Investigation of the State of Water in Hydrating Layered Sodium

This presents an alternative mechanism for alkali−silica reaction (ASR) damage ... monitored for the first time using quasi-elastic neutron scatteri...
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Chem. Mater. 2004, 16, 5042-5050

Investigation of the State of Water in Hydrating Layered Sodium Disilicate in Crystalline and Amorphous Forms by Quasi-Elastic Neutron Scattering John W. Phair,*,† Richard A. Livingston,† Craig M. Brown,‡,§ and Alan J. Benesi| Office of Infrastructure Research and Development, Federal Highway Administration, McLean, Virginia 22101, Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, Department of Materials Science and Engineering, University of Maryland, College Park, Maryland 20742-2115, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802 Received February 3, 2004. Revised Manuscript Received July 5, 2004

The structure and state of water within hydrating crystalline and amorphous sodium disilicate were monitored for the first time using quasi-elastic neutron scattering (QENS) and X-ray diffraction (XRD). The QENS kinetic data collected for the first 12 h of the reaction were fitted to a model consisting of a Lorentzian and a Gaussian function each convoluted with the energy resolution of the instrument. This allowed the QENS signal from water to be associated with two states that included bound and free water, as confirmed by thermogravimetric analyses and 2H NMR. In situ QENS data were collected for a set of sodium disilicate and silica mixtures at a series of discrete temperatures between 20 and 40 °C. The bound water was successfully modeled with first-order reaction kinetics to quantify the hydration of the layered silicate structure within kanemite. First-order reaction kinetics was also used to model the reaction using amorphous starting material, which may also suggest the presence of hydrated layered structure in the reaction product. The presence of the hydrated layered structure in amorphous material was confirmed by XRD analyses. The consequences of this for the alkali-silica reaction (ASR) and its swelling mechanism are discussed. On the basis of these results, a standard test for ASR reactivity of aggregates could be developed using natrosilite as a reagent, and the kinetic parameters as a scale of reactivity.

Introduction The “alkali-silica reaction” (ASR) results in the formation of deleterious products in concrete that cause degradation through their ability to expand in the presence of moisture.1,2 ASR is one of the most destructive phenomena in concrete and has attracted considerable interest from researchers in the past.3 Notwithstanding, it still remains poorly understood in terms of the local atomic structure and chemical composition of its products and their relationship to the physicochemical mechanism of expansion.4 Expansion due to the alkali-silica reaction (ASR) has been established by observing an increase in the volume of ASR gel products relative to the initial volume of the reactants. This has generally been attributed to osmotic swelling, or the imbibition of water by ASR products. Some authors have demonstrated that variation in the composition of ASR gels affects the swelling behavior * To whom correspondence should be addressed. E-mail: John.Phair@ fhwa.dot.gov. Te.: (202) 493 3089. Fax: (202) 493 3086. † Federal Highway Administration. ‡ National Institute of Standards and Technology. § University of Maryland. | The Pennsylvania State University. (1) Struble, L.; Diamond, S. Cem. Concr. Res. 1981, 11, 611. (2) Davies, G.; Oberholster, R. E. Cem. Concr. Res. 1988, 18, 621. (3) Hobbs, D. W. Mag. Concr. Res. 1978, 30, 15. (4) Moranville-Regourd, M. Cem. Concr. Compos. 1997, 19, 415.

of the gels.1 However, the mechanism of expansion is still uncertain because the exact microstructure of alkali-silica gels remains in dispute. The conventional view is that an amorphous structure exists.5 However, it has been recently proposed that a layer silicate structure is present, similar to that observed in the mineral kanemite.6-8 The presence of hydrated layered silicate structures within ASR products could have important ramifications for the mechanism of swelling and the overall expansion potential.9,10 Consequently, the interactions between water and kanemite are of great interest. This paper reports the premier use of quasi-elastic neutron scattering (QENS) to study the transformations during the ASRsbased on a system of reacting amorphous or crystalline sodium disilicate in the presence (5) Gaboriaud, F.; Nonat, A.; Chaumont, D.; Craievich, A. J. Phys. Chem. B 1999, 103, 5775. (6) Tapper, A.; Adrian, R.; Schimmel, G. U.S. Patent No. 5,456,895, 1995. (7) Wieker, W.; Hubert, C.; Heidemann, D.; Ebert, R. Alkali-silica reaction - a problem of the insufficient fundamental knowledge of its chemical base. In Materials Science of Concrete: The Sidney Diamond Symposium; Cohen, M., Mindess, S., Skalny, J., Eds.; The American Ceramic Society: Westerville, OH, 1998; p 395. (8) Cong, X.-D.; Kirkpatrick, R. J. Cem. Concr. Res. 1993, 23, 811. (9) Prezzi, M.; Monteiro, P. J. M.; Sposito, G. ACI Mater. J. 1997, 94, 10. (10) Rodrigues, F. A.; Monteiro, P. J. M.; Sposito, G. Cem. Concr. Res. 1999, 29, 527.

