[Stoves] clay mortar versus cement mortar in stoves

[Crispin]

Dear Christa

>...what the difference in thermal expansion, heat resistance 
>and durability (and other potential benefits) is between 
>clay-based mortar and cement mortar?

The thermal expansion for the brick is probably in the range of 7 to 8 (times ten to the minus 6) and the mortar is about 10 to 11 which is nearly 50% more.

>...cement mortar gives more physical strength to the structure, 
>but might have a problem with expansion, as it is rigid 
>and might crack. 

It is important to have the bricks wet and the mortar relatively wet so the cement runs into the brick wicking into the brick as it does so.  Then the cement should be kept damp for a month if possible.  Using dry mortar is deadly because it dries very quickly and easily separates from the brick later.

>I also suspect that it desintegrates with the heat.

Cement can take quite a lot of heat, but not real flame temperatures.  Above 400 it has problems.  It weakens as it gets hot which may allow it to NOT shear off the brick which is expanding at a lower rate.  In other words a cement-brick joint might last longer if it is hot.  This is not something that can be decided immediately.  It depends on the surface finish of the brick too.

>Clay-based mortar like the people use for the low-cost 
>housing has the advantage of being cheap and locally available...

I agree this is a real possibility.  I would be pleased if you can use ant-hill soil as a mortar and then when it is dry (dry it slowly with a small fire) try making a fire hot enough to get the mortar above 400, perhaps 450 degrees.  That should cause a chemical bonding to take place in the mortar.  I got surprisingly strong bricks from the materials fired at only 400 in Congo.  There is something in the ant-hill soil.

>It might have smiliar expansion coefficients like the 
>bricks for being the same material, 

It should be the identical material.  All or most of the bricks are made from anthill soild are they not?  Someone  said it was regular dirt.  Yes? Always?

Regards
Crispin