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SUMMARYSubjected to the wet surrounding, hydrated cement paste (HCP) exposed to high temperature may exhibit rehydration behavior. This paper presents the influence of the dehydration temperature and the initial water/cement ratio on the rehydration activity of dehydrated cement paste (DCP). Original HCPs were prepared with two water/cement ratios of 0.3 and 0.5, respectively, and cured under the fog-spraying standard condition for 30 days. The DCP powders used were obtained by grinding dry HCP less than 75 m and then subjecting to different temperatures, up to 900 • C. The rehydration properties of DCP were evaluated by the required water for standard consistency, the setting time, the rehydrated compressive strength and the microstructure evolution. X-ray diffraction (XRD) was employed to identify the crystalline phases before and after rehydration. Experimental results showed that the coupled rehydration effect from the dehydrated hydration products and the initially unhydrated cement determined the rehydration behavior of DCP. The rehydration of DCP strongly depended on the dehydration temperature and the water/cement ratio of the original HCP.