1 High-Resolution Nanoprobe X-ray Fluorescence Characterization of

2 Heterogeneous Calcium and Heavy Metal Distributions in Alkali-Activated

3 Fly Ash

4 John L. Provis,*,† Volker Rose,‡ Susan A. Bernal,†,§ and Jannie S. J. van Deventer†

5†Department of Chemical and Biomolecular Engineering, University of Melbourne, Victoria 3010, Australia,

6‡Advanced Photon Source, Argonne National Laboratory, Argonne, Illinois 60439, and §Materials Engineering

7 Department, Composite Materials Group (CENM), Universidad del Valle, Cali, Colombia

8 Received May 1, 2009. Revised Manuscript Received June 27, 2009

9 The nanoscale distribution of elements within fly ash and the aluminosilicate gel products of its alkaline activation10 (“fly ash geopolymers”) are analyzed by means of synchrotron X-ray fluorescence using a hard X-ray Nanoprobe11 instrument. The distribution of calcium within a hydroxide-activated (fly ash/KOH solution) geopolymer gel is seen to12 be highly heterogeneous, with these data providing for the first time direct evidence of the formation of discrete high-13 calcium particles within the binder structure of a geopolymer synthesized from a low-calcium (<2 wt % as oxides) fly14 ash. The silicate-activated (fly ash/potassium silicate solution) sample, by contrast, shows a much more homogeneous15 geopolymer gel binder structure surrounding the unreacted fly ash particles. This has important implications for the16 understanding of calcium chemistry within aluminosilicate geopolymer gel phases. Additionally, chromium and iron are17 seen to be very closely correlated within the structures of both fly ash and the geopolymer product and remain within the18 regions of the geopolymer which can be identified as unreacted fly ash particles. Given that the potential for chromium19 release has been one of the queries surrounding the widespread utilization of constructionmaterials derived from fly ash,20 the observation that this element appears to be localized within the fly ash rather than dispersed throughout the gel21 binder indicates that it is unlikely to be released problematically into the environment.

22 Introduction

23 Fly ash is a byproduct of coal combustion produced at a rate of24 more than 450 million tonnes per annum worldwide1,2 and so25 presents significant challenges in disposal in addition to poten-26 tially highly profitable opportunities for valorization. Fly ash27 consists of particles ranging in size from∼0.1 to>100 μm, which28 are heterogeneousonboth interparticle and intraparticle levels,3-5

29 which makes it a particularly interesting and challenging material30 to characterize. Here, the new hard X-ray Nanoprobe instrument31 operated by the Center for Nanoscale Materials in partnership32 with the Advanced Photon Source, Argonne National Labora-33 tory,6,7 is used to study the nanoscale elemental disposition within34 fly ash particles and the hardened gel products, X-ray amorphous

35aluminosilicate materials termed “inorganic polymers” or “fly ash36geopolymers”,8,9 formed by the reaction of fly ash with alkaline37solutions. Geopolymers are currently being developed as an38environmentally beneficial replacement for Portland cement for39concrete production, offering comparable performance and cost40while reducing greenhouse gas emissions by a factor of approxi-41mately 5.10 Given that cement production is responsible for up to428% of global anthropogenic CO2 emission,2 this provides the43opportunity to reduce CO2 emission by at least tens of millions of44tonnes per annum worldwide. However, a number of aspects of45geopolymer structure are not yet well understood, in particular,46the role of calcium within the geopolymer structure and the47possibility of the release of toxins from the fly ash into the48environment while the material is in use.49There have been a number of previous studies of elemental50distributions within fly ash particles on a variety of length scales,51using several different analytical techniques. Volatile elements are52believed to become enriched at the surfaces of some large fly ash53particles, aswell as in the fine fractions of ashes.11-13Early studies54using particle-inducedX-ray emission (PIXE) in a protonmicrop-55robe showed strong correlations between specific elements, in56particular, a marked correlation between Fe and Ti (as well as57various other elements, including Cr) and a weak correlation58between these elements and Ca.14 Other studies have also been59aimed at classifying fly ash particles into different categories

*Towhom 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 Develop-ment, 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.; 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; Philadelphia, 1993; pp 8.(9) Duxson, P.; Fern�andez-Jim�enez, A.; Provis, J. L.; Lukey, G. C.; Palomo, A.;

van Deventer, J. S. J. J. Mater. Sci. 2007, 42, 2917.

