The Porous Network and its Interface inside Geopolymers as a

Article pubs.acs.org/JPCC

The Porous Network and its Interface inside Geopolymers as a Function of Alkali Cation and Aging Jaroslav Melar,†,∥ Guillaume Renaudin,*,†,§ Fabrice Leroux,‡,§ Adeline Hardy-Dessources,‡,§ Jean-Marie Nedelec,†,§ Christine Taviot-Gueho,‡,§ Elodie Petit,‡,§ Prune Steins,⊥ Arnaud Poulesquen,⊥ and Fabien Frizon⊥ †

Université Clermont Auvergne, ENSCCF, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France Université Clermont Auvergne, Université Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, 63000 Clermont-Ferrand, France § CNRS, UMR 6296, 63177 Aubière, France ∥ Department of Environment Protection Engineering, Tomas Bata University in Zlin, Faculty of Technology, 76272 Zlin, Czech Republic ⊥ CEA, DEN, DTCD/SPDE/LCFI-Marcoule, 30207 Bagnols-sur-Cèze, France ‡

S Supporting Information *

ABSTRACT: A combination of original, powerful characterization techniques was used to make a thorough description of solid geopolymers and of the associated effect of varying the alkali cation sourceNaOH, KOH, or CsOHand aging for up to several years. More specifically, the local and pore structures were progressively determined from atomic local scale up to several nanometers by pair distribution function analysis (PDF), small-angle X-ray scattering (SAXS), and longer correlation concerning the pore network, and possible diffusion and accumulation phenomena were unraveled by thermoporosimetry and electrochemical impedance spectroscopy (EIS), respectively. These complementary observations resulted in a picture of an interface between the mineral and the porous network that was correlated to the solvated alkali cation present in the porous solution. After a short time of a few months, the Na-based geopolymer was found to exhibit a smooth interface built up from small “elementary” particles. This contrasted with the K- and Cs-based geopolymers, which presented developed interfaces arising from hierarchically organized smooth particles forming aggregates of fractal outer surface. This striking difference unraveled by SAXS and EIS is ascribed for the Na geopolymer to the contact of the solvated Na(H2O)x+ cations with the amorphous mineral surface. The K(H2O)x+ and Cs(H2O)x+ solvated cations were an integral part of the porous solution, without direct contact with the mineral surface, thus leading to apparently rough interfaces. Dewatering occurred with time, mostly impacting the Na series. Overall, we obtained a detailed picture of the geopolymer series and their changes in time. The environment generated around the kosmotropic (ordermaking) Na+ alkali cation was more prone to change upon aging toward a non-Debye type relaxation than the initially developed interface supplied by the chaotropic Cs+ alkali cation, which was found to be relatively stable after 5 years.

1. INTRODUCTION Geopolymers are a class of largely X-ray amorphous threedimensional aluminosilicate binder materials, synthesized by the reaction of an aluminosilicate powder with a concentrated alkali metal silicate or hydroxide solution.1 The geopolymerization mechanism is well described2 and agreed upon,3,4 proceeding with a dissolution−polycondensation that yields a gel of a three-dimensional network subsequently turning into a solid-state material through a structural reorganization of the binder.5 The synthesis conditions, particularly the temperature,5,6 and the reactants (aluminosilicate source and the nature and concentration of the alkali ions added to the activation solution)6−9 are of prime importance for the © 2015 American Chemical Society

processes of material gelling and subsequent reorganization in time.10 Geopolymer materials are of interest in civil engineering11 and for immobilizing heavy metals or low- to mid-level nuclear waste.12−15 Obviously, changes in time of the pore structure need to be accurately known because the durability and performance of these materials are directly correlated to the mass transport properties inside the porous network and through the interface.16,17 Pore structure in geopolymers at an Received: March 10, 2015 Revised: July 9, 2015 Published: July 9, 2015 17619

DOI: 10.1021/acs.jpcc.5b02340 J. Phys. Chem. C 2015, 119, 17619−17632

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The Journal of Physical Chemistry C

be of interest to investigate and relate to the migration phenomena. About two decades ago, the high-frequency region was particularly well studied to correlate the electrical response of the concrete to physical characteristics of the microstructure.34−36 The temperature dependency of the relaxation frequency will be compared here with the activation energy of the conductive process for the geopolymer series to disentangle the effect of the cation Na+, K+ versus Cs+, in conjunction with aging. The evolution of the porous network has also been investigated by small-angle X-ray scattering (SAXS). Wellsuited to characterizing porous materials,37,38 SAXS can characterize any state of matter with no special preparation that might damage the morphology of an initially fragile microstructure.39 Also, the alkaline closed environment at the interface between the solid aluminosilicate networks has been characterized using atomic pairs distribution functions (PDF). For geopolymers, structural details are generally difficult to obtain by conventional diffraction methods because only a few, extremely broad Bragg peaks are visible on the XRD diagrams. The X-ray pair distribution function method, by utilizing the full range of information accessible from a powder diffraction experiment that is diffuse, along with Bragg scattering, has become a formidable tool in the analysis of complex or disordered materials in recent years. Hence, valuable insights have been gained into short-to-medium-range order in amorphous or nanocrystalline materials, such as disordered zeolites,40,41 poorly crystalline nanocomposites,42 gels,43 and glasses.44 Recent studies using the PDF method and based on X-ray scattering data collected from synchrotron sources have contributed to a better understanding of geopolymer structures.45,46 The combination of these different techniques allowed us to make a fuller description of the porous network in terms of size, water amount, and interface with the aluminosilicate mineral matrix, and their evolution in time.

early age has already been investigated by gas sorption− desorption isotherms or mercury intrusion porosimetry18,19 and more recently reinvestigated by neutron scattering20 or nanotomography.21 Nevertheless, a full description has not yet been achieved. There remain at least three hurdles: • Owing to the multiscale organization of the geopolymer pore structure and its associated dynamics, a complete understanding of the pore network may be obtained only by a multitechnique approach. • Alkali ions used for geopolymer synthesis have a strong influence on the aluminosilicate networks, but their role on pore structure remains poorly understood. • Data on the long-term behavior of geopolymers in service are still scant.22 Pore structure evolution of geopolymers several years old has not yet been investigated. The present work focuses on geopolymers using a multitechnique cross-study to determine how the alkali cations affect the evolution of the porous and aluminosilicate mineral networks, i.e., aging of the geopolymer inner interface. Besides classical gas sorption and mercury intrusion experiments, thermoporosimetry (TPM) was used here because it fills the gap between the two former techniques by detecting the shift in temperature of the phase transition.23 Because the Kelvin equation, applied to gas sorption branches through the BJH model, for instance, fails for large mesopores, and because mercury intrusion may disrupt the network because of the high pressure required for such a range of sizes, TPM appeared especially promising. Also, because the liquid probe can be chosen at will, it enabled us to study the materials in their native state using residual water. For structural building materials24 and for material containers,25 permeability is closely related to durability, with low permeability being usually indicative of high durability. To characterize these properties and to unravel middle-range order structure such as grain boundaries and phase mixing, electrochemical impedance spectroscopy (EIS) is a well-suited noninvasive, nondestructive method based on measuring the conductivity of migrating charge carriers in an applied electric field. EIS has proved most useful for scrutinizing migration of mobile ions within an openstructure such as zeolite26 or clay.27−29 We note the paucity of information in the literature on the use of AC impedance spectroscopy to characterize dry geopolymer-type material, while much attention is still being paid to the physical origin of the microstructure on fresh geopolymer pastes30 and the specific effects on the kinetics of geopolymerization and of the pore structure formation within the gel. For instance, it was found by EIS investigation that cesium nitrate and cesium sulfate had very similar effects, with a slight retardation of gelling followed by significant disruption of the gel structure, while cesium hydroxide markedly accelerated the gel formation.31 This EIS approach provides a better understanding of the chemical, physical, and mechanical processes that turn initial low-viscous slurries into stone-like solids. However, there is an interest in studying the dry powder to correlate the nominal composition (hydration rate, alkali, framework composition, and substitution) to its porous structure. A recent paper reports the effect of water on the order of magnitude of the dielectric loss,32 while another shows some investigations of nanocrystallized C−S−H and C−A−S−H samples using lowto-medium frequency AC impedance spectroscopy.33 In addition to the conductive aspect, dielectric behavior should

2. STARTING MATERIALS, GEOPOLYMER SYNTHESIS, AND CURING In order to focus on a model system and avoid the precipitation of calcium silicate hydrate phase mixed with geopolymeric materials, we opted to synthesize metakaolin-based geopolymer pastes. Metakaolin, purchased under the brand name Pieri Premix MK from Grace Construction Products, was analyzed by X-ray fluorescence (XRF). Its chemical composition is reported in section SEI1 in the Supporting Information. X-ray powder diffraction (XRPD) analyses and Rietveld treatments47 showed that the metakaolin contained crystalline anatase, kaolinite, and quartz as impurities. Waterglass activating solutions were prepared by simultaneously dissolving in Milli-Q water alkali hydroxide pellets (NaOH, KOH, Prolabo, Rectapur, 98%; CsOH, alfaAesar, 99.9%) and amorphous silica (Rhodia, Tixosil 331), whose main characteristics can be found elsewhere.6 Solutions were then stirred for 15 h, with all containers kept sealed to minimize contamination by atmospheric carbonation. Geopolymer samples were prepared by mixing metakaolin and alkali silicate solution in a standard laboratory mixer (European Standard EN 196-1) at low speed for 1 min and at high speed for 2 min. The material was then transferred to 4 × 4 × 16 cm3 PTFE molds, which were vibrated for a few seconds and sealed from the atmosphere. Samples were cured for 1 day at 20 °C before removal from the mold and stored in airtight bags at room temperature and pressure until tested. A first 17620


