The Effect of Coal Ash Glass Chemistry on the Tailored Design of Waste-Based
Geopolymeric Products
Louise M. Keyte, Grant C. Lukey and Jannie S.J. van Deventer *
Department of Chemical and Biomolecular Engineering, The University of Melbourne, Victoria 3010, AUSTRALIA.
ABSTRACT
Geopolymers can be considered as low-temperature ceramic materials. These materials are formed by the alkali
silicate dissolution of industrial wastes such as coal ash and blast furnace slag. At ambient conditions, the dissolved
aluminium and silicon oxide species polymerise via a condensation reaction and re-precipitate to form the hardened
geopolymer. Geopolymeric materials derived from aluminosilicate wastes such as coal ash can exhibit superior
chemical and mechanical properties to ordinary Portland cement (OPC). The present work will investigate the
relationship between coal ash composition and mechanical properties of various geopolymers. Analytical techniques
including quantitative X-ray diffraction of devitrified coal fly ash enables the prediction of fly ash reactivity in a
geopolymeric system. This can lead to optimisation and tailoring of geopolymer formulations to specific
applications.
1. Introduction
Coal ash is a by-product produced from the burning of pulverised coal in a coal-fired boiler. Minerals including
kaolinite, pyrite and calcite are present in varying concentrations in coal deposits. These minerals do not combust as
they pass through the coal burning flame; however the flame temperature is sufficient for the minerals to melt. As the
melted minerals exit the flame, they are instantly cooled and carried off with the flue gas, and these particles are
known as coal ash. This ash is generally separated into coarse and fine fractions; the fine fraction is referred to as
coal fly ash. Fly ash is a fine-grained, powder particulate material that is carried off in the flue gas and usually is
collected by means of electrostatic precipitators, bag houses, or mechanical collection devices such as cyclones.
Coal fly ash is often incorporated into Ordinary Portland Cement (OPC) as a value adding filler. In this system, coal
fly ash is not highly reactive, but has been shown to improve properties such as workability and durability, and in
some cases up to 40% of the cement binder can be replaced with coal fly ash1. Research has also established that coal
fly ash can be alkali-activated, whereby dissolution of the glassy phases present in the ash results in the formation of
a cementitious binder, referred to as a geopolymer. The potential for superior products to be formed using
geopolymer technology, viz. improved strength, durability and acid resistance, has led to many researchers
investigating ash from a wide variety of countries. From this work it has been established that most fly ashes could
produce geopolymers with superior mechanical properties to that of OPC 2,3.
Currently there is only one commonly used classification system for coal combustion products, including fly ash, as
defined by ASTM C 6 18. Pulverised coal fly ash is separated into two classes known as Class F and Class C 4. The
distinction between these two classes is based on the sum of the total silicon, aluminium and iron (SiO 2 + Al2O3 +
Fe2O3) in the ash. When the sum is greater than 70% (mol), an ash is classified as Class F. Fly ash can be further
classified depending on iron content and alkali cation content. As most coal deposits contain some pyrite or other
iron mineral deposit, fly ash may contain as much as 20% (w/w) iron oxide. Fly ash also generally demonstrates inter
and intra-particle inhomogeneity, making it a very difficult material to analyse as every particle is compositionally
very different and will respond differently to its environment.
Geopolymerisation reactions occur when an aluminium oxide and silicon oxide containing raw material is contacted
with a highly alkaline solution. When using fly ash, the glass structure of the ash is rapidly attacked by hydroxide
ions causing dissolution. At pH > 8, glass dissolution occurs due to deprotonation by hydroxide ions5 (Equation 1):
*
Author to whom correspondence should be addressed: jannie@unimelb.edu.au, Phone: +61 3 8344 6619; Fax: +61 3 8344 7707
Si-OH + OH-  Si-O- + H2O
(1)
Water then attacks the glass via the following reaction (Equation 2):
y
.
Si
y
y
O
y
Si
y
y
+
H2O
.
