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Linnéstrasse 5, D-04103 Leipzig, Germany and IdM an der. Universität Ulm, D-89069 Ulm, Germany. The behavior of nonwatery solvent phases in hydratin...
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Environ. Sci. Technol. 2004, 38, 880-885

Direct Investigation of the Fate of NAPL Contaminations in a Hydrating Cement Matrix by Means of Magnetic Resonance Techniques N I K O L A U S N E S T L E , * ,†,‡ PETRIK GALVOSAS,‡ CHRISTIAN ZIMMERMANN,§ FRANK STALLMACH,‡ AND JO ¨ R G K A¨ R G E R ‡ Universita¨t Leipzig, Abteilung Grenzfla¨chenphysik, Linne´strasse 5, D-04103 Leipzig, Germany and IdM an der Universita¨t Ulm, D-89069 Ulm, Germany

The behavior of nonwatery solvent phases in hydrating cement pastes is of great interest in the context of solidification of wastes containing such phases. In a recent study, the influence of various solvents on the hydration kinetics of cement was studied. In this paper, we present results on the changes in the behavior of the solvent phases themselves during setting of the cement pastes. The methods used in the studies were NMR relaxometry and pulsed field gradient (PFG) NMR diffusometry. To study selectively the behavior of the non-aqueous-phase liquid (NAPL) phases, heavy water was used in the preparation of the cement pastes. The experimental results are in good agreement with the observations from earlier studies concerning the behavior of toluene in hydrating cement. For aliphatic solvents (cyclooctane, n-hexanol), indications for surprisingly large networks of connected droplets in the cement matrices are found.

Introduction Solidification by means of cementitious binders is an important option in the disposal of hazardous wastes (1, 2). The possibility of non-aqueous-phase liquids (NAPL) present as co-contaminants in wastes to be solidified raises the question on the fate of these substances during hydration of the cement binder and the implications of such findings for leaching behavior of the NAPL phase from the solidified waste. In a recent contribution to this journal (3), we reported investigations on the influence of NAPL contaminants on the hydration kinetics of cement pastes. In these experiments, it was found that nonpolar liquids exhibit a relatively small influence on the hydration kinetics, while polar liquids such as n-hexanol or cyclohexanone lead to a very pronounced delay in the hydration process which was found to be much stronger than the delays known from amphiphilic superplasticizers. Furthermore, experimental indications were found that aromatic solvents are distributed in a much finer network in the cement structure than both nonpolar aliphatic and the polar aliphatic liquids. The fine distribution of toluene * Corresponding author phone: ++49 6151 16 2934; fax: ++ 49 6151 16 2833; e-mail: [email protected]. † Present address: TU Darmstadt, Institut fu ¨ r Festko¨rperphysik, Hochschulstrasse 6, D-64289 Darmstadt, Germany. ‡ Universita ¨ t Leipzig. § IdM an der Universita ¨ t Ulm. 880

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in the binder matrix is in accordance with experimental findings from synchrotron tomography, where no indications for homogeneous solvent-filled liquid pockets were found on the micrometer-scale (4). While the direct spatial resolution achievable in tomographic techniques available today is still limited to several hundred nanometers, NMR relaxation and PFG NMR diffusometry allow the indirect observation of structural features much smaller than this by means of their influence on the molecular dynamics of the liquid phases. Changes of the nuclear spin relaxation behavior and the diffusion behavior of liquids confined to small pores have been successfully studied for a wide range of model systems and real samples during the last 20 years (5-7). The water dynamics in hydrating cementitious materials with and without additives received broad attention in this context (8-12). In addition to studies of relaxation and diffusion processes, also results from specialized NMR imaging techniques have been reported for hydrating cements (1316). Here, we apply NMR relaxometry and PFG diffusometry to study the dynamics of the solvent phases in hydrating cement matrices.

