You can increase a mortar's resistance to heat and fire by adding fireclay to the mix. Fireclay, or fire clay, is a type of clay that can withstand extremely high temperatures without breaking down. Fireclay mortar traditionally has been made by adding fireclay to a regular cement mortar mix, often with the addition of lime. But many modern building codes require the use of calcium aluminate instead of cement to make "refractory" mortar, especially in the construction of residential fireplaces.
Add one part fireclay, one part Portland cement, one part lime and three parts sand into a bucket or wheelbarrow, depending on how much mortar you intend to mix. If you are making refractory mortar, use one part aluminum silicate instead of the Portland cement. When using calcium aluminate, make only small batches because the mortar sets quickly.
Check the instructions on the Portland cement or calcium aluminate to see how much water you should add to the mix.
Add about three-fourths of the specified amount of water to the mix, and stir it in thoroughly. Add more of the water while stirring. Stop adding water when the mix is about the consistency of peanut butter. If you are using calcium aluminate, be ready to begin work immediately, because you only have about 15 minutes before the mortar sets.