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part of the experimental impedance Z at a given temperature. In a first approximation, a parallel /R CPEp element was used to reproduce the high-frequency profile while a series +R CPEs was considered for the low-frequency domain. This simple equivalent circuit was adopted because it guarantees data are not misinterpreted and allows comparison with previous data.54,55 The experimental data points obtained in both series were simulated using nonlinear least-squares fits. The errors associated with the determined parameters were within ±5%. Electrical relaxation phenomena were analyzed through different formalizations. 3.6. Small-Angle X-ray Scattering. SAXS measurements using Mo radiation (λ = 0.71 Å) were made on a bench built by Xenocs. The scattered beam was recorded using a large online scanner detector (345 mm diameter, from MAR Research) located 750 mm from the sample stage. The scattered intensities were expressed versus the magnitude of the scattering vector Q = [(4π)/λ] sin(θ/2), where λ is the wavelength of incident radiation and θ is the scattering angle. Silver behenate in a sealed capillary was used as the scattering vector calibration standard. A piece of high-density PE sample was used to calibrate the scattering intensity in absolute units. Glass capillaries of diameter 1.8 mm (for the Na- and K-based geopolymers) and 1.2 mm (for the Cs-based geopolymer) were used. SAXS diagrams were recorded just after setting at room temperature (20 h), at 3 months, and at 6 months. The conditions of preparation are detailed elsewhere.10 All the scattering data are given in absolute intensity.56 3.7. Pair Distribution Function Analysis. PDFs were obtained from X-ray powder diffraction data collected on a Philips X’Pert Pro diffractometer (Bragg−Brentano θ−θ geometry) equipped with an X’Celerator Scientific detector and a Ag anticathode (Kα1 = 0.5594 Å, Kα2 = 0.5608 Å). Powder samples were mounted in a 27 mm diameter sample holder. Data were recorded over the 2° < 2θ < 130° range using variable divergence slits with a constant irradiated sample length of 10 mm and a step size of 0.01671°; the conversion to corresponding 0.03° pseudofixed-slit data was performed using the PANalytical X’Pert HighScore Plus software. The corresponding accessible maximum scattering vector Q magnitude was 20 Å−1, although 15 Å−1 was used as a cutoff value during the PDF analysis. Further processing of data sets was done using the software program PDFgetX257 to generate a normalized and corrected total scattering structure function, S(Q). The Fourier sine transform of the corrected and normalized data then yielded the reduced PDF function, G(r), according to the following equation (with Q = (4π sin θ)/λ, θ the scattering angle, and λ the wavelength of the incident radiation):

group of materials was tested after 2 months of storage (Na1, K1, and Cs1 samples), while a second group was tested after 5 years of aging (Na2, K2, and Cs2 samples). Small-angle X-ray scattering (SAXS) was used to characterize samples at 20 h, 3 months, and 6 months. All the geopolymers were prepared with the same overall formulation, with an Al2O3/SiO2/M2O/H2O molar ratio of 1/ 3.6/1/12 (M = Na, K, and Cs). The Al2O3/M2O molar ratio was adjusted to 1 to maximize the geopolymerization reactions.48,49 The differences were the alkali cation and the age of the materials.

3. EXPERIMENTAL METHODS 3.1. Thermogravimetric Analysis (TGA). TGA was performed on a SetaramTG92 apparatus. About 30 mg of the powdered samples was placed in an alumina crucible, and the measurements were made from room temperature up to 1100 °C with a ramp of 5 °C·min−1, under an air flow of 20 mL· min−1. A blank curve, obtained in the same experimental conditions with the same, empty crucible, was systematically subtracted. 3.2. 27Al and 29Si Solid-State Magic-Angle Spinning (MAS) NMR Spectroscopy. 27Al and 29Si MAS NMR spectra were recorded on a 300 MHz Bruker spectrometer operating at 7.04 T, the Larmor frequencies being 78.21 and 59.62 MHz. Zirconia rotors with 4 mm diameters were spun at 10 kHz in the magic angle spinning (MAS) condition. For 27Al MAS NMR spectra, short radio frequency pulses associated with a 90° pulse of 3.00 μs at 80 W were used, with a recycling time of 5 s. Chemical shifts were not corrected from the second-order quadrupolar effect, which induces a shift to lower frequency, and spectra were calibrated with AlCl3. For 29Si MAS NMR spectra, a 90° pulse of 5.10 μs at 85 W was used, with a recycling time of 60 s. The spectra were referenced to TMS (tetramethylsilane at 0 ppm). 3.3. Isothermal Nitrogen Adsorption−Desorption. The mesoporosity was studied with isothermal nitrogen adsorption−desorption measurements conducted at 77 K on a Micromeritics ASAP 2020 device. Before porosity measurements, freeze−drying was used to minimize degradation of the pore network.50 3.4. Thermoporosimetry. For TPM measurements, water was chosen as the probe solvent for its efficient wetting of the geopolymer surfaces. Because of the presence of residual water in the material, complementary measurements were made with o-xylene. TPM analyses were performed by differential scanning calorimetry (DSC) measurement with a Mettler-Toledo DSC 823e apparatus, using STARe software. Conditions were set according to recommendations in the literature.51 Data from the literature were used for calculations according to Brun et al. for water52 and Nedelec and co-workers for o-xylene.53 3.5. Electrochemical Impedance Spectroscopy. EIS measurements were performed on pressed pellets with a twopoint contacting electrode in the form of a plain capacitor (13 mm diameter). The cell was directly wired to an analyzer. Measurements were analyzed over a temperature domain ranging from −20 °C to +20 °C in the frequency interval 1 Hz to 1 MHz. The data were fitted using either the impedance or admittance functions as defined by adopting an equivalent circuit of Ri and Ci/ni constant phase element (CPE). A minimum number of time constants RiCi were used to reproduce the Z′ + jZ″ spectra, i.e., the real and imaginary

G (r ) =

2 π

∫0

∞

Q [S(Q ) − 1] sin(Qr ) dQ

4. RESULTS 4.1. Chemical Characterization. 4.1.1. Chemical Composition. The elemental chemical analyses of the samples were carried out by X-ray fluorescence (section SEI1 in the Supporting Information), and thermogravimetric analysis was used for the determination of the water content. TGA curves of the geopolymer samples are shown in Figure 1. The main weight losses were observed below 200 °C (corresponding to a dehydration process) and continued up to 800 °C in a less extended manner (corresponding presumably to a dehydrox17621


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using 5 wt % of pure silicon as internal reference (section SEI2 in the Supporting Information). Table SEI2 in the Supporting Information gives the mineralogical composition of the samples. Two kinds of crystalline compounds were present in the sample: well-formed crystalline phases already present in the metakaolin (quartz and anatase) and poorly crystallized phases formed during the geopolymerization process that were alkali cation-dependent (paragonite, trona, muscovite, and pollucite). Figure SEI2 in the Supporting Information shows the Rietveld plot (well-formed crystalline quartz and anatase and poorly crystalline pollucite hydrate) for the Cs2 sample. The Na- and K-based geopolymers contained the equivalent oxy-hydroxide MAl3Si3O10(OH)2 phyllosilicate (paragonite for M = Na, and muscovite for M = K) already present in the early aged samples. The weight percent of this phase slowly decreased in time: from 4.2 to 3.4 wt % and from 5.6 to 4.3 wt % for the Na- and the K-based samples, respectively. The 5 years Na-containing sample was partially carbonated (presence of 5.3 wt % of trona) despite precautions to protect the sample from air exposure. The decarbonation of trona evidently corresponds to the thermal event observed on TGA between 500 and 600 °C (Figure 1). The Cs-based samples behaved differently: whereas the early aged sample contained quartz and anatase, an additional third phase was observed in the 5 years sample (the oxy-hydrate pollucite of the zeolite family). The observation of these poorly crystalline zeolite and clay minerals necessitated long-time XRPD to permit relevant Rietveld analysis. From its bulk chemical composition, its quantitative mineralogical composition, and the electroneutrality principle, a chemical composition for the amorphous geopolymer part of the sample (between 85 and 96 wt % of the whole sample according to the alkali cation and aging) is proposed as follows:

Figure 1. TGA curves of the six geopolymer samples.

