Although expensive per pound, I've found Heatstop brand refractory mortar to perform the best. This is not an endorsement per se, but my preference. The powder comes bagged and ready to mix with water. It contains quantities and materials that one would need to mix their own mortar but the product is consistent.
|pre-cut bricks numbered for position in dome|
This speeds up the process of building and produces very even mortar joints. The stickiness of the mortar allows me to build the dome without forms and clean the interior joints as I build.
Following constructing the entire dome, I coat the exterior with either insulating refractory concrete, or if more mass is needed, add thickness to the dome by mixing grog (ground firebrick) and powdered Heatstop. The grog acts as the aggregate and controls shrinkage. If there are large gaps behind the dome bricks the grog mix will fill them with mortar comparable to the bricks.