[davemac]

I am building a sunken fire pit out of natural stone. I know there are different types of mortar for different applications, does anyone know if there is a special mortar for high heat applications.

[Lanelle]

Yes there is a mortar as well as 'firebrick' to use in fireplace/pit applications. Your local brickyard should be able to help you.

[Stonehenge]

Refractory cement is what you're after, I believe.

[B.lock]

Be careful about refractory mortar in an outside application. Many refractory mortars will erode and are not intended for "outside use". Try Flue-Set by Alsey. It has water and heat resistance properties. We use it with fireplaces.