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Ceramics International 40 (2014) 8479–8488www.elsevier.com/locate/ceramint

Lead immobilization by geopolymers based on mechanicallyactivated fly ash

Violeta Nikolića,n, Miroslav Komljenovića, Nataša Marjanovića,Zvezdana Baščarevića, Rada Petrovićb

aInstitute for Multidisciplinary Research, University of Belgrade, SerbiabFaculty of Technology and Metallurgy, University of Belgrade, Serbia

Received 28 November 2013; received in revised form 13 January 2014; accepted 13 January 2014Available online 21 January 2014

Abstract

In this paper, the effectiveness of geopolymers based on initial and mechanically activated fly ash in immobilization of lead was investigated.Fly ash (FA) was firstly mechanically and then alkali activated at room temperature. The immobilization process was assessed by the means ofthe mechanical and leaching properties of geopolymers. The results indicated that the geopolymers based on mechanically activated FA weremore effective in the immobilization of lead compared to the geopolymers based on the initial FA. Mechanical activation of FA led to asignificant increase in strength and reduced Pb leaching from geopolymers. Higher effectiveness of Pb immobilization was the result of reducedporosity, i.e. higher compactness of geopolymers based on mechanically activated FA.& 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Keywords: C. Geopolymer; Fly ash; Mechanical activation; Immobilization

1. Introduction

High emission of CO2 during Portland cement production (1 tof CO2 per 1 t of Portland cement) [1] is reason enough todevelop and use new inorganic binders – geopolymers. Geopo-lymers are amorphous equivalent of certain zeolitic materials,formed by alkali activation of aluminosilicate materials, usuallymetakaolin or fly ash (FA). The main product of the reaction is analkaline aluminosilicate gel with a three-dimensional structure inwhich both Al and Si are tetrahedrally coordinated. The alkaliions that compensate for the electric charge generated by thesubstitution of Al3þ for Si4þ are housed in the voids of thisthree-dimensional framework [2]. Geopolymers are an alternativeto Portland cement due to excellent mechanical properties [3],chemical resistance and durability in the long term period [4].In addition, many authors [5–10] pointed out the possibility ofusing geopolymers in the solidification/stabilization (S/S) of toxicand radioactive waste.

e front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All ri10.1016/j.ceramint.2014.01.059

g author. Tel.: þ381 11 20 85 047; fax: þ381 11 30 55 289.sses: violeta@imsi.bg.ac.rs,msi.rs (V. Nikolić).

According to the US Environmental Protection Agency [11],S/S process is recognized as “the best demonstrated availabletechnology” (BDAT) for the treatment of waste materialscontaining heavy metals. S/S involves mixing a bindingagent with hazardous waste to reduce the contaminant leach-ability and to convert the hazardous waste into an environmen-tally acceptable form of waste material for land disposal orconstructive use [12]. S/S reduces the contaminant leachability byphysical and chemical mechanisms. The distinction between thepurely chemical and purely physical mechanism is not clear, sincein many cases both operate simultaneously [13].Portland cement is the most commonly used binder for the

S/S purpose [13] either alone or in combination with FA [14].Fly ash has been also used as a low-cost adsorbent in thetreatment of water contaminated with heavy metals [15,16].Fly ash-based geopolymer has higher removal capacity of leadions from aqueous solution compared to raw fly ash [17].Geopolymers are often used in immobilization of heavy metalsin municipal solid waste incineration fly ash [18–21], or otherwastes containing heavy metals [22–24]. In geopolymerssystems, the greatest attention has been paid to lead immobi-lization, since lead adversely affects the setting and hydration

ghts reserved.

V. Nikolić et al. / Ceramics International 40 (2014) 8479–84888480

of binders such as Portland cement. Тhis may be due to theformation of impervious layer around the clinker grains [25].On the other hand, lead is one of the most dangerouspollutants, because it is non-biodegradable and can accumulatein the living systems.