(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.; BragaMarcazzan, G.M.; Pedretti, M.; Grime, G.W.; Baldacci,

A. Atmos. Environ. 1995, 29, 2323.(14) Jak�si�c, M.; Watt, F.; Grime, G. W.; Cereda, E.; Braga Marcazzan, G. M.;

Valkovi�c, V. Nucl. Instrum. Methods Phys. Res., Sect. B 1991, 56-57, 699.

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DOI: 10.1021/la901560h ALangmuir XXXX, XXX(XX), XXX–XXX

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60 according to their composition and/ormicroscopic appearance.15,16

61 Such analysis will not be conducted in this paper, because the very62 fine spatial resolution of the Nanoprobe instrument (∼30 nm) is63 ideally suited to detailed analysis of small regions rather than64 exhaustive samplingof large numbers ofmicrometer-sizedparticles.65 SynchrotronX-ray fluorescence (μ-XRF and tomography) has66 previously been applied to the study of individual coal fly ash67 particles.17-19 However, the products formed by the alkaline68 activation of fly ash have not previously been subjected to this69 type of analysis. Geopolymer gels are known to be heterogeneous70 on a length scale of nanometers to micrometers,9,20-23 but the71 nature of this heterogeneity in fly ash-derived geopolymers is at72 present poorly understood.24-26 Therefore, the primary aim of73 this paper is to analyze the structures of these materials on a74 submicrometer length scale. One complicating factor in the75 generation of construction materials from wastes is that waste76 materials must be either used “as is” or preprocessed; this means77 that the ability to understand the chemistry of fly ash and its78 interaction with alkaline solutions is critical to the successful79 valorization of a wide range of fly ash sources worldwide.80 Additionally, a key question in the reuse or valorization of any81 waste material is whether it contains any toxic or otherwise hazar-82 dous components which will impact its users. In the case of fly ash,83 the element of primary concern is usually chromium. Chromium is84 both highlymobile and toxic in its hexavalent form,which is released85 inpreference to thepoorly solubleCr(III) upon exposureof fly ash to86 alkaline conditions.27 The fraction of chromium which is present as87 Cr(VI) in fly asheshas been shown tovarywidely, froma fewpercent88 in most ashes up to a few rare instances in which 40% of the Cr89 present is hexavalent.28,29 IfCr is being released fromtheashparticles90 duringgeopolymerization, itwould thereforebe expected tobe in this91 problematic form, and this behavior must be understood to ensure92 the safety of geopolymers in general construction applications.

93 Materials and Methods

94 The ash studied here was taken fromGladstone power station,95 Queensland,Australia, derived fromthe combustionof black coal

96for electricity generation, and is Class F according to ASTM97C618. It is usually sold for use as a cement additive and is a98relatively fine ash,with a d50 of 10μmand a d80 of 20μm.The bulk99chemical compositionofGladstone fly ash is given inTable 1, and100powder X-ray diffraction data are given in Figure 1.101It should also be noted that in Figure 1, the iron oxide phases102cannot be specifically identified because of severe peak overlap103between maghemite and a substituted ferrite spinel phase similar104tomagnetite.30Ashes that are low inunburnt carbon, as is the case105here, have previously been observed to have similar amounts of106Fe2O3- and Fe3O4-type phases,31 so it is likely that both are107present here. The importanceofX-raydiffraction is that it enables108the identification of the ferrite spinel, which is able to host Cr3þ as109an isomorphous substituent on the Fe3þ sites, as an important110phase in the fly ash studied.111We prepared fly ash samples for analysis by embedding a small112amount of fly ash in a commercial epoxy binder (Araldite, Selleys,113Australia). We prepared alkali-activated fly ash samples by114reacting the fly ash sample with an 11 M KOH solution115(“hydroxide-activated sample”) at a mass ratio of 0.41, and with116apotassiumsilicate solution [11MKOHand11MSiO2 (“silicate-117activated sample”)] at amass ratio of 0.56. Thesemass ratios gave118identical molar ratios of alkali to fly ash, to enable direct119comparison of the two samples. Samples were cured in sealed120molds at 40 �C for 7 days and then demolded and maintained at121room temperature in sealed plastic bags. Analysis was conducted122on specimens which had been polished to a thickness of 100-123200 μm using successively finer grades of abrasive and with124ethanol as a lubricant to minimize leaching of soluble compo-125nents, based on the procedure used by Lloyd et al.22

