Effects of activator type/concentration and curing temperatureon alkali-activated binder based on copper mine tailings
Saeed Ahmari • Lianyang Zhang • Jinhong Zhang
Received: 23 January 2012 / Accepted: 11 April 2012 / Published online: 24 April 2012
� Springer Science+Business Media, LLC 2012
Abstract This article investigates the effects of activator
type/concentration and curing temperature on alkali-
activated binder based on copper mine tailings (MT). Different
alkaline activators including sodium hydroxide (NaOH),
sodium silicate (SS), and sodium aluminate (SA) at dif-
ferent compositions and concentrations were used and
four different curing temperatures, 60, 75, 90, and 120 �C,
were considered. Scanning electron microscopy/energy-
dispersive X-ray spectroscopy (SEM/EDX), and X-ray
diffraction (XRD) were conducted to investigate the effect
of these factors on the unconfined compressive strength
(UCS), microstructure, and phase composition of the bin-
der. The results indicate that NaOH concentration and
curing temperature are two important factors that affect the
UCS and micro-structural properties of the alkali-activated
MT binder. The optimum curing temperature, i.e., the
curing temperature at the maximum UCS, depends on the
NaOH concentration, lower optimum curing temperature at
smaller NaOH concentration. Addition of aqueous SS to
the NaOH solution can lead to strength improvement, with
the highest UCS obtained at a SiO2/Na2O ratio of 1.0–1.26.
Addition of powder SA to the NaOH solution profoundly
delays the setting at 60 �C but improves the UCS at 90 �C.
The SEM/EDX results show highly heterogeneous micro-
structure for the alkali-activated MT binder as evidenced
by the variable Si/Al ratios in different phases. The XRD
patterns indicate a newly formed crystalline phase, zeolite,
in the 90 �C-cured specimens. The results of this study
provide useful information for recycling and utilization of
copper MT as construction material through the geopoly-
merization technology.
Introduction
Concrete is by far the most widely used construction
material. Each year, more than 10 billion tons of concrete
is produced in the world [1]. However, the popularity of
concrete also carries with it an enormous impact on the
environment. Ordinary Portland cement (OPC) is a major
component of concrete. The manufacturing of OPC not
only consumes significant amount of natural materials and
energy but also releases substantial quantity of green house
gases. To produce 1 ton of OPC, about 1.5 tons of raw
materials are needed and 0.7 ton of carbon dioxide (CO2) is
released to the atmosphere [2]. Worldwide, the cement
industry alone is estimated to be responsible for about 7 %
of all CO2 generated [3–6]. Another drawback for OPC is
that it may not provide the required properties for many
types of structures, such as rapid development of
mechanical strength and high resistance to chemical attack.
Growing environmental awareness and the need to
ensure sustainability of construction materials have led to
efforts to look for alternative materials for OPC [6, 7].
Recently, a new type of ‘‘cement’’, called geopolymer or
inorganic polymer, has attracted the attention of many
researchers. Geopolymer is a synthetic material produced
from the reaction of aluminosilicates with a highly con-
centrated alkaline hydroxide or silicate solution, having
an amorphous polymeric structure with interconnected –
Si–O–Al– bonds [3, 8–12]. Geopolymer not only provides
performance comparable to OPC in many applications, but
S. Ahmari � L. Zhang (&)
Department of Civil Engineering and Engineering Mechanics,
University of Arizona, Tucson, AZ, USA
e-mail: [email protected]
J. Zhang
Department of Mining and Geological Engineering,
University of Arizona, Tucson, AZ, USA
123
J Mater Sci (2012) 47:5933–5945
DOI 10.1007/s10853-012-6497-9
also shows additional advantages such as rapid develop-
ment of mechanical strength, high acid resistance, no/low
alkali–silica reaction (ASR) related expansion, excellent
adherence to aggregates, immobilization of toxic and
hazardous materials and significantly reduced greenhouse
emissions. These characteristics have made geopolymer of
great research interest as ‘‘an ideal material for sustainable
development’’ [12–16, 32]. However, very few researchers
have studied the geopolymerization of mine tailings (MT)
[17–20] despite of their abundance [14, 21, 22] and suit-
ability for geopolymerization considering the high content
of silica and alumina [14, 19, 23, 24].