Lead immobilization by alkali activated FA/blast furnaceslag may be attributed to Pb acting as a network former, anetwork modifier, a charge balancing ion, and as an insolubleprecipitate encapsulated in the structure [26]. Due to highsynthesis pH, the precipitation of hydroxides is the expectedmode of Pb immobilization in geopolymers [27], similar toPortland cement systems [28]. In geopolymers, lead may alsoprecipitate in the lead–silicate form [25]. Many authorsconsider that lead is encapsulated in the amorphous alumino-silicate structure [29–34]. To some extent, lead may contributeto the balancing charge of geopolymers network by replacingNaþ in the stucture [26,35]. Cheng et al. [36] investigated theadsorption of lead on geopolymers and concluded that, whendiffusing into the geopolymer, Pb2þ stays trapped in the pores,thus decreasing the possibility of leaching.

Chemically pure and water soluble compounds are oftenused as a source of heavy metals during S/S investigation[25,30,37]. As heavy metals in real conditions are present inthe less mobile form, the use of these compounds presents the“worst-case scenario” in terms of leaching. The mobility ofheavy metals from S/S matrix was estimated using differentleaching tests. Standard EN 12457 was developed in order toharmonize the standard procedures within the European Unionand it is based on the member countries' procedures. Accord-ing to the defined limits of this standard, waste may beclassified as inert, non-hazardous or hazardous [38].

Recently, a novel method has been used to treat hazardouswaste contaminated by heavy metals. This method is based onthe mechanochemical treatment – the milling technology withhigh mechanical energy input. Reaction induced by mechan-ical energy is called mechanochemical reaction, while a changein the reactivity of the starting material is called mechanicalactivation [39]. Mechanical activation leads to structuralchanges, amorphization of materials and formation of struc-tural defects [40].

Mechanical activation of FA may not only improve adsorp-tion and pozzolanic characteristics of FA, but also the S/Sproperties of matrices based on the Portland cement andmechanically activated FA [41,42]. Onori et al. [43] haveactivated, mechanically and chemically, incinerator bottom ashwith the purpose of improving the reactivity of the ash inPortland cement blends. Also, mechanical and chemicalactivation of phosphorous slag improved properties of cementwith high content of the slag [44]. Mechanochemical treatmentcontributed to the S/S of heavy metals in the municipalsolid waste incineration fly ash [45–47] and in contaminatedsoils [48].

According to the literature, mechanical activation cansignificantly improve the mechanical properties of geopoly-mers [49–51], and enable the synthesis of geopolymers atroom temperature [52,53]. Thus, mechanical activation cansubstitute the thermal activation [40]. Geopolymerization

reaction is very sensitive to the temperature changes andusually requires curing of geopolymers at elevated tempera-ture. This fact is a limiting factor in wider practical use ofgeopolymers.Despite the fact that there are a number of papers related to

the alkali-activated materials, as well as papers related to themechanical activation of FA; the papers that are related to thesimultaneous application of alkali and mechanical activation ofFA are lacking. Recently, improved compressive strength[54,55] and leaching characteristics [55] of alkali activatedmaterials based on mechanically activated slag were con-firmed. However, to the best of our knowledge, there has beenno research on the effects of mechanical activation of fly ashon immobilization ability of the geopolymers based on fly ash.This paper investigates the effectiveness of geopolymers

based on mechanically activated FA in the immobilization oflead. Lead was added in the form of water soluble salt – leadnitrate, during the synthesis of geopolymers. The efficiency oflead immobilization was determined by examining the strengthand leaching behavior of geopolymers. Special attention waspaid to the structural changes of geopolymers which occurredas a result of mechanical activation of the initial FA.

