Construction and Building Materials 44 (2013) 743–750

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Construction and Building Materials

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Durability and leaching behavior of mine tailings-based geopolymerbricks

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.03.075

⇑ Corresponding author. Tel.: +1 520 6260532; fax: +1 520 6212550.E-mail address: lyzhang@email.arizona.edu (L. Zhang).

Saeed Ahmari, Lianyang Zhang ⇑Department of Civil Engineering and Engineering Mechanics, University of Arizona, Tucson, AZ, USA

h i g h l i g h t s

� Systematically studied durability and leaching behavior of MT-based geopolymer bricks.� Immersion in pH = 4 and 7 solutions leads to substantial strength loss.� Immersion in pH = 4 and 7 solutions leads to minor water absorption and weight loss.� MT-based geopolymer bricks effectively immobilize the heavy metals in MT.� FRDM can satisfactorily describe the leaching behavior of heavy metals.

a r t i c l e i n f o

Article history:Received 12 December 2012Received in revised form 19 March 2013Accepted 22 March 2013Available online 24 April 2013

Keywords:Mine tailingsGeopolymerBricksDurabilityHeavy metalsLeaching kinetics

a b s t r a c t

Disposal of mine tailings (MT) in impoundments may have adverse environmental impacts such as airpollution from dust emissions and release of heavy metals to surface and underground water. Geopoly-merization as an environmentally-friendly and sustainable method has been used to stabilize MT so thatthey can be used as construction material. In this paper, the durability and leaching behavior of MT-basedgeopolymer bricks are studied by measuring unconfined compression strength (UCS), water absorption,weight loss, and concentration of heavy metals after immersion in pH = 4 and 7 solutions for differentperiods of time. Microscopic/spectroscopic techniques, SEM, XRD and FTIR, are also employed to investi-gate the change in microstructure and phase composition of MT-based geopolymer bricks after immer-sion in the solutions. To describe the leaching behavior of MT-based geopolymer bricks, the first orderreaction/diffusion model (FRDM) is used to analyze the leaching test data. The results indicate thatalthough there is a substantial strength loss after immersion in pH = 4 and 7 solutions, the water absorp-tion and weight loss are small. The strength loss is mainly due to the dissolution of geopolymer gels asindicated by the microscopic/spectroscopic analysis results. The leaching analyses show that the heavymetals are effectively immobilized in the MT-based geopolymer bricks, which is attributed to the incor-poration of heavy metals in the geopolymer network. The FRDM can satisfactorily describe the leachingbehavior of heavy metals in the MT-based geopolymer bricks and the analysis results indicate that thesolubility or reaction rate is an important factor controlling the leaching behavior.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Mine tailings (MT) are a major waste material generated bymining operations. In current practice, MT are transported in slurryform to and deposited in storage impoundments. Storage of MT insuch impoundments leads to occupation of large areas of land,costly construction and maintenance, and potential environmentaland ecological risks. MT can cause air pollution due to dust emis-sions resulted from surface erosion. MT can also pollute surfaceand underground water due to the leaching of heavy metals. Thesulfide minerals in MT such as pyrite (FeS2), pyrrhotite (Fe1�xS),

and chalcocite (Cu2S) oxidize in the presence of air and water,yielding sulphuric acid and releasing metallic oxides such as FeO[1]. This phenomenon, known as acid mine drainage (AMD), leadsto the drop of pH and results in further leaching of heavy metalssuch as Cd, As and Cu [2]. AMD has caused serious contaminationof surface and underground waters in the United States [3]. There-fore, it is vital to take measures to reduce the risk of environmentalcontamination by MT.

Generally, there are three methods to reduce the potential envi-ronmental hazards imposed by MT: (1) isolation of MT, (2) chem-ical stabilization of MT, and (3) a combination of these twomethods [2,4]. The isolation techniques include containment ofMT from the surrounding environment such as capping the tailingsimpoundment surface. This can be achieved by designing and

Table 1Chemical composition (wt.%) of mine tailings.