The main objective of this study is to investigate the
effects of activator type/concentration and curing tempera-
ture on the mechanical properties and microstructure of
copper MT-based geopolymer. Different activators includ-
ing sodium hydroxide, sodium silicate, and sodium alumi-
nate at different compositions and concentrations were used
and four different curing temperatures, 60, 75, 90, and
120 �C, were considered. The effects of these factors on the
mechanical properties of copper MT-based geopolymer
binders and on the kinetics of dissolution of copper MT were
investigated using unconfined compression tests and leach-
ing analyses, respectively. Scanning electron microscopy/
energy-dispersive X-ray spectroscopy (SEM/EDX) and
X-ray diffraction (XRD) were also performed to investigate
the microstructure and the elemental and phase composition
of the copper MT-based geopolymer specimens prepared at
different conditions.
Experimental study
Materials
The materials used in this investigation include copper MT,
reagent grade 98 % sodium hydroxide (NaOH), aqueous
sodium silicate (SS), powder sodium aluminate (SA), and
de-ionized water. The MT were received in the form of dry
powder from a local mine company in Tucson, Arizona.
Table 1 shows the chemical composition of the MT. It can
be seen that the MT consist mainly of silica and alumina
with substantial amount of calcium and iron. Grain size
distribution analysis was performed on the MT by
mechanical sieving and hydrometer tests following ASTM
D6913 and ASTM D422. Figure 1 shows the particle size
distribution curve of the MT after hand crushing to break
the agglomeration. The mean particle size is around
120 lm with 36 % particles passing No. 200 (75 lm)
sieve. The specific gravity of the MT particles is 2.83.
Figure 2 shows the SEM micrographs of the MT powder.
The MT particles have irregular shapes and some of the
small particles are attached to each other or to the large
particles. The XRD pattern of the MT powder is shown in
Fig. 3. It can be seen that the MT are mainly crystalline
materials consisting of quartz (SiO2), albite (NaAlSi3O8),
sanidine [(K0.831N0.169)(AlSi3O8)], and gypsum (CaSO4).
A weak amorphous phase, centered at about 28�, can also
be seen from the XRD pattern. The amorphous phase is the
main reactive phase for geopolymerization but, as will be
seen later, the crystalline phase also partially reacts to the
alkaline solution.
The sodium hydroxide flakes were obtained from Alfa
Aesar Company in Ward Hill, Massachusetts. The sodium
hydroxide solution is prepared by dissolving the sodium
hydroxide flakes in de-ionized water.
Aqueous SS (SiO2 = 29 %, Na2O = 9 %, and H2O =
62 %) with modulus (SiO2/Na2O) of 3.22 and powder SA
were obtained from Fisher Scientific in Pittsburgh,
Pennsylvania.
Methods
Initially, the agglomerated particles of dry MT were cru-
shed by hand to ensure that all particles pass No. 10
(2.0 mm) sieve. Three types of activator solutions were
used in this experiment: NaOH, mixture of NaOH and SS
Table 1 Chemical composition (wt%) of mine tailings
Chemical compound Contenta (%) SD (%)
SiO2 64.8 2.08
Al2O3 7.08 0.70
Fe2O3 4.33 0.71
CaO 7.52 1.06
MgO 4.06 0.93
SO3 1.66 0.31
Na2O 0.90 0.23
K2O 3.26 0.42
a The values are the average of seven tailings samples
0
10
20
30
40
50
60
70
80
90
100
1 10 100 1000
Particle size (μm)
Per
cent
pas
sing
(%
)
Fig. 1 Particle size distribution of mine tailings
5934 J Mater Sci (2012) 47:5933–5945
123
(NaOH/SS), and mixture of NaOH and SA (NaOH/SA).
The NaOH solution was prepared by dissolving sodium
hydroxide flakes in de-ionized water and stirring for about
5 min. Considering the generated heat, enough time was
allowed for the NaOH solution to cool down before it was
used. Aqueous SS or powder SA, if used, was added to the
NaOH solution and stirred for another 5 min to prepare the
NaOH/SS or NaOH/SA solution. Then the activator solu-
tion was slowly added to the MT and the resulted mixture
was stirred by a mixer for about ten minutes to ensure
sufficient dissolution of silica and alumina in the alkaline
solution. The viscosity of produced pastes increased at
higher NaOH and SS (or SA) concentrations. To prepare
the specimens at consistent workability, the water content
was varied from 27 to 33 %, the higher percentage corre-
sponding to larger amount of SA or SS. The resulted paste
was then placed in cylindrical Plexiglas molds of 34.5 mm
inner diameter and 86.3 mm length (i.e., an aspect ratio of
2.5). The mold was shaken by a vibrator during the casting
to release the trapped air bubbles. Then, the mold was
capped and placed in oven for curing at a specified tem-
perature. The specimens were de-molded after 3 h (24 h at
60 �C due to slow setting) and then placed back in the oven
for 7-day curing. At the 7th day, the specimens are
removed from oven, left in room temperature for 6 h, and
then tested.