126Samples were analyzed using the Center for NanoscaleMateri-127als Hard X-ray Nanoprobe instrument, which operates on Sec-128tor ID-26 at the Advanced Photon Source, Argonne National129Laboratory.6,7 Briefly, this instrument uses two undulators as a

Table 1. Oxide Composition of Gladstone Fly Ash As Given by Bulk X-ray Fluorescence Analysis

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O Cl Cr2O3 Mn3O4 P2O5 SrO TiO2 ZnO LOIa

wt % 47.68 30.28 11.26 1.84 1.41 0.14 0.33 0.47 0.362 0.0138 0.153 0.932 0.146 1.67 0.0225 3.7aLOI is the loss on ignition at 1000 �C.

Figure 1. PowderX-raydiffractiondata (CuKR radiation,PhilipsPW1800, 0.02� 2θ step size, 4 s/step) for the fly ash used in thisinvestigation. M is mullite, Q quartz, and F a combination of ironoxides,mostly a “ferrite spinel” (Fe3O4with substitutionof variouselements ontobothFe2þ andFe3þ sites) but alsopossibly includingmaghemite and other iron oxides with peaks that overlap at theresolution of the instrument used.

(15) Pietersen, H. S.; Vriend, S. P.; Poorter, R. E. P.; Bijen, J. M. In Fly Ash andCoal 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) Gier�e, 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.; Os�an, 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.; vanDeventer, J. S. J. J.Mater. Sci. 2009, 44, 608.(23) Lloyd, R. R.; Provis, J. L.; vanDeventer, 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.

(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.

B DOI: 10.1021/la901560h Langmuir XXXX, XXX(XX), XXX–XXX

Article Provis et al.

130 hard X-ray source, a Si(111) double-crystal monochromator to131 select the photon energy for X-ray excitation of the sample and132 Fresnel zone plate optics for focusing ofX-rays on the sample. An133 active vibration control system based on laser interferometers134 provides accurate positioning. The selected photon energy was135 11.3 keV, which is to some extent limited by the available zone136 plateX-ray optics but did not restrict the utility of the information137 obtained from this study as the elements heavier than Fe that are138 present in fly ash are not generally of significant interest in139 geopolymer chemistry. The photon flux of the focused beam140 was ∼5 � 108 photons/s. Emitted characteristic X-ray fluores-141 cence radiation was detected with a four-element silicon drift142 energy dispersive detector (Vortex ME4). Scanning of the zone143 plate provided high-resolutionmaps of the elemental distribution144 of selected regions of the samples.145 The experimental setup used, operating in air, was capable of146 detecting fluorescence from elements with Z g 18. Particular147 attention was paid to K, Ca, Fe, and Cr in the samples studied148 here. Data for Ti were also collected; however, the levels of Ti149 observed at most points in the samples were very low, so this150 element was not subjected to such detailed analysis. Four full151 spectra are obtained at each point, one for each element of the152 Vortex detector. From these spectra, regions of interest (ROIs) in153 each spectrum are identified (see the Supporting Information),154 and the correspondingROIs for all detector elements are summed155 to provide the total counts for Fe, Ca, Cr,K, andTi at each point.156 To achieve a high energy resolution, the full width at half-157 maximum of each ROI was selected to be ∼150 eV.158 Datawere collected for 2.0 s per point, with a step size of 80 nm159 selected to enable utilization of the high spatial resolution of the160 instrument while mapping an area which is large enough to be161 considered to some extent representative of the sample. The162 narrow (11 μm) depth of focus of the Nanoprobe instrument163 was utilized to obtain accurate and high-spatial resolution data

164from the relatively thick samples studied here. Element-specific165focusing on individual particles within the sample allows unam-166biguous determination of the depth of constituent parts within the167sample. Preliminary test work, presented as Supporting Informa-168tion, using these samples has shown that variation in the focal169depth provides the ability to separately identify particles located170at different depths within the sample, proving that the results171presented here are unlikely to be significantly affected by the172overlap of multiple particles in the z-direction. The sizes of the173square regionsmapped varied from 5.6 to 9.6 μmand are noted in174the captions of the respective figures.

