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Use of Cement-Chelated Solidified MSWI Fly Ash for

Pavement Material: Mechanical and Environmental

Evaluations

Journal: Canadian Geotechnical Journal

Manuscript ID cgj-2017-0007.R2

Manuscript Type: Article

Date Submitted by the Author: 06-Jul-2017

Complete List of Authors: Tang, Qiang; Soochow University, School of Urban Rail Transportation

Zhang, Yu; Soochow University Gao, Yufeng; Hohai University, Laboratory of Ministry of Education for Geomechanics and Embankment Engineering Gu, Fan; Texas A&M University Transportation Institute,

Keyword: Fly ash, freeze-thaw, cement-chelated stabilization/solidification, pavement material, finite element analysis

https://mc06.manuscriptcentral.com/cgj-pubs

Canadian Geotechnical Journal

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Use of Cement-Chelated Solidified MSWI Fly Ash for Pavement Material: Mechanical and

Environmental Evaluations

Qiang Tang, Ph.D.

Associate Professor

School of Urban Rail Transportation

Soochow University

Yangchenghu Campus, Xiangcheng District, Suzhou 215131, China

E-mail: [email protected]

Yu Zhang

Master Candidate

School of Urban Rail Transportation

Soochow University

Yangchenghu Campus, Xiangcheng District, Suzhou 215131, China


Yufeng Gao, Ph.D. (Corresponding Author)

Professor

Key Laboratory of Ministry of Education for Geomechanics and Embankment Engineering

Hohai University

No. 1# Xikang Road, Gulou District, Nanjing 210098, China


Fan Gu, Ph.D.

Postdoctoral Research Associate

Texas A&M Transportation Institute

Texas A&M University System

3135 TAMU, CE/TTI Bldg. 501A, College Station, Texas 77843, USA

Phone: (979) 458-5729, Email: [email protected]

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Abstract:

As a by-product from the incineration of Municipal Solid Waste (MSW), fly ash usually

contains mobile heavy metals that may engender severe pollution for reuse. In this study, fly ash was

solidified with cement and chelating agent to immobilize these polluting elements. The possibility of

using the solidified fly ash for pavement materials was also assessed through mechanical and

environmental perspectives. According to the results, the strength of solidified fly ash was found

proportional to both the cement/fly ash ratio and curing time. This indicated that the increase of fly

ash loading reduced the concentration of products from cement hydration, and thus destroyed the

structure of products of hydration. With the increase of freeze-thaw cycles, the compressive strength

of cement-stabilized fly ash decreased from 7 to 14 days, and then increased from 14 to 28 days.

Subsequently, the finite element analysis showed that placing the solidified fly ash layer as pavement

materials between unbound base course and subgrade was beneficial to prolong the fatigue life and

reduce the rutting distress of asphalt pavements. Finally, the metals leachability of the mixtures was

tested, which showed that leaching concentration decreased as the cement/ash ratio, curing time and

chelating agent content increased.

Keywords:

Fly ash, freeze-thaw, cement-chelated stabilization/solidification, pavement material, finite

element analysis

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Introduction

Incineration is a commonly used method for managing the Municipal Solid Waste (MSW) in the

world. This is because it reduces the waste by about 70% in weight and 90% in volume (Ferreira et al.

2003; González et al. 2016). In Japan, the incineration percentage of solid waste is up to 75% in

2013 (Ministry of the Environment of Japan 2013). Meanwhile, approximately 50% of MSW were

treated by incineration in Europe (e.g., Denmark, Sweden, Switzerland, and Netherlands) (CEWEP

2013). Even though incineration is efficient for treating MSW, one of the shortcomings of this

technology is the production of a significantly large amount of solid residue (e.g., fly ash) (Sukandar

et al. 2009). It is estimated that MSW incinerators generate around 1.2 × 106 tons of fly ash every

year in Western Europe (Fuoco et al. 2005). Due to the high content of heavy metals and the finesse

of their particles, the resulting fly ash is classified as hazardous waste, which requires special

managements (Pesonen et al. 2016; Luna et al. 2011).

Fly ash can be treated in several ways, including cement or geopolymeric solidification,

chemical stabilization using additives, thermal treatment via melting, and hydrothermal techniques

(Wang et al. 2015; Tang et al. 2016). At present, the most common and effective method to handle

MSWI fly ash is stabilization/solidification treatment using cementitious materials, of which

Portland cement has been successfully used to treat hazardous waste for a long time (Chen et al.

