Calcium looping is an emerging CO2 capture technology in which CO2 is captured at high temperature using a calcium-based sorbent, which however suffers from a well-known problem of loss-in-capacity after a few cycles. Water hydration is a potential method that can be applied to reactivate the sorbent on-site. The present work confirmed the effectiveness of periodic water hydration on the reactivation of a limestone and a synthetic CaO/cement sorbent. Furthermore, it was found that the water hydration of calcined limestone was independent of the factors, such as particle size, hydration duration, hydration temperature, and pre-calcination temperature. Also the conversion of hydrated limestone cannot be further enhanced by ultrasonic hydration. In contrast, the synthetic CaO/cement sorbent showed strong dependence on those factors. In addition, the CaO conversion of the synthetic CaO/cement sorbent can be recovered to more than 80% by ultrasonic hydration. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.