Bruce J. Christensen

Solubility behavior of Ca-, S-, Al-, and Si-bearing solid phases in Portland cement pore solutions as a function of hydration time

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement (OPC) and white Portland cement (WPC) pastes were measured during the first 28 days of hydration at room temperature. Saturation indices (SI) with respect to various solid phases known to occur in cement pastes were calculated from a thermodynamic analysis of the elemental concentrations, resulting in good agreement between the two pastes. In agreement with other published work, gypsum was saturated during the first several hours of hydration and then undersaturated thereafter, while portlandite was modestly supersaturated after the first few hours. High levels of supersaturation with respect to ettringite and calcium monosulfoaluminate were calculated, particularly prior to the consumption of gypsum at around 10 h. Results are consistent with published thermodynamic studies that show calcium monosulfoaluminate is metastable with respect to ettringite under normal hydration conditions. Three different ion activity product (IAP) equations for C-S-H were applied to the data. From 10 h onward, each of the IAP values declined gradually over time and the values for the OPC and WPC pastes were in close agreement. The same IAP equations were applied to experimental data from the pure CaO–SiO2–H2O system, resulting in good agreement between the cement paste pore solutions and the equilibrium between portlandite and the upper, or metastable, C-S-H solubility curve. D 2002 Elsevier Science Ltd. All rights reserved.

Comparison of measured and calculated permeabilities for hardened cement pastes

Abstract The experimental and calculated permeabilities of hardened cement pastes were compared. Experimental data for water permeability was obtained from the work of Nyame and Illston in 1980 on neat pastes with water-to-cement ratios ( w c ) between 0.23 and 1.0. Mercury intrusion porosimetry (MIP) and impedance spectroscopy (IS) measurements were performed on equivalently prepared specimens. Then the Katz-Thompson relation was used to calculate permeability. Calculated results track well with experimental data as a function of time, with the experimental value of permeability slightly higher at most times. The correlation between experimental and calculated permeabilities, at all times, are within 1.5 orders of magnitude. The largest differences occurred at late times for the samples with low w c ratio. This calculated permeability is quick, relatively simple and appears to give reasonable results when compared to conventional water intrusion methods.

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.

Interpretation of impedance spectroscopy of cement paste via computer modelling Part I Bulk conductivity and offset resistance

Computer simulation of impedance spectroscopy (IS) of hydrating cement paste, using a three-dimensional, four-phase model, is described. Two puzzling features of experimental IS results, the possible offset resistance in the Nyquist plot and the sharp decrease in normalized conductivity within the first 50 h of reaction, have been studied using the computer simulation model. Insight is provided into these features using the ability of the model to compare quantitatively microstructure and properties. It is concluded that the offset resistance is an experimental artefact, and does not directly relate to microstructure. The drop in conductivity during the first 50 h is shown to be a consequence of a gradual shift from parallel-dominated to series-dominated behaviour of the electrical conductivity, as microstructural modifications take place during hydration, causing the capillary pore structure to become more tortuous. This tortuousity can also explain the high-frequency impedance behaviour in terms of a two-arc response.

EFFECT OF HYDRATION TEMPERATURE ON THE SOLUBILITY BEHAVIOR OF CA-, S-, AL-, AND SI-BEARING SOLID PHASES IN PORTLAND CEMENT PASTES

The concentrations of Ca, S, Al, Si, Na, and K in the pore solutions of ordinary Portland cement and white Portland cement pastes were measured during the first 28 d of curing at temperatures ranging from 5–50 °C. Saturation indices with respect to solid phases known to form in cement paste were calculated from a thermodynamic analysis of the elemental concentrations. Calculated saturation levels in the two types of paste were similar. The solubility behavior of Portlandite and gypsum at all curing temperatures was in agreement with previously reported behavior near room temperature. Saturation levels of both ettringite and monosulfate decreased with increasing curing temperature. The saturation level of ettringite was greater than that of monosulfate at lower curing temperatures, but at higher temperatures there was effectively no difference. The solubility behavior of C-S-H gel was investigated by applying an appropriate ion activity product (IAP) to the data. The IAPCSH decreased gradually with hydration time, and at a given hydration time the IAPCSH was lower at higher curing temperatures.