Chemical compound Contenta (%) Standard deviation (%)

SiO2 64.8 2.08Al2O3 7.08 0.70Fe2O3 4.33 0.71CaO 7.52 1.06MgO 4.06 0.93SO3 1.66 0.31Na2O 0.90 0.23K2O 3.26 0.42

Trace elementsPb 0.000286 0.0007Zr 0.012 0.001Mo 0.022 0.003Zn 0.068 0.009Cu 0.076 0.009Mn 0.163 0.034Ti 0.213 0.006

a The values are the average of seven tailings samples.

0

10

20

30

40

50

60

70

80

90

100

1 10 100 1000Particle size (μm)

Per

cent

pas

sing

(%)

Fig. 1. Particle size distribution of MT powder.

744 S. Ahmari, L. Zhang / Construction and Building Materials 44 (2013) 743–750

constructing a closure system similar to that used for landfills [5].For chemical stabilization, chemicals or cementitious materials areadded to immobilize the heavy metals in MT through physicalencapsulation and/or chemical reactions. In current practice, poz-zolanic materials such as cement and lime are commonly used tostabilize MT [1,6–11] although other materials such as fly ash, slagand aluminum are also studied by researchers [12,13]. The isola-tion and stabilization techniques can also be used simultaneously.For example, the tailings surface can be treated by binders such asorganic polymers, water glass and Portland cement to improve sur-face erosion resistance and reduce water infiltration, and, in themeantime, the hardened surface acts as a capping system whichisolates the underlying tailings from the surrounding environment[14].

Since the stabilization of MT based on the reaction with calciumhas a number of limitations, such as inferior mechanical properties,low acid resistance, poor immobilization of contaminants at highconcentrations, and more importantly high-energy usage andgreenhouse gas emissions related to production of Portland cement[15,16], researchers have studied other stabilization methods tostabilize MT [1,13,17,18]. Of these different methods, geopolymer-ization is a promising one to effectively stabilize MT in an econom-ical and environmentally-friendly way. In this method, geopolymergels are produced on the MT particle surface and the newly formedgels bind the particles together. Geopolymer is an amorphous bin-der with a polymeric network structure consisting of repeatingunits of ASiAOAAlAOA and is formed by alkali activation of silicaand alumina containing materials at high pH and room or slightlyelevated temperature [19]. Geopolymer binder offers superiormechanical properties, excellent durability, and effective immobi-lization of heavy metals [19–28]. An extensive research has beenconducted on fly ash-, slag-, and metakaolin-based geopolymers[21,24,28–32] and MT-based geopolymer has recently attractedattention of researchers worldwide [18,22,33–38]. Van Jaarsveldet al. [18] studied the feasibility of using MT-based geopolymerpaste as a cover system for tailings dam. The results indicated thatfly ash and as much as 65–70% MT can be used to produce geopoly-meric material suitable for capping mine tailings. Pacheco-Torgalet al. [22] evaluated the durability and environmental performanceof calcined tungsten MT-based geopolymer (CTMTG) and reportedthat the CTMTG binder exhibits better durability than Portlandcement binder and the concentrations of released heavy metalsare all below the DIN limits. Giannopoulou and Panias [33] showedthat the compressive strength of mixed fly ash and MT-based geo-polymer increases with the fly ash content and the concentrationsof leached heavy metals in neutral and acidic solutions are all be-low the Greek Standard limits. Silva et al. [34] studied the stabilityof CTMTG immersed in water and reported disintegration after acertain period of time mainly due to deficient geopolymericreaction.

Ahmari and Zhang [39] studied the production of eco-friendlybricks based on geopolymerization of copper MT, focusing on theirphysical and mechanical properties. The results indicate that byproperly selecting the preparation condition (initial water content,NaOH concentration, forming pressure and curing temperature),MT-based geopolymer bricks can be produced to meet the ASTMrequirements on physical and mechanical properties. In this paper,the durability and leaching behavior of the copper MT-based geo-polymer bricks are studied.

2. Experimental

2.1. Materials

The materials used in this investigation include copper mine tailings (MT),reagent grade 98% sodium hydroxide (NaOH), de-ionized water, and BDH AristarPlus (67–70%) nitric acid. The MT were received as 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 of mainly silica and alumina with substantialamount of calcium and iron. Grain size distribution analysis was performed onthe MT using mechanical sieving and hydrometer analysis following ASTM D6913and ASTM D422. Fig. 1 shows the particle size distribution curve. The mean particlesize is around 120 lm with 36% particles passing No. 200 (75 lm) sieve. The spe-cific gravity of the MT particles is 2.83.

