Next: Conclusions Up: Main Previous: Electrical properties of
A possible application of being able to measure the dielectric properties of cement paste is demonstrated in Fig. 13. Figure 13 shows the relative dielectric constant, measured at room temperature, of a white cement paste of w/c ratio 0.4, hydrated for 96 hours, that was subjected to 5 freeze-thaw cycles. The lowest temperature reached in each cycle was decreased on each consecutive cycle. The room temperature dielectric constant tended to drop after each cycle. This is possibly due to the expansive forces created by the freezing pore solution, which may have damaged the C-S-H phase, and thereby reduced the relative dielectric constant of the paste by changing the dielectric properties of the C-S-H phase. Note that in Fig. 13 the value of k drops below the nominal value for pure C-S-H, 103 , as the low temperature end of the freezing cycle is decreased. After a few cycles, however, the dielectric constant of the frozen paste did not continue to decrease with cycling.
Figure 13: Relative dielectric constant vs. end-point temperature for a 96 hour 0.4 w/c white cement paste. The number of freeze-thaw cycles undergone is shown in parentheses for each data point. Each successive cycle reached a lower end-point temperature, which is the abscissa of the graph. Measurements were performed at room temperature when the sample reached equilibrium.
The results shown in Fig. 13 imply that dielectric constant measurements may be sensitive to changes in C-S-H microstructure, during freeze-thaw cycling and during other processes. This can only be true because of the sensitive dependence of the value of k on the microstructure, coming from the dielectric amplification mechanism that has been demonstrated in Part II of this series and further supported by the work of this paper.