Totally, three sodium hydroxide concentrations, 5, 10,
and 15 M, and four curing temperatures, 60, 75, 90, and
120 �C, were used. The SS and SA added specimens were
studied only at 60 and 90 �C, respectively. For the SS and
SA added specimens, the SS to NaOH solution and SA to
solid NaOH mass ratios were in the range of 0.5–2.5 and
0.4–3.1, respectively.
Unconfined compression tests were performed on the
cured cylindrical samples with an ELE Tri Flex 2 loading
machine at a constant loading rate of 0.1 mm/min. The
tests were performed to measure the unconfined compres-
sive strength (UCS) of the geopolymer specimens produced
at different conditions. For each condition, at least three
specimens were tested and the average of the measured
UCS values was used. Before conducting the compression
test, the end surfaces of the specimens were polished to
make sure that they are accurately flat and parallel. In
addition, the end surfaces were lubricated to minimize the
friction between the specimen and the steel platens.
Leaching analysis was performed to investigate the
effects of temperature and NaOH concentration on the
dissolution of silica and alumina species from the MT. 20 g
of MT powder was soaked in 5, 10, or 15 M NaOH
Fig. 2 SEM micrographs of mine tailings powder
10 20 30 40 50 60 70
2θ
G
A
P
A
S SS
S
S
SA
G
P P
P S
Fig. 3 XRD pattern of mine tailings powder (A albite, G gypsum,
P sanidine, S quartz)
J Mater Sci (2012) 47:5933–5945 5935
123
solution with a liquid-to-solid mass ratio of 5. The speci-
mens were kept in 60 or 90 �C oven for 24 h. After 24 h,
the specimens were filtered with a 0.2-lm filter. Finally, a
Perkin Elmer Elan DRC-II ICP-MS was used to measure
the concentration of silicon and aluminum in the filtrate
based on the ICP-MS (inductively coupled plasma mass
spectrometry) technique.
To investigate the effect of activator type/concentration
and curing temperature on the microstructure and the ele-
mental and phase composition of the geopolymer, SEM/
EDX characterization and XRD analysis were also per-
formed. The SEM imaging was performed in SE conven-
tional mode using the FEI INSPEC-S50/Thermo-Fisher
Noran 6 microscope. The freshly failed surfaces from the
unconfined compression tests, without polishing to keep
the fractured surface ‘‘un-contaminated’’ [24], were used
for the SEM imaging. The XRD analysis was performed
with a Scintag XDS 2000 PTS diffractometer using Cu Karadiation, at 2.00�/min ranging from 10.00� to 70.00� with
0.600 s count time.
Table 2 summarizes the combination of variables stud-
ied and the different types of tests conducted.
Results and discussion
Effect of activator type/composition
Effect of aqueous sodium silicate
Mixture of NaOH solution and aqueous SS was used as
activator to investigate the effect of addition of SS on
geopolymerization of MT. Figure 4 shows the variation of
UCS with the SiO2/Na2O ratio for specimens prepared
at 10 M NaOH concentration and cured at 60 �C for
7 days. The UCS increases with the SiO2/Na2O ratio up to
1.0–1.26 and then starts to decrease with higher SiO2/
Na2O. So, the optimum SiO2/Na2O is about 1.0–1.26.
Different researchers have studied the effect of activator
composition for the NaOH and SS mixture on the com-
pressive strength of geopolymers [23, 25–30]. Table 3
summarizes the optimum SiO2/Na2O ratios reported in the
literature. For comparison, the optimum SiO2/Na2O ratio
from this study is also listed in the table. It can be seen that
it is in good agreement with the optimum SiO2/Na2O ratios
obtained by other researchers.