The sodium hydroxide (NaOH) flakes were obtained from Alfa Aesar Companyin Ward Hill, Massachusetts. The sodium hydroxide solution is prepared by dissolv-ing the sodium hydroxide flakes in de-ionized water. The nitric acid (HNO3) wasmanufactured by BDH and supplied by VWR.

2.2. Preparation of brick samples

First, the MT were mixed with NaOH solution. The NaOH solution was preparedby adding NaOH flakes to de-ionized water and stirring for at least five minutes.Due to the generated heat, enough time was allowed for the solution to cool downto room temperature before it was used. The NaOH solution was slowly added tothe dry MT and mixed for 10 min to ensure the homogeneity of the mixture. Thegenerated mixture exhibited varying consistency depending on the initial watercontent. The mixture’s consistency varied from semi-dry to semi-paste as the watercontent increased from 8% to 18%. The mixture was then placed in Harvard minia-ture compaction cylindrical molds of 33.4 mm diameter and 72.5 mm height withminor compaction. The compacted specimens were compressed with an ELE TriFlex 2 loading machine at different loading rates to ensure that the duration toreach the specified forming pressure was about 10 min for all the specimens. Aftercompression, the specimens were de-molded and placed uncovered in an oven forcuring until tested.

0

5

10

15

20

25

30

35

40

0 5 10 15 20 25 30 35

UC

S (M

Pa)

Forming Pressure (MPa)

8

10

12

14

16

18

Initial Water Content (%)

20

25

30

35

0 1 2 3

UC

S (M

Pa)

Forming Pressure (MPa)

Fig. 2. UCS vs. forming pressure for specimens prepared at different initial watercontents and 15 M NaOH concentration and cured at 90 �C for 7 days (from [39]).

Table 2Physical and mechanical properties of geopolymer bricks after immersion in pH = 4and 7 solutions for 4 months.

Specimen 12/25 16/0.5UCS before immersion (MPa) 15.0 27.8

pH 4 7 4 7UCS after immersion(MPa) 6.1 7.0 6.0 6.9UCS loss (%) 59.3 53.3 78.4 75.2Water absorption (%) 7.5 7.0 5.1 6.9Weight loss (%) 7.9 8.6 11.1 11.1

S. Ahmari, L. Zhang / Construction and Building Materials 44 (2013) 743–750 745

2.3. Experiments

The experiments performed in this study consist of unconfined compressiontests, water absorption and weight loss measurements, ICP-MS, SEM, XRD, and FTIR.The specimens were prepared at a NaOH concentration of 15 M, a curing tempera-ture of 90 �C, moisture contents varying from 8% to 18%, and forming pressuresvarying from 0 to 35 MPa. The unconfined compression tests were first performedto measure the 7-day unconfined compressive strength (UCS) of the geopolymerbrick specimens produced at different conditions. For each condition, at least threespecimens were tested and the average of the measured UCS values was used. Theend surfaces of the cylindrical specimens were polished before testing to ensurethat they are accurately flat and parallel. The ELE Tri Flex 2 loading machine wasused for the compression test at a constant loading rate of 0.1 mm/min.

The geopolymer brick specimens prepared at initial water content/formingpressure respectively of 12%/25 MPa and 16%/0.5 MPa (denoted respectively as12/25 and 16/0.5 later on) were selected to study the durability and leaching behav-ior. The specimens were soaked in pH = 4 and 7 solutions for 4 months and then ta-ken out from the solution, weighed, and dried at 110 �C for 24 h. Finally, the driedspecimens were weighed and tested to measure the weight loss and the new UCS.

