Its a baffle over an old fireplace where a gas heater has been installed. A cement based repair was hopeless, builders glue lasted a while but eventually busted when heater used on high for lengthy period. Crack getting bigger so need to get it fixed before a re-installation becomes necessary. Thanks.
go to mitre 10 or place makers and ask an assistant to show you a heat resistant cement that will glue cement board but in saying that gorilla glue is fairlie versatile however companies have spent years discovering the best adhesive for every purpose and I'm conident you will find just the right stuff
Tile thinset. Sanded thinset should be heat resistant when dry, as long as its still a crack and not a break.
you could use a fire cement, that they use to seal coal and wood stoves,it comes already mixed,so you just spread it and level it,and it dries in no time at all,and it is designed for high temperatures
