Minerals Engineering, Vol. 12, No. I, pp. 75-91, 1999
Pergamon
0892-6875(98)00121-6
© 1998 Elsevier Science Ltd
All rights reserved
0892--6875/99/5 - - see front matter
THE POTENTIAL USE OF GEOPOLYMERIC MATERIALS TO IMMOBILISE
TOXIC METALS: PART II. MATERIAL AND LEACHING
CHARACTERISTICS
J.G.S. VAN JAARSVELD ~, J.S.J. VAN DEVENTER ~ and A. SCHWARTZMAN*
§ Department of Chemical Engineering, The University of Melbourne,
Parkville, Victoria 3052, Australia. E-mail: jsj.van_deventer@chemeng.unimelb.edu.au
t School of Physics, The University of Melbourne, Parkville, Victoria 3052, Australia
(Received 21 July 1998; accepted I October 1998)
ABSTRACT
The stabilisation and solidification of waste materials by the technology of
geopolymerisation is fairly unknown and has not been studied in any depth. This paper
presents some experimental evidence as to the physical and chemical characteristics of
geopolymers manufactured from fly ash originating from two different regions. It has
become apparent that these materials could be used for a wide variety of environmental
and other applications such as the immobilisation of heavy metals and the fabrication of
structural products. In this study compressive strength testing, specific surface area
determinations, Transmission Electron Microscopy (TEM), Nuclear Magnetic Resonance
(NMR) and leaching tests were used in characterising a number of geopolymer matrices.
It is also shown how the inclusion of heavy metal ions, alkali metal cations and different
processing conditions affect the physical and chemical characteristics of the final product.
© 1998 Elsevier Science Ltd. All rights reserved
Keywords
Leaching; environmental; recycling; waste processing
~TRODUCTIONANDBACKGROUND
In a previous paper [1] the possible applications of geopolymers in waste processing have been discussed.
These applications can be divided into two categories: (1) Structural products such as concrete replacements
in various environments and (2) Immobilisation systems for heavy metal containment. Experimental
evidence regarding leaching data and physical properties was also presented, although all of it considered
commercially produced geopolymer cements that were mixed with waste products and allowed to set.
Relatively high costs and problems relating to transferring new technology to industry prevented these
commercially produced binders from becoming popular and to date only limited successes have been
achieved [2]. The possibility of producing geopolymeric matrices by utilising waste materials has been
proposed [1] and therefore this paper will present some experimental evidence on the physical properties
and leaching characteristics of geopolymers manufactured almost entirely from waste materials. It should
be kept in mind, however, that the present level of scientific knowledge with regard to geopolymers leaves
much to be desired and even the evidence presented here cannot serve to fundamentally understand these
products although a number of qualitative conclusions can be drawn.
75
76
J.G.S. van Jaarsveld et
al.
Various factors affect the eventual mechanical and chemical properties of the finished product. These
include the thermal history of the clay used, the type and amount of alkali metal cations present, the heavy
metal being immobilised as well as certain physical considerations such as particle size and ease of mixing
of the various reagents. The last two aspects will not be addressed in this paper, as they were fairly similar
for all examples under consideration. It was noted previously [1,3] that the mechanism by which
geopolymerisation is thought to occur involves the dissolution, migration and polymerisation of AI and Si
precursor species as well as a surface reaction on the remaining surface area of undissolved waste particles.
Although this is a very simplistic view of the process it will take some time before sufficient experimental
evidence is available to fully quantify the reaction kinetics of the synthesis reaction.
Depending on the eventual application, the product from such a process should be chemically inert and
physically strong in order to prevent any further leaching of immobilised metals and compounds. Physical
strength is not only an advantage as far as physical encapsulation of toxic materials is concerned, but also
provides for the utilisation of these products in certain building applications. Although physical properties,
such as compressive strength and porosity, can be utilised in distinguishing between different matrices,
leaching tests will usually provide more substantial information regarding the immobilising efficiency,
chemical stability as well as mechanism and kinetics of toxic metal immobilisation in geopolymeric
matrices. Apart from standard environmental leaching tests, specifically designed tests, such as those
discussed in this paper, have not yet been used in characterising geopolymer matrices and remain novel to
a large extent. As was mentioned previously [1] X-ray diffraction techniques are inadequate for studying
the structure of geopolymers, mainly because of their fairly high amorphous content. This problem can be
overcome by using a combination of analytical techniques such as Nuclear Magnetic Resonance (NMR) as
well as Transmission Electron Microscopy (TEM).
