pubs.acs.org/Langmuir © 2009 American Chemical Society
High-Resolution Nanoprobe X-ray Fluorescence Characterization of Heterogeneous Calcium and Heavy Metal Distributions in Alkali-Activated Fly Ash John L. Provis,*,† Volker Rose,‡ Susan A. Bernal,†,§ and Jannie S. J. van Deventer† † ‡
Department of Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia, Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, and §Materials Engineering Department, Composite Materials Group (CENM), Universidad del Valle, Cali, Colombia Received May 1, 2009. Revised Manuscript Received June 27, 2009
The nanoscale distribution of elements within fly ash and the aluminosilicate gel products of its alkaline activation (“fly ash geopolymers”) are analyzed by means of synchrotron X-ray fluorescence using a hard X-ray Nanoprobe instrument. The distribution of calcium within a hydroxide-activated (fly ash/KOH solution) geopolymer gel is seen to be highly heterogeneous, with these data providing for the first time direct evidence of the formation of discrete highcalcium particles within the binder structure of a geopolymer synthesized from a low-calcium (100 μm, which are heterogeneous on both interparticle and intraparticle levels,3-5 which makes it a particularly interesting and challenging material to characterize. Here, the new hard X-ray Nanoprobe instrument operated by the Center for Nanoscale Materials in partnership with the Advanced Photon Source, Argonne National Laboratory,6,7 is used to study the nanoscale elemental disposition within fly ash particles and the hardened gel products, X-ray amorphous *To whom correspondence should be addressed. E-mail: jprovis@unimelb. edu.au. Phone: þ61 3 8344 8755. Fax: þ61 3 8344 4153.
(1) Manz, O. E. Fuel 1997, 76, 691. (2) Humphreys, K.; Mahasenan, M. Toward a sustainable cement industry. Substudy 8: Climate change, World Business Council for Sustainable Development, 2002. (3) Hemmings, R. T.; Berry, E. E. In Fly Ash and Coal Conversion By-Products: Characterization, Utilization, and Disposal IV; McCarthy, G. J., Glasser, F. P., Roy, D. M., Eds.; Materials Research Society: Warrendale, PA, 1988; pp 3-38. (4) Qian, J. C.; Lachowski, E. E.; Glasser, F. P. In Fly Ash and Coal Conversion By-Products: Characterization, Utilization, and Disposal IV; McCarthy, G. J., Glasser, F. P., Roy, D. M., Eds.; Materials Research Society: Warrendale, PA, 1988; pp 45-54. (5) Nugteren, H. W. Part. Part. Syst. Charact. 2007, 24, 49. (6) Maser, J.; Winarski, R.; Holt, M.; Shu, D.; Benson, C.; Tieman, B.; Preissner, C.; Smolyanitskiy, A.; Lai, B.; Vogt, S.; Wiemerslage, G.; Stephenson, G. B. In Proceedings of the 8th International Conference on X-ray Microscopy; Aoki, S., Kagoshima, Y., Suzuki, Y., Eds.; IPAP: Himeji, Japan, 2005; pp 26-29. (7) Shu, D.; Maser, J.; Holt, M.; Winarski, R.; Preissner, C.; Smolyanitskiy, A.; Lai, B.; Vogt, S.; Stephenson, G. B. AIP Conf. Proc. 2007, 879, 1321. (8) Wastiels, J.; Wu, X.; Faignet, S.; Patfoort, G. In Proceedings of the 9th International Conference on Solid Waste Management; Widener University: Philadelphia, 1993; pp 8. (9) Duxson, P.; Fernandez-Jimenez, A.; Provis, J. L.; Lukey, G. C.; Palomo, A.; van Deventer, J. S. J. J. Mater. Sci. 2007, 42, 2917.