175Results and Discussion

176Fly Ash. Figures 2 and 3 show the elemental maps of K, Ca,177Fe, and Cr in two different regions of the fly ash/epoxy sample.178Both regions show the presence of multiple fly ash particles, with179significant differences in composition, and morphologies that are180apparently close to spherical. Hollow particles (Cenospheres) are181commonly observed in fly ashes but were not seen in any of the182samples (either fly ash/epoxy or alkali-activated fly ash) investi-183gated here. A cenosphere-type morphology would be expected to184result in a more uniform distribution of the fluorescence signal185across the particle, as the shells of these particles are generally186quite uniform in thickness.32 It is not possible to tell from the data187obtained here whether the particles are solid spheres or plero-188spheres (Cenospheres filled with smaller particles), although the189relative uniformity of the regions in the centers of the particles190suggests that they are most likely solid. The morphology of the191iron-rich areas within the particles, i.e., whether the iron content192is uniform throughout the particle or present in a dendritic

Figure 2. Elementalmaps of (a) calcium, (b) chromium, (c) iron, and (d) potassium in a 5.6μm� 5.6 μmregion (71 points� 71 points) of thefly ash/epoxy sample.Data are presented in units of total fluorescence counts per point in the spectral region of interest, and the scale used foreach element is shown separately.

(32) Ngu, L.-N.; Wu, H.; Zhang, D.-K. Energy Fuels 2007, 21, 3437.

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193 morphology intermixed with another phase, cannot be deter-194 mined from these data because the hard X-ray beam used has a195 depth of focus of several micrometers as mentioned above. The196 composition presented at each point is therefore an average197 determined across an interaction volume which is much deeper198 than it is wide, which precludes identification of the dendritic199 structures sometimes observed in fly ash particles by back-200 scattered electron imaging.33

201 Figures 2 and 3 each show part of an ∼10 μm fly ash particle202 containing all the Ca, Cr, and Fe. Such particles are well-known to203 comprise a significant fraction of most Class F fly ashes, and204 substitution of Ca2þ and Cr3þ onto the Fe2þ/3þ sites in the ferrite205 spinel phase is expected. The main Fe-rich particle in Figure 2 is206 poor inK,while theparticle inFigure 3 is rich inK.Such variability207 in alkali content between particles is also well-known in fly ashes.13

208 There is no apparent surface enrichment of any of the elements209 studied; in a cross section takenacross the scan region starting from210 the point of highest Fe intensity in Figure 3, the fluorescence211 intensities of all elements decrease according to the same trend212 (to within experimental uncertainty). It is also of interest that213 the chromium and iron sites seem to correspond very closely214 to each other in both scans; this will be discussed in more detail215 below.216 However, there are additional particles observed in each scan217 which are of particular interest in the context of the interactions218 between fly ash and alkaline solutions to form geopolymers. The219 role of iron in geopolymers has been the subject of some discus-

220sion recently,22,23,26,34,35 with some studies showing the presence221of iron to have a negative impact on the formation of reaction222products but others showing little or no effect. The localization of223iron into specific particles within the fly ash but not others224is therefore of potential importance, particularly given that these225other particles identified are sometimes very rich in either226potassium (top right of Figure 2d) or calcium (bottom left of227Figure 3a), and these elements have been identified as being228important in determining fly ash reactivity during geopolymer229formation.36

230Fly Ash Alkali Activation Products. To study the effect of231alkali activation on the different elements present within fly ash,232the ash was reacted with a potassium hydroxide activating solution233and with a potassium silicate activating solution to form two234geopolymer samples, as outlined in Materials and Methods.235Nanoprobe data for these two samples are shown in Figures 4236and 5.237An important point to note in Figures 4 and 5 is that the238concentration of potassium is much higher in the geopolymer than239in the fly ash/epoxy sample, because of the high concentration of240potassium in the activating solutions used. This means that the241regions of newly formed geopolymer binder can be identified as242being the regions which are enriched in potassium alone243(i.e., potassium aluminosilicate). Solvated potassium present in244open pores, which is known to comprise some proportion of245the potassium present in many geopolymers,37 will most likely be246removed during polishing of the samples to a thickness of much247less than a millimeter, and it is likely that this is responsible for the

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) ofthe fly ash/epoxy sample.