2009; Malviya and Chaudhary 2006; Su et al. 2016). Cement acts as an immobilization agent for

trace elements and it develops hardening reactions that increase geotechnical stability of material

mixture in a landfill (Zhang et al. 2016). However, the large quantity of cement required to

endeavour enough mechanical strength to the final monolith is the foremost disadvantage of this

technology, which makes the final mass double in volume (Quina et al. 2010). Meanwhile, traditional

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cement solidification technology presents many other deficiencies, such as weak acid resistance and

poor durability (Jin et al. 2016). In order to solve such problems, chemical stabilization has been

used to immobilize heavy metals without change of material structure (Quina et al. 2010; Bontempi

et al. 2010). This treatment using chelating agents owes to their complexation of heavy metals with

an organic sulphide group (Hong et al. 2000; Sukandar et al. 2009).

Even though the treatments described above can be very effective on fixing the various potential

toxic components of fly ash, another question is raised, namely what to do with the treated residue.

Due to the limited space and high cost of land disposal, ashes have emerged as construction or

geotechnical materials or have been proposed for such applications (Anastasiadou et al. 2012;

Cheeseman et al. 2003). According to Cai et al. (2004), pretreated fly ash could be applied in

cement-treated base layers for roads and pavements without significantly compromising compressive

strength or leaching of salts and heavy metals as investigated in tank leaching experiments. In

Germany, up to 50% of the ash generated from incinerated waste was used for the manufacture of

sound insulation in walls along highways and sublayers of city roads. In India, about 25% of fly ash

is applied to cement production, construction of roads and brick manufacture (Bhattacharjee and

Kandpal 2002). In Denmark, over 72% of the ash was reused for the construction of parking lots,

cycling tracks and other roads (Anastasiadou et al. 2012).

In cold regions, the weathering conditions, which are referred as freeze-thaw cycles, have a

significant effect on both the durability and performance of earthwork structures, such as

embankments and highway projects. Freeze-thaw is considered as one of the crucial causes for

significant damage to earthwork structures (Kamei et al. 2012). However, the strength characteristics

and heavy metal toxicity of fly ash solidified with cement and chelating agent under the

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freezing-thawing cycles have not been systematically studied.

According to the aforementioned research needs, MSWI fly ash is solidified with cement and

chelating agent in this study. The influence of cement content, chelating agent content, curing time

and alternative cycles of freeze-thaw is firstly evaluated in terms of the compressive strength, failure

strain, weight loss and secant modulus loss. The mechanism of freeze-thaw induced deterioration is

then comprehensively investigated. Subsequently, the finite element program is used to investigate

the benefits/disbenefits of application of solidified fly ash in roads. The final section summarizes the

major findings of this study.

Materials and method

Materials

The fly ash used in this study was collected from municipal solid waste (MSW) incineration

facility at the Changshu municipal solid waste landfill in Suzhou, China. The landfill has a total

storage capacity of 3.98 million m3. Approximately 1500 tons (t)/day of MSW was deposited at this

location. Among them, around 900 t/day MSW was incinerated for electricity generation. During the

incineration, the techniques for cleaning flue gas mainly include a semi-dry scrubber, activated

carbon injection, and fabric filter. In this procedure, hydrated lime acts as a neutralizing agent in the

flue gas cleaning system, and is added to the reaction tower to absorb acid gases. Then the fly ash

and the reactants are removed when the flue gas passes through the baghouse. In Changshu landfill

site, over 25 t/day fly ashes were produced during MSW incineration. As a solidified material, the

commercially available Portland cement (i.e., PG325) was prepared. In order to stabilize heavy

metals in the fly ash, chelating agent was used in which dithiocarbamate is the main active group for

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chelating heavy metals.

The maximum and minimum density of the fly ash were measured according to JGS 0162 and

JIS A 1224, respectively. The plastic and liquid limit were measured according to GB/T 50123-1999

by the apparatus (GYS-2, Nanjing Soil Instrument, China). The natural moisture content and water

retention capacity of the fly ash were measured following JIS A 1203 and JGS 0151, respectively

(Tang et al., 2014, 2015). In accordance with JIS A 1218, hydraulic conductivity was obtained using

a permeameter (TST-55, Jingkeyusheng, China). EC and pH of the samples were tested using a

pH/EC meter (PH-2603, Lohand, China) following JGS 0212 and JGS 0211, respectively. In addition,

grain size distribution of fly ash was determined according to GB/T50123-1999. Concerning the

N2-BET adsorption test, it was conducted to analyze specific surface area, average pore diameter and

micropore volume (Diameter<102.12 nm) of fly ash (NOVA2000e, Quantachrome, U.S.). The

elemental and mineral composition of fly ash and cement were analyzed by XRF (JSX-3400R, JEOL,

Japan) and XRD (RAD-2B, Rigaku Corporation, Japan) correspondingly.

