The influence of hydration on the microstructure development of cement paste with bleeding: An NMR study

Bleeding can be described as water coming out from fresh cement paste, which not only affects the homogeneity at fresh stage but also decides the hardened state structure. Previous studies have focused on the determination or simulation of the bleeding extent of fresh cement paste and give objective conclusions. However, there are rare researches about the influence of bleeding on the final structure of cement paste. In this study we have monitored the cement paste with various bleeding caused by water cement ratio and slag replacement. The microstructure development of cement paste from top to bottom was measured by using lowfield NMR with Hahn spin sequence from 6 h to 72 h. Results showed that within 4.4 mm bleeding thickness the bleeding will decrease the slope of the void ratio as a function of distance which suggests a homogeneous structure is formed. However, when the bleeding water thickness is higher than 4.4 mm, hydration would lead to a heterogeneous structure. Detailed discussions on the connection between the bleeding and the final structure are also presented in this paper.