The microstructure of geopolymers synthesised from industrial wastes
Redmond R. Lloyda and Jannie S.J. Van Deventera1
Department of Chemical and Biomolecular Engineering
University of Melbourne, Parkville, Victoria, 3010 Australia
a
Abstract
The microstructure of a geopolymer synthesised from 75% fly ash and 25% ground granulated blast furnace slag,
activated with potassium silicate, was investigated by XRD, SEM and TEM. SEM showed the reaction products to
consist of two phases, one of which had a distinctive dendritic morphology. This phase was attributed to amorphous
calcium silicate hydrate, which was also observed in the TEM. Crystalline phases were observed in the TEM; two
were identified as Quartz and Mullite, from the fly ash. The third crystalline phase could not be identified
unambiguously; the presence of potassium indicates that the phase may have been formed during the
geopolymerisation reaction.
Keywords: Geopolymer, fly ash, slag, SEM, TEM
1. Introduction
Geopolymers are a class of inorganic materials which have promised much but delivered little commercial success
over the past 20 years. Part of this can be attributed to their obvious commercial attractiveness, which has led to
attempts at commercialisation without the necessary fundamental research. As a result, in depth understanding of the
microstructure of these materials remains lacking. The properties of the products formed cover a broad range, but are
generally claimed to be stronger, lighter, more resistant to chemical attack, more thermally stable and potentially
cheaper than conventional building materials such as Portland cement or fired clay products (Davidovits, 1988).
Geopolymers have also been examined as matrices for waste immobilisation (Van Jaarsveld and Van Deventer,
1997) and as low-cost ceramics.
When he coined the term, Davidovits (1982) was referring to a three dimensional network of tetrahedrally
coordinated aluminium and silicon atoms, linked through shared oxygen atoms, with the formal charge due to
tetrahedral aluminium being balanced by alkali metal cations – essentially, the same as zeolites. The key difference,
argued Davidovits, was the lack of long-range order; while zeolites are crystalline, geopolymers are amorphous to xrays. Subsequent work has substantiated these claims, based largely on the lack of clear x-ray diffraction peaks. A
recent electron diffraction study has also been in concordance (Schmücker and McKenzie, 2005). This long-held
belief has been challenged recently by Provis et al. (2004), who believe that the differences between zeolites and
geopolymers are actually smaller than have been claimed. It is noteworthy that geopolymers and zeolites are
synthesised from the same reactants and under very similar conditions. The main significant difference is in the
amount of water present – geopolymer synthesis typically occurs in systems with 10 to 100 times higher ratio of
solids to solution. There is the possibility, therefore, that the geopolymer network is actually an agglomeration of
nano-crystalline zeolites. This has significance, as it provides a basis for thermodynamic examination of the stability
of geopolymers; this is critical if these materials are to be used for building products, where the catastrophic failure
of alumina cements due to delayed “conversion” of the reaction products to a more thermodynamically favoured
state has caused reticence to adopt new cement systems.
When attempting to produce commercially feasible geopolymer products, research has tended away from the systems
favoured by earlier work, i.e. calcined clay as the source of reactive aluminium and silicon, to systems based
predominantly on waste materials. The use of fly ash was pioneered by Wastiels et al. (1993) and further developed
by others (Palomo et al. 1999, Lee and Van Deventer, 2002), and is particularly favourable. A waste material
produced in enormous quantities, sometimes containing heavy metals and radionuclides that can leach into the
1
Author to whom correspondence should be addressed. Tel.: +61-3-8344-6620 Email:jannie@unimelb.edu.au
environment unless handled correctly, can be converted into a product with properties equal to or surpassing Portland
cement. The toxic components can be immobilised, landfill avoided and, by providing an alternative to Portland
cement, the CO2 released from the world’s cement kilns could also be significantly reduced. The benefits are thus
social, environmental and, not least, financial. The microstructure of such systems has been even less well examined,
hindered in part by differences in nomenclature. Some researchers favour the term “alkali-activated fly ash” (Palomo
et al., 1999), presumably in deference to the vast amount of research conducted in the former Soviet Union on alkaliactivated slag systems (Krivenko, 1994). Others prefer “geopolymer”, as the system is similar in that aluminium and
silicon dissolve in alkali silicate solution, then precipitate to form solid products. In the former case, Palomo and
Palacios (2003) make use of fly ash and sodium hydroxide solution, and zeolitic products are identified by x-ray
diffraction. It is not clear whether all the reaction products are crystalline, as only those which are would be
detectable by XRD. In this case the distinction between the chemistry of zeolites synthesised from fly ash (Molina
and Poole, 2004) and alkali-activated fly ash cements disappears – the quantity of water involved, and thus the
formation of products as discrete crystals or as a solid mass, is the distinguishing feature. On the other hand, when
produced from fly ash and an alkaline solution containing dissolved silica, the products are found to be x-ray
amorphous, and the distinction between “zeolite” and “geopolymer” remains.