2. Experimental

2.1. Materials

In this study, FA Kolubara, sample from thermal powerplant “Kolubara” Veliki Crljani, Serbia was used. Sodiumsilicate solution (“Galenika–Magmasil”, Serbia, 13.60% Na2O,26.25% SiO2, 60.15% H2O) was used as an alkaline activator.Starting sodium silicate modulus n¼SiO2/Na2O (mass ratio)was 1.93. Modulus of sodium silicate solution was adjusted byadding NaOH pellets (NaOH p.a. ASC reagent, (min. 98%),“Sigma Aldrich”, Sweden). Pb(NO3)2 was used as heavy metalsource (Pb(NO3)2 p.a., “Centrohem”, Serbia). ConcentratedHNO3 (p.a., “VWR”, France) was used to stabilize the solutionafter filtration.

2.2. Mechanical activation of FA

Mechanical activation of FA was carried out in a planetaryball mill (Fritch Pulverisette type 05 102). FA sample wasmechanically activated in air atmosphere for 15 min [56]. Massratio FA to balls was 1:20, while the rotation speed was380 rpm. Mechanically activated FA will be hereinafterreferred to as MFA.

2.3. Synthesis of geopolymers

2.3.1. Preparation of mortarBased on the previous research [57] the modulus of sodium

silicate solution used as alkaline activator in this study was 1.5.The concentration of the activator was 10% Na2O contentwith respect to the FA mass. Geopolymer mortars wereprepared by mixing FA/MFA with alkali activator and water,and then with sand. Water was added in the amount to obtain

Table 1Chemical composition and particle size distribution of FA Kolubara.

Composition/characteristics investigatedSiO2 62.13Al2O3 17.20Fe2O3 5.95CaO 5.67MgO 2.00SO3 0.67LOI at 1000 1С 2.88

Total 96.50

Particle size

Classification according to ASTM C 618-03 Class FZ63 μm (%) 42.6543–63 μm (%) 5.30r43 μm (%) 52.05

V. Nikolić et al. / Ceramics International 40 (2014) 8479–8488 8481

equal consistency (mortar flow measured on a flow table was12075 mm). FA:sand ratio was 1:3. The molds with threemortar prisms (40 mm� 40 mm� 160 mm) were vibrated on avibrating table to remove air bubbles.

The method of preparation of geopolymers with addition ofPb was similar to the method of preparation of geopolymerswithout Pb. Pre-calculated amount of lead nitrate wasdissolved in the amount of distilled water required to achieveappropriate consistency. The order of geopolymers preparationwas as follows: first the activator was added to the FA, then thelead nitrate solution and finally sand. The content ofPb, accounting for the total mass of FA, was 0.0%, 0.5%and 1.0%.

Water/binder ratio (water represents the total amount ofwater in the system, including water from the activator, whilebinder represents the total fly ash mass and solid part ofactivator) was 0.61 in the case of mortar based on FA, while inthe case of mortar based on MFA it was 0.38. Mortar prismswere cured in a humid chamber (2072 1С; 9075% rel.hum.) for 1 day and 28 days. Mortar samples were used for thecompressive strength measurements. All other tests wereconducted on the paste samples.

2.3.2. Preparation of pasteThe preparation of the geopolymer pastes was performed in

the same manner as the preparation of mortars by mixing theFA/MFA, activator and water or lead nitrate solution (in thecase of geopolymers with Pb added). Water/binder ratio was0.56 for the pastes based on FA and 0.35 for the pastes basedon MFA. Prepared pastes were poured into cylindrical plasticmolds (ø 35� 50 mm). Air bubbles were removed on thevibrating table.

Prepared pastes were cured at room temperature in a humidchamber. After the curing was over, paste samples werecrushed and passed through a 4 mm sieve (for leachingtesting). For other tests pastes were pulverized in isopropylalcohol for 1 h in order to stop further reaction. Afterpulverization, the samples were filtered, rinsed with acetoneand dried at 50 1C.

2.4. Methods of characterization

Chemical composition of the FA sample was determined byclassic chemical analysis – alkali melting. The particle sizedistribution was determined by sieving through meshes of63 μm and 43 μm. Chemical composition, particle size dis-tribution and classification according to ASTM C 618-03 areshown in Table 1.

