7Ba ions are known to immobilise sulfates by forming BaSO 4 . The use of witherite 8 (BaCO 3 ) as a full or partial substitute for gypsum to regulate C 3 A (3CaO•Al 2 O 3 ) 9 hydration was consequently studied with a view to establishing its correct dosage in 10 sulfate-resistant cements presently under development. 11 12The hydration rate of synthetic C 3 A was determined in the presence of varying 13 percentages of gypsum, witherite, and gypsum+witherite by running conduction 14 calorimetry analyses on early age (20-h) samples. The hydration products were 15 subsequently identified with XRD, FTIR and DTA/TG. 16 17The addition of (20-42wt%) witherite to C 3 A neither regulated the speedy reaction of 18 the latter with water nor reacted with the aluminate. Gypsum+witherite blends proved 19 able to regulate C 3 A hydration. The heat flow curves for all the samples studied 20 exhibited an induction period, an indication that gypsum acted as a C 3 A hydration 21 regulator while at the same time reacting with witherite to form barite. 22 23