Ground Granulated Blast-furnace Slag (GGBS, or slag), is another waste material sometimes added to geopolymer
systems. It is known to increase compressive strength and enhance strength development at ambient temperatures
(Davidovits and Sawyer, 1985). The effect of slag addition on metakaolin geopolymers was studied by Yip et al.
(2003). They suggested that crystalline reaction products could be observed, although these results were not
conclusive. The incorporation of GGBS into geopolymers raises further questions regarding microstructure. One
possibility is that Si and Al dissolved from the GGBS are incorporated in the geopolymer, with Ca 2+ balancing the
charge on 2 aluminium tetrahedra. There are analogies with naturally occurring zeolites. Alternatively, Ca 2+ could
precipitate with dissolved silica to form CSH. The formation and distribution of CSH, would significantly affect
properties such as acid resistance.
There remains, therefore, a large gulf in the understanding of the microstructure of waste based geopolymers; in
particular, whether the products are nano-crystalline or amorphous or mixed, and whether Ca, when present, forms
discrete regions of CSH or is incorporated into the aluminosilicate structure. As part of a broader project to examine
the durability of waste based geopolymers we have found it necessary to attempt to resolve these questions. In light
of this, the aim of the present work is to use materials science techniques, in particular scanning and transmission
electron microscopy (SEM and TEM respectively), to characterise the microstructure of a waste based geopolymer
composition typical of those which could be used commercially. The geopolymer chosen consists of 75% fly ash and
25% GGBS, activated with potassium silicate.
2. Materials and Methods
2.1 Materials
Fly ash from the combustion of coal at Gladstone power station was obtained from Pozzolanic Enterprises, Australia.
Ground, Granulated Blast-furnace Slag was obtained from Independent Cement and Lime, Port Melbourne,
Australia. Elemental composition was determined by x-ray fluorescence on a Siemens SRS3000 XRF using fused
lithium tetraborate disks; oxide compositions are presented in Table 1. A Phillips PW1800 XRD using Cu Kα x-rays
was used to collect diffraction spectra, with step size of 0.02º 2θ and acquisition time of 5 seconds per step.
Potassium silicate solution was prepared by dissolving high surface area amorphous silica (Aerosil 200, Degussa) in
potassium hydroxide solution (46wt% solution, Aldrich), adjusted to the desired concentration with distilled water.
The adjusted solution contained 8.5M KOH and 4.2M SiO2. The solution was then sealed in a polyethylene container
and aged for 24 hours at 23ºC before use.
Table 1: Elemental composition (weight %) of fly ash and slag used in this study, determined by XRF.
fly ash
GGBS
Na2O
0.28
0.26
MgO
1.35
6.02
Al2O3
27.83
13.18
SiO2
45.46
32.88
SO3
0.21
3.50
K2O
0.47
0.32
CaO
5.61
40.05
TiO2
1.36
0.66
MnO
0.19
0.40
Fe2O3
11.21
0.32
LOI
2.71
1.19
Both raw materials were devitrified at 900ºC for 24 hours. This provides sufficient thermal energy to allow
crystallisation of the glassy phases present without melting the components, which would allow new phases to form.
Phase analysis was conducted by XRD, and the results are presented in Table 2. Note that for the devitrified
materials the phases listed are in addition to those present in the material as received.
Table 2: Phase analysis of devitrified fly ash and slag.