Materials and Methods To be compatible with previous studies on changes in the behavior of the liquid phases in hydrating cement matrices and in order to have as long relaxation times available for the PFG experiments as possible, the experiments were carried out on commercial white cement (Dyckerhoff weiss CEM I 42.5 R obtained from Dyckerhoff, Wiesbaden). The production and handling of the cement paste samples were described in an earlier paper (3). In contrast to the pastes in that paper, which were all produced using ordinary water, some of the pastes in this study were produced with heavy water (Uvasol 99.8% obtained from Merck, Darmstadt) in order to suppress the NMR signal of the water protons as far as possible. All cement pastes were produced at a water/ cement ratio of 0.33 mL/g. The solvents used in the experiments were analytical grade and obtained from SigmaAldrich subsidiaries. NMR relaxometry was performed on an MRS 6 NMR relaxometer (JSI, Ljubljana, Slovenia) using a simple spinecho technique. Magnetization decay curves were sampled with 40 different echo times between 150 µs and 14 ms, and for each echo time four scans were accumulated. After acquiring a full magnetization decay curve, the data were stored on the computer harddisk and the next magnetization decay curve measurement was started automatically. Data evaluation strategies were described in refs 3 and 17. The PFG NMR experiments without spectroscopic information were performed on the FEGRIS 400 NT spectrometer facility of the University of Leipzig (18). The sample handling techniques used in these experiments were the same as those described in ref 19. As earlier experiments showed a considerable influence of inner magnetic field gradients on PFG NMR experiments in hydrating cement pastes (19, 20), a gradient-compensating five-pulse PFG pulse scheme with alternating gradient directions (21) was used in the diffusometry experiments. The diffusive echo attenuation as a function of the strength of the applied gradient pulse G is given as (21)

Ψ(G) )

S(G) 2 ) exp - γ2δ2G2D 4τ2 + 6τ - δ 3 S(0)

(

(

) exp(- F(G)D) 10.1021/es034444h CCC: $27.50

)) (1)

 2004 American Chemical Society Published on Web 12/03/2003

FIGURE 1. Hydration-time dependence of proton transverse relaxation rates in three white cement pastes with water/cement ratio 0.33. Two pastes, one with heavy water and one with light water, were prepared with added toluene in a quantity of 0.017 mL/g of cement. The third paste contained no toluene and was prepared with heavy water. The observed proton signals arise mainly from H2O, except for the sample with toluene and heavy water, where the toluene signal dominates. The delay in the hydration kinetics of the deuterated sample is mainly due to an isotope effect. with δ denoting the duration of the gradient pulses, τ the time between the 90° pulse and the 180° pulse, τ2 the time between the second and the third 90° pulse, γ the gyromagnetic ratio, G the gradient strength, and S the echo signal amplitude. The self-diffusion coefficients for the pure solvents were measured by means of a standard stimulated echo PFG sequence (22). A further PFG NMR experiment with spectroscopic resolution was carried out on another newly constructed PFG NMR facility in Leipzig which also allows spectrally resolved diffusometry experiments with comparably strong magnetic field gradients as those available in the FEGRIS NT (23, 24).

Results and Discussion In Figure 1 a comparison of the proton transverse relaxation rates for the pore liquid in a toluene-doped cement paste prepared with ordinary water (signal strongly dominated by the water), for toluene in a toluene-containing paste produced with deuterated water (majority of the signal originating from toluene), and for residual water in a paste produced only from the commercial white cement and heavy water (similar background light water signal in all experiments on pastes prepared with heavy water) is given. The amplidude of the signal from the residual water was found to be about 2040% of that from the toluene signal. The most obvious feature of the different data sets in Figure 1 is the strong delay in the hydration kinetics of the deuterated pastes compared to the paste prepared with light water. This difference is due to an isotope effect that has been observed by several groups studying the kinetics of cement hydration (8, 25) but is still not fully understood. This delay prohibits a direct comparison of the relaxation times for the samples prepared with heavy water and the ones prepared from ordinary water at a given hydration time. Comparing the time course of the relaxation rates for the two deuterated samples, we found a very minor delay in hydration in the presence of toluene, as was the case for the protonated pastes (3). Furthermore, it should be mentioned that the strong decrease in the relaxation times for the toluene is quite uncommon for nonpolar liquids in a mineral matrix. Normally, the relaxation times of those liquids tend to be much longer than for polar liquids (26). In Figure 2 a comparison of the average relaxation rates obtained for various types of solvents is given. The average relaxation rates were determined by monoexponential analysis of the initial part of the magnetization decay curves (according to a procedure described earlier (3, 17)). Obviously, toluene experiences a much stronger reduction of the