Table 2 Summary of studied variables and conducted tests
Specimen NH (M) SS (%) SA (%) Water (%) Si/
Al
Na/
Al
Curing temp. (�C) Curing time (days) UCS test XRD SEM/
EDS
5-7-60 5 0 0 27 7.78 0.96 60 7 X
10-7-60 10 0 0 27 7.78 1.92 60 7 X
15-7-60 15 0 0 27 7.78 2.88 60 7 X
5-7-75 5 0 0 27 7.78 0.96 75 7 X
10-7-75 10 0 0 27 7.78 1.92 75 7 X
15-7-75 15 0 0 27 7.78 2.88 75 7 X
5-7-90 5 0 0 27 7.78 0.96 90 7 X
10-7-90 10 0 0 27 7.78 1.92 90 7 X
15-7-90 15 0 0 27 7.78 2.88 90 7 X X X
5-7-120 5 0 0 27 7.78 0.96 120 7 X
10-7-120 10 0 0 27 7.78 1.92 120 7 X
15-7-120 15 0 0 27 7.78 2.88 120 7 X
SS1 10 21.2 0 27 8.82 2.55 60 7 X
SS2 10 19.3 0 27 8.73 2.49 60 7 X
SS3 10 17.3 0 27 8.63 2.43 60 7 X
SS4 10 14.7 0 27 8.50 2.35 60 7 X X X
SS5 10 9.6 0 27 8.24 2.20 60 7 X
10SA1 10 0 4.6 27.0 4.91 1.58 90 7 X
10SA2 10 0 8.8 27.0 3.58 1.42 90 7 X X X
10SA3 10 0 12.6 28.0 2.82 1.33 90 7 X
10SA4 10 0 17.8 31.0 2.08 1.25 90 7 X X X
10SA5 10 0 22.1 33.0 1.67 1.20 90 7 X
NH NaOH, SS aqueous sodium silicate, SA solid sodium aluminate
5936 J Mater Sci (2012) 47:5933–5945
123
The addition of SS to NaOH improves the strength of
the binder because additional silica is provided. It is known
that the aluminum component of an aluminosilicate source
material tends to dissolve more easily than the silicon
component at the early stage. In this case, the dissolved
alumina needs more disolved silica for geopolymerization.
The added SS simply provides such required silica. How-
ever, the improvement due to the addition of SS is only up
to a certain level [36]. This is possibly because too much
sodium silicate hinders evaporation of water and fomation
of polymeric structure by preventing the contact between
the solid material and the activating solution through pre-
cipitation of Si–Al phase [30, 37].
Figure 5 shows the SEM micrographs of the specimen
synthesized with a mixture of 10 M NaOH solution and
aqueous SS at SiO2/Na2O = 1 and cured at 60 �C for
7 days. The geopolymeric matrix is mainly particulate (see
Fig. 5a), in contrast to metakaolin-based geopolymers
[11, 38], and exhibits heterogeneous microstructure indic-
ative of varying degree of reaction (see Fig. 5b). Some
particles are partially reacted on their surface and bonded
to each other by the flaky shape layers which are geo-
polymer gels (see Fig. 5c), while others remain un-reacted
(see Fig. 5d). The EDX analysis results show that Si, Al,
and Na are the main components in both reacted and un-
reacted areas, but there is a noticeable difference between
the Si/Al and Na/Al ratios in the two areas. The Si/Al in the
reacted area is lower than that in the un-reacted one, while
it is the opposite for the Na/Al. The initial Si/Al and Na/Al
ratios are, respectively, 8.5 and 2.35 (See Table 2), while
they are, respectively, 6.8 and 1.7 in the un-reacted area
and 4.9 and 2.1 in the reacted area. So, the Si/Al and Na/Al
ratios decrease in both the un-reacted and reacted areas
from the initial values. The Si/Al in the un-reacted area is
slightly lower than the initial one possibly because some of
silica from the un-reacted area is dissolved in the alkaline
solution and migrated to the reacted area [38]. The Si/Al
ratio in the reacted area is much lower than the initial one
possibly because the initial Si/Al ratio (for the whole
material) is much higher than that of the amorphous phase
(the reactive silica and alumina) which is the main source
for the geopolymer [38–40].
The dissolution of alumina has a major role in the
kinetics of gel formation [40]. Even if large amount of
amorphous silica were available, further geopolymerization
would not take place if the dissolution of alumina stops. In
other words, the dissolution of silica depends on the dis-
solution of alumina [40]. This may also explain why after
leaching of MT in NaOH solution at 60 �C for 24 h, the
Si/Al ratio of the leachate is only 1.85–2.44 despite the
initial high Si/Al ratio of 7.78 for the MT (see Table 4). As
for the Na/Al ratio, its decrease indicates that not all of the
available Na cations participate in the reaction and some of
them might remain un-reacted and appear as precipitate on
the surface. The decrease of Na/Al is also noted by other
researchers [35, 41, 42].
Figure 6 shows the XRD patterns of different specimens
including the one described above. After reaction, the XRD
pattern remains mainly crystalline. This is consistent with
the SEM micrographs in Fig. 5. However, the crystalline
silica exhibits less intense peaks after reaction indicating
that some crystalline silica has participated in geopoly-
merization. The decrease in the intensity of crystalline
peaks can be also due to the addition of soluble silica
[28, 31]. Gypsum as a crystalline peak, which was detected
in the MT, disappears after geopolymerization. This is
most likely because gypsum is locked in the solution pore.
