Mr ILA, pl. send your complete clinker analysis that how will get 2.5 % so3 in cl. because if you have so3 in cl.2.5 % than c3s in cl less than 46% as my experience. than rate of hydration reaction will adjust by addition alkali material. However fused cl with so3 slow retard rate of reaction in zel formation in comp. to natural gypsum. I think you will replace gypsum 75% if your data is correct.