FIGURE 2. Transverse relaxation rates for deuterated white cement samples with a water/cement ratio of 0.33 and various added solvents at 0.017 mL/g of cement. relaxation time during the first few days of hydration than the other solvents used in the study. This suggests confinement of the liquid into a network of much finer pores than in the case of the other solvents, such as cyclooctane, that like toluene exerts only a minor influence on the overall hydration kinetics of the cement (3). This is in good accordance both with the observations in our previous experiments on solvent-doped cement pastes prepared with light water (3) and with the findings of a recent synchrotron tomography study of toluene in hydrating cement (4). Another obvious feature is the much slower increase of the hexanol relaxation rates compared to the nonpolar solvents. Again, a slower change in the n-hexanol-filled pore structure is consistent with the findings in experiments on the hydration kinetics where a very pronounced slowing down of the hydration process was found for pastes with added n-hexanol or other polar liquids. While the recorded relaxation times for n-hexanol and toluene are still undergoing some change after 80 h of observation, the measured relaxation times for cyclooctane seem to reach a plateau value after about 60 h. The development of the relaxation rate as a function of the hydration time could in all cases be fitted with an Avramitype function (27, 28) (lines in Figure 2)

1 1 + Ao(1 - exp(-kthydm) ) T2(thyd) T2ini

(2)

where T2ini, k, m, and Ao were used as fitting parameters. For the exponent m, a value of about 1.5 was found in the case of hexanol. For the other two solvents, the exponents were close to 2. Table S1 in the Supporting Information gives an overview over the complete fitting results on the three curves. As already mentioned above, the data in Figure 2 were obtained by means of a monoexponential analysis of the initial magnetization decays. While this analysis is numerically robust and simple, it fails to account for possible deviations from monoexponential relaxation behavior at longer echo times. As can be seen from the data in Figure 3 (magnetization decay curves after 51 h of hydration), there are pronounced deviations from monoexponentiality in the samples with cyclooctane and n-hexanol at later hydration times. By contrast, the magnetization curves at hydration times up to about 15 h were found to exhibit no notable nonexponentiality for all solvents. A more quantitative analysis of the nonexponentiality was achieved by fitting a stretched exponential model (9, 29)

( (T˜t ) )

M(t) ) A exp -

β

(3)

to the magnetization decay curves. In Figure 4 the exponent β obtained in the fit is plotted as a function of the hydration time for the different samples. As expected on the basis of the shape of the curves given in Figure 3, the exponent β for VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. Echo decay curves for different solvents in deuterated cement pastes of w/ c 0.33 after 51 h of hydration.

FIGURE 4. Exponents obtained in stretched exponential evaluations of the magnetization decay curves for the different solvents under study. Correlation coefficients in all fits were better than 0.993. the toluene sample stays much closer to 1 (suggesting less deviation from exponential behavior) than the exponents obtained for n-hexanol and cyclooctane. Furthermore, for the toluene-containing sample, the decrease of the exponent β sets in at a later time than for the other two samples. Obviously, the polarity of the molecules does not play a major role in the development of the nonexponential behavior. Similar results were obtained when evaluating the nonexponentiality of the magnetization decay curves by means of the Kudryavtsev model (12). Finally, a biexponential analysis of the magnetization decay curves was performed also. The slow relaxation time component obtained in the analyses of the experimental data for cyclooctane and n-hexanol was found to be slower than the original monoexponential relaxation times, while the fast component exhibited similar values such as the ones observed for toluene at later hydration stages. The slow component made up about one-third of the overall signal intensity. It should be noted that the relaxation rates determined in a single-exponential relaxation model are average values between the contribution of the residual water and the NAPL phase. An analysis of the signal amplitudes in the different experiments (like those represented in Figure 1) suggests that in the worst case the contributions from the residual water and from the NAPL might be in the same order of magnitude. Thus, this opens up the question of whether the observed changes in the relaxation rates are just due to averaging between the contributions from the residual water and the actual NAPL phase. A quantitative analysis of the contributions of NAPL and residual water to the echo signal in the deuterated cement pastes turned out to be very difficult: (i) The residual water content in the cement may vary from batch to batch (there is always some macroscopic clustering of cement grains leading to an inhomogeniety in the cement); 882