EXPERIMENTAL
Materials
Fly ash used in the synthesis of matrices El, E2, F1 and G1 was obtained from SASOL at Sasolburg, South
Africa and that used in matrices H1 to H4 from Tarong power station in Queensland, Australia. Both fly
ashes are of coal origin, with particle sizes in the order of 50% smaller than 12 larn and chemical
compositions as shown in Table 1. Kaolinite, grade HR1, was obtained from Commercial Minerals, Sydney,
Australia. In the preparation of matrices E1 and E2, metakaolinite was used, manufactured by calcining the
above-mentioned kaolinite at 600°C for 6 h as described by Madani [4]. All experiments were performed
using the same batches of reagents and starting materials. Distilled water was used throughout.
TABLE 1 Composition of fly ash as determined by fusion and XRF-analysis (mass%)
Element as oxide
SASOL fly ash
Tarong fly ash
CaO
50.1
28.3
8.2
61.4
33.0
0.6
Fe203
4.0
1.1
MgO
TiO2
Na20
2.0
1.5
0.5
0.9
0.4
4.1
0.3
2.0
0.1
0.1
0.0
1.4
SiO2
A1203
K20
SO3
Loss on ignition
Use of geopolymericmaterialsto immobilisetoxic metals: part II
77
Synthesis
Sample preparation was performed as described previously [3] with at least a 7-day waiting period being
observed before any tests were performed. In each case the samples were cast in 50mm cubes, vibrated for
5 minutes and allowed to set at 30°C for 24 hours before being removed from the moulds and kept at room
temperature for another 6 days. Heavy metal cations were added to the reaction mixture during mixing as
a solution of Cu(NO3) 2 or Pb(NO3) 2 in water. Tables 2 and 3 summarise the compositions of the respective
matrices. It could well be noted that the amount of NaOH and KOH used in producing matrices El, E2,
F1, F2 and G1 was chosen such as to provide for equal molar amounts of Na and K present in the structures
of all the matrices in Table 2.
TABLE 2 Compositions of matrices prepared from SASOL fly ash (mass%)
Matrix
Contaminant
E1
E2
FI
F2
G1
Cu
Pb
Cu
Pb
Cu
0.1
0.1
0.1
0.1
0.1
Alkali metal
K O H 5.0
K O H 5.0
NaOH 3.7
NaOH 3.7
KOH 5.0
Clay
Metakaolinite
Metakaolinite
Kaolinite
Kaolinite
Kaolinite
16.0
16.0
15.0
15.0
15.0
Water/Fly ash
mass ratio
A1203/SIO2
mass ratio
0.2
0.2
0.2
0.2
0.2
0.57
0.57
0.57
0.57
0.57
TABLE 3 Compositions of matrices prepared from Tarong fly ash (mass%)
Matrix
H1
H2
H3
H4
Contaminant
Cu
Pb
Pb
Cu
0.1
0.1
0.2
0.2
Alkali metal
KOH
KOH
NaOH
NaOH
5.0
5.0
6.0
6.0
Clay
Water/Fly
ash ratio
A1203/SIO2
ratio
Kaolinite 14.0
Kaolinite 14.0
Kaolinite 14.0
Kaolinite 14.0
0.43
0.43
0.45
0.45
0.57
0.57
0.57
0.57
An~yses
Compressive strength testing
Compressive strength testing was performed as per AS 1012.9 [5] using three 50 mm cubes of each sample
and averaging the experimental values obtained. All samples were tested after 14 days. An Amsler FM 2750
compressive strength testing apparatus was used.