The amorphous hump in the 90 �C-cured specimens
becomes broader and slightly higher. This change in the
60 �C-cured specimen is less evident due to less reactivity
at lower temperature. This is also consistent with the SEM
micrographs (see Figs. 5, 8, 11), as will be discussed in
detail later.
1.0
2.0
3.0
4.0
5.0
6.0
7.0
8.0
9.0
10.0
11.0
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
UC
S (
MP
a)
SiO2/Na2O
Fig. 4 UCS versus SiO2/Na2O ratio for specimens activated with a
mixture of 10 M NaOH and SS and cured at 60 �C for 7 days
Table 3 Summary of optimum composition of alkaline solution
reported in the literature
No. Source material Opimum SiO2/
Na2O
Reference
1 Fly ash 0.5–0.89 [25]
2 Fly ash 1.5 [26]
3 Fly ash 1.0–1.5 [31]
4 Metakaolin 1.5 [11]
5 Fly ash 1.0–1.5 [33]
6 Granulate blast furnace slag 1.0–1.25 [34]
7 Copper mine tailings 1.0–1.26 Current study
J Mater Sci (2012) 47:5933–5945 5937
123
Effect of powder sodium aluminate
Since the Si/Al ratio of the MT is high, the feasibility of
using sodium aluminate (SA) as the activating agent was
studied. The powder SA was mixed with the NaOH solu-
tion at different mass ratios. The specimens synthesized
with the NaOH/SA solution did not show significant
hardening at 60 �C even after a few days. However, the
addition of SA increased the UCS of specimens cured at
90 �C. Figure 7 shows the measured UCS of specimens
synthesized with a mixture of 10 M NaOH and varying
amount of SA and cured at 90� C for 7 days. The UCS
increases with the SA to NaOH mass ratio (A/N) up to
about 1.25 and then starts to decrease. Since the initial
0
2000
4000
6000
8000
10000
12000
14000
Inte
nsity
(cp
s)
keV
O
Fe
Na
Al
Si
Mg K CaS
Si/Al = 4.9Na/Al = 2.1
0
1000
2000
3000
4000
5000
6000
7000
8000
0 1 2 3 4 5 0 1 2 3 4 5
Inte
nsity
(cp
s)
keV
C
O
FeMg
Al
Na
Si
K Ca
Si/Al = 6.8Na/Al = 1.7
S
d
b c
(b)
(c) (d)
(a)
Fig. 5 SEM micrographs and EDX analysis results of aqueous SS
added specimen at SiO2/Na2O = 1 and cured at 60 �C for 7 days:
a low magnification image of whole area; b higher magnification
image of area shown by the square in a; c, d Higher magnification
images, respectively, of reacted and un-reacted areas shown by
squares in b. The EDX spectra are for c and d
5938 J Mater Sci (2012) 47:5933–5945
123
Na/Al does not vary significantly but the Si/Al does
(Table 2), the Si/Al ratio may be responsible for the vari-
ation of the UCS.
Figure 8 shows the SEM micrographs of a specimen
synthesized with10 M NaOH and SA at A/N = 1.25 and
cured at 90 �C for 7 days. The low magnification micro-
graph (Fig. 8a) shows voids of varying sizes, which may be
generated due to the introduction of air bubbles into the
matrix or the evaporation of extra water. Introduction of air
bubbles can be pronounced in the SA added specimens
because SA significantly raises the viscosity of the solution
and makes it more difficult to release the air bubbles. The
higher magnification micrographs show that the micro-
structure is quite different from what observed in the SS
added specimens (see Fig. 5). Unlike the SS added speci-
men, the SA added one mainly consists of densely packed
fine particles which cover the MT particles. This micro-
structure looks similar to the one obtained from geopoly-
merization of geothermal silica by SA [43]. The morphology
of the geopolymeric gel in the SA added specimen also seems
different from that of the SS added one. In the SS added
specimen, the geopolymeric gel looks like tiny flakes and
acts as a binder between the MT particles but in the SA added
specimen, the geopolymeric gel is formed in two ways. First,
the geopolymeric gel looks like a monolithic thin layer and
covers the fine MT or SA particles (see Fig. 8c). The SA fine
particles can be seen attached on the geopolymeric gel layer.
