Hello Hung , if you want to create a geopolymer you need only amorphous powders as binder raw materials, so you can't use calcium sulphate that is crystalline, but if you want to create an alkali activated product and you use fly ash ,low in calcium, you can use gypsum or anhydrite to improve hardening at room temperature. Geopolymer isn't at all an alkali activated material, but is quite different, geopolymer matrix is stronger than alkali activated, of course this comes from my experience of geologist mineralogist and product creator. If you want to do a geopolymer you need to set your recipe using correct raw materials (mineralogy is important not only chemical analisys) and using a correct liquid part. Good luck.