This from Laticrete technician: .inWe have no issue with using our thin set mortar in front of fireplaces, I would probably suggest Stonewall due to its non-sag tendencies. Please keep in mind that the maximum temperature we recommend for mortar and grout is 140 degrees. Temperatures above that you may have to look at a fireplace mortar. .in Thanks, Dan Barbian C&C Phone Technician
Laticrete suggests either their PM (Polymer Modified) Thin-set or the Big Tile And Stone Thin-set. Unfortunately we do not carry these two products but if you go to Laticrete.com then .inWhere to buy, .in you should be able to find these products.