Fireclay Mortar

Buckley Rumford Fireplaces
Fireclay Mortar
2/24/14

ASTM E 1602 on masonry heaters allows and defines "fireclay mortar" - sort of crypticly:
3.2.16 mortar, fire clay—mortar consisting of fine aggregate and fire clay as a binding agent.
The standard guide also defines "soapstone refractory" mortar:
3.2.17 mortar, soapstone refractory - a mixture of powdered soapstone and sodium silicate.
Note that "fireclay mortar" as defined has no cement or binder. It might be okay in very thin joints for firebrick not exposed to mositure. Theroretically it could take on a ceramic set but only if it attains a temperature of over 2,000 deg. F.

Sodium silicate, or water glass, is a pretty good binder but it remains water soluble. Used to bind soapstone powder it would match the soapstone in color. Mixed with fireclay you might get a refractory mortar like HeatStop Premix of other air-setting refractory mortars. Okay, maybe, but the nice thing about HeatStop Premix is that it is tested and meets the ASTM C199 standard. Make it yourself and who know what you get? And ultimately it is still water slouble and leads to efflorecesne which is why we nearly always recommend a hydraulic-setting refractory mortar like HeatStop 50.
Code has gotten fairly rigid in requiring only refractory mortar that meets ASTM C199 for the firebox, throat smoke chamber and flue linings. Code, however doesn't absolutly require a firebrick lining in the the firebox. It just requires the firebox walls to be thicker (10") if you don't use firebrick. And, if you don't use firebrick you don't have to use refractory mortar in the firebox. In other words you could use soapstone and lay it with a home-made soapstone refractory - a mixture of powdered soapstone and sodium silicate. Just make sure the firebox walls are 10" thick. You could, of course, use facebrick and regular Portland mortar but expect it to fall apart soon - which is why the code requires firebox walls to be 10" thick if firebrick and refractory mortar is not used.