10.1021/cm049820r CCC: $27.50 © 2004 American Chemical Society Published on Web 11/03/2004

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Table 1. Composition of Samples Analyzeda

a

sample

sodium disilicate source

SiO2 source

solvent

A B C

δ-Na2Si2O5 (1.81 g) amorphous Na2O.2SiO2‚xH2O δ-Na2Si2O5

amorphous SiO2 (1.19 g) amorphous SiO2 crystalline SiO2

H2O (1.43 g) H2O H2O

Relative mass amount of material used is provided in the brackets.

of water and silica. QENS provides several advantages for measuring reaction kinetics over other methods. It provides a direct method for studying the states of water itself during the reaction and the measurement can be performed in situ without disturbing the sample. While QENS itself does not provide information about the microstructure directly, the nature of the kinetics can be used to put constraints on the possible forms of the microstructure. Recently, it has been hypothesized that the structure of ASR products contains a degree of molecular order that resembles the sheet silicate kanemite. The existence of silicate layers or sheets within ASR products was first observed by Kirkpatrick and coworkers,8 and formally acknowledged as a characteristic feature by Wieker et al.7 Until now, direct synthetic preparation of kanemite-based ASR products for comparing the properties of “real” ASR products with kanemite-based ASR products has not been done. This makes the present study highly novel and valuable. Therefore, to better determine the extent of hydrated silicate layers within ASR products, a series of ASR products were synthesized based on sodium disilicate of both long-range and short-range atomic order, and the kinetics of the reaction was monitored at different reaction temperatures utilizing QENS. The model system investigated in the present study can be summarized by the overall reaction for the formation of kanemite6

δ-(Na2Si2O5) + 2SiO2 + 7H2O f 2[NaHSi2O5‚3H2O] In the industrial synthesis of kanemite, this reaction uses the δ-crystalline isomer of sodium disilicate (or natrosilite, the naturally occurring mineral form) and an amorphous silica source to produce the hydrated layered silicate.6 However, it is unlikely that the δ-crystalline phase of sodium silicate is found in concrete. Subsequently, it is necessary to consider other starting materials that could yield kanemite. ASR products found in concrete often have an amorphous form of sodium silicate present. Furthermore, the silica source may often be quartz. The effect of a crystalline silica source such as quartz would be expected to inhibit the formation of the final product, while the effect of amorphous sodium silicate remains largely unknown.11 Nonetheless, formation of kanemite and its analogues is a complex multi-stage process involving decomposition/dissolution of the starting material, and condensation of small silica species as well as crystallization. Since water plays a vital role in the transformation it is possible to monitor these various processes by measuring changes associated with the water using the QENS technique. (11) Michel, B.; Thiebaut, J.; Wackenheim, C. Bull. Eng. Geol. Environ. 2003, 62, 145.

Obtaining useful information from QENS relies on the differences in the energy spectrum of neutrons scattered by mobile atoms compared to that observed for atoms stationary on the time-scale of the measurement.12 Typically, a broadening around the elastic part of the spectrum occurs for neutron scattering produced by mobile matter compared to structures that appear static on the time-scale of the measurement. Thus, it is possible to quantify the mobile and fixed amounts of a specific phase within a material based on their relative contributions to the total spectrum.13 Since hydrogen has an extremely large incoherent neutron scattering cross section, the neutron scattering signal for a material containing significant quantities of hydrogen will be dominated almost completely by the scattering from that element.14 In materials such as the present system, where the source of hydrogen is restricted to H2O, the neutron scattering signal can, therefore, be directly related to the phases and structure of the water and its hydration reaction products (e.g., silanol -OH or hydrated Na+) within the material. The goals of the present work are two-fold. First, to develop an accurate model of the kinetics of the reaction based on QENS data that may yield parameters characteristic of the formation process that are sensitive to composition and chemical structure. Second, to verify the existence of hydrated layered silicates within products synthesized from amorphous sodium disilicate and other reagents that may be found in concrete. Such information is important, as hydrated silicate layers could potentially provide a major source of swelling in ASR products. Any information learned on this subject will aid the development of subsequent test methods vital to the prevention of ASR. Experimental Procedure Crystalline sodium silicate (δ-Na2Si2O5) was obtained as the commercial product SKS-6 from Clariant Corporation in powder form, and amorphous sodium disilicate was obtained as the commercial product SS-C (SiO2/Na2O) from PQ Corporation in powder form. The amorphous silica used was high purity Merck Silica Gel, 70-230 mesh, and crystalline silica (quartz) was obtained from US Silica as the product Min-uSil 15. The compositions of the reacting mixes and their relative amounts are provided in Table 1. For all QENS experiments, the starting powders were first dry-mixed to ensure both components were evenly dispersed throughout the solid phase. The dry-mixed powder was then combined with H2O at a liquid-to-solid mass ratio of approximately 0.4. After mixing the wetted powder for ∼3 min, the paste was smeared into a thin rectangular aluminum sample holder. A thin strip of Teflon tape covered the inside of the aluminum holder to prevent any reaction with the sample. The resulting thickness of the sample was ∼1 mm. (12) Be´e, M. Quasielastic Neutron Scattering; Adam Hilger: Bristol, 1988; p 25. (13) Livingston, R. A.; Neumann, D. A.; Allen, A. J.; Rush, J. J. Mater. Res. Soc. Symp. Proc. 1995, 376, 459. (14) Sears, V. F. Neutron News 1992, 3, 26.

