Magnesium oxide - search extremegreen or dragon board… might be a few other mfg too. you can get away w/ 1/2" as well vs 5/8" b/c its much more durable than gypsum. only down side is purest MGO is usually mined in china.
ps - ignore the stuff you read about Denmark(incident w/ Chinese mgo), just make sure your mgo is from a reputable mfg. good luck