Leaching tests
Samples submitted to leaching tests were crushed and sieved into particle size fractions, the latter being
leached until equilibrium conditions were obtained. Leaching of each particle size fraction was conducted
using a modified TCLP [3, 6] procedure utilising acetic acid buffered at pH = 3.3 by analytical grade
sodium acetate. The liquid/solid ratio was kept at 1:25 and the temperature controlled at 30°C. Equilibrium
tests on matrices El, E2, F1 and G1 were conducted by two different methods i.e. by the use of stirred
vessels with overhead impellers and also by using horizontally rolled bottles. The latter technique allowed
for equilibrium to be obtained after around 60 hours, with the stirred technique reaching equilibrium in
around 24 hours. In the case of matrices H1 to H4 only the stirring technique was used with sampling being
performed periodically over a 24-hour period. Sampling was conducted by syringe and the total sampling
volume never exceeded 10% of the fluid volume, thus creating an average error of 5%. Concentrations of
Cu and Pb were determined using a Perkin Elmer Optima 3000 ICP-OES, with scandium as internal
standard.
78
J.G.S.
v a n J a a r s v e l d et al.
Specific surface area, infrared analysis and X-ray diffraction
BET surface areas were determined for all samples by using a Micromeritics Flowsorb ASAP 2020 with
a 30/70 ratio of N 2 and He, degassing for 18 hours at 95°C. Infrared spectra were recorded for matrices El,
E2, F1 and G1 on a Mattson Galaxy 2020 spectrometer using the KBr pellet technique (0.5 mg powder
sample mixed with 250 mg of KBr). X-ray powder diffraction data were obtained for all samples using a
Phillips PW 1800 diffractometer with Cu K(x radiation.
Nuclear magnetic resonance and transmission electron microscopy
Nuclear magnetic resonance (NMR) spectra were recorded using the Magic Angle Spinning (MAS)
technique. 29Si and 27A1 spectra were recorded at spinning speeds of 15 kHz (27A1) and 4 kHz (29Si) for
matrices El, F1, F2 and G1. Certain samples were also submitted to investigation by transmission electron
microscopy (TEM) on a JEOL CX 100 TEM fitted with an X-ray microanalysis system. The specimens
were prepared by mechanically grinding the powder in ethanol, followed by ultrasonic agitation and
precipitation on an amorphous carbon film.
RESULTS AND DISCUSSION
General
It is of interest to note that in synthesising the various geopolymer samples, it became very obvious that
mixing will play an important role in any proposed commercial venture involving the large-scale production
of geopolymer related materials. A property such as the rheology of the reacting mixture was quantitatively
observed as being of importance to the properties of the final product and although it fell outside the scope
of this study it will be an important factor that still needs to be investigated in any future study. The two
fly ashes used differed as far as their chemical contents were concerned and especially the amounts of
carbon and calcium that they contained. The main crystalline phases in both cases were mullite and quartz,
although amounts of amorphous silica and alumina compounds were also present. The exact nature and
quantity of these amorphous phases cannot be determined easily although it is widely accepted that they are
responsible for most of the reactivity associated with a specific fly ash [7].
Compressive strength and specific surface area
It is significant to note that the inclusion of Pb instead of Cu serves to strengthen the structure in terms of
compressive strengths achieved (Table 4). This is true in the case of E1 and E2, HI and H2 as well as H3
and H4. In each case the matrices contain equal mass amounts of the heavy metal although the molar
amounts would differ and there would be less Pb ions present in each case than in the corresponding matrix
containing Cu. Table 5 shows that Pb resulted in higher specific surface areas than Cu. However, this is not
at all what would be expected from a structural point of view where the stronger sample could be expected
to have less porosity. It therefore seems a reasonable assumption that although the Pb ion influences the
structure in terms of causing increased porosity this effect is offset by its contribution to structural strength
in another way such as through its much larger ionic radius than that of Cu. It is not clear whether these
ions are finding themselves bonded into the structure or whether the surrounding matrix is just physically
encapsulating them. This issue will be discussed in more detail later.
TABLE 4 Compressive strengths after 14-days (MPa)
Matrix
El
E2
FI
GI
HI
142
H3
[-[4
Contaminant
Cu
Pb
Cu
Cu
Cu
Pb
Pb
Cu
Alkali metal
K
K
Na
K
K
K
Na
Na
Compressivesttens~h
28.1
33.7
43.8
51.4
4.5
7.3
18. !