2.3.1. Leaching testStatic leaching test was performed by soaking MT powder and geopolymer brick

specimens in two different solutions, one at pH = 4 and the other at pH = 7. Staticleaching test was also used by a number of researchers to study the leaching behav-ior of geopolymer binders [30,40–42]. To study the effectiveness of geopolymeriza-tion on immobilization of heavy metals in MT, the leaching test results from the MTpowder and the solid geopolymer specimens were compared with each other. ThepH was monitored during the experiment at least twice a day and was adjustedby adding nitric acid to the solution to keep the pH constant to the predefined value.The choice of nitric acid was motivated by the necessity of compensating the pHraise due to the release of unreacted NaOH from the geopolymer specimen, withoutaltering the leachability of heavy metals through complexation reactions. A liquid tosolid mass ratio of 15 was used for all the specimens throughout the experiment.After specified immersion times, 1, 3, 5, 7, 14, 21, 28, 90 and 105 days, a solutionsample less than 5 ml was taken and then filtered with a 0.45 lm membrane filter.The filtrate was diluted with 1% nitric acid and the concentration of metals in the di-luted extracted sample was measured based on the ICP-MS (inductively coupledplasma mass spectrometry) technique. The total amount of extraction from the solu-tions was less than 5% of the total solution to ensure that the solid to liquid ratio doesnot substantially change during the experiment.

2.3.2. Microscopic/spectroscopic analysesThe change of micro/nanostructure and phase composition of the geopolymer

matrix due to exposure to neutral and acidic environment was studied by perform-ing SEM, XRD, and FTIR analyses on the specimens after UCS test. The SEM imagingwas performed in the SE conventional mode using the FEI INSPEC-S50/Thermo-Fisher Noran 6 microscope. The XRD analysis was performed with a Scintag XDS2000 PTS diffractometer using Cu Ka radiation, at 2.00�/min ranging from 10.00�to 70.00� with 0.600 s count time. FTIR spectra were obtained using Thermo Nicolet370 FTIR/EZ Omnic with a smart performance ATR ZnSe crystal. The spectrometercovers wavelengths from 600 to 4000 cm�1. All the analyses were performed onmultiple specimens and at various locations on each specimen.

3. Results and discussion

3.1. Durability

The effect of initial water content, NaOH concentration, formingpressure and curing temperature on the physical and mechanicalproperties of MT-based geopolymer bricks was studied systemati-cally in [39]. The results indicate that 15 M NaOH concentrationand 90 �C curing temperature are the optimum condition for pro-ducing MT-based geopolymer bricks. The optimum forming pres-sure is related to the initial water content as shown in Fig. 2. Forexample, at initial water content of 16% and 18%, the highest UCSvalues were obtained at a forming pressure respectively of 0.5and 0.2 MPa. The UCS increased with higher initial water content(meaning larger amount of NaOH at the same NaOH concentration)because more geopolymer gels were produced at larger amount ofNaOH. In current study, the durability of only the specimens pre-pared at respectively 12%/25 MPa and 16%/0.5 MPa was studied.

The immersed specimens were monitored continuously. After4 months, no obvious disintegration, efflorescence, or any sign ofcracking was observed on any specimen. Table 2 presents the

measured UCS, water absorption, and weight loss of the specimensafter immersion in the solutions for four months. For both types ofspecimens, the UCS after immersion decreased substantially, 59.3%at pH = 4 and 53.3% at pH = 7 for the 12/25 specimen and 78.4% atpH = 4 and 75.2% at pH = 7 for the 16/0.5 specimen. This is possiblydue to the partial geopolymerization of mine tailings and the chem-ical composition of the formed geopolymer gels [34,43,44]. Similarresults were also reported by Silva et al. [34] on CTMTG whichshowed substantial strength loss after immersion in water mainlydue to insufficient degree of geopolymerization. Duxson et al. [43]argued that geopolymers possessing Si/Al < 1.0 or >5.0 exhibit ten-dency to dissolve in water due to high Na/Al ratio and presence of al-kali in the structure of the geopolymer product and suggested thatthe composition range be narrowed to 1 < Si/Al < 5 and Na/Al nottoo far from 1. In the current study, the initial mixture had a Si/Alof 7.76 and a Na/Al of 0.86 and 1.73 respectively for the 12/25 and16/0.5 specimens. Therefore, in addition to the partial geopolymer-ization, the high Si/Al and unreacted alkali could also be a factor forthe strength loss of the geopolymer specimens after immersion inwater. The Na/Al of the 12/25 specimens was closer to unity thanthe 16/0.5 specimens and thus less strength loss was observed.The other reason for the lower strength loss of the 12/25 specimensis that the compact structure of the 12/25 specimens resulted fromthe higher pre-compression pressure was more resistant to the acidattack than the compact structure of the 16/0.5 specimen resultedfrom the formation of more geopolymer gel. This can be clearly seenfrom Fig. 3 which compares the SEM micrographs of the specimensbefore and after immersion in the pH = 4 solution. For the 12/25specimen, there was not much change in the microstructure afterimmersion. For the 16/0.5 specimen, however, a large amount ofgeopolymer gel was washed off the surface after immersion, leadingto a more porous structure.