Nitrogen adsorption–desorption isotherms and textural char-acteristics of FA, MFA, such as textural characteristic ofappropriate geopolymers were determined using a Micromeri-tics ASAP 2020 instrument. Samples were degassed at 105 1Cfor 10 h under reduced pressure. The specific surface area ofthe samples was calculated according to the Brunauer, Emmett,Teller (BET) method. The volume of the mesopores wascalculated according to the Barrett, Joyner and Halendamethod from the desorption branch of isotherm.

A mineralogical analysis was conducted by X-ray powderdiffraction (RIGAKU X-ray spectrometer KG-3). The diffractionpatterns were recorded using Cu Kα radiation (λ¼1.54178 Å)within 10–501 2θ range, with a rate of 1 1/min. The PCPDFWINsoftware was used for the identification of crystalline phases.The determination of compressive strength of geopolymer

mortars was performed according to SRPS EN 196-1 using theCONTROLS ADVANTEST 9 device.Microstructure of geopolymers was analyzed by scanning

electron microscopy (SEM, “Tescan” VEGA TS 5130 MM).Prior to SEM analysis geopolymer pastes were Au-coated.

2.5. Leaching tests

There are many test methods that simulate the leachingprocess of waste in a landfill or in the environment. In thisway, pollutant content is estimated in order to determine thecharacteristics of the leachant (leaching medium) in contactwith the test material. Standard EN 12457-2 prescribes the useof deionized water as leachant at a liquid to solid (L/S) ratio of10 L/kg for materials with particle size below 4 mm, for 24 h.Testing of Pb leaching from geopolymers (aged 1 day and

28 days) was performed according to a modified standardprocedure EN 12457-2. The speed of rotation was 60 rpminstead of 10 rpm due to the specific characteristics of therotation device used. During the experiments 20 g of samples(dry mass) was mixed with 200 ml of deionized water. After24 h of leaching the samples were vacuum filtered through amembrane filter of 0.45 μm. The same procedure was appliedto the FA and MFA in order to determine whether and towhich extent there was a potential leaching of “native” Pbpresent in the fly ash. The temperature and pH of the eluates(solutions recovered from the leaching test) were recorded. Theeluates were acidified with nitric acid to the value of pH lessthan 2.The inductively coupled plasma optical emission spectro-

meter (ICP–OES, Spectro-Genesis EOP II, Spectro Analytical

Table 2Textural characteristics of FA and MFA.

Specific surface Total pore Average pore

V. Nikolić et al. / Ceramics International 40 (2014) 8479–84888482

Instruments GmbH, Kleve) was used to determine the con-centration of lead in eluates. Results were calculated as mg Pbper kilogram of dry residue.

area (BET) (m2/g) volume (cm3/g) size (nm)

FA 11.6 0.013 5.9MFA 16.5 0.046 13.0

Table 3The mean compressive strength with the standard deviation (given inparentheses) of geopolymers based on FA and MFA after 1 day and 28 days.

Pb content (%) G (N/mm2) G–M (N/mm2)

1 day 28 days 1 day 28 days

0.0 1.18 11.07 30.92 57.85(0.09) (0.86) (1.33) (3.07)

0.5 1.05 10.66 32.15 60.69(0.07) (0.71) (1.48) (3.43)

1.0 0.84 9.78 29.73 58.46(0.05) (0.65) (1.16) (3.10)

3. Results and discussion

3.1. Textural characteristics of FA and MFA

Nitrogen adsorption–desorption isotherms of FA and MFAare presented in Fig. 1. According to the IUPAC (InternationalUnion of Pure and Applied Chemistry) classification, N2

adsorption isotherms can be classified as type IV isotherms –which are generally attributed to mesoporous materials. Themain feature of this type of isotherm is the hysteresis loop,which is associated with capillary condensation taking place inmesopores [58].