Material
fly ash
- as received
fly ash
- devitrified
Phase
Mullite
Quartz
Hematite
Magnetite
Maghemite
Diopside
Gehlenite
Anorthite
Composition
Al6Si2O13
SiO2
Fe2O3
Fe3O4
Fe2O3
MgCaSi2O6
Ca2Al2SiO7
CaAl2Si2O8
Material
GGBS
- as received
Phase
Gypsum
Gehlenite
Quartz
Composition
CaSO4.2H2O
Ca2Al2SiO7
SiO2
GGBS
- devitrified
Åkermanite
Wollastonite
Ca2MgSi2O7
Ca3Si3O9
2.2 Sample preparation
375.0 g of Gladstone fly ash was mixed with 125.0 g of slag in acetone for 10 minutes. After removing the acetone
by evaporation at 105ºC for 2 hours in a laboratory oven, 226.5g of the potassium silicate solution described above
was added and mixed mechanically for 10 minutes. A small quantity of the resulting paste was poured into a
polyethylene container, sealed with tape and cured in a laboratory oven at 65ºC for 48 hours. The sample was
subsequently stored at room temperature.
2.3 Thin section preparation
Preliminary sample preparation was accomplished by polishing small pieces, typically 1mm cubes, of geopolymer to
less than 40μm thickness by use of tripod polisher and successively finer grades of diamond paper. Slotted copper
TEM grids were prepared by cutting in half across the axis of the slot, to which the sample was then attached by
epoxy adhesive. An FEI Nova NanoLab Focused Ion Beam (FIB) system was used to thin the samples to electron
transparency, following deposition of a protective platinum layer on the top of the region of interest.
2.4 Scanning Electron Microscopy
SEM samples were polished with successively finer grades of diamond paste, the ultimate being ¼μm. As the
geopolymer matrix is not conductive a thin layer of platinum was deposited using a Dynavac Xenosput magnetron
sputter coater. SEM was conducted on a Phillips XL30 with a field-emission electron source, operating at 5kV
accelerating voltage.
2.5 Transmission Electron Microscopy
TEM was conducted on a JEOL 2010 electron microscope with a LaB 6 electron source, fitted with a LINK Energy
Dispersive X-ray Spectrometer (EDS). Selected area diffraction patterns were collected from regions of interest first,
followed by image acquisition and finally microanalysis; this order was chosen due to decreasing sensitivity to beam
damage through the sequence.
3. Results
3.1 XRD
The x-ray diffraction spectra of the raw materials and the resultant geopolymer are shown in Figure 1. The raw
materials are predominantly amorphous, with crystalline inclusions; quantitative analysis showed approximately
67% amorphous content for the fly ash and approximately 95% amorphous content for the slag. Phases identified in
the fly ash are Quartz, Mullite, Hematite, Maghemite and Magnetite, while those in the slag are Gypsum, Gehlenite
and traces of Quartz. The intensity and position of the diffraction peaks for the geopolymer spectrum coincide with
those of the fly ash, indicating that the crystalline components play little or no part in the reaction. There is no clear
evidence of new crystalline phases. There is, however, a shift of the broad “amorphous hump” from a centre at ca. 24
º2θ to ca. 30 º2θ. It has been noted that this shift is indicative of geopolymeric reaction (Provis et al., 2004).
550
*
500
#
450
+
#
400
*
#
350
#
#
-
#
#
#
*
^
#+
#
-
#
^
^
#
+
-
*
#
#
#
^
+
-#
*
^
#
*
#
#
#
#
-
^
*
+
+
#
*
+
-
fly ash
200
#
+
#-
^
#
*
^
#
*
300
250
*
#
intensity
+
^
^
+
geopolymer
-#
-
^ *
#
^
#
#
#
*
=< =<
150
=
=
100
=
*
< << =
=<
=<
<
=
<
=
=
GGBS
50
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
degrees 2-theta
Figure 1: X-ray diffractograms of (from bottom), GGBS, fly ash and the resultant geopolymer. Phases identified are Gypsum(=),
Gehlenite (<), Quartz (*), Mullite (#), Magnetite (+), Maghemite (-) and Hematite (^)
3.2 SEM
Examination of polished sections by SEM shows a highly heterogeneous material consisting of reaction products
interspersed with remnants of the original fly ash and slag grains, as shown in Figure 2. Differentiation between fly
ash and slag particles can be made by microanalysis or by morphology: ash particles are generally spherical while
slag particles are angular. At relatively low magnification, the reaction products, labelled “G” in Figure 2, appear as
a homogeneous phase. At higher magnification, as shown in Figure 3, it is clear that the reaction products consist of
at least two phases. This is significant, as geopolymers synthesised from fly ash alone appear to consist of a single
phase. One phase, typical of fly ash geopolymers, is relatively featureless; the other, however, has distinct dendritic
topology, labelled “2” in Figure 3. As the second phase is in close proximity to the remnant of a slag grain, and has
not been observed in samples produced from fly ash alone, it is probably rich in calcium (the primary constituent of
slag). Although microanalysis was not possible, this conclusion is supported somewhat by the contrast observed in
the SEM. Imaging was by secondary electron detector, which produces contrast primarily by topography. As the
samples were polished flat, however, back-scattered electrons would be expected to provide a relatively larger
contribution to contrast. As the back-scattering co-efficient of atoms increases with atomic number, brighter regions
contain constituents with higher atomic number. Calcium has the highest atomic number of the major components of
the geopolymeric reaction products (previous work has shown the iron is not present in the reaction products to a
significant extent).