Fig. 3. SEM micrographs of (a and b) 12/25 specimen, and (c and d) 16/0.5 specimen, respectively before and after immersion in pH = 4 solution for 4 months.

746 S. Ahmari, L. Zhang / Construction and Building Materials 44 (2013) 743–750

The water absorption was small for both types of specimensafter immersion in pH = 4 and 7 solutions. This is mainly becauseboth types of geopolymer specimens had low initial porosity andthe porosity after immersion did not change substantially. Freidin[45] showed that for fly ash-based geopolymer bricks withouthydrophobic additives, the water absorption reached its ultimatevalue, about 25%, within just 1 day. Deboucha et al. [46] reportedwater absorption of about 15% for cemented peat bricks with 3–4% added lime. The ASTM standards require a water absorption

10 20 30 40

PA

AA P P

P

A

G

P

SG

SP

S

2θ

Fig. 4. XRD patterns of MT powder and 16/0.5 specimen before and after immersion in pHP: potassium aluminum silicate (sanidine), S: quartz].

not over 25% for most applications [39]. The MT-based geopolymerbricks easily met the ASTM requirements on water absorption.

The weight loss percentage of the MT-based geopolymer bricksis also low compared to the reported values for Portland cementbased concrete. Olusola and Joshua [47] reported over 16% weightloss for laterized concrete after less than 3 months’ soaking in 5%nitric acid. Pacheco-Torgal et al. [22] reported that the weight lossof Portland cement based concrete immersed in 5% nitric acidcould be over 40% depending on aggregate type.

50 60 70

After immersion 16/0.5 pH=7

After immersion 16/0.5 pH=4

MT powder

Before Immersion 16/0.5

S

S S

P

= 4 and 7 solutions for 4 months [A: sodium aluminum silicate (albite), G: gypsum,

Table 3Infrared (IR) characteristic bands identified in MT powder and geopolymer specimensshown in Fig. 5.

Wave number(cm�1)

Characteristic bands References

800–1200 SiAO stretching vibrations of SiO4 [48–52]872 ACO3 vibrations in CaCO3 [53]

1450 ACO3 vibrations in CaCO3 [54]1650 Bending (m2) mode of HAOAH [48,53]2350 CAO vibrations in CO2 constrained in

amorphous phase[55,56]

2920 CAO vibrations in CO2 constrained inamorphous phase

[55,56]

S. Ahmari, L. Zhang / Construction and Building Materials 44 (2013) 743–750 747

Fig. 4 shows the XRD patterns of the MT powder and the 16/0.5specimen before and after immersion in pH = 4 and 7 solutions.Although the crystalline phases before and after immersion aresimilar, the intensity of the peaks increases after immersion. Thisis more evident for the reflections between 20� and 30� atpH = 4. This confirms the SEM results shown in Fig. 3 – the disso-lution of geopolymer gel leads to exposure of crystalline MT grains.

