Introduction
Bleeding of a fresh cement-based material is a common phenomenon, i.e., water comes out during the initial stage which can be observed as free water on top of the materials. This water is often referred to as bleeding of the cement. In figure 1 the various stage during the bleeding are given schematically.



Fig. 1. Schematic representation of the various stages during bleeding in fresh cement paste: 1) the initial mixing of cement paste resulting in a homogeneous suspension 2) gravitational sedimentation of particles with a small amount of bleeding resulting in a porosity gradient 3) self-weight consolidation forces the water drained out, from the compressible solid skeleton 4) hydration stage where the hydration of cement paste will become dominant


    Results
In this study, we have used NMR to measure fresh cement paste during the bleeding process where we not only focus on the bleeding water, but also on the microstructure development. In this we have compared the measurements as performed by NMR to self-weight consolidation models. As an example the simulated void ration is given in figure 2.


 Fig. 2. A simulation of the void ratio for the finite train model as a function of time in material coordinates

Whereas in the model the void ratio is given as function of the position, with the NMR we have performed measurements of the void ration at 2 fixed positions. In the NMR measurements we have also added a webcam as to correlate the NMR measurements to the amount of water on top of the sample as measured by the webcam. As an example in figure 3 the results are given for OPC-cement. As can be seen the void development over time for the two positions  is different. We have fitted both a small-strain and finite-strain consolidation models for void ratio development.

 


Fig 3. The experimental results in natural coordinates from the NMR measurements of the bleeding of fresh cement paste prepared with Portland cement at a water-to-cement ratio of 0.65: (a) the normalized void ratio at Z=0.22 and 0.75 from the top of the sample. (b) The bleeding depth as determined by the webcam and simulation models. The bleeding depth is expressed as the normalized value which has been divided by the experimental final bleeding depth value  The solid and
dashed lines represent a fit of the small and finite strain model to the data.

As can be seen both models describe the data well. In figure 3 we have also given the amount of bleeding water as function of time, and as can be seen these can also be describe by both models. Moreover we have performed experiments where we have changed the W/C ratio, looked at the influence of slag replacement and water reducer.


    Conclusions
It was found that the NMR data can provide direct insights into the mechanism of the consolidation behaviour of cements pastes. It is seen that as the bleeding water comes out the void ratio at the top almost remains constant, and the void ratio at the bottom gradually decrease until it reaches a constant value. The results suggest that the finite-strain model is more suitable to predict that structure development during a consolidation process


     

Yanliang Ji, Leo Pel, Zhenping Sun, The microstructure development during bleeding of cement paste: An NMR study, Cem. Con. Res. 125, 105866 (2019)

 

Ji. Y., Sun. Z., Yang J., Pel L., Jarmouzi Raja, A., Ge H., NMR study on bleeding properties of the fresh cement pastes mixed with polycarboxylate (PCE) superplasticizers, Construction & Building Materials 240, 117938 (2020)