Figure 2: SEM image of geopolymer microstructure showing spherical ash particles (A), angular remnants of slag grains (S) and
geopolymer phase (G).
Figure 3: Higher resolution SEM image of the same geopolymer, showing the remnants of a slag grain (S), typical geopolymer
phase (1) and dendritic phase (2) specific to slag containing geopolymers.
3.3 TEM
Although multiple phases were observed in the SEM, it was not possible to analyse them further, due to the low
spatial resolution of x-ray microanalysis (a result of beam spreading in the bulk sample). In addition, it was not
possible to examine the reaction products at higher resolution as the grain structure of the conductive platinum
coating dominates images at higher magnification. To overcome this, thin sections were prepared and examined by
TEM.
A low magnification image of the microstructure is shown in Figure 4a. The remnant of the protective platinum layer
added during FIB milling is present at the top. A large quartz particle is present at the centre of the section, identified
by electron diffraction and EDS (not shown). A sphere of unreacted fly ash is present to the right of the quartz
particle, identifiable by its circular cross section.
When the region labelled “a” in Figure 4a was examined at higher magnification, it was found to be amorphous. The
image and diffraction pattern are shown in Figure 4b. Microanalysis showed predominantly Ca and Si, with some Al,
K and Fe.
Figure 4a (left) and b. Figure 4a: Low magnification TEM image of the thin section, showing platinum deposited during FIB
milling at the top (Pt), large Quartz particle (Qz) and the regions analysed at higher magnification (labelled “a”, “b”, and “c”).
Figure 4b: Microstructure of the region observed at “a” in Figure 4a.
The large region (3×5μm) labelled “b” in Figure 4a was found to consist of crystals of approximately 30-60nm and is
shown with diffraction pattern in Figure 5a. Attempts to index the SAD pattern of this phase were unsuccessful.
Microanalysis indicates the presence of Al, Si, K, Ca and Fe.
A third phase with much larger crystals of large aspect ratio was also observed, present at positions labelled “c” in
Figure 4a. An image and SAD pattern are shown in Figure 5b. Microanalysis indicated predominantly Al, with some
Si and traces of Ti and Fe, which, in light of the crystal morphology, allows identification of the phase as Mullite.
The iron-rich crystalline phases detected by XRD were not observed in the TEM. Statistically, this is not altogether
surprising given the minute volume of material observed in a TEM section.
4. Discussion
The x-ray diffraction results presented indicate no new crystalline phases – thus the binding phase of the geopolymer
examined here is either amorphous, or contains crystalline regions too small, or too small in quantity, to be detected
by XRD. This correlates with previously seen results. The results of SEM are important, as two phases are clearly
visible in the reaction products, and they are intimately mixed. The nature of the reaction products of calcium
containing geopolymer systems has been speculated upon before, however this is the first clear evidence of the
presence of more than one phase at the sub-micron level. Previous work (Yip et al., 2003) indicated that calcium rich
and calcium deficient phases coexist in discrete regions hundreds of microns across – such separation of phases
could be explained better by inadequate mixing of the raw materials. In this case, however, the scale of the
intergrowth of phases is much smaller than the size of the particles, and thus independent of mixing. In addition, the
same study showed that the separation of phases disappeared as the alkalinity of the system increased. The present
study, however, used conditions of higher alkalinity than those shown to prevent separation of phases by Yip et al.
Figure 5a (left) and b. Figure 5a: Image and SAD pattern, inset, of the crystalline phase of 30-60nm observed at region “b” of
Figure 4a. Figure 5b: Image and SAD pattern, inset, of the crystalline phase observed at the regions labelled “c” in Figure 4a.