Second, the geopolymeric gel forms on the SA and MT
particle surfaces due to partial dissolution and acts as a binder
between them. In the case of partial dissolution, zeolite is
likely to coexist with the geopolymeric gel as will be dis-
cussed later. Since the fine SA particles are dispersed evenly
within the specimen, they may role as a bridging agent
between MT particles as well (see Fig. 8d). The geopoly-
meric gel was seen to have the first type morphology only in a
few spots, but the second type was dominant. The EDX
Table 4 Results of MT leaching tests at 60 and 90 �C and different
NaOH concentrations
Temperature (�C) 60 90
NaOH (M) 5 10 15 5 10 15
Si (ppm) 71 171 233 1,846 3,970 4,570
Al (ppm) 28 76 121 299 319 550
Si/Al 2.44 2.16 1.85 5.93 11.9 7.98
10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70
2θ
P
15-7-90
10SA2
MT Powder
S
SS4
N A SS P
P S
P SS S
PN
NN
SS
SSS
N
N
N A
PP
P
S
P
PP P
ASS
SS S
S
P P
N
P
A
SS
SS
SP
PP
PG G
A
AP
A
A
AA P
S
SS
S
S
S
S
P
P
P
S
S
S
P
P
P
P
N
N
P
S
Fig. 6 XRD patterns of MT
powder; SS4: binder
synthesized with 10 M NaOH
and aqueous SS at SiO2/
Na2O = 1 and cured at 60 �C
for 7 days; 10SA2: binder
synthesized at 10 M NaOH and
powder SA at A/N = 1.25 and
cured at 90 �C for 7 days; 15-7-
90: binder synthesized at 15 M
NaOH and cured at 90 �C for
7 days (A sodium aluminum
silicate (albite), G gypsum,
N zeolite, P potassium
aluminum silicate (sanidine),
S quartz)
0
2
4
6
8
10
12
14
16
18
0.0 0.5 1.0 1.5 2.0 2.5 3.0
A/N
UC
S (M
Pa)
Fig. 7 UCS versus SA to NaOH ratio (A/N) for specimens synthe-
sized with a mixture of 10 M NaOH and SA and cured at 90 �C for
7 days
J Mater Sci (2012) 47:5933–5945 5939
123
results show that the Si/Al ratio in the first type geopolymer is
2.6 but in the second type is 1.9.
Figure 9 shows the SEM micrographs of the SA added
specimen at A/N = 2.5 and also cured at 90 �C for 7 days.
The specimen at A/N = 2.5 contains more and larger pores
than the specimen at A/N = 1.25 (see Figs. 8a, 9a). This is
because the viscosity of the specimen at A/N = 2.5 is
much higher than that at A/N = 1.25 and thus more and
larger air bubbles are expected to be generated. Similar to
the micrographs in Fig. 8, the MT particles are also sur-
rounded by fine SA particles. However, as the EDX anal-
ysis results indicate, the concentration of Na at A/N = 2.5
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
Inte
nsity
(cp
s)
keV
C
O
Fe As
Al
Na
Si
K Ca
Si/Al = 2.6Na/Al = 2.7
S
0
2000
4000
6000
8000
10000
12000
14000
0 1 2 3 4 5 0 1 2 3 4 5
Inte
nsity
(cp
s)
keV
C
O
Fe As
Al
NaSi
K Ca
Si/Al = 1.9Na/Al = 1.6
S
(a)
c
b
d
(b)
(c) (d)
Fig. 8 SEM micrographs and EDX analysis results of specimen
synthesized with a mixture of 10 M NaOH and SA at A/N = 1.25 and
cured at 90 �C for 7 days: a low magnification image of whole area,
b higher magnification image of the area shown by the square in a;
c higher magnification image of the geopolymeric gel shown by the
arrow in a; and d high magnification image of the area shown by the
square in b. The EDX spectra are for c and d
5940 J Mater Sci (2012) 47:5933–5945
123
is much higher than that at A/N = 1.25, leading to a Na/Al
ratio of 6.5. The high final Na/Al ratio indicates precipi-
tation of un-reacted Na cations as seen in Fig. 9c.
The specimen synthesized with 10 M NaOH and SA at
A/N = 1.25 and cured at 90 �C for 7 days also shows
broader amorphous hump and lower and fewer crystalline
sharps than the MT (see Fig. 6). In addition, a cancrinite
(CAN) type of zeolite [(Na7Ca0.9(CO3)1.4(H2O)2.1 [Si6Al6O24]] is formed in the SA added specimen, as identified by
the sharp peaks at 13.96�, 18.88�, 24.28�, and 27.4� in
the XRD pattern. Formation of zeolite as a co-product
of geopolymerization has also been reported by other
researchers [35, 44–49]. The literature indicates that the Si/
Al and Na/Al ratios, pH, type of activating solution, liquid-
to-solid ratio, and curing temperature are the main factors
affecting the formation of zeolite [50].
