DAP? High heat mortar is a non-combustible silicate cement used for patching chipped firebrick in fireplaces and sealing wood-burning stoves. High heat mortar adheres firmly to the firebrick when wet or dry. High heat mortar can also be used for applications in wood frame construction where an ASTM E136 non-combustible sealant is required, such as sealing around vents, pipes, ducts and chimneys at ceiling and floor level prior to the application of drywall products.