(33) Ramsden, A. R.; Shibaoka, M. Atmos. Environ. 1982, 16, 2191.(34) Fern�andez-Jim�enez, A.; Palomo, A.; Macphee, D. E.; Lachowski, E. E. J.

Am. Ceram. Soc. 2005, 88, 1122.(35) Keyte, L. M. Ph.D. Thesis, University ofMelbourne,Melbourne, Australia,

2008.

(36) Duxson, P.; Provis, J. L. J. Am. Ceram. Soc. 2008, 91, 3864.(37) Duxson, P.; Provis, J. L.; Lukey, G. C.; vanDeventer, J. S. J.; Separovic, F.;

Gan, Z. H. Ind. Eng. Chem. Res. 2006, 45, 9208.

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248 low-potassium region in the bottom right-hand corner of249 Figure 5d.250 Figure 4 can therefore be identified as depicting a large fly ash251 particle centered beyond the bottom right-hand corner of the field252 of view, as shown by the Fe and Cr maps, with a region of253 geopolymer binder in the left-hand side of the image identifiable254 by its elevated potassium content. The calcium appears to have255 become significantly dispersed from its original location within256 the fly ash particle and has intermingled with the geopolymer257 binder. In contrast, the iron appears to have remained essentially258 unaffected by the geopolymerization process and is restricted to259 the area within the remnant fly ash particle. Similar observations260 have been made using elemental mapping in an electron micro-261 scope, although with a spatial resolution approximately 10 times262 lower.23While it is not currently possible to compare “before and

263after” images of the exact same fly ash particle undergoing264geopolymerization to confirm that nomovement has taken place,265the fact that the iron concentration falls away smoothly at the266particle edge suggests that it is unlikely that it has migrated267significantly, if at all, into the geopolymer binder region.268Figure 5 also depicts the edge of an apparently large iron-rich269particle at the top of the field of view, with a remnant calcium-rich270fly ash particle at the bottom left-hand corner and what appears271to be a pore in the right corner, as mentioned above. The calcium272in this sample is also clearly more dispersed than the iron or273chromium. Like the hydroxide-activated case in Figure 4, calcium274appears to have been released from the fly ash particle and275become somehow incorporated into the binder phase. Exactly276what structural role is played by the calcium in the geopolymer277binder is not yet well understood, but it is believed to play an

Figure 4. Elementalmaps of (a) calcium, (b) chromium, (c) iron, and (d) potassium in a 5.6μm� 5.6 μmregion (71 points� 71 points) of thefly ash/KOH sample. (e) Expanded view of part of panel a (in the geopolymer binder region, bottom left corner), showing localized regions ofhigh Ca concentrations (circled), with color scaling changed to highlight this.

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278 important role in determining both the kinetics of geopolymer279 formation and the performance of the final product.23,38,39

280 However, the difference in binder homogeneity between the281 hydroxide-activated and silicate-activated samples can be clearly282 observed by comparing Figures 4 and 5, and this provides a283 critical demonstration of the unique capabilities of the Nano-284 probe instrument. Panels a and d of Figure 4 in particular show a285 distribution of calcium and potassium, respectively, that is highly286 inhomogeneous on a length scale of tens to hundreds of nano-287 meters, while the corresponding panels a and d of Figure 5 show288 much less local variability in composition. The narrow depth of289 focus of the Nanoprobe instrument, as discussed above, provides290 confidence that these are in fact isolated particles and not artifacts291 due to overlapping particles within a large probe volume.292 This observation of inhomogeneous calcium distribution is293 in agreement with the known differences in aluminosilicate294 framework microstructure between silicate-activated and hyd-295 roxide-activated geopolymer materials as observed under elec-296 tron microscopy.21,23,40,41 However, direct comparison of ele-297 mental distributions within these microstructures has not298 previously been presented, and in particular, little emphasis299 has been placed upon the distribution of nonframework species300 as depicted here.301 Figure 4a shows that there are very localized regions of highCa302 content within the region of the sample that can be identified as