Si
y
y
OH
HO
Si
OH
(2)
y
The new Si-OH bonds formed are consequently deprotonated, and the reaction continues. The rate at which glass
dissolution occurs is largely dependent on the composition of the glass present in the fly ash. Vitreous silica type
glass, highly substituted with alkali or alkaline earth metals, will react far more rapidly than glass with a low amount
of these metals. This is because alkali and alkaline earth metals depolymerise vitreous silica networks, and
consequently there are fewer bonds that have to be broken in order for dissolution to occur 6.
These reaction mechanisms are different to those undertaken in coal fly ash substituted cement, as well as for the
dissolution of clays, such as metakaolin. The cement hydration reaction starts at a neutral pH and water must diffuse
into the cement particles. Protons exchange for cations, and hydroxide ions diffuse out in the initial stages of
reaction. With time the cement hydration reactions generate hydroxide ions. This process is considerably slower than
the mechanism described by Equations (1) and (2), as diffusion of the alkali must occur until the cement hydration
reactions start. The fly ash glass dissolution is hence initially controlled by exchange of alkali ions in the glass with
water5. Metakaolin is a calcined clay, not a glass structure, and the mechanisms for dissolution will be different due
to particle size and shape, as well as stability of the alumino-silicate structure.
A great deal of research into the physical and chemical structure of fly ash was undertaken by various researchers in
the 1980s8-13. However, this work was focused on relating fly ash structure to the behaviour of cement/fly ash blends.
Since this time, geopolymer technology has emerged and very little research has been performed on how the physical
and chemical structure of fly ash relates to the performance properties of geopolymer products formed. This paper
will therefore investigate the relationship between the glass composition of fly ash and mechanical properties. A
fundamental understanding of this relationship will therefore enable the tailored design of geopolymers with
consistent and achievable mechanical properties.
2. Experimental Methods
2.1 Materials
Fly ash samples were sourced from Australasia. Fly Ash A, B, C, D and E are all classified as Class F according to
ASTM C 6 18. Fly ash F is classified as Class C according to ASTM C 6 18. Washed and classified sand (Cement
Grade) was obtained from Unimin Australia. Sodium silicate solution (molar ratio SiO 2/Na2O = 2.0, 29.4 wt% SiO2)
was obtained from PQ Australia. Laboratory grade reagents (NaOH, KOH) were obtained from Consolidated
Chemicals, Australia. Distilled water was used throughout.
2.2 Characterisation
Compressive strength results were obtained conforming to ASTM C39 on an ELE compression machine using a
loading rate of 0.9 kN/s. The cube sample dimension was 50 x 50 x 50mm for all geopolymer mortars. All the values
collected were the average of three separate tests, with a standard deviation of less than 5%. Scanning electron
microscopy (SEM) images were obtained using a Philips XL30 SEM. The fly ash samples were adhered to carbon
tape and coated with gold. The fly ash samples were analysed by powder XRD analysis (Phillips PW 1800) using Cu
K radiation, which was generated at 30mA and 40kV with an average wavelength of 1.54184 Å. The samples were
step-scanned at 0.05o 2θ and integrated at the rate of 2 s step -1. Quantitative analysis was performed using BrukerAXS Software: Diffracplus Basic, Evaluation Package comprising the Diffrac plus Basic evaluation programs Eva,
PDFMaint, DQuant, and some auxiliary programs. Optical light microscope images were obtained using an Olympus
SZ 1145 using a Nikon Cool-Pix 990 digital camera. Devitrification of fly ash was performed by calcining samples
at 900oC for 24 hours, with controlled cooling to minimise glass formation.
2.3 Synthesis
For compressive strength analysis, geopolymer mortars were generated using a dry blend of 1 part coal fly ash to 2.1
parts dry, sieved sand and an activating solution containing sodium hydroxide and sodium silicate dissolved in
distilled water. The concentrations of activators were exactly the same for each sample. Pre-blended ash/sand
samples were added to hydroxide/silicate activating solutions that had been cooled to room temperature. All samples
were cured at 40±0.5oC for 12 hours, before being de-moulded and sealed in plastic bags for a further 24 hours
curing at 65oC. Samples were allowed to cool for 2 hours prior to compressive strength testing. The compressive
strength result presented is an average of three measurements taken that fall within ±0.5MPa of each other.