16.5
Use of geopolymedcmaterialsto immobilisetoxic metals: part II
79
TABLE 5 BET surface areas for different matrices (m21g)
Matrix
Contaminant
Alkali metal
Specific surface area
E1
E2
F1
G1
H1
H2
H3
H4
Cu
Pb
Cu
Cu
Cu
Pb
Pb
Cu
K
K
Na
K
K
K
Na
Na
12.1
16.7
8.8
16.4
9.3
12.2
7.8
4.3
Referring to Table 2 it can be seen that the only difference between G1 and E1 is the use of calcined
kaolinite in the case of El. This seems to adversely affect the compressive strength of E1 compared with
that of G1. According to Davidovits [8] the calcining of kaolinite transforms the octahedrally co-ordinated
AI layers into the more reactive tetrahedral form creating metakaolinite and increasing geopolymer
reactivity. E1 would therefore be expected to have a higher compressive strength than the corresponding
matrix manufactured with less reactive kaolinite. This result implies that the concept of geopolymer
synthesis does not entirely depend on dissolution and migration of ions dissolved from mineral surfaces
because A1 ions would find it much easier to dissolve from metakaolinite than from uncalcined kaolinite.
In these results, however, this effect is not reflected in an increased compressive strength and in fact results
in a weaker structure. If it is assumed that there is a reaction taking place on the surfaces of individual
particles and that the layer structure of the uncalcined kaolinite remains intact while particle surfaces bind
to the "bulk" geopolymer phase, these results can be explained. Significant to note is the fact that the
specific surface area of the stronger matrix is again larger, implying that the thermal processing of the
kaolinite results in a totally different final structure, which suggests a combination of dissolution-migrationpolymerisation as well as surface-solid state reactions taking place.
In the case of F1 and G1 the exact same molar amounts of respectively NaOH and KOH were used and
resulted in GI having a higher compressive strength although its specific surface area was smaller than that
of F1. The proposed explanation for these contradictory observations is that Na and K form part of the final
structure, effectively creating two different geopolymers. This observation is also in accordance with existing
knowledge of zeolite chemistry [9].
Comparing the compressive strengths achieved for those samples manufactured from the SASOL fly ash
(Table 2) and those from the Tarong fly ash (Table 3) it is evident that the first group exhibits generally
much higher compressive strengths and this can be attributed to a number of factors: (a) the higher CaO
content [10] of the SASOL fly ash (b) a lower water to fly ash ratio used in the samples made from the
SASOL fly ash (c) different alkali metal content of the two fly ashes (d) different A1 and Si content and
(e) different solubilities of the AI and Si precursors. The increased carbon content of the SASOL fly ash
could be responsible for a decrease in strength as well as increased porosity although it seems to be more
than compensated for by the presence of Ca and the fact that less water was used in the initial mix design
(Tables 2 and 3).
X-ray diffraction (XRD) and electron microscopy
As mentioned earlier [1] the study of geopolymers using X-ray diffraction is made difficult by the fact that
a large part of the structure is amorphous to x-rays. Comparing Figures 1 and 2 with Figures 3 and 4 shows
the relatively large amorphous content between 20 and 40 degrees 20. This is much more pronounced in
the case of E1 and E2 than in F1 and G1, mainly because of an increased amorphous contribution by
unreacted metakaolinite used in the synthesis of E1 and E2. The amount of crystallinity present is caused
by quartz and mullite phases present in the fly ash. It can be seen, however, that a certain amorphous
content is present in all of the samples and this was further investigated by electron diffraction. As far as
80
J.G.S.
v a n J a a r s v e l d et al.
E1 and E2 are concerned the different contaminant metal included in each structure did not seem to make
any difference to the crystalline part of the spectra and it is therefore assumed that the metal finds itself
bonded into the amorphous part of the matrix. Further proof of this is supplied by the electron diffraction
studies as will be discussed below.
2500
2000
1500
0
1000
500
0
I
I
I
I
I
I
I
I
I
10
20
30
40
50
60
70
80
90
Degrees
(2-Theta)
Fig.1 X-ray diffraction spectrum of matrix El.
2500
2000
IO
O
1500
1000
500
0
!
I
I
I
I
!
!
I
I
10
20
30
40
50
60
70
80
90
Degrees
(2-Theta)
Fig.2 X-ray diffraction spectrum of matrix E2.
3500
3000
2500
e:S
O
O
2000
1500
1000
500
0
I
I
10
20
I
30
I
40
Degrees
I
50
,
60
(2-Thsta)
Fig.3 X-ray diffraction spectrum of matrix F1.
i
l0
i
80
!