At relatively low pressures there was a slight increase inadsorption, which was attributed to the filling of micropores.With the further increase in the relative pressure (P/PoE0.8) asignificant increase in adsorption pressure was observed,indicating the presence of mesopores and macropores in theFA and MFA samples.

Mechanical activation during 15 min caused the fragmenta-tion of the FA particles and increased specific surface area(BET). Specific surface area of MFA was higher by approxi-mately 42% compared to FA (Table 2). The values of the totalpore volume (total porosity) and the average pore size were alsohigher in MFA compared to FA. This is probably due to the factthat FA partly consists of cenospheres. Cenospheres are thehollow particles in fly ash [59], which were mainly destroyedduring the mechanical activation [49,60]. Mechanical treatmentincreased the content of poor shape FA particles [60].

3.2. Characterization of geopolymers

3.2.1. Compressive strength of geopolymersThe mean compressive strength with standard deviation of

the geopolymers based on FA (G) and MFA (G–M), with andwithout Pb added is shown in Table 3. The compressive

Fig. 1. N2 adsorption–desorption isotherms of FA and MFA.

strength of G–M after 1 day was almost 30 times higher thanthe compressive strength of G. Similarly, the compressivestrength of geopolymers at 28 days was nearly six times higherfor G–M than for G. Also, G–M аged 1 day achieved almostthree times higher compressive strength than G aged 28 days.The higher compressive strength of G–M compared to G wasdue to the enhanced geopolymerization reaction. The alterationin geopolymerization is a result of combined effect of FAparticle size and change in reactivity due to mechanicalactivation [53]. Mechanochemical treatment reduced the par-ticle size of FA and increased the surface area available forgeopolymerization. In the previous works [57,61], investiga-tions on the effects of fly ash particle size on compressivestrength of geopolymers showed that the finer particle size, thehigher compressive strength.The addition of Pb slightly decreased the compressive

strength of G. In the case of G–M, the addition of 0.5% Pbhas led to a small increase in compressive strength. Consider-ing the standard deviation data and the fact that the addition ofPb does not significantly affect the compressive strength valuesof geopolymer mortars, no definite conclusion (on how Pbaddition affects the mechanical properties) can be drawn.However, independently of Pb content, the compressivestrength of geopolymer mortars based on the initial FA andmortars based on mechanically activated FA is clearly visible.According to the US Environmental Protection Agency [62]

the minimum strength of the S/S product required for thelandfill is 0.35 MPa. In the UK, the minimum compressivestrength at 28 days for final disposal is 1 MPa [63]. As can beseen from the shown results, all samples met the requiredcompressive strength. While G after 1 day of aging at room

Table 4Specific surface area and average pore size of geopolymers.

Component Pb content(%)

Specific surfacearea (m2/g)

Average poresize (nm)

G 0.0 33.1 17.90.5 40.1 18.11.0 41.5 18.8

G–M 0.0 78.3 5.60.5 65.8 6.41.0 67.7 6.9

V. Nikolić et al. / Ceramics International 40 (2014) 8479–8488 8483

temperature reached the strength of about 1 MPa, G–Mexceeded (almost 100 times) the required strength value.

3.2.2. Porosity of geopolymersAccording to the IUPAC classification, porosity is generally

classified as macroporosity (pore sizes exceeding 50 nm),mesoporosity (pore size between 2 and 50 nm) and micro-porosity (pore size less than 2 nm) [64]. In geopolymers,macropores represent the gaps between unreacted fly ashparticles. Mesopores are typical pores between geopolymerphases, while micropores exist within the gel network [18].

