Kos flue cement is a high temperature (to 1250âºc) ready mixed fireproof mortar paste, designed for the safe installation/jointing of type a 1 (bsen 1857) concrete flue liners (previously known as class 1) for use with domestic fires & stoves e.g. coal, wood burning or gas (including flame effect). kos flue cement may also be used as a general purpose fire cement in most applications where direct flame contact occurs.