90
81
Use of geopolymericmaterialsto immobilisetoxic metals:part II
4000
3500
3000
,!!
¢:
:l
o
(,,1
2500
2000
1500
1000
500
0
0
I
I
I
I
!
!
!
I
I
10
20
30
40
50
60
70
80
90
D e g r e e s (2-Theta)
Fig.4 X-ray diffraction spectrum of matrix G1.
Results from the electron diffraction experiments are qualitatively summarised in Tables 6 to 8 for F1, G1
and E1 respectively. In each case the structure, relative abundance and constituent elements of each phase
are indicated. In most cases it is possible to quantify most of these phases in terms of d-spacings and semiquantitative compositional information, although such a discussion will form part of a different study and
falls outside the scope of the present paper. The electron diffraction study concentrated on newly formed
phases and with reference to Figure 5 this can be explained by noting the relatively large unreacted fly ash
particles surrounded by newly formed geopolymer phase. Most diffractograms were recorded avoiding
obvious unreacted fly ash, kaolinite and metakaolinite particles. In every case the calculated d-spacings
proved whether a phase was new or belonging to unreacted components of the initial mix, all of whose dspacings are well documented in the literature. Each phase presented in Tables 6 to 8 therefore constitutes
a newly formed structure. Matrix F1 (Table 6) consisted of two main phases and a number of less-abundant
phases. Although the two main phases were both amorphous and consisted of essentially the same elements,
they were in fact different, containing different ratios of Si and A1. It is also significant to note that the most
abundant crystalline phase does not contain any of the added Cu and in fact most of the Cu is contained
in the amorphous phases. The presence of Ca in each of the amorphous phases suggests that Ca could be
fulfilling a charge-balancing role together with K and Na. Although F1 was synthesised with NaOH the
amorphous phases almost exclusively contain K as well, suggesting that K is favoured in a charge-balancing
environment to Na and this could explain why matrices containing primarily K seem to have higher
compressive strengths compared with those synthesised with primarily NaOH. In the case of G1 (Table 7)
and E1 (Table 8) most new phases seem to be amorphous, especially in the case of El, where no semiamorphous or micro-crystallinity was detected. In each case the matrices were synthesised using KOH and
in each case the added heavy metal appeared in the amorphous phase. The phases described here also differ
from those in the traditional cement field through their fairly high AI content, because traditional cement
hydration products usually consist of phases containing either Si or A1. The reason for this can be ascribed
to the fact that in most geopolymer mixes an abundance of alkali-metal ions are present which enables the
A1 to substitute Si without losing the electronic neutrality of the structure.
TABLE 6 Qualitative electron diffraction results for F1
Structure
Abundance
Composition
Amorphous
Amorphous
Crystalline
Amorphous
Amorphous
Amorphous
Polycrystallinv
Semi-Crystalline
High
High
Medium
Medium
Low
Low
Low
Low
A1, Si, K, Ca, Cu
AI, Si, K, Ca, Cu
Al, Si, Na, K, Ca, Fe
Al, Cu
Al, Si, Na, K, Ca
Al, Si, Na, K, Ca, Cu, Fe
Al, Si, Na, K, Ca, Cu
Al, Si, Cu
82
J. G. S. van Jaarsveld et
al.
TABLE 7 Qualitative electron diffraction for G1
Structure
Amorphous
Semi-amorphous to
microcrystalline
Amorphous
Abundance
High
High
Composition
A1, Si, K, Ca, Cu
A1, Si, K, Ca, Cu
Medium
AI, Si, Na, K, Ca, Fe, Ti
TABLE 8 Qualitative electron diffraction for E1
Structure
Amorphous
Amorphous
Amorphous
Abundance
High
High
Medium
Composition
AI, Si, Na, K, Ca, Cu, Ti
Al, Si, K, Na, Ca, Cu
A1, Si, K, Ca, Cu
Fig.5 TEM micrograph of matrix G1.
The micrograph in Figure 6 shows what was typically found for matrices E1 and E2 where the grinding of
the sample during preparation for electron microscopy resulted in the amorphous phase being dislodged from
the fly ash particles. This is not the case for matrix G1 in Figure 5 where the amorphous phase seems quite
securely bonded to the surface of the fly ash particles, even after sample grinding, resulting in a structure
with a much higher compressive strength.