The total pore volume and pore size distribution ofgeopolymers G and G–M are presented in Fig. 2. The totalpore volume of G–M was smaller compared to G, regardless ofPb content (Fig. 2a). Differential pore size distribution curvesof G (Fig. 2b) are characterized by a broad peak. Themaximum of this peak reflects the size of the majority ofpores and it is located at approximately 30 nm. G–M showed abimodal pore size distribution, i.e. two peaks in the differentialcurves were observed. Less prominent of these two peaks isassigned to the larger pores (40 nm). The second peak wasclearly visible and corresponds to the pores of smaller diameter(about 4 nm). On the other hand, the increase in the Pb contentresulted in the increase of the total pore volume, which wasmore noticeable in G. These results are consistent with theresults of the compressive strength of geopolymers: geopoly-mers G–M had a lower porosity, thus developing superiormechanical properties in comparison to G.

Mechanical activation increased the reactivity of FA,resulting in a higher degree of reaction during geopolymeriza-tion. Increased reactivity led to the formation of larger amountsof the geopolymer gel, as the main reaction product. This gelfilled the cavities between unreacted fly ash particles and porespaces, thus refining the size of these pores. There were lessunreacted particles in the structure of G–M compared to G dueto the increased surface area and increased reactivity of theMFA. The total specific pore volume of the geopolymersdepends on the FA particle size. The finer FA particle sizeresults in the denser paste and hence the reduced pore volumein the specimens [65].

In the cement case, the porosity is divided into gel pores andcapillary pores [66]. The “gel pores” are formed within the gel,

Fig. 2. Pore size distribution of the geopolymers (aged 28

and their diameter is in the range of a few nanometers.Capillary pores correspond to the spaces originally filled withwater, and not filled by reaction products. The size of capillarypores range from a few nanometers to several dozen ofmicrometers. The size of air voids ranges from several dozenof micrometers to millimeter size [67]. In practice, there is anoverlap in gel and capillary pore systems in the range of about5–20 nm. The formation of capillary pores is characteristic ofcement systems, while their formation in geopolymers is lessdistinct, because the gel takes up most of the space [68].Despite the fact that the BJH method cannot give informa-

tion about pores 4100 nm, this method clearly confirmed theshifting of the differential curve peak towards lower values ofpore diameter in the case of G–M samples (Fig. 2b). Due to themechanical activation of FA, the pore size of geopolymersbased on MFA was significantly reduced, and thus the totalporosity. The peak shifting of differential curves from meso-pores to micropores suggested the formation of larger amountsof reaction products (gel).Table 4 shows the results of the investigation of specific

surface area and average pore size of geopolymers G andG–M. Specific surface area of G–M was higher more than twotimes compared to the specific surface area of G. Due to themechanical activation of FA, the average pore size of G–Mwas three times lower than of G. In G and G–M, addition of Pbled to an increase in the average pore size in comparison togeopolymers without Pb added, which was also observed byother authors [6,29]. The increase in geopolymers porosity dueto Pb addition resulted in poorer mechanical properties

days): (a) total pore volume and (b) differential curves.

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(especially in the case of geopolymers G), as confirmed by thecompressive strength results (Table 3).

It should also be noted that the mechanical activation of FAreduced the amount of water needed for the geopolymerspreparation. It is well known that the binder system with thehigher water content has a higher porosity [69].

3.2.3. Scanning electron microscope analysis (SEM)The SEM micrographs of geopolymers based on FA and

MFA at the same magnification are shown in Fig. 3. Thedifference in microstructure in terms of compactness isdistinctly evident. The microstructure of G–M (Fig. 3b andd) was more homogenous and less porous compared to G(Fig. 3a and c). The higher compactness of G–M may beexplained by the formation of greater amount of aluminosili-cate gel [53]. The improved geopolymerization take place due

Fig. 3. SEM of g

to the increased surface area and increased reactivity of MFAparticles.The increase in porosity of geopolymers due to Pb addition

was clearly visible on the SEM geopolymers based on FA(Fig. 3a and c). Meanwhile, due to significantly lower porosityof G–M, the differences in porosity of G–M with and withoutPb added were small and could not be observed by SEM(Fig. 3b and d).