Interpretation of the impedance spectroscopy of cement paste via computer modelling - Part II Dielectric response

Dielectric properties of cement pastes are measured using impedance spectroscopy, and the effective dielectric constants of the low frequency bulk arcs are reported. The unusually high values thereby obtained, and their dependence on reaction time and water:cement ratio, are explained by the presence of microstructural features that serve to amplify the dielectric constants of the individual material phases. The dielectric properties of three-dimensional cement paste models and of simple two-dimensional models of the hypothesized microstructural features are analysed. The model results provide insight into the proposed dielectric amplification mechanism in real cement paste.

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.

Microstructural analysis of young cement pastes using impedance spectroscopy during pore solution exchange

Abstract Impedance spectra were collected in situ during the exchange of the aqueous phase with an organic solvent in young (1–28 days old) ordinary portland cement (OPC) pastes at two water-to-cement ratios ( w/c = 0.35 and 0.70 ). The high initial values of dielectric constant (103−105) and the rapid decline upon exchange support a model of dielectric amplification where layers of calcium silicate hydrate (C-S-H) gel separate capillary pores. When the liquid in pastes of w/c = 0.35 was exchanged with isopropanol, the residual dielectric constants were typically between 102−103, indicating that C-S-H has a dielectrically amplified microstructure. After methanol exchange, however, the residual dielectric constant was much smaller, 10–100. This suggests there are two kinds of water within C-S-H gel in pastes of w/c = 0.35 —that which is exchangeable by both solvents (capillary pores) and that which is exchangeably by methanol but not isopropanol (gel pores). For pastes of w/c = 0.70 , differences due to solvent types were neglible. These observations suggest that the nature of C-S-H varies with w/c ratio.

Deterioration of the nitrogen BET surface area of dried cement paste with storage time

Abstract Gas sorption surface area measurements have been important for developing microstructural models for the structure of calcium-silicate-hydrate (C-S-H), the major hydration product of portland cement. However, surface area measurements using the nitrogen BET method have historically had high variability and have not always been reproducible. In this study, the variability of the nitrogen BET surface area of hardened cement paste (HCP) was investigated by varying the D-drying time and by storing the dried cement for various times before the BET measurement. The surface area of the samples was found to decrease with increasing storage time, and the surface area values became independent of the initial D-drying time after about 2 weeks. The storage time accounted for most of the observed variation in the data.

Early containment of high-alkaline solution simulating low-level radioactive waste in blended cement

Abstract Portland cement blended with fly ash and attapulgite clay was mixed with high-alkaline solution simulating low-level radioactive waste at a one-to-one weight ratio. The pastes were adiabatically and isothermally cured at various temperatures and analyzed for phase composition, total alkalinity, pore solution chemistry, and transport properties as measured by impedance spectroscopy. The total alkalinity is characterized by two main drops. The early one corresponds to a rapid removal of phosphorus, aluminum, sodium, and to a lesser extent potassium from the pore solution. The second drop from about 10 h to 3 days is mainly associated with the removal of aluminum, silicon, and sodium. Thereafter, the total alkalinity continues to decrease, but at a lower rate. All pastes display a rapid loss in fluidity that is attributed to an early precipitation of hydrated products. Hemicarbonate appears as early as 1 h after mixing and is probably followed by apatite precipitation. The hemicarbonate is unstable, however, and decomposes at a rate that is inversely related to the curing temperature. At high temperatures, a sodalite-type zeolite appears at about 10 h after mixing. At 30 days the stabilized crystalline composition includes zeolite, apatite and other minor amounts of CaCO 3 , quartz, and monosulfate. The impedance behavior correlates with the pore solution chemistry and X-ray diffraction data. The normalized conductivity of the pastes displays an early drop followed by a large decrease from about 12 h to 3 days. At 3 days the permeability of the cement-based waste as calculated by the Katz-Thompson equation is over three orders of magnitude lower than that of Ordinary Portland cement paste. A further decrease in the calculated permeability is not apparent. This particular cement-based system provides rapid stabilization/solidification of the waste material. The transport of waste species is reduced by probable incorporation into apatite, zeolite, and other solid phases.