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aluminosilicate materials termed “inorganic polymers” or “fly ash geopolymers”,8,9 formed by the reaction of fly ash with alkaline solutions. Geopolymers are currently being developed as an environmentally beneficial replacement for Portland cement for concrete production, offering comparable performance and cost while reducing greenhouse gas emissions by a factor of approximately 5.10 Given that cement production is responsible for up to 8% of global anthropogenic CO2 emission,2 this provides the opportunity to reduce CO2 emission by at least tens of millions of tonnes per annum worldwide. However, a number of aspects of geopolymer structure are not yet well understood, in particular, the role of calcium within the geopolymer structure and the possibility of the release of toxins from the fly ash into the environment while the material is in use. There have been a number of previous studies of elemental distributions within fly ash particles on a variety of length scales, using several different analytical techniques. Volatile elements are believed to become enriched at the surfaces of some large fly ash particles, as well as in the fine fractions of ashes.11-13 Early studies using particle-induced X-ray emission (PIXE) in a proton microprobe showed strong correlations between specific elements, in particular, a marked correlation between Fe and Ti (as well as various other elements, including Cr) and a weak correlation between these elements and Ca.14 Other studies have also been aimed at classifying fly ash particles into different categories (10) Duxson, P.; Provis, J. L.; Lukey, G. C.; van Deventer, J. S. J. Cem. Concr. Res. 2007, 37, 1590. (11) Linton, R. W.; Loh, A.; Natusch, D. F. S.; Evans, C. A.; Williams, P. Science 1976, 191, 852. (12) Linton, R. W.; Williams, P.; Evans, C. A. Anal. Chem. 1977, 49, 1514. (13) Cereda, E.; Braga Marcazzan, G. M.; Pedretti, M.; Grime, G. W.; Baldacci, A. Atmos. Environ. 1995, 29, 2323. (14) Jaksic, M.; Watt, F.; Grime, G. W.; Cereda, E.; Braga Marcazzan, G. M.; Valkovic, V. Nucl. Instrum. Methods Phys. Res., Sect. B 1991, 56-57, 699.
Published on Web 07/14/2009
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Provis et al. Table 1. Oxide Composition of Gladstone Fly Ash As Given by Bulk X-ray Fluorescence Analysis SiO2
Al2O3
Fe2O3
CaO
wt % 47.68 30.28 11.26 1.84 a LOI is the loss on ignition at 1000 °C.
MgO
SO3
Na2O
K2O
Cl
Cr2O3
Mn3O4
P2O5
SrO
TiO2
ZnO
1.41
0.14
0.33
0.47
0.362
0.0138
0.153
0.932
0.146
1.67
0.0225
according to their composition and/or microscopic appearance.15,16 Such analysis will not be conducted in this paper, because the very fine spatial resolution of the Nanoprobe instrument (∼30 nm) is ideally suited to detailed analysis of small regions rather than exhaustive sampling of large numbers of micrometer-sized particles. Synchrotron X-ray fluorescence (μ-XRF and tomography) has previously been applied to the study of individual coal fly ash particles.17-19 However, the products formed by the alkaline activation of fly ash have not previously been subjected to this type of analysis. Geopolymer gels are known to be heterogeneous on a length scale of nanometers to micrometers,9,20-23 but the nature of this heterogeneity in fly ash-derived geopolymers is at present poorly understood.24-26 Therefore, the primary aim of this paper is to analyze the structures of these materials on a submicrometer length scale. One complicating factor in the generation of construction materials from wastes is that waste materials must be either used “as is” or preprocessed; this means that the ability to understand the chemistry of fly ash and its interaction with alkaline solutions is critical to the successful valorization of a wide range of fly ash sources worldwide. Additionally, a key question in the reuse or valorization of any waste material is whether it contains any toxic or otherwise hazardous components which will impact its users. In the case of fly ash, the element of primary concern is usually chromium. Chromium is both highly mobile and toxic in its hexavalent form, which is released in preference to the poorly soluble Cr(III) upon exposure of fly ash to alkaline conditions.27 The fraction of chromium which is present as Cr(VI) in fly ashes has been shown to vary widely, from a few percent in most ashes up to a few rare instances in which 40% of the Cr present is hexavalent.28,29 If Cr is being released from the ash particles during geopolymerization, it would therefore be expected to be in this problematic form, and this behavior must be understood to ensure the safety of geopolymers in general construction applications.