ylation process). The Na-based geopolymers presented a small thermal event between 500 and 600 °C (more visible for the 5 years Na2 sample) attributable to dehydroxylation or decarbonation. Results are summarized in Table SEI1 in the Supporting Information for the six geopolymer samples studied. As expected, the main elementary oxides were silicon oxide, aluminum oxide, alkali oxide, and water. The sum of these four elementary oxides corresponded to more than 99 mol % of the whole samples. The targeted Al/M ratio of 1 was relatively well attained (experimental values between 0.84 for Na1 and 1.09 for Na2), and the targeted Al/Si ratio of 0.56 was systematically exceeded (between 0.65 for K2 and 0.72 for Cs1). The water contents in the samples were very close to 55 mol % in the early aged samples Na1, K1, and Cs1, against a value close to 45 mol % in the 5 years samples Na2, K2, and Cs2. Aging of the sample led to weak dewatering (loss of relatively “free” water molecules filling the capillaries, as indicated by the difference in the TGA curves below 200 °C between the 2 months and the 5 years series). The bulk compositions deviated slightly from the composition of the geopolymer itself, with the presence of crystalline phases in small quantity. The quantitative mineralogical analysis was performed by Rietveld refinement of the XRPD patterns

Figure 2. MAS NMR spectra recorded on the six geopolymer samples: (a) 29Si nucleus and (b) 27Al nucleus. 17622


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signals were quite similar for the 2 months series, with a signal close to −80 ppm, corresponding to a mix of Q3(3Al) and Q4(4Al) components (expected near −72 and −85 ppm, respectively).61 Aging of the Na- and K-based geopolymers induced a shift toward lower values (around −84 ppm), corresponding to the aluminosilicate network condensation with formation of Q4(4Al) entities only, contrary to the Csbased geopolymer, which displayed an unmodified 29Si MAS NMR spectrum. Similar results were obtained with 27Al MAS NMR spectroscopy: the Na-based geopolymer signal changed from 55.7 ppm (Na1) to 54.6 ppm (Na2); the K-based geopolymer signal changed from 55.9 ppm (K1) to 54.6 ppm (K2), while no change was observed with the Cs-based geopolymer (55.5 ppm for Cs1 and 55.7 ppm for Cs2). The main signal of the 27Al MAS NMR spectra (close to 60 ppm) was typical of AlIV and could be ascribed to the geopolymer part of the sample, while the weak signal close to 0 ppm could be attributed to AlVI from the crystalline phases. 27Al MAS signals close to 55 ppm correspond to Q4(0Al) in compliance with the “Loewenstein” exclusion rule stating that linkage between AlO4 tetrahedra are energetically unfavorable in aluminosilicates.61,62 Contrary to the Cs-based samples, an aging effect was then observed for both Na- and K-based geopolymer amorphous mineral matrix corresponding to a tetrahedra condensation. 4.2. Pore Structure Characterization. Pore sizes were determined by both TPM (section SEI5 in the Supporting Information) and gas sorption.63 The presence of residual water that can be strongly bound to the materials, and the possible partial destruction of the porous network during desorption, are limitations to the application of gas sorption, but globally the results were consistent, even though values differed appreciably. Another source of variation for numerical values is the use of data related to pure water for TPM, whereas the solution present is known to be rich in alkali cations. Na-based geopolymers were characterized by two pore families with little evolution with aging (slight shift toward bigger pores with aging time). K-based geopolymers presented one pore family only whose size increased upon aging. The case of the Cs-based geopolymer was different: one family similar to K-based geopolymer after a few months (Cs1) with no evolution upon aging. All the data are gathered in Table 1. All

Na1 ⇒ SiAl 0.71Na 0.85O3.20 (OH)0.57 (H 2O)1.97 Na2 ⇒ SiAl 0.65Na 0.60O3.16 (OH)0.23 (H 2O)1.37 K1 ⇒ SiAl 0.69K 0.73O3.16 (OH)0.48 (H 2O)1.84 K2 ⇒ SiAl 0.65K 0.64O3.11(OH)0.37 (H 2O)0.94 Cs1 ⇒ SiAl 0.72Cs0.75O2.97 (OH)0.98 (H 2O)1.97 Cs2 ⇒ SiAl 0.72Cs0.70O3.11(OH)0.64 (H 2O)1.37

The whole samples were composed of about 90 wt % of geopolymer with the following average composition: SiAl0.70A0.80O3.10(OH)0.70(H2O)1.90 for the 2 months series (Na1, K1, and Cs1) and SiAl0.65A0.65O3.10(OH)0.40(H2O)1.20 for the 5 years series (Na2, K2 and Cs2). The average calculated molar ratios were: Al/Si = 0.67 (targeted value 0.56) and Al/M = 0.9 (targeted value 1.0). Calculations agree with the dewatering observed upon aging (decreased water and hydroxyl molar ratios), indicating either a decrease in the porous network volume or its dewatering with no volume modification. 4.1.2. Aluminosilicate Network. The aluminosilicate network of the geopolymer part of the samples was studied by infrared spectroscopy (section SEI3 in the Supporting Information), Raman spectroscopy (section SEI4 in the Supporting Information), and MAS NMR spectroscopy on 27 Al and 29Si nuclei (Figure 2). Infrared spectra of the samples are shown in Figure SEI3 in the Supporting Information. The spectral range corresponding to O−H stretching vibrations (Figure SEI3a) were extremely similar for all the samples, with one main broad band around 3475 cm−1 and a shoulder around 3250 cm−1, indicating that the dewatering effect previously observed on the TGA curve with aging did not modify the wide range of hydrogen bond strengths (i.e., dewatering of capillary water). A weak signal close to 3750 cm−1 was also observed, which can be attributed to O−H stretching from silanol.58,59 The spectral range from 800 to 2000 cm−1 (Figure SEI3b) showed the bending vibration of water around 1650 cm−1, the modes of vibration of carbonate groups (the out-of-plane bending ν2 and the asymmetric stretching ν3 modes from carbonate included in the geopolymer, as well as the ν3 mode from the crystalline trona Na2CO3·NaHCO3·2H2O phase at 1460 cm−1 for the Na-based geopolymers) and the vibrations of the aluminosilicate network. Weak differences between the spectra are detailed in section SEI3 in the Supporting Information and suggest that aging had a structuring effect (condensation of the Si- and Al-tetrahedra network) for the Naand K-based geopolymers but not for the Cs-based geopolymer. Raman spectra were mainly composed of vibrations from the minor crystalline anatase phase (section SEI4 in the Supporting Information). The aluminosilicate network vibrations observed close to 1050 cm−1, attributed to the silicate symmetric stretching mode,60 characterize a slight increase in the tetrahedra condensation (increase in Q4 intensity and decrease in Q2 intensity) when the alkali radius decreased. The 29Si and 27Al MAS NMR spectra shown in Figure 2 are in agreement with infrared and Raman spectroscopy results. An aging effect was observed for both Na- and K-based geopolymers, but not for the Cs-based geopolymers (in agreement with infrared observations detailed in section SEI3 in the Supporting Information). For the 29Si MAS NMR spectroscopy, the signal was centered at −79 ppm for Na1 versus −83 ppm for Na2, −80 ppm for K1 versus −84 ppm for K2, and −81 ppm for Cs1 versus −81 ppm for Cs2. The three

Table 1. Textural Characteristics of Samples Determined by Thermoporosimetry and by Nitrogen Sorption Isotherms thermoporosimetry

nitrogen sorption

sample

pore radius (nm)

pore radius (nm)a

specific surface (m2/g)b

Na1 Na2 K1 K2 Cs1 Cs2

2 and 4 2 and 5 3 6 2 −

12 14 8 16 5 5

44 16 124 43 166 135

a

Pore size distributions calculated with the BJH method from desorption branch. bSpecific surface estimated by BET method.

the samples showed an adsorption isotherm of type IV behavior related to mesoporous materials or a mixture of type II and type IV (see K samples after aging in Figure 3) that would indicate a dual mesoporous−macroporous structure. Nevertheless, the Na-containing geopolymer differed significantly from Cs-containing geopolymers by the absence of a flat region above the hysteresis loop. The K1 sample acted like the two Cs17623


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Figure 3. Evolution of the nitrogen adsorption (dark blue curves)−desorption (pink curves) isotherms of the various geopolymers.

Figure 4. Evolution of the SAXS spectra as a function of time (∼20 h, ∼3 months, and ∼6 months) for (a) Na-based geopolymer, (b) K-based geopolymer, (c) and Cs-based geopolymer; (d) comparison of the SAXS results according to the alkali activator for 180 days.