3.
RESULTS AND DISCUSSION
The oxide compositions of the six fly ashes studied in the current work are presented in Table 1. By comparing fly
ashes A – F, it can be seen that the aluminium and silicon content varies considerably, as well as the iron oxide and
calcium oxide content. This is expected and demonstrates that no two ashes are similar in composition, and therefore
each ash will behave differently during geopolymerisation, resulting in products of varying properties.
Table 1. Oxide Compositions of Fly Ash Samples as Determined by XRF (wt %)
Fly Ash
A
B
C
D
E
F
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
TiO2
MnO
Fe2O3
LOI
0.36
0.00
3.23
0.09
0.19
1.05
1.33
0.00
2.11
0.07
2.04
2.06
26.79
28.44
30.10
23.00
30.41
18.10
45.83
64.62
48.16
71.90
42.97
47.49
1.57
0.00
1.04
0.13
0.19
0.45
0.12
0.01
0.09
0.04
0.06
1.01
0.84
0.27
1.55
0.39
0.26
0.40
4.16
0.09
4.74
0.10
3.19
19.11
1.21
1.62
1.59
1.27
1.87
0.91
0.17
0.00
0.05
0.01
0.25
0.03
11.62
0.43
3.82
0.57
14.14
6.32
6.00
4.52
3.52
2.43
4.43
3.07
For geopolymerisation, the amount of aluminium and silicon available for reaction will determine the strength of the
product formed. Factors including particle size and available surface area are also important; however the actual
quantity of material that can react in an alkali-silicate system will determine the amount of species which can
dissolve to contribute to the geopolymer framework. Quantitative XRD can determine the types of crystalline phases
present and the approximate quantity. These phases will generally be unreactive in a geopolymer system as they are
resistant to alkali attack. Table 2 presents the type and amount of crystalline phases present in the ashes studied. As
expected, the results in Table 2 show that fly ashes A – F contain predominantly mullite and quartz phases, as well as
varying quantities of other crystalline materials including maghemite, calcite and hematite. It is important to note
that other crystalline phases may also be present in concentrations of <0.5%.
Table 2. Crystalline phase concentrations of Fly Ash A - F (wt %), as determined by Q-XRD (±5%).
Fly Ash
A
B
C
D
E
F
Mullite
Quartz
Maghemite
Calcite
Hematite
12
25
23
15
2
8
4
7
11
14
17
9
4
0
0
0
7
0
0.5
0
0
0
0.5
0
0
0
0
0
2
0
The results in Table 1 and Table 2 can be used to calculate the amount of aluminium and silicon in the glassy phases
of fly ash. As discussed previously, the glass phases of fly ash are thought to undergo dissolution in an alkaline
medium, and therefore contribute the aluminium and silicon species for geopolymerisation. The available silicon and
aluminium per gram of material are presented in Table 3. The compressive strengths of geopolymer mortar samples
synthesised are also presented in Table 3.
Table 3. Compressive Strength (MPa) of geopolymer mortars generated using fly ash samples.
Fly Ash
n(Al)glass
n(Si)glass
Compressive
Strength (MPa)
A
B
C
D
E
F
0.357
0.206
0.266
0.254
0.358
0.242
0.640
0.842
0.511
0.881
0.607
0.602
32.7
5.7
73.9
9.8
30.5
67.5
The results in Table 3 show that there is no correlation between the amount of aluminium and silicon in the glass and
the compressive strength of the geopolymer formed. For example, fly ash C contains considerably less silicon and
aluminium than fly ash A, yet it exhibited almost double the compressive strength. These results are unexpected, as it
is generally accepted in the geopolymer community that more aluminium and silicon available for dissolution will
result in more geopolymer gel formation, and thereby a hardened product of greater mechanical strength.
