The replacement of phosphogypsum for natural gypsum significantly influenced the early hydration behaviour of OPC. The impurities present in phosphogypsum strongly retarded the hydration of OPC. Addition of set accelerators — calcium formate, calcium nitrate and sodium molybdate — improved the hydration characteristics of OPC containing phosphogypsum. Calcium nitrate was found to be the most effective set accelerator, whereas calcium formate and sodium molybdate showed a moderate acceleration. There was no direct correlation between set acceleration and strength development of the cement pastes. The cement samples with calcium nitrate gave very low strength development at early ages, but very high strength values were obtained with calcium formate at 90 days. © 1993, Thomas Telford Ltd. All rights reserved.