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C-S-H percolation

As was discussed above, the percolation of the combined cement:C-S-H phase is what determines the set point, or the rigidity threshold. The model described above also, however, predicts a percolation threshold for the C-S-H phase by itself. The prediction is that the C-S-H phase, independent of water:cement ratio, will percolate when its volume fraction is about 20% [19]. This will be fairly early in the hydration process, but well after the set point. Since this percolation threshold is not usually connected to a physical observable at this stage, it is difficult to experimentally validate this prediction. Recently, however, electrical measurements at low temperature (-40^oC) have been able to confirm this prediction. Both the capillary pores and the C-S-H pores serve as conducting channels, since they are filled with the electrolytic pore solution [27]. At lower temperatures, the water in the larger capillary pores freezes while that in the smaller C-S-H pores does not. In this case, the electrical conductivity of the capillary pores is turned off, and the only conductivity comes from the C-S-H pores. If the C-S-H phase is not percolated, then the conductivity should be very small. Figure 7 shows the conductivity at a temperature of -40^oC, normalized by the pure C-S-H conductivity and plotted vs. volume fraction of C-S-H. There is a clear percolation threshold at around 20% volume fraction, validating the predictions of the model [28]. There is also excellent agreement between model and experimental values for all the volume fractions of C-S-H examined. The simple fact that the conductivity goes up with hydration shows that at this temperature, the conductivity is coupled to the growing C-S-H phase, not the shrinking capillary porosity phase. For more on this topic

Figure 7: Showing the electrical conductivity of the cement paste at −40ºC, plotted vs. the volume fraction of C-S-H. The solid symbols are model results, and the open symbols represent experimental values.