Materials and Methods The ash studied here was taken from Gladstone power station, Queensland, Australia, derived from the combustion of black coal (15) Pietersen, H. S.; Vriend, S. P.; Poorter, R. E. P.; Bijen, J. M. In Fly Ash and Coal Conversion By-Products: Characterization, Utilization, and Disposal VI; Day, R. L., Glasser, F. P., Eds.; Materials Research Society: Warrendale, PA, 1990; pp 115-126. (16) Giere, R.; Carleton, L. E.; Lumpkin, G. R. Am. Mineral. 2003, 88, 1853. (17) Somogyi, A.; Janssens, K.; Vincze, L.; Vekemans, B.; Rindby, A.; Adams, F. Spectrochim. Acta, Part B 2000, 55, 1039. (18) Golosio, B.; Simionovici, A.; Somogyi, A.; Camerani, C.; Steenari, B. M. J. Phys. IV 2003, 104, 647. (19) Vincze, L.; Somogyi, A.; Osan, J.; Vekemans, B.; T€or€ok, S.; Janssens, K.; Adams, F. Anal. Chem. 2002, 74, 1128. (20) Provis, J. L.; Lukey, G. C.; van Deventer, J. S. J. Chem. Mater. 2005, 17, 3075. (21) Duxson, P.; Provis, J. L.; Lukey, G. C.; Mallicoat, S. W.; Kriven, W. M.; van Deventer, J. S. J. Colloids Surf., A 2005, 269, 47. (22) Lloyd, R. R.; Provis, J. L.; van Deventer, J. S. J. J. Mater. Sci. 2009, 44, 608. (23) Lloyd, R. R.; Provis, J. L.; van Deventer, J. S. J. J. Mater. Sci. 2009, 44, 620. (24) Rees, C. A.; Provis, J. L.; Lukey, G. C.; van Deventer, J. S. J. Langmuir 2007, 23, 8170. (25) Rees, C. A.; Provis, J. L.; Lukey, G. C.; van Deventer, J. S. J. Langmuir 2007, 23, 9076. (26) van Deventer, J. S. J.; Provis, J. L.; Duxson, P.; Lukey, G. C. J. Hazard. Mater. 2007, A139, 506. (27) Narukawa, T.; Riley, K. W.; French, D. H.; Chiba, K. Talanta 2007, 73, 178. (28) Huggins, F. E.; Najih, M.; Huffman, G. P. Fuel 1999, 78, 233. (29) Goodarzi, F.; Huggins, F. E. Energy Fuels 2005, 19, 2500.
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LOIa 3.7
Figure 1. Powder X-ray diffraction data (Cu KR radiation, Philips PW1800, 0.02° 2θ step size, 4 s/step) for the fly ash used in this investigation. M is mullite, Q quartz, and F a combination of iron oxides, mostly a “ferrite spinel” (Fe3O4 with substitution of various elements onto both Fe2þ and Fe3þ sites) but also possibly including maghemite and other iron oxides with peaks that overlap at the resolution of the instrument used.
for electricity generation, and is Class F according to ASTM C618. It is usually sold for use as a cement additive and is a relatively fine ash, with a d50 of 10 μm and a d80 of 20 μm. The bulk chemical composition of Gladstone fly ash is given in Table 1, and powder X-ray diffraction data are given in Figure 1. It should also be noted that in Figure 1, the iron oxide phases cannot be specifically identified because of severe peak overlap between maghemite and a substituted ferrite spinel phase similar to magnetite.30 Ashes that are low in unburnt carbon, as is the case here, have previously been observed to have similar amounts of Fe2O3- and Fe3O4-type phases,31 so it is likely that both are present here. The importance of X-ray diffraction is that it enables the identification of the ferrite spinel, which is able to host Cr3þ as an isomorphous substituent on the Fe3þ sites, as an important phase in the fly ash studied. We prepared fly ash samples for analysis by embedding a small amount of fly ash in a commercial epoxy binder (Araldite, Selleys, Australia). We prepared alkali-activated fly ash samples by reacting the fly ash sample with an 11 M KOH solution (“hydroxide-activated sample”) at a mass ratio of 0.41, and with a potassium silicate solution [11 M KOH and 11 M SiO2 (“silicateactivated sample”)] at a mass ratio of 0.56. These mass ratios gave identical molar ratios of alkali to fly ash, to enable direct comparison of the two samples. Samples were cured in sealed molds at 40 °C for 7 days and then demolded and maintained at room temperature in sealed plastic bags. Analysis was conducted on specimens which had been polished to a thickness of 100200 μm using successively finer grades of abrasive and with ethanol as a lubricant to minimize leaching of soluble components, based on the procedure used by Lloyd et al.22 Samples were analyzed using the Center for Nanoscale Materials Hard X-ray Nanoprobe instrument, which operates on Sector ID-26 at the Advanced Photon Source, Argonne National Laboratory.6,7 Briefly, this instrument uses two undulators as a (30) Winburn, R. S.; Lerach, S. L.; McCarthy, G. J.; Grier, D. G.; Cathcart, J. D. Adv. X-Ray Anal. 2000, 43, 350. (31) Chaddha, G.; Seehra, M. S. J. Phys. D: Appl. Phys. 1983, 16, 1767.
