What product should I use to bond the surface of the firebox of my fireplace. A few of the bricks are loose. I plan to put a gas log set in it.

Do not use QUIKRETE® Zip and Mix – FastSet Repair Mortar, which is a high strength product. If the mortar used for repairs is too strong, there is potential to damage the existing mortar and bricks. QUIKRETE® Zip and Mix – FastSet Repair Mortar should also not be used on the surface of the firebox where the repair area will be exposed to temperatures over 200 °F. QUIKRETE® Zip and Mix – FastSet Repair Mortar contains Portland cement, which can degrade in temperatures as low as 200 °F. Products that will directly be in contact with the high heat, should be made of refractory cement, rather than the Portland cement used in QUIKRETE® Zip and Mix – FastSet Repair Mortar. Refractory cement is designed to be able to withstand exposure to high temperatures.