Fig. 5 shows the IR spectra of the MT powder and the 12/25 and16/0.5 specimens before and after immersion in pH = 4 and 7 solu-tions. The identified IR characteristics are summarized in Table 3.There is no evident difference between the spectra of specimensimmersed in pH = 4 and 7 solutions, which confirm the results pre-sented earlier. However, the spectra exhibit a significant differencebetween before and after immersion. For the MT powder, the bandthat centers around 1000 cm�1 corresponds to the stretchingvibrations of SiAOAT (T = Al or Si) bonds. After geopolymerization,this band shifts toward lower wave numbers. In the meanwhile,the weak shoulder localized at 1070 cm�1 becomes stronger, whichis considered as a footprint for geopolymerization. After immer-sion, the shoulder becomes weaker and the SiAO related bandbecomes sharper and shifts slightly to a higher wave number indi-cating that the amorphous geopolymer gel is dissolved and theunderlain crystalline phase is exposed. The geopolymerization also

22,6003,1003,600

wave number

MT Powder

After immersion 16/0.5 pH = 4

After immersion 16/0.5 pH=7

Before immersion 16/0.5

2,6003,1003,600

wave number

Before Immersion 12/25

MT Powder

After immersion 12/25 pH = 4

After immersion 12/25 pH=7

(a)

(b)

Fig. 5. IR spectra of MT powder, and (a) 12/25 and (b) 16/0.5 specimens

results in a new band around 1450 cm�1, which is attributed tocarbonates. After immersion, this band disappears because the car-bonates dissolve in the solution.

3.2. Leaching behavior

3.2.1. Effectiveness of immobilization of heavy metalsTable 4 shows the concentration of different metals leached

from the MT powder and the brick specimens after immersion in

6001,1001,600,100

(cm-1)

6001,1001,6002,100

(cm-1)

before and after immersion in pH = 4 and 7 solutions for 4 months.

Table 4Concentration (ppm) of leached metals from MT powder and 12/25 and 16/0.5 specimens after immersion in pH = 4 and 7 solution for 90 days.

pH Na Mg Al K Ca Cr Mn Fe Co Ni Cu Zn As Se Mo Cd Pb

MT 7 5.9 36.9 0.2 7.1 359 0.1 1.0 0.0 0.1 0.1 0.0 0.0 0.1 0.0 0.13 0.0 0.04 11.1 497 1.2 28.6 2998 0.0 8.8 1.4 0.0 0.0 3.9 1.9 0.0 0.1 0.0 0.0 0.0

12/25 7 2952 1.2 0.4 101 78.0 0.0 0.2 0.0 0.1 0.0 0.1 0.1 0.7 0.0 0.7 0.0 0.04 3740 8.8 1.3 124 69.3 0.0 0.1 0.9 0.0 0.0 0.3 0.1 0.1 0.0 0.7 0.0 0.0

16/0.5 7 4135 1.6 0.6 132 87.3 0.0 0.5 0.0 0.1 0.0 0.0 0.0 0.8 0.0 0.8 0.0 0.04 4858 0.6 0.6 592 46.7 0.0 0.0 1.4 0.0 0.0 0.2 0.2 0.1 0.1 0.8 0.0 0.0

EPA limit NA NA NA NA NA 5.0 NA NA NA 5.0 NA NA 5.0 1.0 NA 1.0 5.0DIN NA NA NA NA NA NA NA NA NA NA 2.0–5.0 2.0–5.0 NA NA NA NA NAGreek NA NA 2.5–10.0 NA NA NA 1.0–2.0 NA NA 0.2–0.5 0.25–0.5 2.5–5.0 NA NA NA NA NA

748 S. Ahmari, L. Zhang / Construction and Building Materials 44 (2013) 743–750

pH = 4 and 7 solutions for 90 days. The threshold concentrationsregulated by different standards are also shown in the table. Theconcentration of leached metals is consistent with the chemicalcomposition and the content of trace elements shown in Table 1.The MT powder contains substantial amount of Fe, Ca, Mg, K andNa but only trace amount of Mn, Cu, Zn and Mo. The MT powderexhibits high leachability for Ca, Mg, K, Na and trace elementsMn, Cu and Zn in acidic condition. However, Fe does not showconsiderable leaching despite its high concentration in the MTpowder. In copper MT, Fe mainly exists as pyrite (FeS) or chalcopy-rite (CuFeS2) and during AMD process, it may oxidize and yield FeO(ferrous iron) or Fe2O3 (ferric iron). Considering the low solubilityof Fe in the current experiment, Fe3+ is most likely to be thedominant valence since Fe3+ is less soluble. For the MT powder,the concentration of leached Mn and Zn at pH = 4 exceeds thethreshold concentrations regulated by some of the standards. Forthe geopolymer brick specimens, however, these heavy metalsare effectively immobilized and exhibit concentrations signifi-cantly lower than the standard limits and those of MT powder.For both MT powder and brick specimens, the concentration ofmany leached metals at pH = 4 is higher than that at pH = 7 be-cause most metals have higher solubility at acidic condition [57].The concentration of released As and Mo from the brick specimensis higher than that from the MT powder indicating that geopoly-merization has an adverse effect on the immobilization of theseelements. The major reason could be that the natural pH of thegeopolymer gel is alkaline and As and Mo exhibit higher solubilityin alkaline condition [58–60].