It is proposed that the amorphous phase “a” observed in TEM, shown in Figure 4b, correlates with phase “2” in
Figure 3. The presence of a significant amount of potassium in the TEM microanalysis indicates that the phase is
newly formed, as neither of the raw materials contains appreciable levels of potassium. The high content of Ca and
Si in this phase and the amorphous diffraction pattern indicate CSH. In the absence of microanalysis in the SEM it is
difficult to say which of phases “1” and “2” is calcium rich, and therefore correlates with phase “a”. The contrast
observed in Figure 3 indicates that phase “2” is calcium rich, and therefore correlates with phase “a”; this is
supported by the absence of phase “2” from geopolymers synthesised without slag, and therefore with little calcium.
The phase present at “b” in Figure 4a cannot be identified unambiguously; the presence of diffraction spots in Figure
5a proves that it is nano-crystalline, but before it can be claimed as newly formed it is necessary to examine the
possibility that this phase was present in the fly ash or slag before reaction. The absence of reflections from the x-ray
diffraction spectra of the ash and slag cannot be considered conclusive, as new crystalline phases could not be
identified in the diffraction spectrum of the geopolymer reaction products either. Phase “b” is thus either too small,
or present in too small quantity, to diffract x-rays measurably. The phase analysis of the devitrified fly ash, shown in
Table 2, indicates Diopside, Anorthite, Gehlenite, Åkermanite and Wollastonite in addition to the crystalline phases
already discussed. These materials, therefore, are present in the raw materials as nano-crystals within amorphous
regions, and could be the source of the crystals observed in Figure 5a. Of these, Diopside and Åkermanite can be
discounted immediately due to the absence of magnesium in phase “b”, while the presence of aluminium precludes
Wollastonite. It remains possible, therefore, that phase “b” is Anorthite or Gehlenite; on the contrary, the presence of
potassium in the TEM microanalysis again indicates that the phase was formed due to reaction in the presence of
potassium. Single crystal electron diffraction studies are required to clarify this. It is unclear why these crystals are
not visible in the x-ray diffractogram. The crystals are large enough to diffract x-rays, although the diffraction peaks
would be broadened due to the relatively small crystal size. Broadening would be expected to be more severe if the
unit cell was large, as is the case for zeolites, for example. A combination of low abundance and broad peaks could
render the crystals undetectable by XRD.
It is surprising that the expected amorphous alkali aluminosilicate geopolymer phase was not observed in the TEM.
This phase, which should correlate with phase “1” in Figure 3, would be expected to be the predominant phase. The
relatively small amount of material examined in the TEM could be the cause of this; an alternative is that calcium is
incorporated into the aluminosilicate phase as well as the CSH phase. This would render the phases difficult to
distinguish in the TEM, both being amorphous and both containing calcium. The key difference would be in
aluminium content – CSH can contain a maximum Al:Si ratio of approximately 0.28 (Faucon et al. 1999), whereas
this ratio can be higher (theoretically as high as 1) for geopolymers. Further investigation, for instance by
STEM/EELS, is necessary to map the distribution of aluminium and calcium, and thus of the phases which are
expected. The abundance of the crystalline aluminosilicate phase “b” is too low to be attributed to the expected
aluminosilicate “geopolymer” phase.
The identification of CSH in geopolymers synthesised from fly ash and slag is significant; while providing
confidence in the stability of one of the reaction products (CSH is the main constituent of Portland concrete) it will
have an effect on the resistance of these materials to attack by acids. Acid attack is a complex phenomenon, and it is
difficult to predict the effect that CSH will have on the resistance of geopolymers to acid attack. This is the subject of
ongoing work.
5. Conclusion
A detailed examination of a geopolymer produced from fly ash and slag has been conducted by XRD, SEM and
TEM. The reaction products have been found to be x-ray amorphous, in line with the findings of other workers in the
field. The intergrowth of two distinct phases was observed by SEM; it was suggested that one of these phases is
amorphous CSH. The presence of CSH will affect the resistance of these materials to attack by acids, although the
exact result is difficult to predict. There is some evidence that crystalline reaction products were formed, although
these products could not be identified unambiguously. The inability to detect new crystalline phases by x-ray
diffraction can be explained by a combination of small size and low abundance.
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Acknowledgments:
The financial support of the Particulate Fluid Processing Centre is gratefully acknowledged, as is the skill of Ms. Julin Peng in the collection of TEM images and SAD patterns. We also acknowledge the technical, scientific and
financial assistance from the NANO-MNRF.