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Table 2. Characteristic X-ray Diffraction Reflections of Samples A, B, C, and SKS-6 as Displayed in Figures 1 and 2 sample A

sample B

sample C

SKS-6

d (Å) ((0.005)

relative intensity

d (Å) ((0.15)

relative intensity

d (Å) ((0.005)

relative intensity

d (Å) ((0.005)

relative intensity

10.212 9.466 5.118 4.011 3.765 3.637 3.434 3.159 3.098 2.474 2.377

v. intense weak intense v. intense Intense intense intense weak weak intense weak

10.145 8.650 7.194 3.363

weak weak weak w-broad

4.921 4.265 3.962 3.776 3.346 3.312 2.456 2.425 1.821 1.542

weak intense intense intense v. intense intense(sh) intense intense intense intense

10.264 6.837 6.002 4.902 4.164 3.944 3.839 3.764 3.616 3.431 3.294 3.009 2.833 2.445 2.418 2.092

v. weak v. weak intense intense intense v. intense intense v. intense intense weak intense intense intense intense v. intense intense

The sample temperature was monitored continuously and kept constant with a thermostat at either 20, 30, or 40 ( 0.5 °C. QENS measurements were performed on the Fermi chopper time-of-flight spectrometer (FCS)15 at the NIST Center for Neutron Research. The experimental configuration followed that used previously for cementitious materials, consisting of an incident neutron wavelength of 4.8 Å and a sample-todetector distance of 2.29 m, yielding an energy resolution of 0.146 meV.16 Data from detectors covering a range of 1.9 to 2.4 Å-1 in the scattering vector, Q, were averaged to obtain a statistically meaningful estimate of the scattering function. For each sample, data were collected for approximately 14 h, with each data point counted over a 30 min interval. After conducting the QENS experiments, the samples were analyzed by X-ray diffraction (XRD). All XRD analyses utilized Cu KR (λ ) 1.541838 Å) radiation generated at 40 mA using 40 kV subtracting instrumental background. Specimens were prepared as random powder mounts and scanned at step rates varying between 0.02° s-1 and 0.04° s-1 and 2θ values range from 5 to 70°. For calorimetry, a TA Instruments 2920 Modulated DSC was used with Thermal Solutions software to monitor the isothermal heat evolution of the reaction. Samples and references were placed inside aluminum hermetic pans. The states of water within the sample were also investigated by thermal analysis utilizing a TA Instruments SDT 2960 for simultaneous DTA-TGA, and the data were analyzed using TA Instruments software. Deuterated proton (2H) NMR measurements were conducted on a 6.98 T Chemagnetics CMX-300 spectrometer operating in the quadrature mode at 45.65 MHz for 2H at 21 ( 1 °C. A quadrupole echo pulse sequence was used to obtain 2H spectra at 45.65 MHz (π/2 ) 1.9 µs, τ1 ) 30 µs, τ2 ) 25 µs), while the T1 values were determined with the inversion recovery pulse sequence. Samples used for 2H NMR experiments were synthesized using 2H2O instead of H2O, but in the identical molar amount as prescribed in Table 1.

Results 1. Characterization of Reaction Products. X-ray diffractograms of the reaction products (formed at 40 °C) along with that of SKS-6 are presented in Figure 1. Table 2 provides the characteristic X-ray diffraction reflections (d-spacing values) of the reaction products (samples A, B, and C) and SKS-6 with their relative intensities as displayed in Figure 1. The peaks obtained for sample A are identical to those observed in the (15) Copley, J. R. D.; Udovic, T. J. J. Res. Natl. Inst. Stand. Technol. 1993, 98, 71. (16) Thomas, J. J.; FitzGerald, S. A.; Neumann, D. A.; Livingston, R. A. J. Am. Ceram. Soc. 2001, 84, 1811.

literature, thus indicating complete kanemite formation.6 Sample B does not exhibit any of the sharp peaks observed for kanemite and the presence of only a few diffuse peaks suggest the material is mostly amorphous. Sample C, on the other hand, is slightly more complicated since it displays peaks attributed to unreacted quartz and SKS-6 (δ-Na2Si2O5). The peaks associated with kanemite are present in sample C, but at a far greater reduced intensity compared to sample A, indicating incomplete or significantly reduced kanemite formation.

