and I will be lining most of that with a commercial fire brick.

While we are on the topic, How large of a batch box could one make? Is there a point where the material is too far away from the entrance to the riser to work properly?