Welcome to Permies, Mark!
Concrete is not generally thought of as "fireproof" because the intense heat of a fire can cause chemical reactions to take place in the concrete that weaken it. That said, there are refractory mortars that are useful for bonding chimney bricks together and they can be considered "fireproof". Generally they have a higher clay content and are made with more
water than regular mortars so that they are runnier and make for a thinner bond.
Here is a link with recipes for refractory mortar.