3.2.4. X-ray diffraction analysis (XRD)X-ray diffractograms of FA and G, with and without Pb

added are shown in Fig. 4a, while the diffractograms of MFAand G–M, with and without Pb added are shown in Fig. 4b.The following crystalline phases were identified: quartz

(PDF# 79-1906) as the most prominent phase, anhydrite(PDF# 72-0916), feldspar (PDF# 89-8574) and hematite

eopolymers.

Fig. 4. X-ray diffractograms of samples: (a) FA and G, and (b) MFA and G–M.

Fig. 5. EN 12457-2 soluble Pb from FA, MFA and geopolymers G and G–Maged 1 day.

V. Nikolić et al. / Ceramics International 40 (2014) 8479–8488 8485

(PDF# 87-1164). Crystalline phases, which were recorded inthe initial fly ash, were also present in the mechanicallyactivated FA. However, due to the mechanical activation ofFA, a reduction in the peak intensities of some crystallinephases, especially feldspar and anhydrite were observed. Thereduction in the peak intensities can be attributed to the partialamorphization [45,48]. In geopolymers, the presence ofsecondary calcite (PDF# 47-1743), as the result of carbonationof the samples during the curing process, was observed.

In the geopolymer samples with Pb added, crystalline phasesof lead (such as lead–hydroxide or lead–silicate) were notdetected. This fact could indicate that Pb did not formchemical bonds with the aluminosilicate phase of geopoly-mers. However, it should be noted that a relatively smallamount of Pb (maximum of 1% Pb in relation to the mass FA)was used. According to the previous work [25] addition ofmore than 3% of Pb in the form of Pb(NO3)2 to thegeopolymers based on FA causes the formation of lead–silicatephases Pb3SiO5 detectable by X-ray analysis.

3.2.5. LeachingThe results of Pb leaching from geopolymer pastes aged 1

day and 28 days, according to EN 12457-2, are shown inFigs. 5 and 6, respectively.

Although both FA and MFA met the required limit for inertwaste, less Pb was leached from FA than from MFA. Theleaching test applied to the fly ash (particle size much smallerthan 4 mm) can overestimate the Pb content. The mechanicalactivation increases the specific surface area of FA, thusincreasing the solubility of “native” Pb present in the fly ash.It can be seen from Figs. 5 and 6 that “native” Pb leaching ishigher from geopolymers without Pb added (G – 0.0% Pb,G–M – 0.0% Pb) than from fly ash (FA, MFA). It can besupposed that the conditions existing during the alkali activa-tion caused an increase in the solubility of “native” Pb species.Factors contributing to this phenomenon were probably thehigh pH and an excess of Naþ ions from the activator.Recently, a group of authors investigated the mercury immo-bilization by FA based geopolymers and came to a similarfinding [70].

In accordance with the limits specified by EN 12457-2,G–M aged 1 day with the Pb content r1 mass% could beconsidered as non-hazardous waste, while equivalent G couldbe classified as a hazardous waste. In the case of geopolymerswhere the content of Pb added was 1%, leachable Pb exceededthe limit acceptable for wastes to be disposed at the landfill(Fig. 5). It should be noted that the geopolymer pastes werecured at room temperature, and curing process was notcompleted after 1 day. This especially refers to the geopoly-mers based on the initial FA, which had low strength (Table 3).It can be clearly seen from Fig. 6 that all geopolymers aged

28 days complied with the relevant limits for non-hazardouswaste. However, the leachable Pb concentration from the G–Mwas still significantly less than from G.Many authors [7,19] emphasize the importance of pH value

on the degree of leaching of immobilized toxic ions. In thisstudy, the solutions obtained after leaching were highly alka-line due to the release of free alkali in the aquatic environment,but in all cases the pH values were similar (11.90–12.59).Thus, one could not say that the pH value had a decisiveinfluence on the differences in the leaching behavior of thegeopolymers. Based on the presented results it can beconcluded that the reduced Pb leaching from G–M with

Fig. 6. EN 12457-2 soluble Pb from FA, MFA and geopolymers G and G–Maged 28 days.

