What is the correct mortar to use when joining the concrete slabs to support the oven base? I have a 12 pound tub of HWI Sairset. But my chimney kit also came with four 10-ounce tubes of DAP Hi-Temp black mortar. I assume I ought not use plain mortar. Shall I save the DAP product for joining the casted oven parts and use the Sairset for joining the slabs? Are the products interchangeable?
In this case, plain mortar is what’s called for. The slabs do not take any heat, and the concrete block wall underneath is a level surface. You are using the mortar primarily to seal gaps and as a little “peace of mind” insurance—the design is such that the three slabs would stay up there by themselves if you weren’t adding anything on top of them.
In practice, you really do want something that will “glue” them together. But there is no need for any of the pricier refractory materials you mention. Standard mortar is cheap, and it does the best job for that area.