Everbuild kos fire cement everbuild pckosflue3 kos flue cement is a high temperature to 1250c ready mixed fireproof mortar paste designed for the safe installationjointing of type a1 bsen1857 concrete flue liners previously known as class 1 for use with domestic fires and stoves eg 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 everbuild pckosflue3 kos flue