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With the proper selection of electrical properties for the microstructural phases, the impedance spectrum of cement paste can be quantitatively modeled. Results from the model suggest that the changes in the impedance spectra that accompany hydration are due to changing amounts, distribution, and arrangements of phases. The main features of the impedance spectrum of cement paste result from a composite response of microstructural phases and are not the result of interactions or interfaces between these phases.
The decrease in normalized conductivity that accompanies the hydration reaction can be explained by an increase in the tortuosity of the capillary pore structure without requiring a premature disconnection, or de-percolation, of porosity. The tortuosity of the capillary pore structure, intertwined with the conducting C-S-H phase, can also give rise to two-arc behavior. The model indicates the existence of a high frequency arc due to the response of pore fluid, which had been interpreted previously as an offset resistance [4,5,6,7,8,9,10,11,12]. The high frequency behavior of the impedance spectrum is better explained by the existence of a second arc, rather than a series arrangement of the conducting phases giving rise to an offset resistance with physical meaning.
Fitting the model to experimental results by varying the electrical parameters of the various phases leads to the conclusion that C-S-H has a relative dielectric constant of the order of 500, and a conductivity approximately 100 times smaller than pore fluid. This conductivity is obviously expected to scale with pore fluid conductivity, as C-S-H probably derives its conductivity from having its nanometer-scale pores filled with conductive pore solution.
Finally, the use of microstructure models, accompanied by accurate computations of properties like impedance spectra, allows the researcher to go beyond simple series and parallel ideas, enabling more quantitative and detailed understanding of microstructure-electrical property relationships to be achieved.