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Figure 2. Elemental maps of (a) calcium, (b) chromium, (c) iron, and (d) potassium in a 5.6 μm 5.6 μm region (71 points 71 points) of the fly ash/epoxy sample. Data are presented in units of total fluorescence counts per point in the spectral region of interest, and the scale used for each element is shown separately.
hard X-ray source, a Si(111) double-crystal monochromator to select the photon energy for X-ray excitation of the sample and Fresnel zone plate optics for focusing of X-rays on the sample. An active vibration control system based on laser interferometers provides accurate positioning. The selected photon energy was 11.3 keV, which is to some extent limited by the available zone plate X-ray optics but did not restrict the utility of the information obtained from this study as the elements heavier than Fe that are present in fly ash are not generally of significant interest in geopolymer chemistry. The photon flux of the focused beam was ∼5 108 photons/s. Emitted characteristic X-ray fluorescence radiation was detected with a four-element silicon drift energy dispersive detector (Vortex ME4). Scanning of the zone plate provided high-resolution maps of the elemental distribution of selected regions of the samples. The experimental setup used, operating in air, was capable of detecting fluorescence from elements with Z g 18. Particular attention was paid to K, Ca, Fe, and Cr in the samples studied here. Data for Ti were also collected; however, the levels of Ti observed at most points in the samples were very low, so this element was not subjected to such detailed analysis. Four full spectra are obtained at each point, one for each element of the Vortex detector. From these spectra, regions of interest (ROIs) in each spectrum are identified (see the Supporting Information), and the corresponding ROIs for all detector elements are summed to provide the total counts for Fe, Ca, Cr, K, and Ti at each point. To achieve a high energy resolution, the full width at halfmaximum of each ROI was selected to be ∼150 eV. Data were collected for 2.0 s per point, with a step size of 80 nm selected to enable utilization of the high spatial resolution of the instrument while mapping an area which is large enough to be considered to some extent representative of the sample. The narrow (11 μm) depth of focus of the Nanoprobe instrument was utilized to obtain accurate and high-spatial resolution data Langmuir 2009, 25(19), 11897–11904
from the relatively thick samples studied here. Element-specific focusing on individual particles within the sample allows unambiguous determination of the depth of constituent parts within the sample. Preliminary test work, presented as Supporting Information, using these samples has shown that variation in the focal depth provides the ability to separately identify particles located at different depths within the sample, proving that the results presented here are unlikely to be significantly affected by the overlap of multiple particles in the z-direction. The sizes of the square regions mapped varied from 5.6 to 9.6 μm and are noted in the captions of the respective figures.
Results and Discussion Fly Ash. Figures 2 and 3 show the elemental maps of K, Ca, Fe, and Cr in two different regions of the fly ash/epoxy sample. Both regions show the presence of multiple fly ash particles, with significant differences in composition, and morphologies that are apparently close to spherical. Hollow particles (Cenospheres) are commonly observed in fly ashes but were not seen in any of the samples (either fly ash/epoxy or alkali-activated fly ash) investigated here. A cenosphere-type morphology would be expected to result in a more uniform distribution of the fluorescence signal across the particle, as the shells of these particles are generally quite uniform in thickness.32 It is not possible to tell from the data obtained here whether the particles are solid spheres or plerospheres (Cenospheres filled with smaller particles), although the relative uniformity of the regions in the centers of the particles suggests that they are most likely solid. The morphology of the iron-rich areas within the particles, i.e., whether the iron content is uniform throughout the particle or present in a dendritic (32) Ngu, L.-N.; Wu, H.; Zhang, D.-K. Energy Fuels 2007, 21, 3437.
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Figure 3. Elemental maps of (a) calcium, (b) chromium, (c) iron, and (d) potassium in a 9.6 μm 9.6 μm region (121 points 121 points) of the fly ash/epoxy sample.
morphology intermixed with another phase, cannot be determined from these data because the hard X-ray beam used has a depth of focus of several micrometers as mentioned above. The composition presented at each point is therefore an average determined across an interaction volume which is much deeper than it is wide, which precludes identification of the dendritic structures sometimes observed in fly ash particles by backscattered electron imaging.33 Figures 2 and 3 each show part of an ∼10 μm fly ash particle containing all the Ca, Cr, and Fe. Such particles are well-known to comprise a significant fraction of most Class F fly ashes, and substitution of Ca2þ and Cr3þ onto the Fe2þ/3þ sites in the ferrite spinel phase is expected. The main Fe-rich particle in Figure 2 is poor in K, while the particle in Figure 3 is rich in K. Such variability in alkali content between particles is also well-known in fly ashes.13 There is no apparent surface enrichment of any of the elements studied; in a cross section taken across the scan region starting from the point of highest Fe intensity in Figure 3, the fluorescence intensities of all elements decrease according to the same trend (to within experimental uncertainty). It is also of interest that the chromium and iron sites seem to correspond very closely to each other in both scans; this will be discussed in more detail below. However, there are additional particles observed in each scan which are of particular interest in the context of the interactions between fly ash and alkaline solutions to form geopolymers. The role of iron in geopolymers has been the subject of some discus(33) Ramsden, A. R.; Shibaoka, M. Atmos. Environ. 1982, 16, 2191. (34) Fernandez-Jimenez, A.; Palomo, A.; Macphee, D. E.; Lachowski, E. E. J. Am. Ceram. Soc. 2005, 88, 1122. (35) Keyte, L. M. Ph.D. Thesis, University of Melbourne, Melbourne, Australia, 2008.