In an attempt to gain further understanding of the effect of coal ash glass chemistry on the properties of
geopolymeric materials, selected ash samples were subjected to different separation processes, namely density
classification (Fly Ash A) and particle size classification (Fly Ash F). It is possible that both processes may result in
an ash that is less variable, leading to predictable mechanical properties. In particular, classification of the ash by
density will separate out iron containing particles, especially maghemite, which is unreactive in a geopolymer
system. Particles with large quantities of crystalline inclusions should also have a higher density than spherical glass
particles with low quantities of crystalline inclusions. Therefore, increasing the amount of glassy phases in the fly
ash fractions should assist in increasing the strength of the samples, as more material will be available for reaction.
A Mozely MGS table was employed to perform the gravity separation, and as a consequence, the reference ash (ie.
unclassified fly ash A) must be water treated to be compared with the fractions obtained from the separation. The
fine and coarse fractions were collected for two separate runs. For the first run, the mass yield of the fine fraction
(A1F) was 90.9% (w/w) and the mass yield of the coarse fraction (A1C) was 9.1% (w/w). For the second run, the
mass yield of the fine fraction (A2F) was 75.7% (w/w) and the mass yield of the coarse fraction was 24.3% (w/w).
The oxide compositions of the fractions were determined using XRF and phase determination and quantification
were performed using quantitative XRD. The results are presented in Table 4 and Table 5. The available silicon and
aluminium per gram of material are presented in Table 6, along with compressive strengths of geopolymer mortar
samples synthesised.
Table 4. Oxide Compositions of Classified Fly Ash A Samples as Determined by XRF (wt %)
Fly Ash
A (ref)
A1Fa
A1Cb
A2Fc
A2Cd
a
b
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
TiO2
MnO
Fe2O3
LOI
0.28
0.35
0.27
0.34
0.27
1.35
1.24
1.38
1.25
1.43
27.84
28.95
22.41
27.36
22.30
45.56
48.42
40.88
47.44
37.76
0.53
1.41
1.83
1.46
1.71
0.21
0.10
0.19
0.18
0.19
0.47
0.91
0.66
0.86
0.64
5.61
2.95
6.10
3.70
5.02
1.36
1.03
1.51
1.10
1.48
0.19
0.11
0.24
0.13
0.31
11.21
6.45
16.44
8.28
22.40
5.39
8.08
8.09
7.90
6.49
A1F = 1st pass, fine fraction, mass yield = 90.9%; c A2F = 2nd pass, fine fraction, mass yield = 75.7%
A1C = 1st pass, coarse fraction, mass yield = 9.1%; d A2C = 2nd pass, coarse fraction, mass yield = 24.3%
Table 5. Crystalline phase concentrations of Fly Ash A, A1F, A1C, A2F and A2C (wt %), as determined by Q-XRD (±5%).
Fly Ash
A (ref)
A1F
A1C
A2F
A2C
Mullite
Quartz
Maghemite
Calcite
Hematite
4
4
4
4
2.5
12
12
9
12
7
4
1
8
2
7
0.5
0.4
0.2
0.4
0.4
0
1
4
1
5
Table 6. Compressive Strength (MPa) of mortars generated using fly ash samples
Fly Ash
n(Al)glass
n(Si)glass
Fe2O3 (wt
%)
Compressive
Strength (MPa)
A
A1F
A1C
A2F
A2C
0.357
0.399
0.313
0.368
0.338
0.640
0.683
0.571
0.667
0.553
11.21
6.45
16.44
8.28
22.40
35.4
24.9
35.5
-
The results in Table 6 demonstrate the poor correlation between the amount of aluminium and silicon in the glassy
phases, and the compressive strength of the samples. The fine fraction collected from the first run of the density
separation of Fly ash A (A1F) had a greater aluminium and silicon content than the sample generated from Fly ash
A, as well as considerably less iron, however yielded a lower compressive strength. The separation of this ash clearly
demonstrates that removing the iron rich phases in fly ash A does not improve the compressive strength of the
resultant geopolymer mortar. The inhomogeneity of fly ash is such that the glassy particles are equally likely to be
associated with crystalline particles as without. It is clear that the only benefit density separation can achieve is a
reduced fly ash bulk density.
