You can mix any gypsum based product togetherin different ratios just get different results(magic mixes) play have fun lol.... wouldnt advise adding cement in tho to accelerate especially in base coats causes top coat to blister if done same day. And in top coats when dry makes it very soft and powdery.I allways keep a water drum of dirty finish water to use as an accelerator for both base coats and top coats(for the small jobs) time is money