Chip Friedline

allen lumley wrote:Matthew Nistco :As the mortar used to assemble the bricks is actually used to hold the bricks (universally ) apart from each other, we can
use a Lime mortar, rather than a Portland cement based mortar. Lime based mortar cleans off of old bricks easily, and holds up to the
Rocket Mass Heaters R.M.H.s, high Temperatures with out failure much better than the Portland.

Again, if you are strongly thinking about a R.M.H. in your future, look at my comment above, and get your copy of Rocket Mass Heaters at
cobcottage.com

The mortar performs two functions, it acts as a gap filler for the irregular nature of the brick surfaces,and for an oven, fire box, or flue should be installed with as small an exposure to the flame as possible, the. Other function of the mortar is to provide stress relief, mortar should always be weaker than the bricks it is used with.

A completely lime based mortar is not waterproof until it has aged by fire or time (chemical change). A mixture by volume 3-1-1-1 of fine sand, fireclay, hydrated lime, and Portland cement will be waterproof and stable at high temps. Yes the Portland will burn out at some point but it will keep the mortar waterproof and stable until the lime takes over as the binder.

Chip

"We will see ... I am currently researching cast-able insulating refractory. "

When doing your investigation into cast able refractory consider adding stainless reinforcement needles they will add significant strength and resistance to cracking.

A simple low cost refractory mortar can be made from 3-1-1-1 sand, fire clay, hydrated lime, and Portland cement, all measured by volume. Add water to get a creamy peanut butter consistency.

You may also want to consider stepped rings instead of flat that way you could avoid the need for seals as the laminar up-flowing nature of flue gases would create a self sealing labyrinth.

The rings could also be constructed with a tongue (T) top, and groove (G) bottom built into the casting to have the same effect.

Chip