Immobilization of heavy metals can take place chemicallythrough incorporation into the geopolymeric structure as charge-balancing cation and/or physically through encapsulation withinthe geopolymer gel [61]. It is believed that the chemical stabiliza-tion dominates the physical encapsulation because the structuralbreakdown of the geopolymer gel does not result in significant re-lease of the heavy metals.

3.2.2. Kinetics of leachingIn this section, the leaching behavior of Al, Fe, Cu, and Zn is

studied based on the first-order reaction/diffusion model. Thismodel explains the leaching of species out of solid specimensthrough a simplified mechanism, which has been shown to be suf-ficiently accurate for solid wastes [30,59]. The model consists ofthe first order reaction model (FRM) which involves dissolutionof the species at the solid–liquid interface and the bulk diffusionmodel (BDM) which accounts for transportation of the dissolvedspecies through the porous medium. The governing differentialequations for the one-dimensional FRM and BDM are as follows:

dQdt¼ �kQ

@C@t¼ D

@2C@x2

ð1Þ

where Q, k, C, and D denote respectively the amount of solublecontaminant in the solid waste, the reaction rate, the concentrationof the contaminant at time t and position x, and the coefficient ofdiffusion. The obtained concentrations from both equations can besuperimposed to account for the dissolution/diffusion phenomenonas in many systems, the leaching behavior is dominated by both dis-solution and diffusion. The combined solution to the FRM and BDMmodels is called the first order reaction/diffusion model (FRDM).Suzuki et al. [62] and Zheng et al. [30] successfully predicted theleaching kinetics by using the FRDM model as the following:

M ¼ Q 0½1� expð�ktÞ� þ 2SC0Dobstp

!12

ð2Þ

where M, Q0, S, C0, Dobs are respectively the cumulative concentra-tion of the contaminant, the initial amount of the contaminant,the surface area, the total concentration of the contaminant in thesolid specimen, and the observed diffusivity. Dobs represents the ef-fect of physical barrier due to transport through the tortuous poresand chemical retardation due to sorption on the solid phase. To sep-arate physical from chemical retardation, effective diffusivity (Deff)which represents only the effect of tortuous pores and their connec-tivity on the transport of contaminants can be used. In case of nosorption, Dobs and Deff are identical. However, in the case of linearsorption, Dobs can be obtained by multiplying Deff by a factor, whichindicates the chemical retardation [63]. Zheng et al. [30] employedthe FRDM by using Deff and introducing two factors accounting forthe chemical and physical retardation. In this study, the chemicaland physical contributions to Dobs are separated by introducing aphysical retardation factor (fp) and a chemical diffusion parameter(Dc) that depends only on the diffusing contaminant. By doing so,Eq. (2) is reduced to the following:

M ¼ Q 0½1� expð�ktÞ� þ 2SC0Dcfptp

� �12

ð3Þ

Using the non-linear regression method with Microsoft Excel solver,Eq. (3) was applied to fit the measured concentration vs. time data.First, Dc was back-calculated by fitting Eq. (3) to the measured con-centration data of the MT powder with the assumption of fp = 1. Theobtained Dc was then substituted in the equation and fp was back-calculated by fitting Eq. (3) to the measured concentration data ofthe brick specimen. The back-calculated parameters are summa-rized in Table 5 and some of the fitting curves are shown in Fig. 6.It can be seen that Eq. (3) can fit the experimental data very well.

