Is pre mixed heat mortar ok?

[Remember Wingmen?]

Hi folks, I'd really love some feedback about why the FB pompei instructions say not to use pre mixed heat resistant mortar for pizza ovens - their quote is below. I have a product called Rubbedin Hot Spot Redi-Mortar and its directions suggest that it's appropriate for BBQ's, fireplaces, chimneys etc.

"Finally, makes sure that you do not use a wet, premixed
fireplace mortar with water-soluble binder. There are a
number of mortar brands designed for in-door fireplace
installation, where the mortar does not cure to a fully-cured,
waterproof state. This is the wrong product for pizza oven
installation."

Thanks heaps guys.

[Lburou]

No answer in five weeks

I'll take a stab at it. The obvious point is that your oven is outside and could get wet at some point. With the mortar described in your quotation, your mortar joints would go weak and the oven could crack or fall. I think the issue is to make sure the mortar you use won't be affected by moisture after you put it in your oven. The 'home brew' mortar is what I plan to use.