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respect to G was a result of reduced porosity of G–M, achievedby mechanical activation of FA. Mechanical activation resultedin the increased surface area and increased reactivity of FA.Increased reactivity of FA led to the formation of more gel asthe main reaction product. This gel filled the pore system,reducing the total pore volume of geopolymers. Reducedporosity due to the formation of larger amount of gel in G–M is responsible for better immobilization of Pb. It is wellknown that the pore structure, water and ions transportdetermine the resistance of the material to external influences[68]. Also, the diffusion process during leaching depends onpore size distribution [18], physical properties [35] andstability of the matrix [30]. Microstructural properties of abinder include the porosity, tortuosity and extent of percolationof the pore network [71].

Although there was no evidence that could indicate theformation of a new Pb-phase (Fig. 4), a possibility of chemicallybonded Pb, beside the physically bonded Pb, should not beexcluded, since a small amount of Pb was added to the geopolymermatrix, and consequently a small amount of potentially formedPb-phase. Pb bonded in geopolymer matrix is “invisible” for X-rayanalysis. It is possible that divalent Pb ions to some extent have acharge balancing role in three-dimensional network of geopolymer[26,35]. Geopolymer matrix has a limited capacity for the heavymetals before the deterioration of the structure and increasedleaching [6]. Independently of the mechanism of lead immobiliza-tion, lead within the less porous and more stable geopolymermatrix based on MFA is less subjected to leaching.

It should also be borne in mind that mechanical activationincreases the adsorption capacity of FA [41]. In this way, thenumber of adsorption sites is increased in the MFA comparedto FA, which can affect the more efficient bonding of heavymetals during geopolymerization.

4. Conclusion

In this paper, the effectiveness of geopolymers based onmechanically activated fly ash (FA) in immobilization of Pbwas investigated in comparison to geopolymers based oninitial fly ash.

Fly ash was firstly mechanically and then alkali activated atroom temperature. Lead was added in the form of watersoluble salt – lead nitrate, during the synthesis of geopolymers.The efficiency of lead immobilization was determined byexamining the strength and leaching behavior of geopolymers.Mechanical activation of FA caused an increase in the FA

surface area, which resulted in the increased reactivity of FAduring geopolymerization, and increased leaching of Pb fromFA itself. Mechanical activation of FA also caused changes inthe pore structure (increase of gel pores content) and reducedporosity of ensuing geopolymer. Consequently, geopolymerbased on mechanically activated FA showed much higherstrength and lower susceptibility to Pb leaching with respect togeopolymer based on the initial FA.The presented results clearly confirmed the higher effective-

ness of geopolymers based on mechanically activated FA inthe immobilization of lead.Mechanical activation of FA can substitute the thermal activa-

tion which is of great importance for practical applications ofgeopolymers, especially in the case of S/S treatment. Mechanicalactivation of FA prior to geopolymer synthesis has not only thepotential of reducing the level of toxicity of hazardous waste, butgiven the high values of compressive strength obtained, geopoly-mers based on MFA can be used in selected constructionapplications such as barriers for other hazardous wastes.

Acknowledgments

This research was financially supported by the Ministry ofEducation, Science and Technological Development of the Repub-lic of Serbia through Project TR 34026 and COST Action TU1301. The authors would like to thank MSc Ivona Janković–Častvan from the Department of Inorganic Chemical Technology,Faculty of Technology and Metallurgy, University of Belgrade forporosity investigations. The valuable advice of Professor Dr.Miroslav Nikolić (Plant and Soil Laboratory, Institute for Multi-disciplinary Research, University of Belgrade) in ICP–OEStechnique and the results interpretation is also acknowledged.

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