Water reducers became so important in concrete, that they might be thought of the "fifth" ingredient. Admixtures that contains microsilica (condensed silica fume) are used to satisfy superior strength and small permeability demands. A change from ASTM C494 Kind A to Kind D water lowering and set retarding combine might https://www.tjcyindustrialchem.com/news/cement-additives-and-concrete-admixtures-key-differences.html