Nix the 'Tufa for bonding stonework. You simply need a good basic mortar mix that maybe has a high content of Latex modifier and is sealed well. Take a look at the Glossary on the page below for a definition of mortar. It is basically Portland cement, sand & water, usually with a little extra lime for stickiness and to lower the strength of the mix below that of the material being bonded. Any admixtures may be added to address specific concerns such as freeze/thaw or water penetration.

All My Best --- Tango

Here is a link that might be useful: Cement, Concrete & Hypertufa Glossary