Figure 1. X-ray diffractogram of the starting material, SKS6, and the three reaction products defined in Table 1 formed after curing at 40 °C for 12 h. The individual patterns are offset by 1000 units for greater visibility.

Of most interest is the lowest angle region in the XRD patterns, as this indicates the extent of layered silicate structures within the samples. Figure 2 presents the basal peak region for SKS-6 and the reaction products (formed at 40 °C) while Table 2 provides the d-spacing values for the peaks with their relative intensities. There is a major peak at ∼10.2 Å for sample A which represents the ordered basal spacing of silicate sheets (010) within kanemite. Sample B also exhibits this peak (d ≈ 10.15 Å) but of significantly reduced intensity, suggesting limited layer structure is present despite lacking regular order. This is seen in addition to two

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Figure 4. Mass and rate of change of temperature plotted against temperature for the TGA run of sample A formed at 20 °C. Mass is normalized to that of dehydrated sample. Figure 2. X-ray diffractogram of the basal peak region for the starting material, SKS-6, and the three reaction products defined in Table 1 cured at 40 °C for 12 h.

Figure 3. TGA curves of the three reaction products defined in Table 1 formed at 20 °C. Mass is normalized to that of dehydrated sample.

slight humps in this region centered around 7.19 and 8.65 Å and an intense peak at d ≈ 3.36 Å in sample B. The implication of this is that structured layers can occur in silicate compounds other than those containing a complete kanemite structure. Although two humps are also observed in Sample C, they are of reduced intensity compared to those in sample B. More importantly, there is no clear peak around 10.2 Å which would be expected if hydrated silicate layers were present in the sample.17 Finally, the occurrence of the hydrated silicate layer peak in SKS-6 would also seem to suggest that the SKS-6 was contaminated with a small portion of kanemite or hydrated silicate layers. However, the magnitude of this peak is too low for the contaminant to significantly affect the experimental results. Further characterization of the reaction products was achieved utilizing thermogravimetric analysis (TGA) and 2H NMR to determine the states of water within the samples. Figure 3 presents TGA curves for the samples obtained after complete reaction at 20 °C, and Table 3 provides a summary of the TGA results. In (17) Garvie, L. A. J.; Devouard, B.; Groy, T. L.; Camara, F.; Buseck, P. R. Am. Mineral. 1999, 84, 1170.

Table 3. Summary of TGA Results sample

step 1

step 2

step 3

A B C

64-105 °C 50-95 °C 35-103 °C

105-143 °C 95-310 °C 103-141 °C

143-310 °C 141-310 °C

Figure 3, three steps of water loss could readily be observed for sample A, the product formed by reacting SKS-6 and amorphous SiO2. While it is possible more states of water exist, they could not be resolved from the present experimental configuration. Step 1 is associated with interlamellar water, which was observed in all samples, while steps 2 and 3 are associated with the release of water from hexagonal rings. Previously, up to four states of water have been observed to exist in wet kanemite samples by TGA.18 This includes two states associated with the interlamellar space (an interlamellar monolayer of water molecules as well as another trapped within the vacancies of the folded SiO3OH hexagonal rings),19 as well as water molecules adsorbed on the external surface or in the intercrystalline region. Not all these forms of water were observed in the present system, which is considerably drier. Assigning specific steps in the TGA curve to one particular state of water is challenging given the potential overlap of different species for any one given step. Consideration is now given to the TGA curves for the products resulting from the reaction of SKS-6 with quartz, as well as sodium disilicate with amorphous SiO2. While we may assign three states of water to sample C, such a distinction becomes unclear for sample B (Figure 3 and Table 3), where no step above 150 °C is apparent. The absence of a well-defined step 2 in sample B suggests there are fewer states of water than in either sample A or C. Unlike previous data for wet kanemite, 18 no significant amounts of surface water in the present samples were observed. A greater understanding of the states of water associated with the synthesized kanemite may be obtained by considering Figure 4, which displays the effect of temperature on both the mass and rate of change of temperature for the TGA run of sample A. According to Figure 4, there are clearly two peaks centered at (18) Hayashi, S. J. Mater. Chem. 1997, 7, 1043. (19) Beneke, K.; Lagaly, G. Am. Mineral. 1977, 62, 793.