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Figure 5. Experimental PDFs for metakaolin (top left plot) and the geopolymers (bottom left plots) with the PDFs of the geopolymers in the 2−4 Å range (bottom right plots): comparison of 2 month and 5 year samples). The major contributions are labeled according to White et al.46

(Euclidian-type), the scattered intensity decreases as a function of q−4, while for an interface presenting roughness (fractaltype), the power dependence q−n (Porod slope) is in the 3 ≤ n ≤ 4 range, as for Vycor glass.66,67 On the other hand, for the Kand Cs-based geopolymers, Figure 4b,c, the evolution of the interface was more complex. The scattering curves did not present a power law dependency over the entire q-domain but did present a convex curvature. The scattering intensity increased with time, which means that either the quantity of interfaces per diffused volume increased or a part of the interstitial solution evaporated, entailing a change in the electronic contrast. The convex curvature is visualized by a disruption of the linear trend in the form of a hump centered at position qi. The position of this hump was found to be alkalidependent as well as time-dependent. On a size scale, it can be interpreted as particles of size domain lower than 2π/qi corresponding to the first linear disruption at qi = 3 nm−1. These “elementary” Euclidian-type particles of size ≈2 nm aggregated to form a larger domain of size observed at the centered hump, equal to qi(K) = 0.5 nm−1 regardless of the time, and from qi(Cs) = 1 to 0.75 nm−1. These aggregates presented an extended interface because a strong deviation in q−4 was observed above this structuring (linear trend at lower q than the associated hump). This can be explained by a fractal dimension of the outer surface of the aggregates.68 A stronger deviation was observed for Cs than for K-based geopolymers, indicating the presence of a fractal developed surface in the former series. Moreover, the Porod regime (q−4) appeared at higher scattering vector when the alkali cation turned to being more chaotropic (Cs+ > K+ > Na+), indicating an associated size domain range (aggregates and pores): Cs+ < K+ < Na+. The quantity of interface using the Porod equation is difficult to assess because the electronic contrast between the solid geopolymer and the poral solution is unknown. On the other hand, the mean pore size may be approached with Figure 4d.

containing samples, whereas the aged K2 sample acted like the two Na-containing samples, indicating that the three alkalibased samples presented three distinct behaviors. The shape of the hysteresis was characteristic of cylindrical pores for Na1 and of interconnected pores for the K1 and Cs1 samples. The Brunauer−Emmett−Teller (BET)64 specific surface areas presented in Table 1 were dependent on the alkali cation used: the higher the alkali ion radius, the higher the specific surface (44, 124, and 166 m2·g−1 for Na1, K1, and Cs1 respectively). Specific surface area also decreased with aging. The decrease was marked for Na2 and K2 (3-fold) and lower for Cs2 (in agreement with the similar pore radius for Cs1 and Cs2 samples). Contribution of microporosity to SSA was also evaluated using the model of Harkins and Jura.65 The microporous surface fraction was higher for Na > K > Cs and increased with aging (i.e., as specific surface decreased). This is clearly due to possible shrinking of the porous network over time. Exact pore size determination is not easy, but both techniques indicated an increase in average pore radius as the alkali cation radius decreased (or as alkali cation hydration energy increased) and with aging. 4.3. Structure Formation of the Geopolymer. The evolution of the structure of the geopolymers was assessed by SAXS as a function of time: just after setting (20 h), 3 months, and 6 months. Figure 4a−c present the scattering intensity as a function of time for the three alkali-based geopolymers. The measured signals consisted of a transition regime at low q value and a Porod regime at higher q value due to the inner porous network of the geopolymers. Figure 4d presents the same results for t = 6 months by plotting the intensity in Porod’s representation, i.e., I·q4 as a function of the scattering vector (q). For the Na-based geopolymer, the intensity weakly evolved during the densification of the material. From q = 2 nm−1 down to 0.25 nm−1, a stable power law dependency q−3.7 was observed (Figure 4a). Generally, for smooth interface 17625


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Figure 6. Nyquist representation of Z″−Z′ (ω) curve for K2O-geopolymer at −18, −14, −9, −2, +4, +10, and +19 °C. Left-hand plot shows the high-frequency part.

∼2.7 Å in K1 and K2 was ascribed to K−OH2O bonds; the rather intense peak near ∼3.2 Å in Cs1 and Cs2 was probably due to both T−T and Cs−OH2O correlations. Upon aging, the dewatering of the material may lead to rearrangement of the solvated alkali cations in the porous network. The remarkable similarities observed in short-tomedium range order between Cs and K geopolymers after 2 months and 5 years suggest that only minor atomic rearrangements in these geopolymer structures occurred upon aging (Figure 5, right), presumably because these two alkali cations present weak hydration energies (with a limited number of neighboring water molecules) and are not sensitive to dewatering. On the other hand, for Na1 and Na2, an evolution was observed in the M−OH2O peak upon aging with a split into two peaks, at r ∼ 2.32 Å and r ∼ 2.66 Å, attributed to the dehydration of the sodium cation. As water was expelled from the geopolymer, the dewatering of the highly solvated Na+ cation (due to large hydration energy) induced a decrease in its number of neighboring water molecules, which moves it closer to the oxygen anion from the mineral matrix. In this configuration, Na+ cations were located at the interface of the porous network with Na−OH2O interatomic distances of about 2.32 Å (from water molecule in the porous network) and Na− OO2− interatomic distance of about 2.66 Å (from oxygen anions in the mineral network). The formation of Na−O−T linkages implies little reorganization of the mineral network, with reorientation of the Si- and/or Al-tetrahedra contributing to the modification observed for the signal attributed to T−T at ∼3.1 Å: appearance of the Na−T contribution. The fact that only the Na-based sample exhibited a structural evolution upon aging is consistent with the strong hydration energy of sodium relative to potassium and cesium ions.71 4.4. Electrochemical Impedance Spectroscopy. Duplicated electrochemical impedance spectroscopy data were recorded for the geopolymer series. Selected Nyquist plots, imaginary part as a function of the real part of the impedance Z″ versus Z′(ω), are displayed in Figure 6 for the K2Ogeopolymer. The impedance plots are dependent on the temperature of measurement. For low-temperature measurements, a depressed semicircle was observed at high frequency in association with a sloping straight line in the lower-frequency region. When the temperature increased, the semicircle was barely observed, and only a straight line was visible (Figure 6, left). The feature at high ω and lower temperatures was due to the response of the bulk and the associated dielectric

The peak observed in the Porod representation may be related to the mean pore size distribution. To gain insight into the mean pore size (diameter) according to the alkali activator, the SAXS data were modeled by a sphere model convoluted by a log-normal law.69 The correlation length assimilated to the mean pore diameter was then equal to 5.6 ± 2.0 and 3.8 ± 1.4 nm for the K- and the Cs-based geopolymer, respectively, after 6 months of aging. For 2 months of aging, this correlation length was equal to 2.8 ± 1.2 nm for the Cs geopolymer. These results agree well with the mean pore size obtained from adsorption−desorption gas method. The decrease in the mean pore size with the increase in the radius of the alkali activator may be explained by the chaotropic nature of the alkali cations. As explained by Steins et al.,10 during the dissolution of the metakaolin, the size of the alumino-silicate oligomers is alkalidependent, with Cs < K < Na. Consequently, small-sized oligomers may move more freely in the fresh paste, and when the geopolymer network appears, the densification of the geopolymer network results in a finer mean pore size (and also aggregate size). Figure 5 displays the X-ray pair distribution functions (PDFs) of metakaolin-based geopolymers together with the PDF of the starting metakaolin used for the synthesis. The PDF obtained for metakaolin was quite similar to that recently published by White et al.46 attributing the first peak with a large maximum centered at ∼1.65 Å to the nearest-neighbor T−O correlation, where T denotes Si- and Al-tetrahedra,44 while the O−O correlation contributes to the peak at r ∼ 2.65 Å and the T−T correlation is located at r ∼ 3.11 Å; peaks beyond 3 Å are the result of multiple pairs. The PDFs of the geopolymer samples resemble that of metakaolin. The T−O peak position at r ∼1.65 Å remains the same whatever the alkali cation (M) present in the structure, thus indicating similar T−O bond characteristics. In addition to the O−O correlation, the M− OH2O correlation resulting from the solvated alkali cation also contributes to the X-ray PDF of the geopolymers in the same range of r (close to 2.5 Å). Also, when going from Na to K to Cs, the correlation between M−OH2O pairs is expected to shift to a higher r value and to increase in intensity because of the increase in M−O bond length and greater X-ray scattering cross section, respectively. Consistent with the M−OH2O bond lengths reported in the literature for solvated cations70,71 and considering the interpretation of PDFs on geopolymers reported elsewhere,45,46 the peak at ∼2.5 Å in Na1 and Na2 was attributed to Na−OH2O correlation, and the shoulder at 17626


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indicates ZK ≅ R; n = 1 indicates ZK ≅ C). The refinements are summarized in Figure SEI6-2 in the Supporting Information. The K values were found to increase with temperature for all three alkali cations, while n decreased with temperature. The latter parameter is related to the semicircle Z′ vs. Z″(ω) depletion by n = 1 − (2θ/π) and to a fractal dimension of the interface by n ≈ (Ds − 1)/2 (where Ds is an average of interfacial fractal dimension in the bulk). Such an approach is often tentatively associated with the pore size distribution and compared to fractal BET theory and to neutron scattering study of hydrating cements.72 The exponent n = 1 is interpreted as a continuous Euclidian medium with Ds = 3 and associated with a perfect semicircle, indicating a Debye-type relaxation. Smaller n values yield a smaller Ds that can be interpreted by some developed inner interfaces, thus deviating from the Debye case. A lower fractal dimension of the pore surfaces ranging from 2.2 to 2.3 was observed for the Cs geopolymer, shifting to slightly higher values (2.4−2.6) after 5 years. The Na geopolymer exhibited a value ranging from 2.8 to 2.6 close to a compact microstructure, but this decreased after 5 years, while the exponent n for the K-series remained unchanged with aging (Figure SEI6-2 in the Supporting Information). The variation in temperature has to be understood by a more compact microstructure due to the frozen state of the water molecules (decreasing the contrast between the mineral part and the porous network), thus resulting in an increase in n (associated with a decrease in capacitance). Dielectric loss, also called the dissipation factor tan δ, was preferred here to determine the temperature dependence of the relaxation time, noted as τR, and the resulting energy ER (see comments in section SEI 6 in the Supporting Information). The dielectric relaxation times, τR, were determined from the maximum position of the dissipation factor tan δ(ω) (Figure 8). This approach was preferred because it is model-free and

phenomena (see below), while increasing the temperature revealed the diffusion and the interface between sample and blocking electrode, both partly hindered at low ω for the lower temperatures. The frontier between the two typical features displayed a relative minimum in Z″(ω) absolute values, enabling us to calculate the resistance of the sample. This was refined using the equivalent circuit over the entire frequency domain (see Experimental Methods). As no special care was taken to avoid dehydration (during measurement), which might have damaged the porous network by the loss of water molecules, the spectral response above room temperature is not considered here (see Figure SEI6-1 in the Supporting Information and associated comment). As expected, the refined resistance was lowered (shift of the cusp Z″−Z′min to a smaller value) when the temperature increased, corresponding to an increase in ionic conductivity. Its associated temperature dependency expressed in the Arrhenius relation log(σT) versus 1000/T is reported in Figure 7 and followed a linear variation in all cases. The thermally

