I'm building a new fireplace with a 2cm granite surround (around the firebox opening). This leaves (by design) a 1/2" gap between the back of the granite and the face of the firebox & smoke dome (gap is several inches wide x 1/2" thick).
My question is: To seal off this gap (to avoid a "false chimney" fire hazard), what type of grout should I use? Will the same standard thinset that I use to attach granite-to-fireplace work, or do I need some type of special heat-resistant thinset or mortar?
Or, instead, or in conjunction with, is there a caulk that is suitable for high temperatures that would work in this application?