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Figure 5. 2H quadrupole echo spectrum of a sealed sample of kanemite synthesized from 2H2O after 2 h. The arrows indicate the powder diffraction pattern.

approximately 85 and 180 °C, respectively. The first and larger peak at 85 °C is associated with the interlamellar water, while the second and smaller peak is associated with hydroxyls on the surface and within the sample. According to the 2H NMR data, water experiences a complete liquid to immobile transition during the formation of kanemite. The 2H quadrupole echo spectrum obtained at 45.65 MHz for kanemite prepared with 2H O is provided in Figure 5. A large peak, which is 2 assigned to the free 2H2O phase, dominates the spectrum. A much smaller 2H powder pattern (as indicated by the arrows in Figure 5) is also present at the shoulder of the major peak, and is probably associated with the silanol -OH in kanemite.20 The shortening of the H2O (aqueous phase) 2H T1 relaxation time observed during the formation of kanemite has been attributed not to the incorporation of water molecules into the solid matrix as amorphous and crystalline hydration products, but to the transition of free liquid water into an immobile state.21,22 This is because the T1 relaxation times are too short to merely indicate the presence of a rigid hydrate.22 The observed 2H NMR T relaxation values fell from 13 ms at 2 h after 1 preparation to 3.9 ( 0.2 ms at several days after preparation. A more accurate interpretation is, therefore, to ascribe the relaxation times to the tetrahedral H jumps observed in immobile states of water.21-23 On the basis of both calorimetry data and NMR data that follow the formation of kanemite (sample A), it is appropriate to classify water as existing in either one of two types: free or immobile (including silanol OH). 2. QENS Data Fitting. The QENS data describe a scattering function in reciprocal space, which is the time and spatial Fourier transform of the pair correlation (20) Lausch, M.; Speiss, H. W. J. Magn. Res. 1983, 54, 466. (21) Rakiewicz, E. F.; Benesi, A. J.; Grutzeck, M. W.; Kwan, S. J. Am. Ceram. Soc. 1998, 120, 6415. (22) Wittebort, R. J.; Usha, M. G. J. Am. Ceram. Soc. 1988, 110, 5668. (23) Fujara, F.; Wefing, S.; Kuhs, W. F. J. Chem. Phys. 1988, 88, 6801.

Phair et al.

function that describes the state of matter in real space. Thus, to select a function for fitting the QENS data it is necessary to have some information about the state or states of matter. A hydrous material, like ASR products, consists of an intimate mixture of liquid and solid phases. The pair correlation function for atoms translating in a liquid is simply the diffusion equation, for which the Fourier transform is the Lorentzian function.12 Owing to limits on the accessible time scales and the energy resolution of the QENS, the immobile water is described in reciprocal space simply as a Gaussian function. Thus, ASR products can be modeled with the sum of a Gaussian function and one or more Lorentzians.24-26 This interpretation is preferred to alternatives such as fitting the data to a stretched exponential,27,28 since the fitting parameters appear to have a more direct physical meaning. Determination of the suitability of either a single Lorentzian (two peak)24 or a double Lorentzian (three peak)16 model to fit to the data rests on the extent to which time and temperature affect the full width at halfmaximum (fwhm) of the Lorentzian peak. For instance, Thomas et al. relied on the observation of a sharp decrease in the bound water index (BWI) when the temperature was abruptly changed to justify utilizing a three-peak model for fitting the QENS data.16 The shift was ascribed to the fact that a single-Lorentzian function was modeling the combined response from more than one population of water. In addition, the fwhm of the Lorentzian representing free water decreased significantly over time, which substantiated the hypothesis that more than one population of water was contributing to the Lorentzian peak. The two-peak model of Fitzgerald et al.24 used to describe the formation of kanemite and its related samples as a function of time can be represented as the following incoherent scattering function:

{

Sinc(Q,ω) ) C0 + Aδ(ω ) 0) +

[

B

]} [( )

]

Γ 1 X exp(-ω2/2σ2) (1) 2 π(Γ + ω ) σx2π 2

where Q is the scattering vector (Q ) [4π/λ]sin(θ)), where λ is the incident neutron wavelength and θ is the scattering angle); ω is the energy transferred from the scattered neutron, and C0 is a Q independent flat background, which can be determined at the start of each run. Also, Γ is the Lorentzian fwhm, σ is the energy resolution of the instrument, and the coefficients A and B provide the number density of bound and free hydrogen atoms, respectively. Fitting of the data essentially involves varying the three parameters A, B, and C0 to yield a minimum value for χ2 between the fit and data. Peak widths for the free water were kept at (24) Fitzgerald, S. A.; Neumann, D. A.; Rush, J. J.; Bentz, D. P.; Livingston, R. A. Chem. Mater. 1998, 10, 397. (25) Fitzgerald, S. A.; Neumann, D. A.; Rush, J. J.; Kirkpatrick, R. J.; Cong, X.; Livingston, R. A. J. Mater. Res. 1999, 14, 1160. (26) FitzGerald, S. A.; Thomas, J. J.; Neumann, D. A.; Livingston, R. A. Cem. Concr. Res. 2002, 32, 409. (27) Damasceni, A.; Dei, L.; Fratini, E.; Ridi, F.; Chen, S.-H.; Baglioni, P. J. Phys. Chem. B 2002, 106, 11572. (28) Fratini, E.; Chen, S.-H.; Baglioni, P.; Bellissent-Funel, M.-C. J. Phys. Chem. B 2002, 106, 158.