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(ii) there is the possibility of slow exchange between water contained in hydration products of the residual water formed during the storage of the cement and the heavy water in the paste; such an exchange process will most probably also be dependent on the progress of the cement hydration reactions, i.e., the elution of protonated water into the pore fluid may be modulated by the presence of the different solvents; (iii) both the signals from residual water and from the NAPL are affected by diffusive signal attenuation in internal magnetic field gradients in the freshly prepared paste; this effect later decreases, but it may do so in different ways for water and NAPL; (iv) in order to follow the hydration process with a decent temporal resolution and a good signal/noise ratio, the repetition time for the spin-echo experiment was chosen as 1 s, which does not necessarily allow for full longitudinal relaxation of the NAPL component. Therefore, the transverse relaxation rates alone do not yet provide a safe means to asses the state of the NAPL in the hydrating cement pastes. To gain further insight into the distribution of the solvents in the hydrating cement matrix, the time dependence of the self-diffusion coefficient was studied by means of the PFG diffusometry method (30). In the PFG-NMR experiments, there is an inherent relaxation-time-weighting (19) which leads to a nearly complete suppression of the contributions originating from residual water at later hydration stages and for longer diffusion times also at the beginning of the experiments (typical longitudinal relaxation times for NAPL phases in the freshly prepared cement paste are on the order of several 100 ms, for water at about 100 ms in a freshly prepared white cement paste at w/c 0.33). However, there is a notable contribution of HDO diffusion to the signal measured in the diffusometry experiments performed during the first hours of the hydration. This is obvious from the comparison between the diffusion coefficients of the free bulk solvents and the diffusion coefficients measured during the first hours of hydration, which is given in Table 1. By contrast, for toluene (which has a self-diffusion coefficient close to that of HDO), the ratio between the initially measured diffusion coefficient in the paste and the bulk value is even lower than for water in a protonated cement paste (19). This suggests a higher tortuosity for the toluene-filled parts of the sample than for the overall pore network in a freshly prepared cement paste. This would be the case if the toluene is initially mainly concentrated close to the pore walls. A spectroscopically resolved PFG NMR experiment may help to separate the signals originating from residual water and those from the solvent phase. This option for PFG experiments with strong gradients has only become available very recently (23, 24). A cement sample prepared with toluene and heavy water was studied with this setup. Toluene was used in the experiment as the two spectral lines with their given ratio in the intensities should allow an easier discrimination from the water line than just a single line or a complicated spectrum. In Figure 5 the spectra obtained after about 3.5 h and about 46 h of hydration are given for two different values of F(G). The observed NMR lines of toluene exhibit strong inhomogeneous line broadening (by about 1.8 ppm) and are shifted to higher chemical shift values (i.e., higher resonance frequencies) compared to the chemical shift values in the bulk liquids by about 1.9 ppm (literature values for toluene chemical shifts (31), 2.3 and 7.2 ppm; extracted from the spectra, 4.1 and 9 ppm). Both the shift in the resonance frequencies and the inhomogeneous line broadening can be understood as the result of the magnetic susceptibility variation in the cement paste with the shift representing the average field offset and the inhomogeneous broadening its standard deviation. Due to the massive line broadening, only the peak at highest chemical shifts rep-

TABLE 1. Initial Diffusion Coefficient in PFG without Spectral Resolution and Corresponding Bulk Diffusion Coefficients of the Solvents under Study solvent

n-hexanol cyclooctane toluene water (H2O paste, w/c 0.33)

Dbulk bulk diffusion coefficient [m2/s] (temp. 304 K)

D1h diffusion coefficient in 1 h old paste

10-10

1.87 × 5.25 × 10-10 2.45 × 10-9 2.64 × 10-9

10-10

3.54 × 7.3 × 10-10 1.04 × 10-9 1.37 × 10-9

D1h/ Dbulk 1.88 1.38 0.42 0.52

D1d diffusion coefficient in 1 day old paste 10-11

19.57 × 2.31 × 10-10 2.81 × 10-10 6 × 10-11

D1d/Dbulk 0.28 0.44 0.11 0.022

a The observation of diffusion coefficients higher than the bulk values suggests a superposition with a signal originating from faster-diffusing HDO protons.

FIGURE 6. Diffusion coefficients measured at 3 ms observation time of various solvents in a hydrating white cement paste of w/c 0.33 as a function of the hydration time. For toluene, no sufficient signal intensity is available for measurements at the later hydration stages. The step in the diffusion data for hexanol is significant and occurs before the onset of notable changes in the hexanol relaxation times.