Figure 7. Arrhenius representation for the geopolymer series: ●, ○ for Na-; ■, □ for K-; and ▲, △ for Cs-samples. Filled and unfilled marks are for series aged 2 months and 5 years, respectively.

activated process of migrating species can be classified according to the activation energy as follows: Na+ (0.53, 0.66 eV) > K+ (0.46, 0.48 eV) > Cs+ (0.31, 0.32 eV) for 2 months and 5 years series, respectively. The activation energy (Ea) is a combination of the energy of defect formation and ion migration. For geopolymers, Ea should correspond to the migration of the solvated alkali cations through the porous network. For Na-based geopolymers, it underlined a conduction process that is highly dependent on the temperature, even more pronounced after aging, while for the other two alkali cations, K+ and Cs+, Ea remained near-identical. The trend in the conductivity between series went σ(Cs+) > σ(K+) > σ(Na+), with an ion conductivity of ∼10−4 S·cm−1 for Cs samples at room temperature and 1 order of magnitude lower for Na samples. This trend was even more pronounced when the temperature fell, evidently because of the difference in the activation energy between alkali cations. Scarcely reported for geopolymer-type materials, dielectric behavior was addressed. The refinement of the spectra by the equivalent circuit was conducted using nonideal capacitors that are a combination of a capacitive−resistive component, also termed constant phase element (CPE). The capacitance ZK as parallel element deviates from ideality, ZK = (jKω)−n, where K is the capacitance value and n the deviation from ideality (n = 0

Figure 8. Variation of tan δ(ω) for K-geopolymer and correlation of activation energy between ion conductive and dielectric relaxation process (inset).

may represent the most probable relaxation time, because the peak maximum is indicative of transition from long- to shorterrange mobility of charge carriers. The measurement of the dissipation factor gives an idea of the amount of electrical energy converted to heat by the geopolymer series. Similarly to Figure 7, the associated relaxation frequency values f R were determined at the frequency corresponding to the peak maximum and present a linear inverse dependence on temperature (Figure SEI6-4 in the Supporting Information). On a time scale, this can be physically interpreted with the time 17627


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Figure 9. Argand M″−M′(ω) for (a) Na-, (b) K-, and (c) Cs-geopolymers. All the series (2 months and 5 years) are reported over the reversible temperature domain. (M′ and M″ units: Ω rad s−1).

M″ = ωZ′

involved for the dipoles to reorganize, and shorter relaxation times are indicative of faster dynamics at the interfaces. The activation energy of the dielectric relaxation, ER, is calculated based on the following Arrhenius type equation: f R = f 0 exp(−ER/kT). Values are gathered in Table SEI6 in the Supporting Information. It is also interesting to compare Ea with ER activation energies obtained from the conductive and dipole relaxation processes (Figure 8, inset). The dependence in frequency of the dielectric properties of the geopolymer series may be interpreted as being due to polarization effects at the fractal pore inner interfaces. From the comparison between series members, Ea = ER for Na geopolymer for both aging times, while Ea > ER for K1 and Cs1, with the difference narrowing in time, because Ea ≅ ER for K2 and Cs2. Finally, loss modulus, M″, versus electric modulus, M′, also called Argand M″−M′(ω) plots were qualitatively analyzed. It is known that by adopting the electric modulus formalization for the interpretation of bulk relaxation properties, some advantages over other treatments may be observed, as the conductivity relaxation becomes prominent because of the suppression of the electrode effects.55 Conductivity and dielectric constant are obtained by converting the resistance and capacitance from the impedance data and are plotted in the frequency domain. Electric modulus is then defined as the inverse quantity of the complex permittivity by M* = M′ + jM″ = 1/ε* = jωZ* with

where M′, M″, ε*, Z* and ω are the real part and imaginary part of the electric modulus, complex permittivity, complex impedance, and pulsation, respectively. It is then of interest to compare the alkali-activated geopolymer series by their associated dielectric relaxation strength, which further informs us on the effect of interfacial polarization resulting from conductive mechanism (Figure 9). M″ versus M′(ω) curves exhibit a variably depressed semicircle. Close to ideal for the Na geopolymer, it is again the signature of a Debye behavior obtained in the case of small rigid molecules and molecular quasi-liquid continuous medium. The largely depressed semicircle for the two other alkali-activated samples indicates a deviation from Debye relaxation. Any deviation in the form of skewed semicircles, i.e., the dielectric relaxation strength (variation of diameter on x-axis in Figure 9), is a measure of the alignment of the dipoles inside the geopolymer according to the relation (M′ − ΔM/2)2 + M″2 = (ΔM/2)2, where ΔM = M∞ − Ms is the dielectric relaxation strength, M∞ the electric modulus at high frequency (ω → ∞), and Ms the electric modulus at zero frequency (ω → 0). A semicircle is described for all the series as a function of the temperature. The dielectric relaxation strength, ΔM, was found to be low and constant in temperature for the K and Cs geopolymer samples, with a slight change in the low-ω tail. Upon aging, the diameter of the semicircle was maintained, indicating that the dipole fluctuation for the two series remained identical. The general feature was

M′ = −ωZ″ 17628


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network and the porous network were also alkali-dependent and evolved upon aging. The interface topology was investigated with SAXS and EIS techniques. Treatment of the SAXS signals showed, over the q-domain studied, a smooth Euclidian-type interface for “elementary” particles forming aggregates with a fractal-type interface (smaller and rougher when alkali cation radius increased). Evolution of the interface topology, during the first 6 months, is difficult to characterize because of the dewatering process, implying the modification of the electronic contrast between the two interface sides. Nevertheless, SAXS interpretations agree with the specific surface area evolution (the fractal-type interface was correlated with an increase in the specific surface area value: increase from Na, K, and Cs, and decrease with time) and are consistent with EIS interpretations. For instance, for the 2 months samples, a fractal dimension (Ds values close to 2.2, extracted from the EIS data treatment) was observed for the Cs geopolymer, and an almost Euclidian-type interface was obtained for the Na geopolymer (Ds value close to 2.8). This agrees with SAXS measurements with a continuous Euclidian medium in the absence of larger aggregates for the Na series, while fractal aggregates were observed for the Cs series deviating strongly from the q−4 trend. At low q values, scattering curves did not reach a plateau, indicating a finite structure in that size domain. Thus, agglomerates of size larger than 25 nm (q lower cutoff of 0.25 nm−1) cannot be ruled out. Interestingly, SAXS provides a picture of a continuous medium for the Na series for which the only singularity comes from the “elementary” particles. A higher hierarchy was observed for the K and Cs series, “elementary” particles forming bigger aggregates with a scale factor between aggregate to agglomerate of about 3 and up to 8 for Cs and K series, respectively. Such a small scale factor corresponds to a small difference in the associated capacitance, too small to be visualized on a Nyquist diagram. Thus, this result explains why the depressed semicircle can be considered as one R||C element and not the sum of elements resulting from a hierarchically organized matter on sharply different size domains. With aging, the difference between samples receded, in agreement with specific surface area values and evolutions. Na geopolymer and Cs geopolymer fractal dimensions reached approximately that of the K geopolymer (which did not evolve over time: Ds ∼ 2.5). We note that SAXS experiments here give interesting information on the mean pore diameter, but quantification of the interface depends on the electronic density modeling for both the mineral and the porous networks (which are subject to dehydration). Interestingly, the ion migration was found to be irreversibly disconnected, because of the departure of water molecules, already above +19 °C. TGA demonstrated that the dehydration process occurred just above room temperature. The interstitial medium was found to be highly sensitive to an increase in temperature (its effect was already observed just above room temperature). It is roughly in agreement with recent data reported for alkali-activated metakaolin-based geopolymer where most of the water is “free water”, located in large pores or water hydrating chargebalancing cations.74 The authors show that the amount of water is the most dominant factor affecting density and open porosity, but in contrast to the behavior of the present aged samples (stored at constant RH), it was observed that most of the “free” water evaporated from the geopolymer in ambient conditions during extended aging. The progressive emptying of the porous network of alkali solution leads to a decrease in the conductivity combined with an increase in the activation energy. This

drastically different for the Na geopolymer. Much larger than for K and Cs samples, the diameter was markedly increased and the semicircle largely depressed for the Na sample after 5 years. This agrees well with the decrease of n for the 5 years Na geopolymer, which showed a deviation from an initial Debye case at two months. The diameter, ΔM, was proportional to the number density of fluctuating dipole.73 Thus, the strong modification for the Na geopolymer after 5 years indicates a change in the number of fluctuating dipoles, probably due to the dehydration leading to a modification of the environment of the less solvated Na+ cations.