Infrared (IR) and nuclear magnetic resonance (NMR)
In order to gain a better structural understanding, infrared analyses were conducted on El, E2, F1 and G1
and NMR spectroscopy on El, F1, F2 and G1. The main feature of all IR spectra in Figures 7 to 10 is the
central peak between 1010 and 1040 cm -~ that is attributed to the Si-O-Si or A1-O-Si asymmetric
Use of geopolymericmaterials to immobilise toxic metals: part II
83
Fig.6 TEM micrograph of matrix El.
stretching mode [11]. In the case of F1 and G1 this peak can be found at exactly 1031 cm -~ and it is
reasonable to assume that the presence of K or Na would not affect its locality. On the other hand in the
case of E1 and E2 this peak is found at 1025 and 1012 cm -~ respectively, indicating that the contaminant
metal does affect the structure and suggesting that it might be part of the structure and not just physically
encapsulated. The peak found at 543 cm -~ in both Figure 9 and 10 has previously been assigned to bending
of Si-O-AI where the AI is in octahedral co-ordination [4] confirming the fact that some kaolinite particles
remain unreacted. E1 and E2 exhibit a peak each at 559 cm -1 that has been assigned to be originating from
double-ring structures formed by Si and A1 tetrahedra [11]. In all four spectra a small broad peak can be
found between 700 and 800 cm -~ although this is more pronounced in the case of E1 and E2 than for F1
and G1. The reason is found in the fact that this region is associated with bonds containing tetrahedrally
co-ordinated A1 and specifically Si--O-AI. Again this agrees with the earlier conclusions that structures
manufactured from metakaolinite will contain relatively more four co-ordinated A1, not only because of
unreacted metakaolinite but also because of easier dissolution of A1 before and during synthesis. This also
indicates that in the case of F1 and G1 a degree of dissolution takes place either during or after synthesis
as this is the only way that will allow for tetrahedrally co-ordinated A1 to become part of the structure.
1.5
1025
•
u
c-
1
467
o
,~ 0.5
!
I
!
I
I
I
500
1000
1500
2000
25OO
30O0
Wavelength (era-l)
Fig.7 Infrared spectrum of matrix El.
84
J.G.S. van Jaarsveld et
al.
1.5
1012
0
/'~
459
0.5
I
I
!
I
I
!
500
1000
1500
2000
2500
300O
Wavelength (cm-1)
Fig.8 Infrared spectrum of matrix E2.
2.5
2
1031
1010~
=8 1.5
0
.Q
1
0.5
0
I
!
I
I
I
I
500
1000
1500
2000
2500
3000
Wavelength (cm-1)
Fig.9 Infrared spectrum of matrix FI.
2.5
1031
2.
~ 1.5
1010]~
472
0.5
w
1
!
!
I
i
o
500
1000
1500
2000
2500
3000
Wavelength (¢m-1)
Fig.10 Infrared spectrum of matrix G1.
Comparing Figures 11 to 14 and Figures 15 to 18 for matrices El, F1, F2 and G1 it is evident that both
the 29Si and the 27A1 MAS NMR spectra are very similar for F1, F2 and G1. The 29Si and 27A1 spectra for
E1 differ from these, mainly because of the relatively higher content of tetrahedrally co-ordinated AI. Figure
15 suggests that E1 contains considerably more tetrahedrai AI, denoted by the 58.5 ppm shift, than six coordinated AI, indicated by the 0.1 ppm shift [12]. This is in accordance with what has previously been
suggested. The spectra of F1, F2 and G1 exhibit the same peak shifts of 58.5 and 2.2 ppm indicating a
mixture of 4 and 6 co-ordinated A1, again confirming and supporting the earlier findings from the TEM and
IR analyses. It is worth noting that different alkali and heavy metal ions do not seem to influence either the
29Si or 27A1 spectra. The 29Si spectra for E1 (Figure 11) contain two main shifts i.e. at -90 and -104.2 ppm.
