Hello Brian, I believe you would find the following book extremely helpful. And it will save you a lot of time and money, from making lots of first time build errors. Well worth the read for sure:
http://www.newsociety.com/Books/R/The-Rocket-Mass-Heater-Builder-s-Guide
The heat riser can indeed be constructed of high temperature brick, either dense fire brick with added exterior insulation wrap, or insulated fire brick (IFB) which requires no additional insulation. Whichever brick is used it is important that the air spaces (leaks) between the "stacked" brick be sealed. For that most folks use a simple mortar made from masonry sand and clay, or even clay "slip". Constructed with such mortar, the heat riser (and all combustion unit brick work for that matter) can still be disassembled later if desired, and most of the mortar reclaimed if desired.
Sand for bench fill should have clay added, which is necessary to fill all the tiny insulating air spaces between the grains of sand for a more suitable thermal mass. This too, can be later crushed, removed, re-hydrated, and used again when the stove is moved to its new location etc.
There are lots of multiple bits of info. like this and much much more contained in the builder's guide book. Highly recommended reading.