Alkaline silicates (if used in proper dosage) improve the
strength at lower temperature (60 �C) (see Fig. 4) but
alkaline aluminates contribute to the strength at higher
temperature (90 �C). This can be explained by the Si/Al ratio
and its dependence on temperature. Although the Si/Al ratio
of the original MT is 7.78, the available silica and alumina for
0
2000
4000
6000
8000
10000
12000
0 1 2 3 4 5
Inte
nsity
(cp
s)
keV
O
Al
Na
Si
K
Si/Al = 1Na/Al = 6.5
S
b
(a)
c(b)
(c)
Fig. 9 SEM micrographs and EDX analysis results of specimens
synthesized with a mixture of 10 M NaOH and SA at A/N = 2.5 and
cured at 90 �C for 7 days: a low magnification image of whole area;
b higher magnification image of area shown by square in a; and
c higher magnification image of area shown by square in b. The EDX
spectrum is for c
0
5
10
15
20
25
30
60 75 90 105 120
UC
S (
MP
a)
Temperature (°C)
5
10
15
NaOH (M)
Fig. 10 UCS versus curing temperature for specimens synthesized at
different NaOH concentrations and cured for 7 days
J Mater Sci (2012) 47:5933–5945 5941
123
geopolymerization can be very different depending on the
activator alkalinity and temperature. The leaching tests on
MT at 60 �C for 24 h result in a Si/Al ratio of 1.85 to 2.44 for
the leachate at different alkalinity levels; but the corre-
sponding Si/Al ratio at 90 �C ranges from 5.93 to 11.9 (see
Table 4). Therefore, the Si/Al ratio of the leachate at 60 �C is
only slightly smaller than the optimum ratio for geopoly-
merization, which is 2.0-3.0, but the Si/Al ratio of the
leachate at 90 �C is significantly larger. This explains why
the addition of SA does not help improve the strength at
60 �C but contributes to the strength at 90 �C. On the other
hand, the addition of SS at 60 �C results in increase of the Si/
Al ratio and makes it closer to the optimum Si/Al ratio and
thus improves the UCS.
Effect of NaOH concentration/curing temperature
Figure 10 shows the variation of UCS with curing tem-
perature at different NaOH concentrations. The effect of
NaOH concentration on the UCS depends on the curing
temperature. At curing temperature of 60 �C, the UCS
slightly increases with higher NaOH concentration. At
curing temperature of 75 �C, the UCS first increases with
the NaOH concentration from 5 to 10 M and then
decreases from 10 to 15 M. At curing temperature of
90 �C, there is a significant jump for the UCS when the
NaOH concentration increases from 10 to 15 M. At
120 �C, however, the rate of increase for the UCS with the
NaOH concentration becomes slower. The effect of NaOH
concentration on the UCS of alkali-activated MT has been
discussed in more detail in [51, 52].
It can also be seen from Fig. 10 that elevated curing
temperature results in increase of UCS up to a certain level
and then decrease of UCS. The improving effect of curing
temperature depends on the NaOH concentration. At lower
alkalinity, geopolymerization is less sensitive to curing
temperature [53]. The optimum curing temperature for 5
and 10 M NaOH is about 75 �C, while for 15 M NaOH it is
around 90 �C. The effect of curing temperature on the UCS
of geopolymer has been studied by many researchers
[26–28, 39, 54–56]. The optimum curing temperature
depends on both the alkaline concentration and the source
material. In general, higher alkaline concentration and lower
source material reactivity will have higher optimum curing
temperature. Table 5 shows the optimum curing tempera-
tures reported in the literature and obtained from this study.
The improving effect of curing temperature below the
optimum one is mainly due to the higher solubility of alu-
minosilicate minerals in the alkaline solution. At higher
temperature, the silica and alumina species are more likely to
dissolve and larger amount of Si and Al will be available for
geopolymerization. The effect of curing temperature on the
solubility of aluminosilicates can be clearly seen in Table 4.
The weakened strength of geopolymer above the optimum
temperature is mainly due to the fast polycondensation and
rapid formation of geopolymeric gel which hinders further
dissolution of silica and alumina species [57, 59].
