Many dealers across the US confuse “premixed fireclay” with premixed refractory mortar. They are not the same!
Fireclay is basically powdered Firebrick… a 50lb bag of firebrick powder. Premixed “fireclay” is NOT fireclay - it is refractory mortar.
If a dealer tells you that you can substitute the fireclay for the premixed refractory mortar, you can not. It is not the same material. In addition, you will have a heck-of-a time trying to get the pail of premixed refractory mortar to mix with the silica sand, lime and Portland cement.