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Figure 6. Value of fwhm of the Lorentzian peak obtained from the 2-peak fit of the QENS data for sample A as a function of time.

Figure 7. Value of fwhm of the Lorentzian peak obtained from the 2-peak fit of the QENS data for sample B as a function of time.

constant values for 20, 30, and 40 °C and were taken from previous measurements on the same instrument.16 Utilizing the parameters determined from the data fits, it is possible to describe the state of water in the sample in terms of one parameter referred to as the bound water index (BWI), which may be defined in terms of the number densities A and B as24

BWI )

A (A + B)

(2)

The kinetics of hydration can then be accurately quantified as the changes in the BWI with time. In the present system, both two-peak and three-peak fits were attempted. First, utilizing a two-peak fit produced a Lorenztian with a time-independent fwhm for the formation of sample A (kanemite) as seen in Figure 6. When the three-peak fit was used, the proportion of water present as the constrained species was insignificant, typically lying below 2%. This is substantially different from the 20% of constrained water previously determined for Portland cement.15 On this basis it would appear that the two-peak model is adequate to fit the QENS data for this system. Values of fwhm obtained from the two-peak fit for the formation of samples B and C were not constant with reaction time. According to Figures 7 and 8, fwhm values for both samples decreased with increasing time, suggesting that the two-peak fit may be inadequate for these samples. However, subsequent fitting of the QENS data for samples B and C with the three-peak model did not improve the situation, and decreasing fwhm values were observed again. Despite the slowing of water with time possibly indicating the immobilization of water from a “glasslike” state, the two-peak fit still produced a better (lower χ2) fit than for a stretched exponential. Although the thermogravimetric and 2H NMR data (see Figures 3-5) used to characterize the states of H2O may not be directly related to QENS data (different time scale of motion), when the data are considered in unison one obtains a useful indication as to the states of water in the formation of kanemite. Consequently, the most reasonable interpretation of the QENS data is by way of a two-peak model, which assumes the hydration of layered silicates involves two states of water, one free

Figure 8. Value of fwhm of the Lorentzian peak obtained from the 2-peak fit of the QENS data for sample C as a function of time.

and one bound or immobile. For a consistent interpretation of the data, the two-peak model will be applied to samples A, B, and C. This approximation allows comparisons to be made between the chemically divergent samples as to reaction kinetics and the presence of any kanemite-like layer silicate hydration within their respective microstructures.29,30 The fact that a 2-peak model was successfully used for kanemite while a 3-peak model was used for cement reflects the similarities and differences between the systems. A 3-peak model is necessary for cement to account for the significant amount of water in the layers next to the surface of the gel particles. The fractal surface area of the gel is extremely large, on the order of 400 m2/g,31 and hence the water associated with it constitutes a considerable fraction of the total water in the system. On the other hand, the kanemite layer particles would have a much lower specific area. 3. Rate of Silicate Layer Hydration. Plots of BWI of the various reaction systems as a function of time (29) Almond, G. G.; Harris, R. K.; Franklin, K. R. J. Mater. Chem. 1997, 7, 681. (30) Hanaya, M.; Harris, R. K. J. Mater. Chem. 1998, 8, 1073. (31) Rarick, R. L.; Bhatty, J. I.; Jennings, H. M. Surface Area Measurement Using Gas Sorption: Application to Cement Paste. In Materials Science of Concrete IV; Skalny, J., Mindess, S., Eds.; The American Ceramic Society: Westerville, OH, 1995; p 1.

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Figure 12. Schematic diagram of the binding of silicate layers in kanemite. Figure 9. Bound water index (BWI) for the formation of sample A as a function of time. All curves were fitted for Langmuir adsorption kinetics.

