Effect of BaCO3 on C3A hydration

Ba ions are known to immobilise sulfates by forming BaSO4. The use of BaCO3 as a full or partial substitute for gypsum to regulate C3A (3CaO·Al2O3) hydration was consequently studied with a view to establishing its correct dosage in sulfate-resistant cements presently under development. The hydration rate of synthetic C3A was determined in the presence of varying percentages of gypsum, BaCO3, and gypsum + BaCO3 by running conduction calorimetry analyses on early age (up to 20 h) samples. The hydration products were subsequently identified with XRD, FTIR and DTA/TG. The addition of (20–42 wt.%) BaCO3 to C3A neither regulated the speedy reaction of the latter with water nor reacted with the aluminate. Gypsum + BaCO3 blends proved able to regulate C3A hydration; the heat flow curves for the mixes studied exhibited an induction period, an indication that gypsum acted as a C3A hydration regulator whilst at the same time reacting with BaCO3 to form barite.