The latter has been attributed to Si atoms connected in four directions via oxygen linkages to 3 Si and 1
A1 atom or to only 4 Si atoms, the so called Si(1AI) or Si(1A1) sites [13]. The shift at -90 ppm is again
Use of geopolymericmaterialsto immobilisetoxic metals: part II
85
associated with Si(3A1) sites and it is apparent that not many Si(4A1) sites (-80 to - 9 0 ppm) are present
in the spectra of El. This is, however, not the case with the 29Si spectra of F1, F2 and G1 where the two
main shifts are -87.3 and -91.9 ppm, indicating an abundance of Si(4AI) and Si(3A1) sites. In terms of
structural properties this could explain why the compressive strengths achieved with matrices F1, F2 and
G1 were higher than that of E1 and E2. The classical model of a geopolymer as proposed by Davidovits
[8] theoretically contains mainly Si(4A1) sites and as such F1, F2 and G1 could be regarded as resembling
this model more closely than E1 and E2. The MAS NMR investigation presented here is, however, very
simplistic and a more thorough investigation of geopolymers manufactured from waste materials will be
necessary to fully quantify all the structural characteristics.
-90 ppm
-
I
I
0
- 100
ppm
.
I
-200
Fig.ll 29Si MAS NMR spectrum of El.
-91.9 ppm
-87.3 ppm
I
0
I
-I00
~m
I
-200
Fig.12 2aSi MAS NMR spectrum of F1.
86
J.G.S. van Jaarsveld
et al.
-91.9 ppm
-87.3 ppm
I
I
-100
0
I
-200
ppm
Fig.13 29Si MAS NMR spectrum of F2.
-91.9 ppm
-87.3 ppm
I
0
I
-100
ppm
I
-200
Fig.14 29Si MAS NMR spectrum of G1.
Use of geopolymericmaterials to immobilise toxic metals: part II
58.5 ppm
I
200
I
0
~m
I
-200
Fig.15 27A1 MAS NMR spectrum of El.
2.2 ppm
58.5 ppm
I
I
200
0
ppm
I
-200
Fig.16 27A1 MAS NMR spectrum of F1.
87
88
J . G . S . van Jaarsveld et al.
2.,' ppm
58.5 ppm
I
200
I
0
ppm
I
-200
Fig.17 27A1MAS NMR spectrum ofF2.
2.2 ppm
58.5 ppm
I
200
I
0
ppm
I
-200
Fig.18 27A1 MAS NMR spectrum of G1.
Use of geopolymeric materials to immobilise toxic metals: part II
89
Leaching
In a previous study [3] the potential of geopolymer binders for the immobilisation of heavy metals was
discussed and environmental leaching results presented. Kinetic leaching tests were conducted with sampling
done periodically until equilibrium was reached. Matrices El, F1 and G1 contain identical amounts of Cu
and therefore leaching results can be compared on the same basis. Figure 19 shows the leaching curves for
five particle size fractions of H2 and for the sake of simplicity all other leaching tests will be summarised
in terms of the equilibrium values obtained. These results are presented in Tables 9 and 10.
10
9
8
A
E
Q.
Q.
7
v
I::
6
g
5
o
c
Q
oe,.
o
0
,.Q
n
• 212-600
•--.m~ 600-1000
~
1000-1700
1700-2360
2360-28O0
x
4
3
2
1
0
200
0
400
600
800
Time (min)
1000
1200
1400
1800
Fig. 19 Leaching curve of H2. Leaching in acetic acid buffered at pH 3.3, solid/liquid ratio 1:25, 30°C and
mixing speed of 200 rpm.
TABLE 9 Equilibrium concentrations achieved during leaching (ppm)
Matrix
E1
E2
F1
G1
TABLE 10
Matrix
H1
H2
H3
I"t4
Element
Cu
Pb
Cu
Cu
212/600 ~tm
Stirred
Rolled bottle
22
23
17
12
30
34
24
20
1700/2360 ~tm
Stirred
R o l l e d bottle
28
33
22
17
17
17
11
9
Equilibrium concentrations for different particle size fractions using stirred leaching
configuration (ppm)
Contaminantt
212/600
I.tm
60011000
gm
100011700
gm
170012360
rtm
2360/2800
gm
Cu
Pb
Pb
Cu
26.1
9.1
14.8
17.1
21.2
8.9
13.3
14.1
20.3
8.6
11.6
11.1
19.2
8.4
9.8
7.5
18.5
8.2
9.2
6.3
90
J.G.S. van Jaarsveldet
al.