Figure 10. Bound water index (BWI) for the formation of sample B as a function of time. All curves were fitted for Langmuir adsorption kinetics.

bonding through O atoms of adjacent SiO4 sheets of the type Si-O-H‚‚‚OsSi.18,32 However, this model has been disputed on the grounds that the closest distance between O atoms of adjacent Si layers is 5.97 Å, which is probably too large for such bonding to be feasible.17 An alternative model has therefore been suggested in which the general hydrogen bond of the type Si-OH‚‚‚OsSi still exists, but the hydrogen participating in the bond is associated with the H of the H2O coordinated to Na+ rather than as a surface silicate silanol Si-OH.17 The O, however, remains associated with the silicate sheets. The present work interprets the BWI to be the water associated with the octahedral hydration spheres surrounding the sodium ions and involved in the direct binding of two silicate layers through surface hydroxyls (the BWI also includes the hydroxyl groups). This type of water is illustrated in the schematic diagram of Figure 12, during the formation of kanemite. Since the octahedrally bound sodium ions form a layer found between the silicate layers, it is useful to consider the formation of the octahedrally hydrated sodium layers as a measure of the overall rate of silicate layer hydration. The rate of reaction appears to follow firstorder reaction kinetics, but it may also be useful to consider a more specific relationship for interpreting the data. Figure 9 presents the BWI for the formation of sample A in which the data were fitted to the Langmuir surface adsorption reaction, which is a slight modification to the general equation for first-order kinetics. Langmuir adsorption kinetics, or the rate of hydration of one surface layer, can be expressed as follows33:

dθ ) ka(1 - θ)C - kdθ dt

Figure 11. Bound water index (BWI) for the formation of sample C as a function of time. Only the curve at 40 °C could be fitted for Langmuir adsorption kinetics; the other two curves are linear.

are presented in Figures 9-11. The crystal structure of kanemite consists of alternating silicate and hydrated sodium sheets,30 strongly hydrogen-bonded SiOH groups, and interlayer (crystal) water.32 Initially, it was hypothesized that the two sheets were connected by hydrogen (32) Wieker, W.; Heidemann, D.; Ebert, R.; Tapper, A. Z. Anorg. Alleg. Chem. 1995, 621, 1179.

(3)

where θ is the fraction of available sites that have reacted (equivalent to the fraction of monolayer formed), C is the concentration of the adsorbing species (H2O), and ka and kd refer to the respective rate constants of the competing adsorption and desorption processes. By integrating eq 3 it is possible to express the fractional coverage, θ, as a function of time, t

θ(t) ) k′(1 - e-kobst)

(4)

where k′ ) C/(C + kd/ka) and kobs ) kaC + kd. Equation 4 applies to systems in which full monolayer coverage is attained. In instances of incomplete monolayer cover(33) Karpovich, D. S.; Blanchard, G. J. Langmuir 1994, 10, 3315.

Water Within Hydrating Layered Sodium Disilicate

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Table 4. Fit Parameters ((1 SD) Obtained for the Kinetic Curves Presented in Figures 9-11 sample

k′

m

kobs

A - 20 °C A - 30 °C A - 40 °C B - 20 °C B - 30 °C B - 40 °C C - 20 °C C - 30 °C C - 40 °C

0.367 ( 0.005 0.459 ( 0.011 0.491 ( 0.039 0.496 ( 0.055 0.582 ( 0.006 0.613 ( 0.012

2.167 ( 0.032 1.771 ( 0.044 1.666 ( 0.134 1.468 ( 0.047 1.259 ( 0.012 1.190 ( 0.024

0.240 ( 0.001 0.419 ( 0.015 0.769 ( 0.058 0.053 ( 0.009 0.174 ( 0.007 0.423 ( 0.013

0.722 ( 0.057

1.127 ( 0.007

0.053 ( 0.006

age, both k′ and kobs can be determined fitting to the following equation:

θ(t) ) k′(m - e-kobst)

(5)

where m is a constant related to the extent of monolayer coverage (and also the residual OH in kanemite) and a small value of k′ indicates that the rate of desorption is greater than the rate of adsorption. To determine whether Langmuirian kinetics apply to the present reactions, the BWI for the reaction was plotted against time as a function of time at 20, 30, and 40 °C. Figure 10 provides a plot of BWI for the formation of sample A (kanemite) as a function of time. Clearly, all curves in Figure 9 could be successfully fitted to Langmuir adsorption kinetics, and the fit parameters ((1σ) are provided in Table 4. On this basis, it is evident that increasing the temperature increased the rate of adsorption as well as the surface coverage of the monolayer. The existence of this process in the other samples was investigated. Figure 10 indicates that the BWI for the formation of sample B could be fitted to Langmuir adsorption kinetics. Again, both the rate of adsorption and the surface layer coverage increased with increasing temperature. Compared to sample A, sample B had large values for k′ which suggests that the rate of desorption was slightly greater for the hydration process than sample A. The fact that kobs values for sample B are generally smaller than they are for sample A is consistent with this fact and suggests that the overall rate of reaction is smaller. For sample C (Figure 11), only the curve at 40 °C could be fitted to Langmuir adsorption kinetics. The curves at 20 and 30 °C could be fitted with a straight line, possibly reflecting k′ is small enough that the linear approximation to the exponential applies. The likelihood of a significant amount of hydrated silicate layers in sample C at 20 and 30 °C is remote since substantial dissolution of quartz is improbable at these temperatures.11 For sample C at 40 °C, k′ is larger than that in either sample A or B, suggesting the rate of adsorption has decreased (or the rate of desorption has increased). Discussion The hydration of silicate layers within ASR products presents a potentially new method of expansion that has largely been overlooked by cement researchers. ASR is generally associated with reactive silica being present within the aggregate. Amorphous or opaline silica are usually the most reactive forms of silica and, therefore, must be investigated to confirm the validity of any proposed theory. These experimental results demon-