Another type of separation process is classification by particle size. It was determined that the fraction below 38 m
of Fly ash F was found to contain significantly higher amounts of calcium, than the fraction above 38 m. It is
generally accepted that as the calcium content of fly ash increases, so too does the compressive strength. The oxide
compositions as well as phases present for each fraction are presented in Table 7 and Table 8. The available silicon
and aluminium per gram of material is presented in Table 9, along with compressive strengths of geopolymer mortar
samples synthesised.
Table 7. Oxide Compositions of Fly Ash Samples as Determined by XRF (wt %)
Fly Ash
F
F > 38 m
F < 38 m
Na2O
MgO
Al2O3
SiO2
P2O5
SO3
K2O
CaO
TiO2
MnO
Fe2O3
LOI
1.05
0.59
1.28
2.06
1.34
2.41
18.10
17.04
18.60
47.49
59.02
41.36
0.45
0.30
0.54
1.01
0.09
0.77
0.40
0.36
0.42
19.11
12.81
22.96
0.91
0.98
0.83
0.03
0.03
0.04
6.32
4.38
7.13
3.07
3.06
3.66
Table 8. Phase concentrations of Fly Ash F and separated fractions of Fly Ash F (wt %), as determined by Q-XRD (±5%).
Fly Ash
F
F > 38 m
F < 38 m
Mullite
Quartz
Lime
Amorphous
8
5
12
9
5
10
0
1
1
83
89
77
Table 9. Compressive Strength (MPa) of mortars generated using fly ash samples
a
b
Fly Ash
n(Al)glass
n(Si)glass
Compressive Strength (MPa)
F
F > 38 ma
F < 38 mb
0.242
0.264
0.224
0.602
0.877
0.441
67.5
74.8
F > 38 m = The fraction of Fly Ash F with particle size greater than 38 m, mass yield = 33%
F < 38 m = The fraction of Fly Ash F with particle size less than 38 m, mass yield = 67%
It can be seen from Tables 7 to 9 that the fraction of fly ash F that was collected below 38m had considerably less
aluminium and silicon than the sample generated using Fly Ash F, yet yielded a stronger product. This may possibly
be attributed to the higher concentration of calcium in this fraction, but is clearly not a function of aluminium or
silicon content in the fly ash.
In an attempt to understand the poor correlation between the aluminium and silicon content of the glassy phases in
fly ash and the compressive strength, the physical and chemical morphology was investigated. Scanning Electron
Microscopy (SEM) is a useful tool for observing fly ash particle morphology, including the spherical nature of most
fly ash particles (Figure 1a), plerospheres (Figures 1b and 1c), and the crystalline content of some particles (Figure
1d). Magnetic particles can also be detected when a particle with an irregular surface is located and the electron
beam is moved closer during magnification. The beam will repel these particles and they will shift position.
(a)
(b)
(c)
(d)
Figure 1. (a) Fly Ash A particle morphology demonstrating mainly spherical shape of particles; (b) Large plerosphere in Fly Ash
A; (c) Small plerosphere in Fly Ash A; (d) Fly Ash A particle with visible crystals.
Although not shown, all of the fly ash samples investigated displayed similar morphology when observed using
SEM. This is to be expected, and indicates that fly ash in general has similar physical characteristics (when observed
under SEM). The fly ash samples also had similar particle size distributions. The complexity and variability of ash
therefore results from significant differences in glass chemistry. This is supported by the results of the current study.
Fly ash glass chemistry has been extensively studied and it is well known that fly ash particles display both
interparticle and intraparticle inhomogeneity7,8. This unique characteristic can be observed using an optical
microscope. Optical micrographs of Fly ash A and Fly ash F are shown in Figure 2 demonstrating both interparticle
and intraparticle inhomogeneity of the fly ash.