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sion recently,22,23,26,34,35 with some studies showing the presence of iron to have a negative impact on the formation of reaction products but others showing little or no effect. The localization of iron into specific particles within the fly ash but not others is therefore of potential importance, particularly given that these other particles identified are sometimes very rich in either potassium (top right of Figure 2d) or calcium (bottom left of Figure 3a), and these elements have been identified as being important in determining fly ash reactivity during geopolymer formation.36 Fly Ash Alkali Activation Products. To study the effect of alkali activation on the different elements present within fly ash, the ash was reacted with a potassium hydroxide activating solution and with a potassium silicate activating solution to form two geopolymer samples, as outlined in Materials and Methods. Nanoprobe data for these two samples are shown in Figures 4 and 5. An important point to note in Figures 4 and 5 is that the concentration of potassium is much higher in the geopolymer than in the fly ash/epoxy sample, because of the high concentration of potassium in the activating solutions used. This means that the regions of newly formed geopolymer binder can be identified as being the regions which are enriched in potassium alone (i.e., potassium aluminosilicate). Solvated potassium present in open pores, which is known to comprise some proportion of the potassium present in many geopolymers,37 will most likely be removed during polishing of the samples to a thickness of much less than a millimeter, and it is likely that this is responsible for the (36) Duxson, P.; Provis, J. L. J. Am. Ceram. Soc. 2008, 91, 3864. (37) Duxson, P.; Provis, J. L.; Lukey, G. C.; van Deventer, J. S. J.; Separovic, F.; Gan, Z. H. Ind. Eng. Chem. Res. 2006, 45, 9208.
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Figure 4. Elemental maps of (a) calcium, (b) chromium, (c) iron, and (d) potassium in a 5.6 μm 5.6 μm region (71 points 71 points) of the fly ash/KOH sample. (e) Expanded view of part of panel a (in the geopolymer binder region, bottom left corner), showing localized regions of high Ca concentrations (circled), with color scaling changed to highlight this.
low-potassium region in the bottom right-hand corner of Figure 5d. Figure 4 can therefore be identified as depicting a large fly ash particle centered beyond the bottom right-hand corner of the field of view, as shown by the Fe and Cr maps, with a region of geopolymer binder in the left-hand side of the image identifiable by its elevated potassium content. The calcium appears to have become significantly dispersed from its original location within the fly ash particle and has intermingled with the geopolymer binder. In contrast, the iron appears to have remained essentially unaffected by the geopolymerization process and is restricted to the area within the remnant fly ash particle. Similar observations have been made using elemental mapping in an electron microscope, although with a spatial resolution approximately 10 times lower.23 While it is not currently possible to compare “before and Langmuir 2009, 25(19), 11897–11904
after” images of the exact same fly ash particle undergoing geopolymerization to confirm that no movement has taken place, the fact that the iron concentration falls away smoothly at the particle edge suggests that it is unlikely that it has migrated significantly, if at all, into the geopolymer binder region. Figure 5 also depicts the edge of an apparently large iron-rich particle at the top of the field of view, with a remnant calcium-rich fly ash particle at the bottom left-hand corner and what appears to be a pore in the right corner, as mentioned above. The calcium in this sample is also clearly more dispersed than the iron or chromium. Like the hydroxide-activated case in Figure 4, calcium appears to have been released from the fly ash particle and become somehow incorporated into the binder phase. Exactly what structural role is played by the calcium in the geopolymer binder is not yet well understood, but it is believed to play an DOI: 10.1021/la901560h
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Figure 5. Elemental maps of (a) calcium, (b) chromium, (c) iron, and (d) potassium in a 9.6 μm 9.1 μm region (121 points 115 points) of the fly ash/potassium silicate sample.