303containing geopolymer binder. While some of these high-Ca304regions overlap with the fly ash particle visible in the Fe map305(Figure 4c), theK concentration gradient suggests that the central306part of the field of viewcontains both geopolymer and remnant fly307ash phases overlaid on each other,with both being sampled due to308the penetration depth of the hardX-rays.Considering the absence309of small discrete high-Ca regions within any other fly ash particles310observed here, these can be confidently identified as being located311within the geopolymer binder. The K is also somewhat inhomo-312geneously distributed, but not to the same extent as Ca.313A physical interpretation of this may therefore be proposed:314calcium released from the fly ash particles into the very highly315alkaline (pH>14) solution reacts rapidly with the hydroxide ions316present, nucleating as particles of Ca(OH)2 which are then317enclosed by the binder as it forms. This has been proposed as an318explanation for the behavior and structure ofmetakaolin geopoly-319mers containing calcium silicate minerals,38 but the formation of320such discrete nanoscale particles has not been observed before321now. The relative rates of release of calcium, silicon, and alumi-322num will also play some role here, with the formation of calcium323(alumino)silicate hydrate and calcium aluminate hydrate phases324also possible under some circumstances,42 but the chemistry of the325hydroxide-activated system studied here suggests that Ca(OH)2 is326the phase most likely to form. No newly formed crystalline327calcium-containing phases are able to be identified in the sample328by X-ray diffraction, which may mean that the high-calcium329precipitates are crystallographically disordered or may simply be330because they are small (a few tens of nanometers in size) and331comprise only a very minor fraction of the total sample volume.

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) ofthe fly ash/potassium silicate sample.

(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; Bı́lek, V.,Ker�sner, Z., Eds.; Brno, Czech Republic, 2008; pp 589-597.(40) Steveson, M.; Sagoe-Crentsil, K. J. Mater. Sci. 2005, 40, 4247.(41) Fern�andez-Jim�enez, A.; Palomo, A. Cem. Concr. Res. 2005, 35, 1984.

(42) Yong, S. L. Ph.D. Thesis, University of Melbourne, Melbourne, Australia,2009 (in progress).

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332 By comparison, Figure 5a, consistent with the scans of other333 regions of the silicate-activated sample (data not shown), does not334 show localized high-Ca regions. Instead, the Ca appears to be335 “smeared” around the edges of the fly ash particles, suggesting336 that it has been released from the particles and has been able to337 diffuse more uniformly into the surrounding region before the338 system hardened. It cannot be stated conclusively whether this339 region contains any discrete calcium silicate hydrate phases that340 have separated from the alkali aluminosilicate geopolymer bin-341 der, but the homogeneity of the elemental distributions of bothCa342 and K in Figure 5 suggests that this has not occurred. Previous343 work has shown the absence of such phase separation on a length344 scale of several micrometers in a similar system,23 and the results345 presented here enable a further reduction in the upper bound on346 the size of any segregated calcium silicate hydrate regions in this347 material to less than 100 nm. Given that X-ray photoelectron348 spectroscopic analysis ofmetakaolin-derived geopolymer systems349 does indicate nanoscale phase segregation in these materials,39,42

350 but extensive X-ray diffractometry studies have not shown any351 indication of crystalline hydrate formation, it appears increas-352 ingly likely that any segregation present is taking place on a length353 scale of at most a few nanometers.354 Disposition of Chromium in Fly Ash Geopolymers. As355 outlined in Introduction, a key concern in the use of fly ash in356 construction materials is the potential availability of chromium357 from the hardened materials. This is particularly the case for358 geopolymers, where the fly ash content is much higher than in359 Portland cement-based materials. The identification of a strong360 correlation between unreacted fly ash particles and chromium361 in the geopolymer samples studied here is therefore significant362 in determining whether these materials will meet widespread363 approval in themarket. The importance of this particular correla-364 tion is that if the chromium is restricted to the unreacted (and365 generally not very porous) ash particles embedded within the366 geopolymer binder, it is much less likely to be accessible to the367 pore networkof the geopolymer and is therefore less susceptible to368 leaching.Also, if the chemical state of the chromiumwithin fly ash369 particles is resistant to leaching under the highly alkaline condi-370 tions prevailing during geopolymerization, it is likely to be capable371 of resisting removal under less aggressive service conditions.372 It has previously been observed27,43 that the level of available373 chromium inAustralian fly ashes is in general low. Themaximum374 leaching takes place in alkaline environments, and more alkaline375 ashes are usually more susceptible to Cr leaching. The Gladstone376 fly ash used here is considered quite an alkaline ash,44 meaning377 that its exposure to highly alkaline conditions during geopoly-378 merization is likely to represent close to a worst-case scenario in379 terms of release of Cr from an Australian fly ash. The Gladstone380 fly ash used here contains 0.0138 wt % (138 ppm) chromium on381 an oxide basis according to bulk X-ray fluorescence analysis382 (Table 1), but Narukawa et al.27 have observed that the concen-383 trations of total Cr and available Cr in Australian fly ashes show384 at most a weak correlation.385 Figure 6 shows the correlation between Fe and Cr concentra-386 tions in the unreacted fly ash and in the two geopolymer samples.387 The data are normalized to remove background effects, with the388 lowest concentration of each element in each sample region389 plotted as 0 and the highest as 1. The diagonal line on each plot390 therefore represents an exact correspondence between Cr con-

