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Conclusions

The dielectric response of hydrating cement paste is dependent on the developing microstructure and the arrangement of microstructural phases. Impedance simulations from digital image-based models have successfully reproduced the main features of the dielectric response of cement paste, and provide valuable insight into the relationship between microstructure and measured impedance. The current distributions and effective conductivities obtained through the digital image technique are excellent examples of how the data obtainable from computer modeling can be used in understanding observed phenomena.

The extremely high effective dielectric constants determined from the bulk arcs of cement paste may be explained by an amplification mechanism produced by microstructural arrangements in hydrating cement paste. Amplification could result from relatively large capillary pores, D and thin product layers, d, that block or re-direct current flow in the impedance experiment. It is thus possible to produce dielectric amplification without disconnecting, or de-percolating, the conductive, capillary pore phase. In fact, higher effective dielectric constants may be obtained from microstructures in which the conductive phase remains connected. This is consistent with what is known about the continued presence of continuous capillary pores in cement pastes that have achieved high degrees of hydration [23].

Moderate amplification of the effective dielectric constant is found in the three dimensional, digital image-based model of cement paste, probably due to the finite resolution (1 pixel = 1 µm) of the pixel-based model. Higher values of keff would be obtained if the resolution were to be increased. The I and T models suggest that the capillary pore structure is highly tortuous, and, at early reaction times, thin product layers extend into capillary pores and connect reacting cement grains.

The drop in keff with degree of hydration can be explained by the changing microstructural features that control dielectric amplification. If D is taken as a measure of the size of the pore fluid-filled capillary pores, and d is taken as a measure of the size of the reaction product thicknesses separating or extending into these pores, then as the reaction products grow, capillary pore sizes are reduced (D decreases), and reaction products extending into the pore space thicken (d increases). Both of these effects serve to decrease amplification. The increase in effective dielectric constant with water:cement ratio, at equal degrees of hydration, is a result of larger capillary pores being present in the microstructure at the higher water:cement ratios, but with nearly the same value of product thickness, d. Thus the amplification ratio, D/d, will be increased as water:cement ratio increases at roughly equal degrees of hydration.


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