FIGURE 5. (A) Spectra obtained on a hydrating deuterated cement sample containing 0.0017 mL of toluene/g of cement at two different hydration times and F(G) values. (B) Achievable signal/noise ratios did not allow sufficiently stable fitting of the three inhomogeneously broadened lines corresponding to methyl, HDO, and aromatic protons for an automatic spectrally selective evaluation of the diffusion behavior. While the spectra at late hydratation times could be reasonably fitted with the two toluene lines (half line width, 1.89 ppm), fitting the spectra obtained at earlier hydration stages needs another line corresponding to HDO (individual toluene and HDO contributions represented by thin lines; half line widths, 1.7 ppm for toluene, 2.9 ppm for HDO; intensity ratio, 6:1). resenting the aromatic protons can be clearly seen in all spectra while the HDO and methyl peaks overlap very strongly. Fitting the spectra with 3 Gaussians and taking into account the fixed intensity ratio between the aromatic and methyl peaks in the toluene did not allow a numerically sufficiently stable discrimination between the water and toluene contributions to the signal to allow for a reliable analysis of the two component’s diffusion coefficients. The necessary increase in the number of signal accumulations in order to allow a reliable line fitting would require unrealistically long measuring times of several hours for a single-diffusion decay curve. As changes in the diffusion behavior of the hydrating sample will already be quite prominent on this time scale, the further development of spectrally resolved diffusometry experiments for solvents in hydrating cement was abandoned. Spectrally nonresolving PFG NMR experiments with a short observation time of 3 ms could be performed during the first 2 days of hydration for all samples. After that time, the signal/noise ratio in the toluene-containing samples

becomes too poor for reliable diffusometry experiments due to the short transverse relaxation times. Also, the longitudinal relaxation times for toluene in the cement matrix are relatively short, so that no studies of the observation-time dependence of the self-diffusion coefficient could be performed over a wide range of observation times. For the other solvents, both the transverse relaxation time and the longitudinal relaxation time are long enough to carry out PFG NMR experiments over a wide range of observation times even after long hydration times of several weeks (the T1 values of n-hexanol at 400 MHz are still on the order of 600 ms even in a sample hydrated for three months). In Figure 6 the short-time diffusion coefficients obtained for the toluene, n-hexane, and cyclooctane are plotted as a function of the hydration time. As can be seen from the graph, all solvents exhibit a similar decrease of the self-diffusion coefficient during the first 10-20 h of hydration. For later hydration times, the self-diffusion coefficient in the cyclooctane sample approaches a plateau value at about one-fourth of the value observed in the fresh paste. This coincides with the behavior of the transverse relaxation time, which also settles to a plateau value after about 40-50 h. By contrast, the diffusion coefficients measured for n-hexanol are found to decrease continuously for at least 70 h. The diffusion coefficient measured after about 2 months is even much lower. Similarly, the diffusion coeffients for toluene were found to decrease throughout the observation time of up to 100 h. Despite the poor signal/noise ratio at the later hydration times, it can be clearly seen from the experimental data that the decrease of the diffusion coefficient of toluene is more pronounced than the one observed for n-hexanol. This stronger decrease of the diffusion coefficient can be attributed to two effects: (1) The bulk value of the self-diffusion coefficient for toluene is much bigger than that for n-hexanol (2.54 × 10-9 VOL. 38, NO. 3, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 7. Diffusion coefficients for cyclooctane in a deuterated cement paste at w/c 0.33 after various hydration times as a function of the mean diffusive shift s(t) ) x2D(t)t corresponding to the observation times under study.

FIGURE 8. Observation-time dependence of hexanol diffusion coefficients in a deuterated white cement sample after 3 months of hydration. The lines in the graph correspond to fits of a 1/t dependence with an offset term over all data points and over the diffusion times of 10 ms and more, respectively.