391centrations and Fe concentrations. It is apparent from Figure 6392that the correlation between the locations of these two elements in393all samples is very strong, particularly in the intermediate con-394centration region.395The data presented in Figure 6 were obtained from two to three396regions on each sample, the regions shown in Figures 2-5 and397additional regions on each sample whose full fluorescence maps398are not presented in this paper, but the trends across regions399appear to be very consistent. It is obviously somewhat perilous to400attempt to represent a heterogeneous sample by analyzing a small401number of small regions, but this consistency in the observed402trends brings some measure of confidence in the analysis pre-403sented here. Similar consistency was not always observed in404the correlations between other element pairs, in particular, the

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 ineach panel correspond to the different regions mapped in eachsample. The region shown in Figure 2 is region A2; Figure 3corresponds to region A3, Figure 4 to region B2, and Figure 5 toregion C2. Full elemental maps for the other regions (A1, B1, andC1) were collected but are not presented in this paper. Data werenormalized so that the highest concentration of each element ineach region is displayed as 1 and the lowest as 0.

(43) Jankowski, J.; Ward, C. R.; French, D. Preliminary assessment of traceelement mobilisation from Australian fly ashes, Research Report 45, CooperativeResearch Centre for Coal in Sustainable Development, 2004.(44) Pathan, S.M.; Aylmore, L. A. G.; Colmer, T. D. J. Environ. Qual. 2003, 32,

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405 Fe-Ca and Fe-K pairs, reflecting the highly heterogeneous406 distribution of Ca and K across the ash particles.407 Interestingly, both panels a and b of Figure 6 (the fly ash and408 the hydroxide-activated sample, respectively) appear to show a409 number of data points falling in a region where the Fe concentra-410 tion is very low but the Cr concentration is significant (up to 25%411 of the highest Cr fluorescence intensity recorded), and thismay be412 identified as being of some concern in terms of the chromium413 beingmobile. However, Figure 6c does not show any such points.414 Thismay simply be due to having selected twomapping regions in415 the silicate-activated sample which happened not to contain any416 regions of excess chromium content, or it may be due to the less417 aggressive silicate activating solution used in this instance. Given418 that such Cr-enriched regions were observed in the ash itself and419 that the chemistry of the other samples tested was consistent420 across the different regions analyzed, the former explanation421 seems more likely, but further work is necessary to provide422 decisive evidence one way or the other.423 However, regardless of the exact situation in the silicate-424 activated sample presented in Figure 6c, some important informa-425 tion can be obtained from Figure 6. In particular, it is observed426 that even a geopolymer formed by activation of an alkaline fly ash427 by a hydroxide solution, which should be considered close to a428 worst-case scenario as far as chromium release is concerned, does429 not contain significantly more mobile chromium (defined here as430 chromium which is not associated closely with iron) than the431 original fly ash. Given that the use of fly ash activated by an alkali432 hydroxide solution is becoming increasingly widespread in various433 applications in Europe and elsewhere,45-47 this observation is of434 some significance. The data presented here provide some insight435 into this behavior, but it would be necessary to confirm these436 observations by a technique which is sensitive to Cr oxidation437 states, such as spatially resolved X-ray absorption near-edge438 spectroscopy (μ-XANES), to fully confirm these suggestions.439 However, it should also be noted that many commercial440 incarnations of geopolymer concrete incorporate at least a441 moderate level of blast furnace slag, which accelerates the setting442 process and also gives added performance benefits in the final443 hardened product.10,36,38 Blast furnace slag contains sufficient444 sulfide to generate a reducing environment, which is capable of445 reducing mobile Cr(VI) to relatively immobile Cr(III). It has446 recently been shown that the addition of 0.5 wt % Na2S447 (simulating the reducing environment generated by slag) to a fly448 ash geopolymermatrix containing elevated levels ofCr(VI) added449 asNa2CrO4 reduces the leaching rate of chromiumverymarkedly450 under both acidic and alkaline conditions.48,49 It may therefore451 be expected that, even if Cr is released from the fly ash into452 the binder region of these materials, its reduction to Cr(III)453 and consequent immobilization will minimize the potential for