important role in determining both the kinetics of geopolymer formation and the performance of the final product.23,38,39 However, the difference in binder homogeneity between the hydroxide-activated and silicate-activated samples can be clearly observed by comparing Figures 4 and 5, and this provides a critical demonstration of the unique capabilities of the Nanoprobe instrument. Panels a and d of Figure 4 in particular show a distribution of calcium and potassium, respectively, that is highly inhomogeneous on a length scale of tens to hundreds of nanometers, while the corresponding panels a and d of Figure 5 show much less local variability in composition. The narrow depth of focus of the Nanoprobe instrument, as discussed above, provides confidence that these are in fact isolated particles and not artifacts due to overlapping particles within a large probe volume. This observation of inhomogeneous calcium distribution is in agreement with the known differences in aluminosilicate framework microstructure between silicate-activated and hydroxide-activated geopolymer materials as observed under electron microscopy.21,23,40,41 However, direct comparison of elemental distributions within these microstructures has not previously been presented, and in particular, little emphasis has been placed upon the distribution of nonframework species as depicted here. Figure 4a shows that there are very localized regions of high Ca content within the region of the sample that can be identified as (38) Yip, C. K.; Lukey, G. C.; Provis, J. L.; van Deventer, J. S. J. Cem. Concr. Res. 2008, 38, 554. (39) Provis, J. L.; Yong, S. L.; Duxson, P.; van Deventer, J. S. J. In 3rd International Symposium on Non-Traditional Cement and Concrete; Bı´ lek, V., Kersner, Z., Eds.; Brno University of Technology: Czech Republic, 2008; pp 589-597. (40) Steveson, M.; Sagoe-Crentsil, K. J. Mater. Sci. 2005, 40, 4247. (41) Fernandez-Jimenez, A.; Palomo, A. Cem. Concr. Res. 2005, 35, 1984.
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containing geopolymer binder. While some of these high-Ca regions overlap with the fly ash particle visible in the Fe map (Figure 4c), the K concentration gradient suggests that the central part of the field of view contains both geopolymer and remnant fly ash phases overlaid on each other, with both being sampled due to the penetration depth of the hard X-rays. Considering the absence of small discrete high-Ca regions within any other fly ash particles observed here, these can be confidently identified as being located within the geopolymer binder. The K is also somewhat inhomogeneously distributed, but not to the same extent as Ca. A physical interpretation of this may therefore be proposed: calcium released from the fly ash particles into the very highly alkaline (pH >14) solution reacts rapidly with the hydroxide ions present, nucleating as particles of Ca(OH)2 which are then enclosed by the binder as it forms. This has been proposed as an explanation for the behavior and structure of metakaolin geopolymers containing calcium silicate minerals,38 but the formation of such discrete nanoscale particles has not been observed before now. The relative rates of release of calcium, silicon, and aluminum will also play some role here, with the formation of calcium (alumino)silicate hydrate and calcium aluminate hydrate phases also possible under some circumstances,42 but the chemistry of the hydroxide-activated system studied here suggests that Ca(OH)2 is the phase most likely to form. No newly formed crystalline calcium-containing phases are able to be identified in the sample by X-ray diffraction, which may mean that the high-calcium precipitates are crystallographically disordered or may simply be because they are small (a few tens of nanometers in size) and comprise only a very minor fraction of the total sample volume. (42) Yong, S. L. Ph.D. Thesis, University of Melbourne, Melbourne, Australia, 2009 (in progress).