5. DISCUSSION The global composition of the alkali-activated samples remained relatively constant with aging. The greatest part of the samples (about 90 wt %) was composed of the amorphous aluminosilicate geopolymer matrix with a composition close to SiAl0.70M0.70O3.10(OH)0.60(H2O)x for all the samples (M = Na, K and Cs). Minor crystalline phases were present: quartz (less than 1 wt %) and anatase (about 5 wt %) from initial metakaolin. A third nanocrystalline phase was formed during t h e g e o p o l y m e r iz a t i o n p r o c e s s : a p h y l l o s il i c a t e MAl3Si3O10(OH)2 for the Na- and the K-based samples, and pollucite Cs2Al2Si4O12·2H2O (zeolite family) for the Cs-based samples. The main difference was the aging-dependent water amount: x about 1.9 (about 55 mol %) for the 2 months series, and x about 1.2 (about 45 mol %) for the 5 years series. All the samples were stored in airtight bags, but dewatering was still evidenced between 2 months and 5 years corresponding to a weight loss of about 6 wt % (from ∼22 wt % of water for Na1 to ∼15 wt % for Na2, from ∼20 wt % of water for K1 to ∼10 wt % for K2, and from ∼15 wt % of water for Cs1 to ∼10 wt % for Cs2). Infrared spectroscopy showed no evolution of the O−H stretching region, indicating that dewatering during aging was not discriminative in terms of the nature of the water molecules (or H-bond strength). The water amount in the sample was apparently not alkali-dependent. IR, Raman, and NMR MAS spectroscopic techniques indicated that the amorphous mineral matrix was not alkali-dependent for the 2 months samples. The two smaller cations presented a weak tetrahedra network condensation between 2 months and 5 years, whereas no evolution was observed for the Cs-based geopolymer during the same time. The pore sizes were characterized by thermoporosimetry, nitrogen adsorption−desorption isotherms, and SAXS. Each technique has its own experimental limitations, but the different results provide a good representation of the porous network and of its evolution with aging. It is difficult to give absolute values for the mean pore diameters: 5−10 nm is a coarse scale order. The pore diameter decreased as the alkali cationic radius increased (i.e., when the alkali cation hydration energy decreased). The effective radius to consider here is not the alkali cationic M+, species but the solvated M(H2O)x+ species, which have opposite changes considering sodium, potassium, and cesium (because x decreases from Na to K to Cs, in correlation with their hydration energies). On the other hand, chaotropic (Cs+) and kosmotropic (Na+) character of the alkali cation should certainly impact the porous network formation during the geopolymerization process. The effect of aging was to increase the pore size and even out the different pore networks. Simultaneously, a decrease in the specific surface area (alkali-dependent) was systematically observed between the 2 months and 5 years series. These features indicate that the interface between the amorphous mineral 17629


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and/or waste immobilization, it is necessary to achieve the best possible representation of the porous network and of its interface with the amorphous mineral network. New results upon aging effects (from two months to five years) and alkali activators (Na, K, and Cs) extend earlier results.68 The complex description of the porous network due to closed and open porosity, dewatering with aging, anf high sensitivity of the samples on heating above room temperature implies the use of several time- and size-dependent techniques. The alkali ion mobility investigated through EIS supplied new important information characteristic of the solid−solid and solid−liquid interfaces. Interpretations were based on previous observations on the porous network structure (thermoporosimetry, nitrogen adsorption−desorption isotherms, and SAXS) and on the solid amorphous aluminosilicate network condensation (IR, Raman, NMR MAS, and PDF). The role of the alkali cation was predominant in the pore structure organization, its formation during the geopolymerization process and its evolution with aging. The global chemical composition was not agingdependent: SiAl0.70M0.70O3.10(OH)0.60(H2O)x. Nevertheless the x decrease from ∼ 1.9 for the 2 months series to ∼1.2 for the 5 years series induced a dewatering of the porous network, where the solvated alkali cations were located. The geopolymers studied exhibited a clear difference in their pore structure organization, with an apparent deviation from ideality when going from Na+ to K+ and Cs+ cations that should be correlated with an increase in the specific surface area. The interpretation as the fractal dimension of the developed interface is then possible but not trivial. Fractal dimension determined by EIS is directly correlated to the quantity of interface. Similarly, fractal interface is unraveled by SAXS, providing a picture of a continuous Euclidian medium for the Na series and developed outer surface of aggregates for the K and Cs series. The aging dewatering is accompanied by a decrease in the specific surface area of the geopolymer whatever the alkali cation. According to the alkali cation and the amount of water present in the porous network, the interface may be fairly smooth, as for kosmotropic Na+ cations, or largely developed, as in the case of the bigger chaotropic Cs+ cations. Our results highlight the need to further characterize the formation of the porous network during the geopolymerization process to better anticipate their potential interest as inorganic scavengers or structural building blocks.

indicates that the migrated species are the solvated alkali cations. After two months of networking, the Cs-based geopolymer presented the smallest Ea, the greatest ionic conductivity, and the least compact microstructure, in association with ER smaller than Ea. Conversely, Na cations resulted in a compact network with a higher Ea and smaller ionic conductivity and associated with a Debye-type dielectric relaxation. The K-based geopolymer ranged between these two cases. After 5 years, conductive and dielectric characteristics for the K-based geopolymer remained almost unchanged, while the Na network deviated significantly from ideality as observed on the Argand plot. For the Cs network, the conductive characteristics remained similar, but with a slight decrease in the interface quantity (fractal dimension in agreement with specific surface area evolution), thus promoting the activation energy of the dielectric relation, ER, at the same level as Ea. This is understandable as dewatering upon aging induced modifications at the interfaces for the Na-based geopolymer. For the kosmotropic Na alkali, departure of water molecules resulted in a slower conduction−diffusion mechanism that was more temperature-dependent, while the disruption of the water molecule network had a limited effect on a chaotropic type alkali cation-like Cs+. Intuitively, it may be tentatively suggested that the kosmotropic character of Na alkali through its hydration rate helps to stabilize a homogeneous compact network but at the same time is sensitive to hydration change. We may conclude that an environment generated around an order-making alkali is more prone to change/disorder upon aging than an initial relatively disordered one supplied by Cs alkali, which was found to be nonsensitive to aging. Similarly, PDF analyses showed almost no evolution for the two K and Cs geopolymers between the 2 months and the 5 years series. In contrast, the Na+ local environment showed modifications associated with dewatering. Whereas a single Na−OH2O interatomic distance was observed for the 2 months Na geopolymer presenting water filled capillaries, two distinct interatomic Na−O distances were determined for the dewatered 5 years sample. These two Na−O interatomic distances are attributed to oxygen from water molecules still present in the porous network and to nonbridging oxygen from the amorphous mineral network composed of Al- and Sitetrahedra. The expulsion of some of the water molecules from the porous network in the Na geopolymer modified the interface, on which solvated Na+ cations were bound for the 5 years Na geopolymer. Concerning the chaotropic smaller solvated M(H2O)x+ entities (for K and Cs), the movement of charge carriers into the porous network was weakly disturbed by the interface topology. The interface for the K and Cs geopolymers is then assimilated to a variably developed fractal surface.

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ASSOCIATED CONTENT

S Supporting Information *

Results from characterization of the mineral part of the geopolymers (X-ray fluorescence spectroscopy, X-ray powder diffraction, infrared spectroscopy, and Raman spectroscopy); thermoporosimetry curves, together with X-ray pair distribution functions; and a complete description of the results from electrochemical impedance spectroscopy. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.5b02340.

