Hydrated Lime in Refractory Motar

From:  Traditional Oven

Refractory mortar mixing formulas

10 : 3 : 1.5 - Sand, Calcium Aluminate cement, Fireclay.

If you found hard locating refractory cement don't worry, here is one mixing formula with Portland cement plus lime available in ordinary building stores.

There are two Portland cement types, get the common gray in color, the other one is white decorative used for around pools etc. where a white effects are required.

Lime is calcium it takes over the cement in hot conditions (Portland will gradually burn out, it is used only for the mortar to set while working.)

10 : 6 : 2 : 3 – Sand, Fire Clay, Portland cement, and Lime.

First mix all dry ingredients well. Then in small amounts start adding clean water at room temperature while continuing mixing.

Mix into peanut butter consistency.

and...

Mix it in wheelbarrow or straight in a bucket. After mortar looks mixed well, leave it for couple of minutes and then remix one more time. It will have slight thicker consistency after 2 mins., you may have to add a SPOON of water and mix to a peanut butter consistency. Prepare smaller quantity at the time, e.g. for one dome arch. Amount on the picture is nearly for all 3 aches, for making complete rectangular dome ceiling. ... Hoy, Rado that stuff looks quite mushy in there, oi it is mushy is it not? ... Oy ye, ye mate it is mushy, it always is mushy, ye.

The key info is "Lime is calcium it takes over for the cement in hot conditions (Portland will gradually burn out, it is used only for the mortar to set while working.)"

Now I wonder if the strength of the Portland cement bond matters.   And how long is "gradually"      If the cement bonds remain in the outer, cooler, part of the shell then it should.  But no idea if that is true.  May vary from one furnace design to the next.

According to the text the lime bond withstands heat better than the Portland cement but does not give numbers.