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By comparison, Figure 5a, consistent with the scans of other regions of the silicate-activated sample (data not shown), does not show localized high-Ca regions. Instead, the Ca appears to be “smeared” around the edges of the fly ash particles, suggesting that it has been released from the particles and has been able to diffuse more uniformly into the surrounding region before the system hardened. It cannot be stated conclusively whether this region contains any discrete calcium silicate hydrate phases that have separated from the alkali aluminosilicate geopolymer binder, but the homogeneity of the elemental distributions of both Ca and K in Figure 5 suggests that this has not occurred. Previous work has shown the absence of such phase separation on a length scale of several micrometers in a similar system,23 and the results presented here enable a further reduction in the upper bound on the size of any segregated calcium silicate hydrate regions in this material to less than 100 nm. Given that X-ray photoelectron spectroscopic analysis of metakaolin-derived geopolymer systems does indicate nanoscale phase segregation in these materials,39,42 but extensive X-ray diffractometry studies have not shown any indication of crystalline hydrate formation, it appears increasingly likely that any segregation present is taking place on a length scale of at most a few nanometers. Disposition of Chromium in Fly Ash Geopolymers. As outlined in Introduction, a key concern in the use of fly ash in construction materials is the potential availability of chromium from the hardened materials. This is particularly the case for geopolymers, where the fly ash content is much higher than in Portland cement-based materials. The identification of a strong correlation between unreacted fly ash particles and chromium in the geopolymer samples studied here is therefore significant in determining whether these materials will meet widespread approval in the market. The importance of this particular correlation is that if the chromium is restricted to the unreacted (and generally not very porous) ash particles embedded within the geopolymer binder, it is much less likely to be accessible to the pore network of the geopolymer and is therefore less susceptible to leaching. Also, if the chemical state of the chromium within fly ash particles is resistant to leaching under the highly alkaline conditions prevailing during geopolymerization, it is likely to be capable of resisting removal under less aggressive service conditions. It has previously been observed27,43 that the level of available chromium in Australian fly ashes is in general low. The maximum leaching takes place in alkaline environments, and more alkaline ashes are usually more susceptible to Cr leaching. The Gladstone fly ash used here is considered quite an alkaline ash,44 meaning that its exposure to highly alkaline conditions during geopolymerization is likely to represent close to a worst-case scenario in terms of release of Cr from an Australian fly ash. The Gladstone fly ash used here contains 0.0138 wt % (138 ppm) chromium on an oxide basis according to bulk X-ray fluorescence analysis (Table 1), but Narukawa et al.27 have observed that the concentrations of total Cr and available Cr in Australian fly ashes show at most a weak correlation. Figure 6 shows the correlation between Fe and Cr concentrations in the unreacted fly ash and in the two geopolymer samples. The data are normalized to remove background effects, with the lowest concentration of each element in each sample region plotted as 0 and the highest as 1. The diagonal line on each plot therefore represents an exact correspondence between Cr (43) Jankowski, J.; Ward, C. R.; French, D. Preliminary assessment of trace element mobilisation from Australian fly ashes, Research Report 45, Cooperative Research Centre for Coal in Sustainable Development, 2004. (44) Pathan, S. M.; Aylmore, L. A. G.; Colmer, T. D. J. Environ. Qual. 2003, 32, 687.
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Figure 6. Correlation between Fe and Cr concentrations in (a) Gladstone fly ash, (b) hydroxide-activated fly ash geopolymer, and (c) silicate-activated fly ash geopolymer. Different symbols in each panel correspond to the different regions mapped in each sample. The region shown in Figure 2 is region A2; Figure 3 corresponds to region A3, Figure 4 to region B2, and Figure 5 to region C2. Full elemental maps for the other regions (A1, B1, and C1) were collected but are not presented in this paper. Data were normalized so that the highest concentration of each element in each region is displayed as 1 and the lowest as 0.
concentrations and Fe concentrations. It is apparent from Figure 6 that the correlation between the locations of these two elements in all samples is very strong, particularly in the intermediate concentration region. The data presented in Figure 6 were obtained from two to three regions on each sample, the regions shown in Figures 2-5 and additional regions on each sample whose full fluorescence maps are not presented in this paper, but the trends across regions appear to be very consistent. It is obviously somewhat perilous to attempt to represent a heterogeneous sample by analyzing a small number of small regions, but this consistency in the observed trends brings some measure of confidence in the analysis presented here. Similar consistency was not always observed in the correlations between other element pairs, in particular, the DOI: 10.1021/la901560h
