alternatives to cob?

John Elliott wrote:Urban area? How about used bricks? Rather than sculpt your RMH out of cob, if you can score some old solid bricks, you could assemble it LEGO-style. If you want it to be a little more permanent, then you can hold it in place with some refractory mortar (regular mortar with clay added).

posted Yesterday 1:14:30 PM by Allen Lumley in the thread 'Heat riser, duct/flue questions'
...All of the stove pipe buried within the R.M.H. / Thermal bench should be considered to be a sacrificial form around which your cob, your Perlite And Clay Slip are placed and a few bricks are stacked. As such we do not care whether they last weeks or years, here we can go with the cheapest grade of thin wall piping we can find, the lighter the better, galvanized pipe having no inherent advantage!

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

Chip Friedline wrote:

allen lumley wrote:Matthew Nistco :

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

Would you say that your 3-1-1-1 mixture is a good cob alternative for use in a greenhouse? I am building a RMH in a 10x26 greenhouse, the mass will be the soil, I need a suitable substitute to cover the exposed feeding tube and would rather not use cob as the environment will have a high RH value.

Katherine Burelle wrote:

Chip Friedline wrote:
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.

Would you say that your 3-1-1-1 mixture is a good cob alternative for use in a greenhouse? I am building a RMH in a 10x26 greenhouse, the mass will be the soil, I need a suitable substitute to cover the exposed feeding tube and would rather not use cob as the environment will have a high RH value.

allen lumley wrote:Since what you want should fit in about 4-6 5gal. buckets you Do Not Want A Truck Load...

Perlite (2 bags ), can be found in Builders Supply places...

Builders sand will take somewhere around 5 cubic yards...

Glenn Herbert wrote:The bell type heat exchanger you participated in (a "half-barrel bell" to be precise), or any other mass that consists of a large hollow box, is generally easier to build than the original duct-in-cob style, and has much less friction so draft may be better.

Matthew Nistico wrote:

Katherine Burelle wrote:

Chip Friedline wrote:
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.

Would you say that your 3-1-1-1 mixture is a good cob alternative for use in a greenhouse? I am building a RMH in a 10x26 greenhouse, the mass will be the soil, I need a suitable substitute to cover the exposed feeding tube and would rather not use cob as the environment will have a high RH value.

I can't really answer your question, and I don't know if Chip will even see it, 8 years later.  I would point out two things, however:

1) What was the point of Chip's comment in the first place?  Why should I need to be concerned with how waterproof is the mortar on my RMH?  It is an interior appliance.

2) See Erica's comment above on strictly avoiding lime or Portland cement in an RMH, in favor of clay/sand mortar or just monolithic clay/sand (i.e. cob).  Anything from Ernie or Erica I would take as The expert opinion when it comes to RMH tech!  Doesn't mean there isn't room for different ways of doing things, but she sounded pretty definite about the dangers of these materials in an RMH build.