This paper presents a study, including experimental and mechanism analysis, on investigating the effect of agglomeration of a recycled cement additive on the hydration and microstructure development of cement based materials. The recycled additive is firstly produced form waste hardened cement paste subjected to high temperature environment, namely original dehydrated cement paste (DCP). Then, to reduce the amount of agglomerated particles, the original DCP is dispersed by the method proposed in this study. The experimental results show that due to the agglomeration of the DCP particles, much free water can be trapped into the flaky layers, which cause that the cementitious system with original DCP need more water to maintain the normal consistency. Additionally, due to the nucleation effect of small DCP particles, the hydration and microstructure development of cement can be effectively promoted, which directly cause the enhancement of the mechanical properties of the cementitious system. However, this promotive effect of the fine particles in DCP could be obviously reduced by its particle agglomeration. (C) 2013 Elsevier Ltd. All rights reserved.