S. J. Ford

Interpretation of the impedance spectroscopy of cement paste via computer modelling - Part III Microstructural analysis of frozen cement paste

The d.c. conductivity, σ, and low-frequency relative dielectric constant, k, of Portland cement paste were monitored, using impedance spectroscopy, during cooling from room temperature down to -50 °C. Dramatic decreases in the values of σ and k, as great as two orders of magnitude, occurred at the initial freezing point of the aqueous phase in the macropores and larger capillary pores. This result provides strong experimental support for the dielectric amplification mechanism, proposed in Part II of this series, to explain the high measured low-frequency relative dielectric constant of hydrating Portland cement paste. Only gradual changes in the electrical properties were observed below this sudden drop, as the temperature continued to decrease. The values of σ and k of frozen cement paste, at a constant temperature of -40 °C, were dominated by properties of calcium-silicate-hydrate (C-S-H) and so increased with the degree of hydration of the paste, indicating a C-S-H gel percolation threshold at a volume fraction of approximately 15%–20%, in good agreement with previous predictions. Good agreement was found between experimental results and digital-image-based model computations of σ at -40 °C. Freeze-thaw cycling caused a drop in the dielectric constant of paste in the unfrozen state, indicating that measurements of k could be useful for monitoring microstructural changes during freeze-thaw cycling and other processes that gradually damage parts of the cement paste microstructure.

Assignment of features in impedance spectra of the cement-paste/steel system

Abstract This paper presents a systematic study of the features seen in typical Nyquist plots (-imaginary vs. real impedance) for cement-paste/steel systems and discusses the assignment of each feature to its appropriate origin, e.g., bulk, contact, interface, product layer, etc. Assignments are made based upon as many considerations as possible—dc measurements, sample geometry, capacitance, local chemical modifications, alternative electroding schemes, etc. In addition to three distinct arcs from lowest (mHz) frequency to highest (MHz) frequency (due to product layer, interfacial reaction, and bulk, respectively), a fourth arc is sometimes observed between the bulk and interface arcs. When this occurs in paste-only systems, this arc is attributable to imperfect electrodes due to drying/shrinkage. In composite systems, e.g., cement with conductive chopped fibers added, this arc is clearly a “bulk” feature and an important indicator of microstructural inhomogeniety.

Experimental limitations in impedance spectroscopy: Part V. Apparatus contributions and corrections

Overlooked apparatus (cabling, leads, sample holder) contributions to experimental impedance spectra can be significant, especially at high frequencies, and can obscure the true sample response. Instrumental limitations are discussed and high-frequency artifacts arising from apparatus contributions are investigated as they pertain to accurate impedance measurements of materials systems. Remediation strategies are presented, including geometrical adjustments, lead shielding, and null-correction procedures.

Experimental limitations in impedance spectroscopy: Part I — simulation of reference electrode artifacts in three-point measurements

Impedance studies on test circuits demonstrated that three-point measurements are susceptible to voltage divider distortion/artifacts if the reference electrode impedance approaches the analyzer input impedance. These features can be avoided by using an instrument with a high input impedance or a reference electrode with a low input impedance. A correction procedure is also presented.

Dielectric amplification in cement pastes

Dielectric amplification (dielectric constants >80) is observed in cement pastes at early ages. Standard nonlinear least squares fitting routines yield artificially large “capacitances” when constant phase elements are employed. Instead, capacitance vs. frequency analysis provides reliable evidence of dielectric amplification. A physical model system consisting of a polycarbonate box with electrodes at each end, divided into two compartments by a polycarbonate barrier with a single hole, and filled with electrolyte solution, simulates the impedance response in young cement pastes. The barrier represents hydration products whereas the hole represents the constriction between two adjacent capillary pores, i.e., the pore network remains percolated. Dielectric amplification is inversely proportional to barrier thickness, i.e., it decreases as the barrier (product phase) thickens. Impedance spectra from real pastes vs. water/cement (w/c) ratio and during freezing or solvent exchange (to preferentially reduce the conductivity of the capillary pores) exhibit significant dielectric amplification, even after freezing or exchange, suggesting that C-S-H gel also has a dielectrically amplified microstructure. Advanced Cement Based Materials 1997, 5, 41–48.

Experimental limitations in impedance spectroscopy: Part VI. Four-point measurements of solid materials systems

Four-point impedance spectroscopy of solid materials systems is severely hampered by unavoidable voltage-divider effects associated with the reference electrodes. As demonstrated by test circuit studies and experiments with Pt/YSZ/Pt cells (with embedded silver reference electrodes), high impedance reference electrodes can produce distorted and erroneous impedance data. The relationships between these data and the sample properties (conductivity, dielectric constant) and instrument limitations (input impedance/capacitance) are derived. Successful four-point impedance measurements on conductive systems require large effective dielectric constants, which may be unattainable in bulk solids, but are often associated with internal interfaces.

Electrode configurations and impedance spectra of cement pastes

Electrode effects on impedance spectra of cement pastes were investigated by two-, three-, and four-point measurements without a potentiostat over the frequency range 0.01 Hz–10 MHz. Electrode immittance effects arising from highly resistive/capacitive contacts cannot be fully corrected by nulling procedures. Two-point measurements are much more susceptible to such effects than three- or four-point measurements. The three- and four-point results on pastes suggest that there is negligible high-frequency “offset” resistance, and that bulk paste arcs are not significantly depressed below the real axis in Nyquist plots. The important impedance-derived equivalent circuit parameters are bulk resistance and capacitance; offset resistance and arc depression angle may not be physically meaningful parameters. Whereas all electrode configurations give reliable values of bulk paste resistance, only the three-point configuration provides the total paste/electrode dual arc spectrum involving a single electrode. Multielectrode (three- or four-point) measurements may be necessary to establish the true bulk paste dielectric constant.

Experimental limitations in impedance spectroscopy of materials systems

Using resistor-capacitor networks, sources of experimental artifacts in impedance spectroscopy were investigated, such sources include machine limitations, rig/cabling contributions at high frequencies, and artifacts due to high impedance reference electrodes and their geometrical placement. In the instance of electrode placement, computer simulations with a pixel-based model were in agreement with the experimental observations. Remedies for these artifacts such as rig shielding/grounding, geometrical adjustments, and null corrections are also discussed.

Combined bulk and interfacial studies of the cement/steel system by impedance spectroscopy

Three arcs, corresponding to the bulk, interfacial, and electrode responses, are apparent in the impedance spectrum of cement paste / reinforcing steel systems. The 3-point impedance technique, without potentiostat, is capable of simultaneously and continuously monitoring these three responses over several orders of frequency (MHz-mHz). The high frequency (MHz) bulk arc provides such parameters as diffusivity and permeability of the paste, which in turn have implications for the ingress of corrosive species, such as chloride ions, known to greatly affect the corrosion rate of reinforcing steels. The double layer arc (Hz) provides insight into the nature of the near interfacial zone of the paste/ steel system, where the higher degree of porosity may affect corrosive behavior. The low frequency (mHz) electrode arc is related to the passive nature of the steel. Oxide film thickness can be inferred, as well as corrosion rates in active, passive, and pitting regimes.