mortar and cement

I'm asking this question for my husband.

His mortar mix for the RMH core was fire clay and sand. His question is, can he use portland cement for the bell?

Hi Leigh;
What style of RMH are you building, a Walker stove, or have you started building a Batchbox?
If you are building a Batchbox, what size and shape is the bell?
Is it a double-skin or single-skin?

The lower portions of the bell could use a regular concrete mortar.
The upper sections, particularly with a 6" Batch riser, will exceed temp ratings for concrete.
Concrete starts to degrade above 200F and over 1000F failure is assured.
Sakrete makes a high-heat mortar that could work at the top of a bell, but I have never tried it.
They claim a working temperature of 2500F

Hi Thomas! We're building a double shoebox batachbox (DSR2) with a bell ISA of approximately 57 sq. ft. with an arched roof. The idea is to use this as a baking oven. Single skin.

Dan's question is largely based on material availability, of which are local choices are slim.

Is fireclay hard to find?
Is it overly expensive?
Fireclay should cost $7-12 for a 50# sack

The sakrete high heat mortar should work and they sell it at Loews

Regular concrete might last for the walls but not for the roof of the bell and several layers down.
Putting super wool as an insulator on top would help but you would lose the ceiling ISA by insulating.

The fireclay here runs about $18 for 50#. It's a bit of a drive to get, but doable. He did find the Sakrete mortar you mention at Lowe's for $60 for 50#. He still has some fire clay left, though, so he said he'd just continue with that.

Thank you for your help!

I built most of the shell of my Walker stove using refractory mortar which is rated up to 950°C (about 1750°F)  I've a feeling that inside the core it could get hotter than that.
The core I put together with clay/sand in about a 1:3 mix, which is what Matt recommends.  I've also used that for the top couple of layers of brick on the shell, a) in case it gets hot enough that it's an issue, which I doubt but mainly b) so that if I need to I can get it apart.

If you use cement based refractory mortar it bonds much harder to the brick, which means you won't be able to take it apart should you want to, or not at all easily.

However, this thread handily allows me to pose my own mortar question:

I've found the adhesion of the clay mortar to the bricks once it's dried out to be much lower than cement-based mortar.  It is also prone to shrink when it dries, especially thick lumps of it.  The adhesion issue came to light when I tried to drill into a brick which was fixed with clay mortar to fit one of the door hinge points, the vibration of the drill knocked the brick loose.  I had to finish drilling it and then re-stick it in place.  

Has anyone got a way to increase the bond strength of clay/sand mortar to brick?  I foresee the possibility of recurring repairs, for example in the brick top surround to the hotplate, which might get knocked and the bricks loosened.

Obviously it's not yet been lit, as it's not finished so I don't know whether once it's been heated and thus dried more the mortar gets harder or more secure.

Standard mortar mix for brick pizza ovens is 3.1.1.1, Sand, Fireclay, Portland cement and Hydrated lime.
This mix is good for around 4- 500c max and has been used on thousands of wood fired ovens.