Can this be used to build a outside fireplace

Thank you for the question. Loctite PL 500 Landscape Block Adhesive is not a fireproof or fire-resistant product. It is not classified as heat resistant and would not be recommended for use in or around an outside fireplace where temperatures may exceed 150°F, for safety reasons. The product is not flammable once fully cured however the heat generated from the fireplace or fire pit can cause the adhesive in close proximity to melt and if exposed, burn. Mortar would be the preferred product that should be used in or on a fireplace / fire pit where heat and safety are concerned.