454harm to the biosphere during the use of slag-containing geo-455polymer concrete.

456Conclusions

457Elemental mapping of fly ash and fly ash-derived geopolymers458using hard X-ray fluorescence mapping with nominal 30 nm459spatial resolution has been shown to provide valuable information460regarding the structures of these materials. The calcium distribu-461tion in a hydroxide-activated fly ash geopolymer gel is highly462heterogeneous, with discrete high-calciumparticles observedwith-463in the binder structure. The exact chemical identity of these regions464cannot be confirmed; however, the fact that they are at most a few465tens of nanometers in size and highly enriched in calcium suggests466that they may be Ca(OH)2 precipitates which nucleate as Ca2þ is467released from the fly ash particles into the activating solution468which contains a very high concentration of free OH-. The469existence of such structures within geopolymers has previously470been the subject of much speculation, but this work provides the471first direct evidence confirming their existence. Calcium in the472silicate-activated fly ash geopolymer is much more dispersed473within the gel structure andappears to have spreadmore gradually474from the fly ash particles during geopolymer formation. Calcium475is also seen to have been released in preference to iron from the fly476ash particles which are rich in both elements.477The locations of chromium and iron are very closely correlated478within the structures of both fly ash and the geopolymer product,479suggesting that the chromium is present as Cr(III) within a ferrite480spinel phase which is relatively resistant to alkali attack. This is481important because chromiumcontamination is a potential issue in482the valorization of fly ash, and the data indicate that the level of483readily available and toxic chromium in a fly ash-derived geopo-484lymer will be significantly lower than the total chromium content485of the precursor fly ash.

486Acknowledgment. Use of the Center for Nanoscale Materials487was supported by theU.S.Department of Energy,Office of Science,488Office of Basic Energy Sciences, under Contract DE-AC02-48906CH11357. Use of the Advanced Photon Source was supported490by theU.S.Department of Energy,Office of Science,Office of Basic491Energy Sciences, under ContractDE-AC02-06CH11357. This work492was funded by the Australian Research Council (ARC), including493partial funding from the Particulate Fluids Processing Centre, a494SpecialResearchCentreof theARC,and throughDiscoveryProject495grants. Thework of S.A.B.was supported by travelling scholarships496from Colciencias and from the Walter Mangold Trust. Travel497funding for J.L.P. was supplied by the Australian Synchrotron498ResearchProgram.We thankDr. J€orgMaser,Dr.RobertWinarski,499Dr. Martin Holt, and Ms. Claire White for assistance with experi-500ments on the Nanoprobe instrument.

501Supporting Information Available: An example of one of502the spectra obtained from the Vortex detector elements with503the ROIs identified and a more detailed discussion and504validation of the probe volume of the Nanoprobe instru-505ment. Thismaterial is available free of charge via the Internet506at http://pubs.acs.org.

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Rev. Ing. Constr. 2007, 22, 75.(47) Fern�andez-Jim�enez, A.; Palomo, A.; Pastor, J. Y.; Martı́n, A. J. Am.

Ceram. Soc. 2008, 91, 3308.(48) Zhang, J.; Provis, J. L.; Feng, D.; van Deventer, J. S. J. J. Hazard. Mater.

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H DOI: 10.1021/la901560h Langmuir XXXX, XXX(XX), XXX–XXX

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