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: lyzhang@email.arizona.edu

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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