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Fe-Ca and Fe-K pairs, reflecting the highly heterogeneous distribution of Ca and K across the ash particles. Interestingly, both panels a and b of Figure 6 (the fly ash and the hydroxide-activated sample, respectively) appear to show a number of data points falling in a region where the Fe concentration is very low but the Cr concentration is significant (up to 25% of the highest Cr fluorescence intensity recorded), and this may be identified as being of some concern in terms of the chromium being mobile. However, Figure 6c does not show any such points. This may simply be due to having selected two mapping regions in the silicate-activated sample which happened not to contain any regions of excess chromium content, or it may be due to the less aggressive silicate activating solution used in this instance. Given that such Cr-enriched regions were observed in the ash itself and that the chemistry of the other samples tested was consistent across the different regions analyzed, the former explanation seems more likely, but further work is necessary to provide decisive evidence one way or the other. However, regardless of the exact situation in the silicateactivated sample presented in Figure 6c, some important information can be obtained from Figure 6. In particular, it is observed that even a geopolymer formed by activation of an alkaline fly ash by a hydroxide solution, which should be considered close to a worst-case scenario as far as chromium release is concerned, does not contain significantly more mobile chromium (defined here as chromium which is not associated closely with iron) than the original fly ash. Given that the use of fly ash activated by an alkali hydroxide solution is becoming increasingly widespread in various applications in Europe and elsewhere,45-47 this observation is of some significance. The data presented here provide some insight into this behavior, but it would be necessary to confirm these observations by a technique which is sensitive to Cr oxidation states, such as spatially resolved X-ray absorption near-edge spectroscopy (μ-XANES), to fully confirm these suggestions. However, it should also be noted that many commercial incarnations of geopolymer concrete incorporate at least a moderate level of blast furnace slag, which accelerates the setting process and also gives added performance benefits in the final hardened product.10,36,38 Blast furnace slag contains sufficient sulfide to generate a reducing environment, which is capable of reducing mobile Cr(VI) to relatively immobile Cr(III). It has recently been shown that the addition of 0.5 wt % Na2S (simulating the reducing environment generated by slag) to a fly ash geopolymer matrix containing elevated levels of Cr(VI) added as Na2CrO4 reduces the leaching rate of chromium very markedly under both acidic and alkaline conditions.48,49 It may therefore be expected that, even if Cr is released from the fly ash into the binder region of these materials, its reduction to Cr(III) and consequent immobilization will minimize the potential for (45) Kovalchuk, G.; Fernandez-Jimenez, A.; Palomo, A. Fuel 2007, 86, 315. (46) Palomo, A.; Fernandez-Jimenez, A.; Lopez-Hombrados, C.; Lleyda, J. L. Rev. Ing. Constr. 2007, 22, 75. (47) Fernandez-Jimenez, A.; Palomo, A.; Pastor, J. Y.; Martı´ n, A. J. Am. Ceram. Soc. 2008, 91, 3308. (48) Zhang, J.; Provis, J. L.; Feng, D.; van Deventer, J. S. J. J. Hazard. Mater. 2008, 157, 587. (49) Zhang, J.; Provis, J. L.; Feng, D.; van Deventer, J. S. J. Cem. Concr. Res. 2008, 38, 681.
11904 DOI: 10.1021/la901560h
Provis et al.
harm to the biosphere during the use of slag-containing geopolymer concrete.
Conclusions Elemental mapping of fly ash and fly ash-derived geopolymers using hard X-ray fluorescence mapping with nominal 30 nm spatial resolution has been shown to provide valuable information regarding the structures of these materials. The calcium distribution in a hydroxide-activated fly ash geopolymer gel is highly heterogeneous, with discrete high-calcium particles observed within the binder structure. The exact chemical identity of these regions cannot be confirmed; however, the fact that they are at most a few tens of nanometers in size and highly enriched in calcium suggests that they may be Ca(OH)2 precipitates which nucleate as Ca2þ is released from the fly ash particles into the activating solution which contains a very high concentration of free OH-. The existence of such structures within geopolymers has previously been the subject of much speculation, but this work provides the first direct evidence confirming their existence. Calcium in the silicate-activated fly ash geopolymer is much more dispersed within the gel structure and appears to have spread more gradually from the fly ash particles during geopolymer formation. Calcium is also seen to have been released in preference to iron from the fly ash particles which are rich in both elements. The locations of chromium and iron are very closely correlated within the structures of both fly ash and the geopolymer product, suggesting that the chromium is present as Cr(III) within a ferrite spinel phase which is relatively resistant to alkali attack. This is important because chromium contamination is a potential issue in the valorization of fly ash, and the data indicate that the level of readily available and toxic chromium in a fly ash-derived geopolymer will be significantly lower than the total chromium content of the precursor fly ash. Acknowledgment. Use of the Center for Nanoscale Materials was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC0206CH11357. Use of the Advanced Photon Source was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract DE-AC02-06CH11357. This work was funded by the Australian Research Council (ARC), including partial funding from the Particulate Fluids Processing Centre, a Special Research Centre of the ARC, and through Discovery Project grants. The work of S.A.B. was supported by travelling scholarships from Colciencias and from the Walter Mangold Trust. Travel funding for J.L.P. was supplied by the Australian Synchrotron Research Program. We thank Dr. J€org Maser, Dr. Robert Winarski, Dr. Martin Holt, and Ms. Claire White for assistance with experiments on the Nanoprobe instrument. Supporting Information Available: An example of one of the spectra obtained from the Vortex detector elements with the ROIs identified and a more detailed discussion and validation of the probe volume of the Nanoprobe instrument. This material is available free of charge via the Internet at http://pubs.acs.org.
Langmuir 2009, 25(19), 11897–11904