0.5 mm
100 m
Figure 2. Optical Microscope Images: (a) Fly Ash A, particle size fraction -58m +38m; (b) Fly Ash F, particle
showing glass phase intraparticle heterogeneity.
The colour of a fly ash particle reveals a great deal of information about its particular chemistry. For example, some
black particles will generally be unreactive as they will be either: (1) magnetite - a non-glassy component of fly ash;
(2) a glass with potentially less reactivity than other types of vitreous silica due to high iron content; or (3) carbon - a
non-glassy component of fly ash. It is the combination of these different coloured particles in fly ash that contribute
to the overall appearance of a coal fly ash sample. Consequently, if it were possible to separate fly ash particles
based on colour, then effective separation of particles into reasonably homogeneous fractions would be possible, as
the colour of the particle is an indication of the chemical composition. This would then enable the formation of
geopolymer products of consistent mechanical strength.
Although coal fly ash is an extremely complex material due to inter- and intraparticle heterogeneities, it is these very
characteristics that allow physical observation of the compositional differences between particles. Coal fly ash with a
larger particle size and lower concentrations of spheres can also be expected to be less reactive as small spherical
particles generally have a high glassy composition, as glass formation is favoured for small particles as they quench
more rapidly than larger particles9. Retention of spherical shape is also more likely due to rapid quenching, with no
time available for the particle to collapse in on itself. Physical characterisation methods could further be used as
relatively simple methods of determining compositional changes in fly ash. For example, classification of coal fly
ash to remove iron particles would result in a change in the concentration of black particles observed under an optical
microscope.
These observations however, do not explain the differences in reactivity of the fly ash samples tested in relation to
their compressive strength. As the results of the present work show, the fly ash particles all have a similar particle
size and morphology, and the crystalline inclusions can be determined from quantitative XRD. Separation processes
such as density separation of fly ash A showed a reduction in iron containing particles, however the compressive
strength remained effectively unchanged.
The chemical complexity of fly ash is a direct result of the raw material composition and process conditions from
which it is formed. It is known from glass literature that glasses generated from such minerals as those present in
coal can result in a variety of phases being formed, depending on the concentrations of the minerals on each coal
particle. As each coal particle passes through the burner, the particles melt and cool without coming into contact with
other particles. As a result of poor mixing of particles (ie. individual mineral particles), the inhomogeneity that can
be observed in fly ash occurs. Each particle can therefore be considered to contain glass phases of different
chemistry. By understanding the individual glasses which form, it should be possible to determine the overall
reactivity of the different fly ashes, thereby enabling the tailored design of geopolymer products.
As discussed previously, glass dissolution chemistry in alkali hydroxide solutions proceeds via Equation 1 and 2, and
modifications to the glass structure, will impact on the dissolution chemistry of the particle. It is not possible to
determine the chemistry of all of the different types of glass possible in a fly ash fraction as an infinite combination
of minerals may be present in each coal particle. However, it may be possible that some of the aluminium and silicon
present in fly ash is contained within glass phases that have a higher resistance to alkali attack. This may be the
reason why a poor correlation between aluminium and silicon content and compressive strength is observed in the
current work.
4. CONCLUSIONS
Coal fly ash is a glassy particulate material with crystalline inclusions displaying inter- and intraparticle
inhomogeneity. This investigation into the relationship between coal fly ash glass chemistry and compressive
strength has established a poor correlation between aluminium and silicon content in the glass phases of fly ash and
the compressive strength of the geopolymers formed. Classification methods including density and particle size
separation were investigated in an attempt to gain a further understanding of the relationship between glass chemistry
of fly ash and mechanical properties. However no clear correlation between aluminium and silicon content of
classified ash and material properties could be determined. It is thought that the tailored design of geopolymers to
produce materials with consistent compressive strength can best be achieved by blending fly ashes of known
reactivity. This will allow the utilisation of fly ashes that are currently unable to produce geopolymers with adequate
structural compressive strength.
Acknowledgments
The financial support of the Particulate Fluids Processing Centre (a Special Research Centre of the Australian
Research Council) is gratefully acknowledged.
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