Figure 11 shows the SEM micrographs and EDX analysis
results of the specimen synthesized at 15 M NaOH and cured
at 90 �C for 7 days. The lower magnification micrograph
(see Fig. 11a) shows varying size voids, which might be
formed due to entrapped air bubbles or evaporated extra
water. As seen in Fig. 11b, there are both partially reacted
and un-reacted MT particles. The morphology of the geo-
polymeric gel in this specimen (Fig. 11c) looks different
from that of the SS or SA added specimens although the SA
added one is cured at the same temperature. In the NaOH
only specimen, a thicker layer of geopolymeric gel covers the
MT particles and the geopolymeric matrix has a denser
structure [39, 59].
The EDX analysis results indicate a noticeable differ-
ence between the Si/Al values of the reacted and un-reacted
areas. In the reacted area, the Si/Al ratio (5.4) is lower than
the initial one (7.8). As discussed earlier, this may be due
to the difference between the initial Si/Al ratio and that
of the amorphous phase. The high Si/Al ratio in the
un-reacted area (12.4) may be caused by the dissolution
and migration of Al to the reacted area.
The XRD pattern of the above-mentioned specimen
(also shown in Fig. 6) looks similar to that of the SA added
specimen. The amorphous peak broadening and emergence
of zeolitic peaks are the XRD characteristics of NaOH
activated MT at 90 �C. However, the main difference in the
XRD characteristics between the specimens activated with
only NaOH and those with NaOH/SA is the size of zeolitic
peaks, which are smaller in the NaOH only activated one.
This is consistent with the above discussion about the
dependence of the formation of zeolite on both temperature
Table 5 Summary of optimum curing temperature reported in the
literature
Source
material
Optimum curing
temperature (�C)
NaOH
concentration
(M)a
Reference
Metakaolin 35 4.3 [57]
Natural zeolite 40 7 [28]
Glass cullet 40 5–10 [58]
Class C fly ash 60 8.1 [26]
Class F fly ash 75 7.5 [39]
Class F fly ash 80 7 [27]
MT 75 5 and 10 Current study
MT 90 15 Current study
a Equivalent NaOH concentration is presented in the case that stoi-
chiometric molar ratio of the alkaline cation or the mixture of NaOH
and SS is used in the literature
5942 J Mater Sci (2012) 47:5933–5945
123
and Si/Al ratio. For this specimen, due to the higher Si/Al
ratio, less amount of zeolite is generated.
Conclusions
The effect of activator type/composition and curing tem-
perature on the mechanical properties, microstructure, and
elemental and phase composition of alkali-activated copper
MT is studied in this article. Based on the experimental
results, the following conclusions can be drawn:
(1) NaOH concentration and curing temperature are
two important factors that affect the UCS and
micro-structural properties of alkali-activated MT.
The optimum curing temperature, i.e., the curing
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
0 1 2 3 4 5
Inte
nsity
(cp
s)
keV
O
Fe As
Al
Na
Si
K Ca
Si/Al = 5.4Na/Al = 2.7
S0
2000
4000
6000
8000
10000
12000
14000
0 1 2 3 4 5
Inte
nsity
(cp
s)
keV
O
AsAl
Na
Si
K
Si/Al = 12.4Na/Al = 3.1
SC
CaC
(a)
b
c
(b) d
(c) (d)
Fig. 11 SEM micrographs and EDX analysis results of specimen
synthesized at 15 M NaOH and cured at 90 �C for 7 days: a low
magnification image of whole area; b higher magnification image of
area shown by square in a; and c, d higher magnification images,
respectively, of the reacted and un-reacted areas shown by squares in
b. The EDX spectra are for c and d
J Mater Sci (2012) 47:5933–5945 5943
123
temperature at the maximum UCS, depends on the
NaOH concentration, lower optimum curing temper-
ature at smaller NaOH concentration.
(2) Addition of aqueous SS to the NaOH solution can
lead to strength improvement. The highest UCS is
obtained at a SiO2/Na2O ratio of about 1.0–1.26.
(3) Addition of powder SA to the NaOH solution
profoundly delays the setting at 60 �C but improves
the UCS at 90 �C. The SA to NaOH ratio (A/N)
corresponding to the highest UCS is about 1.25 for
specimens cured at 90 �C.
(4) The SEM/EDX results indicate a heterogeneous
matrix for the alkali-activated MT. The matrix is
denser at higher curing temperature due to formation
of larger amount of geopolymer gel.
(5) The XRD patterns at the optimum conditions show
change in both amorphous and crystalline phases.
Formation of both zeolite and geopolymer improves
the UCS at elevated curing temperatures.
Acknowledgements This work is partially supported by the
National Science Foundation under Grant No. CMMI-0969385, the
University of Arizona Faculty Seed Grants Program, and a local mine
company in Tucson, AZ.
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