6. CONCLUSION The present study, based on a set of complementary experimental techniques, provides a fine description of the geopolymer interface developed between amorphous mineral and porous networks. The characterization of the porous network and its corresponding interface is essential for geopolymers because it concerns a large part of the material.75−77 The interface is the developed surface in direct contact with the surrounding media and environment. For the durability aspect of a mineral binder, or for ion exchange properties of sorbent materials used for catalytic application

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 00 33 4 73 40 73 36. Fax: 00 33 4 73 40 70 95. Notes

The authors declare no competing financial interest. 17630


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(20) Maitland, C. F.; Buckley, C. E.; O’Connor, B. H.; Butler, P. D.; Hart, R. D. Characterization of the Pore Structure of Metakaolinderived Geopolymers by Neutron Scattering and Electron Microscopy. J. Appl. Crystallogr. 2011, 44, 697−707. (21) Provis, J. L.; Rose, V.; Winarski, R. P.; van Deventer, J. S. J. Hard X-ray Nanotomography of Amorphous Aluminosilicate Cements. Scr. Mater. 2011, 65, 316−319. (22) Xu, H.; Provis, J. L.; van Deventer, J. S. J.; Krivenko, P. V. Characterization of Aged Slag Concretes. ACI Mater. J. 2008, 105, 131−139. (23) Nedelec, J. M.; Grolier, J. P. E.; Baba, M. Thermoporosimetry: a Powerful Tool to Study the Cross-linking in Gels Networks. J. Sol-Gel Sci. Technol. 2006, 40, 191−200. (24) Gordon, L. E.; Provis, J. L.; van Deventer, J. S. J. Durability of Fly Ash/GGBFS based Geopolymers Exposed to Carbon Capture Solvents. Adv. Appl. Ceram. 2011, 110, 446−452. (25) Cheng, T. W.; Lee, M. L.; Ko, M. S.; Ueng, T. H.; Yang, S. F. The Heavy Metal Adsorption Characteristics on Metakaolin-based Geopolymer. Appl. Clay Sci. 2012, 56, 90−96. (26) Ohgushi, T.; Sakai, Y. Movements of Ions in Zeolite A Containing Hydrogen Ion. J. Phys. Chem. C 2007, 111, 2116−2122. (27) Logsdon, S. D.; Laird, D. A. Dielectric Spectra of Bound Water in Hydrated Ca-smectite. J. Non-Cryst. Solids 2002, 305, 243−246. (28) Logsdon, S.; Laird, D. Cation and Water Content Effects on Dipole Rotation Activation Energy of Smectites. Soil Sci. Soc. Am. J. 2004, 68, 1586−1591. (29) Logsdon, S. D. Activation Energies and Temperature Effects from Electrical Spectra of Soil. Soil Sci. 2008, 173, 359−367. (30) Hajimohammadi, A.; Provis, J. L.; van Deventer, J. S. J. Effect of Alumina Release Rate on the Mechanism of Geopolymer Gel Formation. Chem. Mater. 2010, 22, 5199−5208. (31) Provis, J. L.; Walls, P. A.; van Deventer, J. S. J. Geopolymerization Kinetics. 3. Effects of Cs and Sr Salts. Chem. Eng. Sci. 2008, 63, 4480−4489. (32) Cui, X. M.; Liu, L. P.; He, Y.; Chen, J. Y.; Zhou, J. A Novel Aluminosilicate Geopolymer Material with Low Dielectric Loss. Mater. Chem. Phys. 2011, 130, 1−4. (33) Leroux, F.; Russias, J.; Cau-dit-Coumes, C.; Frizon, F.; Renaudin, G. Low to Medium Frequency AC Impedance Spectroscopy Investigations of Nanocrystalline Calcium Silicate Hydrate Dried Powders. J. Am. Ceram. Soc. 2011, 94, 2680−2687. (34) Christensen, B. J.; Coverdale, R. T.; Olson, R. A.; Ford, S. J.; Garboczi, E. J.; Jennings, H. M.; Mason, T. O. Impedance Spectroscopy of Hydrating Cement-based Materials: Measurements, Interpretation, and Application. J. Am. Ceram. Soc. 1994, 77, 2789− 2804. (35) Christensen, B. J.; Mason, T. O.; Jennings, H. M. Influence of Silica Fume on the Early Hydration of Portland Cements Using Impedance Spectroscopy (IS). J. Am. Ceram. Soc. 1992, 75, 939−945. (36) Gu, P.; Xie, P.; Fu, Y.; Beaudoin, J. J. A.C Impedance Phenomena in Hydrating Cement Systems: Frequency Dispersion Angle and Pore Size Distribution. Cem. Concr. Res. 1994, 24, 86−88. (37) Levitz, P.; Ehret, G.; Sinha, S. K.; Drake, J. M. Porous Vycor Glass: the Microstructure as Probed by Electron Microscopy, Direct Energy Transfer, Small Angle Scattering, and Molecular Adsorption. J. Chem. Phys. 1991, 95, 6151−6191. (38) Spalla, O.; Lyonnard, S.; Testard, F. Analysis of the Small Angle Intensity Scattered by a Porous and Granular Medium. J. Appl. Crystallogr. 2003, 36, 338−347. (39) Spalla, O.; Barboux, P.; Sicard, L.; Lyonnard, S.; Bley, F. Influence of Insoluble Elements on the Nanostructure of Water Altered Glasses. J. Non-Cryst. Solids 2004, 347, 56−58. (40) Martinez-Inesta, M. M.; Lobo, R. F. Investigation of the Negative Thermal Expansion Mechanism of Zeolite Chabazite Using the Pair Distribution Function Method. J. Phys. Chem. B 2005, 109, 9389−9396. (41) Martinez-Inesta, M. M.; Peral, I.; Proffen, T.; Lobo, R. F. A Pair Distribution Function Analysis of Zeolite Beta. Microporous Mesoporous Mater. 2005, 77, 55−66.

ACKNOWLEDGMENTS J.M. thanks ENSCCF (Ecole Nationale Supérieure de Chimie de Clermont-Ferrand) for financial support for a six-month doctoral internship.

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REFERENCES

(1) Davidovits, J. Geopolymers − Inorganic Polymeric New Materials. J. Therm. Anal. 1991, 37, 1633−1656. (2) Shi, C.; Fernandez Jimenez, A.; Palomo, A. New Cements for the 21st Century: The Pursuit of an Alternative to Portland Cement. Cem. Concr. Res. 2011, 41, 750−763. (3) Duxson, P.; Fernandez-Jimenez, A.; Provis, J. L.; Lukey, G. C.; Palomo, A.; van Deventer, J. Geopolymer Technology: the Current State of the Art. J. Mater. Sci. 2007, 42, 2917−2933. (4) Rees, C.; Provis, J. L.; Lukey, G. C.; van Deventer, Deventer J. S. J. Attenuated Total Reflectance Fourier Transform Infrared Analysis of Fly Ash Geopolymer Gel Aging. Langmuir 2007, 23, 8170−8179. (5) Fernandez-Jimenez, A.; Palomo, A.; Sobrados, I.; Sanz, J. The Role Played by the Reactive Alumina Content in the Alkaline Activation of Fly Ashes. Microporous Mesoporous Mater. 2006, 91, 111−119. (6) Poulesquen, A.; Frizon, F.; Lambertin, D. Rheological Behavior of Alkali-actived Metakaolin During Geopolymerization. J. Non-Cryst. Solids 2011, 357, 3565−3571. (7) Duxson, P.; Lukey, G. C.; Separovic, F.; van Deventer, J. S. J. The Effect of Alkali Cations on Aluminium Incorporation in Geopolymeric Gels. Ind. Eng. Chem. Res. 2005, 44, 832−839. (8) Fernandez-Jimenez, A.; Palomo, A. Caracterisation of Fly Ashes. Potential Reactivity as Alkaline Cements. Fuel 2003, 82, 2259−2265. (9) Lee, W. K. W.; van Deventer, J. S. J. Structural Reorganisation of Class F Fly Ash in Alkaline Silicate Solutions. Colloids Surf., A 2002, 211, 49−66. (10) Steins, P.; Poulesquen, A.; Diat, O.; Frizon, F. Structural Evolution During Geopolymerization, from Early Age to Consolidated Material. Langmuir 2012, 28, 8502−8510. (11) van Deventer, J. S. J.; Provis, J. L.; Duxson, P.; Brice, D. G. Chemical Research and Climate Change as Drivers in the Commercial Adoption of Alkali Materials. Waste Biomass Valorization 2010, 1, 145−155. (12) Palomo, A.; Palacios, M. Alkali-actived Cementitious Materials: Alternative Matrices for the Immobilization of Hazardous Wastes: Part II. Stabilisation of Chromium and Lead. Cem. Concr. Res. 2003, 33, 289−295. (13) Xu, J. S.; Zhou, Y. L.; Chang, Q.; Qu, H. Q. Study on the Factors of Affecting the Immobilization of Heavy Metals in Fly Ashbased Geopolymers. Mater. Lett. 2006, 60, 820−822. (14) Berger, S.; Frizon, F.; Joussot-Dubien, C. Formulation of Caesium Based and Caesium Containing Geopolymers. Adv. Appl. Ceram. 2009, 108, 412−417. (15) Desbats-Le Chequer, C.; Frizon, F. Impact of Sulphate and Nitrate Incorporation on Potassium- and Sodium-based Geopolymers: Geopolymerization and Materials Properties. J. Mater. Sci. 2011, 46, 5657−5664. (16) Monteiro, P. J. M.; Kirchheim, A. P.; Chae, S.; Fischer, P.; MacDowell, A. A.; Schaible, E.; Wenk, H. R. Understanding the Nano and Micro Structure of Concrete to Improve its Durability. Cem. Concr. Compos. 2009, 31, 577−584. (17) Frizon, F.; Gin, S.; Jegou, C. Mass Transfer Phenomena in Nuclear Waste Packages Advances in Transport Phenomena. Wang, L. Q., Ed.; Springer-Verlag: Berlin Heidelberg, 2009; Vol. 1, pp 31−133. (18) Sindhunata; Provis, J. L.; Lukey, G. C.; Xu, H.; van Deventer, J. S. J. Structural Evolution of Fly Ash-based Geopolymers in Alkaline Environments. Ind. Eng. Chem. Res. 2008, 47, 2991−2999. (19) Duxson, P.; Provis, J. L.; Lukey, G. C.; Mallicoat, S. W.; Kriven, W. M.; van Deventer, J. S. J. Understanding the Relationship between Geopolymer Composition, Microstructure and Mechanical Properties. Colloids Surf., A 2005, 269, 47−58. 17631