It is significant to note that for E1 and E2 the Pb ions always seem to leach slightly more than the Cu,
regardless of the particle size and leaching method used. This fact is probably due to the larger surface area
measured in E2 compared with E1 although comparison of F1 and G1 does not follow the same trend where
Cu leaching from G1 is generally less than from F1, with the latter having half the specific surface area.
On the other hand G1 has a higher compressive strength and one would expect a higher abrasion resistance
than in the case of El, E2 or F1. The leaching of Cu from El, F1 and G1 follows the same trend as their
respective compressive strengths and this suggests that abrasion plays a role in liberating some of the metal
during leaching. An increase in equilibrium leaching values with decreasing particle size is a feature that
is found for all matrices and this further supports the above statement that some liberation occurs form
particle surfaces, whether through mechanical or chemical means. It was shown previously [3], however,
that a certain amount of leaching also occurs as a result of pore diffusion and this aspect needs to be
investigated further. The differences in leaching of Cu from F1 and G1 as well as differing surface areas
and strengths again point to the fact that the type of alkali metal cation has an important influence on all
aspects of the final structure. This is further supported by the fact that equilibrium values for the rolled
bottle experiments were generally higher than those found for the stirred type method. This is most probably
due to the longer leaching time (60 hours) as well as the fact that abrasion between particles and the side
of the vessel is much more severe than in the case of the latter method leading to higher degree of physical
breakdown. Visual observation of the solid residue from the leaching experiment also confirmed this.
Table 10 shows that increased leaching from smaller particle sizes holds for the range of matrices and
particle sizes investigated although certain trends appear to be different from those obtained for El, E2, FI
and G1. As was previously reported [3] the immobilisation of Pb in matrices not containing metakaolinite
seems to be more efficient than for Cu and this could be in part attributed to differences in ionic radius or
chemical interaction with the other matrix forming components during synthesis resulting in slightly
different roles played by Cu and Pb in the final product. It was mentioned earlier that Cu is present in most
of the newly formed amorphous phases. Unfortunately this could not be confirmed for lead due to
equipment limitations. One could safely assume, however that any matrix will have a limited capacity for
the amount of heavy metal its structure can tolerate before structural failure will cause increased leaching
of that metal. This point is illustrated by comparing the differences in leaching of Cu and Pb from matrices
HI, H2, H3 and H4. Although H1 and H2 were synthesised with KOH, and H3 and H4 with NaOH, the
latter group also contains double the amount of heavy metal introduced into the first. At smaller particle
sizes H3 releases less Pb than the Cu released by H4 although this trend is reversed when the larger particle
size fractions are considered. This trend is not present in the case of H1 and H2 where the Cu always
leaches more than the Pb. Referring to Table 5 the specific surface area of Cu containing matrices H1 and
H4 is smaller than that of their Pb-containing counterparts although the latter seem to generally have a
higher immobilisation efficiency.
CONCLUSIONS
The study and structural understanding of geopolymers derived from waste materials will become more
essential as the number of commercial applications of these materials increases. From this study it is not
only apparent that a multitude of possible applications exist but also that this technology can be applied to
waste sources from different parts of the world. From a structural point of view it is possible to manufacture
materials with fairly high compressive strengths and the capacity to also immobilise heavy metals. It was
also shown that total dissolution of the waste materials is not necessary and that the solidified waste material
consists of an amorphous phase bonded to the surfaces of unreacted waste particles. In the case where a
system such as this is utilised for the immobilisation of heavy metals, the immobilisation proceeds through
a combination of physical encapsulation and chemical bonding into the amorphous phase of the matrix. It
does not seem as if the heavy metal influences the basic tetrahedral building blocks of the structure although
it influences the structure in a physical manner such as to alter the compressive strength and specific surface
area. Finally the presence of Si(4AI) sites is preferred if a strong durable product is desired although this
situation can only be achieved where substantial amounts of charge balancing ions are available. K and Na
usually fulfil this role although geopolymers manufactured from fly ash containing some Ca compounds
have higher compressive strengths than those without. It is finally worth noting that both the physical and
Use of geopolymericmaterialsto immobilisctoxic metals:part I1
91
chemical properties of the final product will be interdependent on not only the waste materials used in
synthesis but also the curing conditions as well as effects introduced by the presence of any heavy metals
immobilised in the structure.
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