strate that hydrated silicate layers appear using either a crystalline/layered or amorphous sodium disilicate source. The hydrated layers observed in the latter are considerably less ordered and structured than those observed in kanemite, making complete characterization and quantification difficult. Nonetheless, the present work provides a novel kinetic approach to determining the extent of layer hydration and formation in the amorphous material, assuming layer hydration occurs by a mechanism similar to that observed for kanemite. The sensitivity of the QENS technique for this reaction suggests the possibility of a standard test for ASR reactivity of aggregates. This would consist of reacting a sample with a standard solution containing natrosilite and measuring the kinetics by QENS measurements. The reactivity of a given aggregate would then be quantified in terms of kobs, which can be related directly to its ASR susceptibility. Traditionally, the expansion of ASR gels has been explained in terms of classical osmotic theory.34 Calcium silica hydrate-shell theory and calcium silica hydrate/ alkali exchange theory have also been put forth as mechanisms of expansion.35,36 The recent trend to explain expansion has been to combine classic osmotic swelling theory with double layer theory for amorphous structures.9,37 The present work supports the important role that alternative surface forces have in controlling the swelling within ASR gels. The unhydrated layered silicate has a basal spacing of 8.615 Å while the hydrated form has a spacing of around 10.27 Å.17 This represents a volume increase of almost 20% in one dimension so it could clearly have significant physical consequences for expansion in a material. In the present work, sample A would be expected to swell much more than sample B, largely due to the greater volume fraction of the hydrated layer silicates. In real systems also, the extent of swelling will depend on the content of hydrated silicate layers within the material. Measurement of the swelling potential of these layered silicates and amorphous silicate structures has not yet been done. Clearly, the role of expansion through silicate layer hydration as a factor in contributing to ASR swelling is an area of research that has largely been unexplored. Mention must also be made of the fact that the presence of hydrated silicate layers in kanemite is an idealized model of what actually occurs in real systems experiencing ASR. However, studying and characterizing kanemite formation has allowed a similar approach to quantify its formation in amorphous systems, which can then be more closely related to real systems. Conclusions QENS was confirmed as a useful technique for analyzing the kinetics of hydration of a series of layered silicates starting from both amorphous and crystalline sodium disilicate. A two-peak model consisting of both (34) Hansen, W. C. J. Am. Concr. Inst. 1944, 15, 213. (35) Chatterji, S.; Jensen, A. D.; Thaulow, N.; Christensen, P. Cem. Concr. Res. 1986, 16, 246. (36) Chatterji, S.; Thompson, J. L.; Knudsen, T. Cem. Concr. Res. 1988, 18, 363. (37) Prezzi, M.; Monteiro, P. J. M.; Sposito, G. ACI Mater. J. 1998, 95, 3.

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Gaussian and Lorentzian functions was used to fit the data and allowed the hydration process to be described in terms of bound and free water over the first 12 h. Thermogravimetric and 2H NMR analyses confirmed the validity of this model. By fitting the bound water index with first order reaction kinetics, specifically the Langmuir adsorption equation, it was possible to obtain quantitative information about the hydration of layered silicate structures, including an observed rate constant, kobs. Hydrated layered silicate structures were observed when kanemite was synthesized from either an amorphous or crystalline sodium disilicate source as confirmed by X-ray diffraction. A standard test for ASR reactivity could be based on the use of natrosilite as a reagent and compared in terms of kobs, the apparent rate constant.

Phair et al.

Acknowledgment. We are indebted to Paul Stutzman at NIST for conducting XRD analyses on the samples. Certain commercial equipment, instruments, or materials (or suppliers, or software, etc.) are identified in this paper to foster understanding. Such identification does not imply recommendation or endorsement by the Federal Highway Administration or the National Institute of Standards and Technology, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose. This work was performed while J.W. Phair held a National Research Council Research Associateship Award at the Turner-Fairbank Highway Research Center (FHWA). CM049820R