Article

The Journal of Physical Chemistry C (42) Petkov, V.; Parvanov, V.; Trikalitis, P.; Malliakas, C.; Vogt, T.; Kanatzidis, M. G. Three-Dimensional Structure of Nanocomposites from Atomic Pair Distribution Function Analysis: Study of Polyaniline and (Polyaniline)0.5V2O5.1.0H2O. J. Am. Chem. Soc. 2005, 127, 8805− 8812. (43) Schmücker, M.; Schneider, H. Structural Development of Single Phase (Type I) Mullite Gels. J. Sol-Gel Sci. Technol. 1999, 15, 191− 199. (44) Petkov, V.; Billinge, S. J. L.; Shastri, S. D.; Himmel, B. Polyhedral Units and Network Connectivity in Calcium Aluminosilicate Glasses from High-energy X-Ray Diffraction. Phys. Rev. Lett. 2000, 85, 3436−3439. (45) Bell, J. L.; Sarin, P.; Driemeyer, P. E.; Haggerty, R. P.; Chupas, P. J.; Kriven, W. M. X-ray Pair Distribution Function Analysis of Potassium Based Geopolymer. J. Mater. Chem. 2008, 18, 5974−5981. (46) White, C. E.; Page, K.; Henson, N. J.; Provis, J. L. In Situ Synchrotron X-ray Pair Distribution Function Analysis of the Early Stages of Gel Formation in Metakaolin-based Geopolymers. Appl. Clay Sci. 2013, 73, 17−25. (47) Rodriguez-Carvajal, J. PROGRAM FullProf.2k, version 3.20; Laboratoire Léon Brillouin (CEA-CNRS): Saclay, France, 2005; Fullprof.2k manual available on http://www-llb.cea.fr/fullweb/fp2k/ fp2k_divers.htm. (48) Barbosa, V. F. F.; MacKenzie, K. J. D.; Thaumaturgo, C. Synthesis and Characterisation of Materials Based on Inorganic Polymers of Alumina and Silica: Sodium Polysialate Polymers. Int. J. Inorg. Mater. 2000, 2, 309−317. (49) Gao, X. X.; Autef, A.; Prud’homme, E.; Michaud, P.; Joussein, E.; Rossignol, S. Synthesis of Consolidated Materials from Alkaline Solutions and Metakaolin: Existence of Domains in the Al−Si−K/O Ternary Diagram. J. Sol-Gel Sci. Technol. 2013, 65, 220−229. (50) Gallé, C. Effect of Drying on Cement-based Materials Pore Structure as Identified by Mercury Intrusion Porosimetry: A comparative Study Between Oven-, Vacuum-, and Freeze-drying. Cem. Concr. Res. 2001, 31, 1467−1477. (51) Landry, M. R. Thermoporometry by Differential Scanning Calorimetry: Experimental Considerations and Applications. Thermochim. Acta 2005, 433, 27−50. (52) Brun, M.; Lallemand, A.; Quinson, J. F.; Eyraud, C. A New Method for the Simultaneous Determination of the Size and Shape of Pores: the Thermoporometry. Thermochim. Acta 1977, 21, 59−88. (53) Hardy Dessources, A.; Hartmann, S.; Baba, M.; Huesing, N.; Nedelec, J. M. Multiscale Characterization in Hierarchically Porous Hybrid Materials. J. Mater. Chem. 2012, 22, 2713−2720. (54) Keddam, M.; Takenouti, H.; Novoa, X. R.; Andrade, C.; Alonso, C. Impedance Measurements on Cement Paste. Cem. Concr. Res. 1997, 27, 1191−1201. (55) Gatos, K. G.; Martinez Alcazar, J. G.; Psarras, G. C.; Thomann, R.; Karger-Kocsis, J. Polyurethane Latex/Water Dispersible Boehmite Alumina Nanocomposites: Thermal, Mechanical and Dielectric Properties. Compos. Sci. Technol. 2007, 67, 157−167. (56) Bösecke, P.; Diat, O. Small-angle X-ray Scattering at the ESRF High-brilliance Beamline. J. Appl. Crystallogr. 1997, 30, 867−871. (57) Qiu, X.; Thompson, J. W.; Billinge, S. J. L. PDFgetX2: a GUIdriven Program to obtain the Pair Distribution Function from X-ray Powder Diffraction Data. J. Appl. Crystallogr. 2004, 37, 678−678. (58) Tohoué Tognonvi, M.; Rossignol, S.; Bonnet, J. P. Effect on Alkali Cation on Irreversible Gel Formation in Basic Medium. J. NonCryst. Solids 2011, 357, 43−49. (59) Yu, P.; Kirkpatrick, R. J.; Poe, B.; McMillan, P. F.; Cong, X. Structure of Calcium Silicate Hydrate (C-S-H): Near-, Mid-, and Farinfrared Spectroscopy. J. Am. Ceram. Soc. 1999, 82, 742−748. (60) Renaudin, G.; Russias, J.; Leroux, F.; Cau-dit-Coumes, C.; Frizon, F. Structural Characterization of C-S-H and C-A-S-H Samples - Part II: Local Environment Investigated by Spectroscopic Analyses. J. Solid State Chem. 2009, 182, 3320−3329. (61) Massiot, D.; Messinger, R. J.; Cadars, S.; Deschamps, M.; Montouillout, V.; Pellerin, N.; Veron, E.; Allix, M.; Florian, P.; Fayon,

F. Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR. Acc. Chem. Res. 2013, 46, 1975−1984. (62) Loewenstein, W. The Distribution of Aluminium in the Tetrahedra of Silicates and Aluminates. Am. Mineral. 1954, 39, 92−96. (63) Barret, E. P.; Joyner, L. G.; Halenda, P. P. The Determination of Pore Volume and Area Distributions in Porous Substances. I. Computations from Nitrogen Isotherms. J. Am. Chem. Soc. 1951, 73, 373−380. (64) Brunauer, S.; Emmet, P. H.; Teller, E. Adsorption of Gasses in Multimolecular Layers. J. Am. Chem. Soc. 1938, 60, 309−319. (65) Harkins, W. D.; Jura, G. An Absolute Method for the Determination of the Area of a Finely Divided Crystalline Solid. J. Am. Chem. Soc. 1944, 66, 1366−1373. (66) Levitz, P.; Ehret, G.; Sinha, S. K.; Drake, J. M. Porous Vycor Glass: The Microstructure as Probed by Electron Microscopy, Direct Energy Transfer, Small-angle Scattering, and Molecular Adsorption. J. Chem. Phys. 1991, 95, 6151−6161. (67) Schaefer, D. W.; Keefer, K. D. Structure of Random Porous Materials: Silica Aerogel. Phys. Rev. Lett. 1986, 56, 2199−2202. (68) Schaefer, D. W.; Justice, R. S. How Nano are Nanocomposites ? Macromolecules 2007, 40, 8501−8517. (69) Steins, P.; Poulesquen, A.; Frizon, F.; Diat, O.; Jestin, J.; Causse, J.; Lambertin, D.; Rossignol, S. Effect of Aging and Alkali Activator on the Porous Structure of a Geopolymer. J. Appl. Crystallogr. 2014, 47, 316−324. (70) Mahler, J.; Persson, I. A Study of the Hydration of the Alkali Metal Ions in Aqueous Solution. Inorg. Chem. 2012, 51, 425−438. (71) Babu, C. S.; Lim, C. Theory of Ionic Hydration: Insights from Molecular Dynamics Simulations and Experiment. J. Phys. Chem. B 1999, 103, 7958−7968. (72) Niklasson, G. A.; Berg, A.; Brantervik, K.; Hedberg, B.; Nilsson, L. O. Dielectric Properties of Porous Cement Mortar: Fractal Surface Effects. Solid State Commun. 1991, 79, 93−96. (73) Frunza, L.; Schönhals, A.; Frunza, S.; Parvulescu, V. I.; Cojocaru, B.; Carriazo, D.; Martin, C.; Rives, V. Rotational Fluctuations of Water Confined to Layered Oxide Materials: Nonmonotonous Temperature Dependence of Relaxation Times. J. Phys. Chem. A 2007, 111, 5166− 5175. (74) Lizcano, M.; Gonzalez, A.; Basu, S.; Lozano, K.; Radovic, M. Effects of Water Content and Chemical Composition on Structural Properties of Alkaline Activated Metakaolin-based Geopolymers. J. Am. Ceram. Soc. 2012, 95, 2169−2177. (75) Bortnovsky, O.; Dedecek, J.; Tvaruzkova, Z.; Sobalik, Z.; Subrt, J. Metal Ions as Probes for Characterization of Geopolymer Materials. J. Am. Ceram. Soc. 2008, 91, 3052−3057. (76) O’Connor, S.; MacKenzie, K. J. D.; Smith, M. E.; Hanna, J. V. Ion Exchange in the Charge-Balancing Sites of Aluminosilicate Inorganic Polymers. J. Mater. Chem. 2010, 20, 10234−10240. (77) Skorina, T. Ion Exchange in Amorphous Alkali-Activated Aluminosilicates: Potassium based geopolymers. Appl. Clay Sci. 2014, 87, 205−211.

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The Porous Network and its Interface inside Geopolymers as a

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