vs 1.87 × 10-10 m2/s at the measurement temperature of 304 K). Therefore, in toluene, a larger part of the pore structure is sampled by the diffusive movement of the molecules at the same observation time. (2) As the relaxation times show, there is a much more pronounced increase in the surface/volume ratio for the toluene sample than for the hexanol. Therefore, a higher tortuosity of the pore system can be expected than in the case of hexanol. Another feature of the n-hexanol diffusion data is the step structure marked by an arrow in Figure 6. While no significant changes can be seen in the n-hexanol relaxation time data (despite the contribution of some residual water to the signal), there is a similar feature in the water NMR relaxation behavior of protonated samples (see figure S1 in the Supporting Information). The diffusion coefficient measured at an observation time of 3 ms corresponds to a diffusive shift on the order of about 2 µm for a liquid with a self-diffusion coefficient on the order of 10-9 m2/s. Increasing the observation time to longer values will increase the mean diffusive shift of the molecules observed in the experiments, which corresponds to contribution of structures on greater length scales to the restriction of the diffusive motion. An especially strong decrease in the diffusion coefficient can be expected in a system of unconnected pores with the size a where the measured diffusion coefficient in the case decreases proportional to 1/t with the observation time t for a2 , Dt (30). Such a decrease of the diffusion coefficient could be observed in our experiments for liquid fractions that are confined to homogeneous droplets with a size of several micrometers or less in the cement stone matrix. While the data for the diffusion of cyclooctane as a function of the mean diffusive shift length at various observation times given in Figure 7 clearly exhibit a decreasing trend for all observation times studied, they do not follow the 1/t behavior expected for isolated simple droplets over the range of observation times covered in the experiment (see Figures S2 and S3 in the Supporting Information). Rather, both the strongly decreased values of the short time diffusion coefficient (see Figure 7) and the slower decrease of the diffusion coefficient at longer observation times suggest the confinement of the non-aqueous phase to highly tortuous mazelets (3) with structure sizes on the submicrometer scale. The maximal diffusive shifts possible within these mazelets are obviously bigger than 10 µm as we still see a sublinear decrease of the diffusion coefficient with the reciprocal observation time even up to 500 ms. At the long diffusion times and hydration times under consideration in this discussion, possible HDO contributions to the signal can be excluded due to the very short relaxation times of water after

those hydration times, and all the detected signals actually originate from the cyclooctane. This suggests the existence of diffusion paths allowing a net displacement of at least 20 µm within the liquid-filled mazelets. As no clear 1/t dependence of the apparent diffusion coefficient is seen even for the longest observation times of 500 ms used in the experiments, the maximal diffusion paths within the mazelets might be even longer. However, it should be noted that the observed diffusion data correspond to an average diffusion coefficient. Therefore, we cannot conclude on the basis of the present results whether they arise from mazelets of similar size or from a difference between a high number of small mazelets and several bigger mazelets. A diffusion behavior suggesting the confinement of the solvent phase into mazelets instead of simple droplets was also observed for n-hexanol (Figure 8). Due to the lower value of the diffusion coefficient in this case, the length scale mapped by the time-dependent diffusion coefficient is shorter (see also Figure S4 in the Supporting Information). From the diffusion coefficients observed in a sample after 3 months of hydration, we can again only give a lower boundary for the possible maximal diffusive path length in the mazelets as fitting a 1/t dependence to the data is only possible with an offset term. The experimental results suggest this maximal diffusion length to be at least 4 µm. Due to the poor signal/noise ratio resulting from the dramatically decreasing relaxation times in the toluene samples, similar studies of the diffusion coefficient in toluene are presently not feasible. However, both the much stronger decrease of the relaxation times and the development of a steeper observation-time dependence of the diffusion coefficient after 1 day of hydration compared to the other solvents indicate a pore network with a higher tortuosity at a short length scale. To conclude, we have presented direct NMR data on the dynamics and transport properties of non-aqueous solvent phases in hydrating cement matrices. The experimental results again correspond very well with recent results from synchrotron tomography on hydrated samples and from NMR relaxometry on the hydration kinetics in the presence of solvents: A fundamentally different behavior for aromatic solvent phases is observed compared to aliphatic solvents. The pore network into which the aromatic phase is incorporated exhibits much shorter length scales and a higher tortuosity than the networks into which the other solvents are confined. However, also the other solvents have been found to be confined to a tortuous mazelet network rather than to isolated droplets with a simple topology. A clear resolution of the maximal length scales for transport within the non-aqueous liquid network is not feasible on the basis

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of the present data. However, it is obvious from the data that this length scale is bigger than the original pore sizes in a freshly prepared cement phase. This together with the observation of pronouncedly biexponential relaxation behavior suggests a mesoscopic phase separation occurring during the onset of the hydration process for the nonaromatic solvents. For further elucidation of this phase separation process, synchrotron tomography of solvent-doped pastes during the respective hydration steps might be an interesting option. For aromatic solvents, no indication of a similar phase-separation on the micrometer scale was found.

Acknowledgments Parts of this work were funded by the German Science Foundation under the auspices of the Graduiertenkolleg “Physikalische Chemie der Grenzfla¨chen” and the SFB 294 at the University of Leipzig and the project grant BA-1592/ 1-1 at the TU Munich.

Supporting Information Available Table S1 and graphs S1 to S4 as mentioned in the article. This material is available free of charge via the Internet at http://pubs.acs.org.

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Received for review May 6, 2003. Revised manuscript received October 27, 2003. Accepted November 3, 2003. ES034444H

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