Dc varies significantly with contaminants indicating that chem-ical retardation is an important factor affecting the leaching behav-ior of the brick specimens. Except for Al, fp does not change largelywith the specimen type, indicating that the 12/25 and 16/0.5 spec-imens have about the same effective porosity for contaminants tomigrate.

Table 5Summary of back-predicted FRDM parameters for Al, Cu, Fe and Zn at pH = 4.

Element Specimen Q0 (mg/kg) k (1/h) fp Dc (m2/h) Dobs (m2/h) R2 (%)

Al 12/25 0.140 0.053 8.154E�01 2.846E�05 2.32E�05 96.216/0.5 0.462 0.012 3.810E�03 2.846E�05 1.08E�07 94.2

Cu 12/25 0.209 0.017 9.703E�04 1.851E�07 1.80E�10 99.316/0.5 0.075 0.122 1.204E�03 1.851E�07 2.23E�10 98.3

Fe 12/25 0.383 0.004 8.244E�04 3.580E�01 2.95E�04 98.116/0.5 1.553 0.004 1.825E�03 3.181E�01 5.80E�04 99.6

Zn 12/25 0.060 0.011 NA NA NA 95.116/0.5 0.189 0.004 NA NA NA 93.8

0.00

0.05

0.10

0.15

0.20

0.25

0.30

Con

cent

ratio

n (p

pm)

Cu (pH=4)

12/25 Measured12/25 predicted16/0.5 measured16/0.5 predicted

0.000.200.400.600.801.001.201.401.60

0 20 40 60 80 100 120

Con

cent

ratio

n (p

pm)

Time (days)

Al (pH=4)

12/25 Measured12/25 predicted16/0.5 measured16/0.5 predicted

0.000.200.400.600.801.001.201.401.601.802.00

Con

cent

ratio

n (p

pm)

Fe (pH=4)

12/25 Measured12/25 predicted16/0.5 measured16/0.5 predicted

0.000.020.040.060.080.100.120.140.160.180.20

Con

cent

ratio

n (p

pm)

Zn (pH=4)

12/25 Measured12/25 predicted16/0.5 measured16/0.5 predicted

0 20 40 60 80 100 120Time (days)

0 20 40 60 80 100 120Time (days)

0 20 40 60 80 100 120Time (days)

Fig. 6. Measured and predicted concentrations of heavy metals at pH = 4 by FRDM.

S. Ahmari, L. Zhang / Construction and Building Materials 44 (2013) 743–750 749

The k for Fe has the lowest value indicating that it has the low-est reaction rate with the solution. So the low leachability of Fe, asdiscussed earlier, is attributed to its low chemical reaction rate.This is further evidenced by the same low level leachability of Fein the MT powder as in the brick specimens, which indicates thephysical encapsulation has not significant effect. On the otherhand, Cu exhibits higher reaction rate and higher chemical retarda-tion. This implies that the leaching kinetics of Cu is dominated byreaction and its lower immobilization efficiency is due to its higherreaction rate. The back-predictions of the measured concentrationsof Zn show that the contribution from the diffusion part of Eq. (3) iszero. In other words, the BDM or the reaction part of the equationfits into the measured curve. This means that the leaching of Znfrom MT-based matrix is controlled by early-stage chemical reac-tion. Zn might come from the outer surface of the specimen so thatit does not face any physical or chemical barrier.

4. Conclusions

The durability and leaching behavior of MT-based geopolymerbrick specimens were studied. Based on the experimental results,the following conclusions can be drawn:

� The UCS of the MT-based geopolymer bricks decreases substan-tially after immersion in pH = 4 and 7 solutions for four months;but the water absorption is relatively small and weight loss issmaller than that for Portland cement-based binder. Thestrength loss is mainly due to the high initial Si/Al ratio andthe partial geopolymerization of MT.� The heavy metals are effectively immobilized in the MT-based

geopolymer bricks. The effective immobilization can be due tothe incorporation of heavy metals in the geopolymeric networkor physical encapsulation.� The first-order reaction/diffusion model can be used to satisfac-

torily describe the leaching of heavy metals in MT-based geo-polymer bricks. It indicates that the solubility or reaction rateis an important factor controlling the leaching behavior.

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