A BaCO3 slurry, containing radioactive C-14, is produced during the reprocessing of spent nuclear fuel. This slurry is encapsulated in a Portland-blastfurnace slag composite cement. The effect of BaCO3 on the hydration of OPC and Portland-blastfurnace slag cements has been studied in this work. Samples containing a simulant BaCO3 slurry were cured for up to 720 days at 20 and 60 degrees C and analysed by XRD. SEM(EDX) and ICC. BaCO3 reacted with OPC to precipitate BaSO4 from a reaction between soluble sulfate and BaCO3. Calcium monocarboaluminate subsequently formed from the carbonate released. The monocarboaluminate precipitated as crystals in voids formed during hydration. At 60 degrees C in OPC, it was not identified by XRD, suggesting the phase is unstable in this system around this temperature. In the Portland-blastfurnace slag cements containing BaCO3, less monocarboaluminate and BaSO4 were formed, but the hydration of BFS was promoted and monocarboaluminate was stable up to 60 degrees C. (C) 2010 Elsevier Ltd. All rights reserved.