Preparations of samples

Both fly ash and Portland cement were dried under 105°C for at least 24 hours by using oven

(101-A, Leao, China) to ensure the satisfied quality during the experiment, since there might be some

moisture content in them. Subsequently, they were cooled to room temperature in a desiccator. The

fly ash was then added with the stabilizing agent, which was diluted by adding water, with the

concentration of 3%, 6%, 9% (w/w) each. The mixture was stirred and chelated thoroughly, allowed

to settle for 24 h at room temperature, dried at 105°C and then pulverized. After that, the chelated fly

ash was homogenized with the cement at four different ratios of cement/fly ash (10%, 20%, 30% and

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40%) by using a blender for 2-3min. Water was added slowly into the dry mix to promote hydration

and the moisture contents maintained at 60%. After the mixing procedure, the mixture was

transferred into the moulds (70 mm× 70 mm× 70 mm, Jianyi, China), which were then sealed with a

polyethylene membrane to avoid severe dehydration. Afterwards, the mixtures were left undisturbed

for 24 h at room temperature (23 ± 2°C) and high relative humidity (>50%). Three replicates were

analyzed for each trial. After the period of the initial setting, the mixtures were demoulded and cured

(90 ± 2% humidity, 20 ± 2°C) for 7, 14, 28 days in a curing box (HBY-15B, Donghua, China).

In this study, the cyclic freeze-thaw test was conducted in the accelerated freeze-thaw testing

apparatus. In a freeze-thaw cycle, the temperature of samples cooled from 10°C and was lowered in

1 h with a constant cooling rate of 20°C/h. It was kept constant for 2 h at -10°C and increased in 1 h

with a constant heating rate of 20°C/h. It was kept constant for 2 h at 10°C. The samples were

subjected to the required number of freeze-thaw cycles (up to 5 cycles and 10 cycles) because

ultimate stresses of treated samples under lower confining pressure do not almost change in the

freeze-thaw cycle more than 3 (Kamei et al. 2012b; Wang et al. 2007). They were then tested to

evaluate how freeze-thaw cycles affect the durability of fly ash stabilised with cement. The 70 mm

cubes were used to examine the weight loss, secant modulus loss, compressive strength loss and

failure strain at every 5 freeze-thaw circulations. All experiments were performed on three specimen

replicates. The average values were used for the discussion of the test results.

The measurement of compressive strength property was conducted according to the GB/T

50081 (2002) method. The following Eq. (1) was used to calculate the compressive strength:

A

Ffcc = (1)

where fcc is the compressive strength (MPa), F is the maximum load (N) and A is the area of the

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cube loading face (mm2).

The measurement of failure strain was determined by Eq. (2):

L

Lf

∆=ε (2)

where fε is the failure strain (%), L∆ is the displacement at the maximum of stress (mm),

L is the height of the sample (mm).

The following Eq. (3) was used to calculate the weight loss:

%100×−

=∆o

non

W

WWW (3)

where ΔWn is the weight loss of specimens at every 5 freeze–thaw cycles (%),Wo is the average

weight of concrete specimens before freeze–thaw cycles (kg) and Wn is the average weight of

concrete specimens at every 5 freeze–thaw cycles in water (kg).

The secant modulus is a deformation parameter, which was determined by Eq. (4):

f

2/150

2ε

σ=E (4)

Where 50E is secant modulus, 2/1σ is compressive stress (MPa), when compression strain is

half of the failure strain and fε is failure strain (%).

According to the test procedure, the deterioration of the specimens was investigated by

determining the compressive strength loss, failure strain, weight loss and secant modulus loss.

Compressive strength test method

Whether samples under standard curing condition or after accelerated freeze-thaw cycles, both

of them were prepared for compression test. The unconfined compression tests were conducted as

per ASTM D 2166-91 using a microcomputer controlled electronic testing machine (LDS-50,

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Chenda, China) with a vertical displacement rate of 1 mm/min. The strength (MTS 10/GL) had a

normal loading capacity of 50 kN, following by Koohestani et al. (2016). Total maximum loads were

recorded at the point of fracture and the compressive strength was determined using the formula

P=F/A, where P is the compressive strength (MPa), F is the total maximum load (N), and A is the

area of loaded surface (mm2). Based on the accuracy of the experiment, all of these tests were

conducted in replicate and the average values were reported. All of samples were prepared in

duplicate.

Finite element modeling of pavements with/without solidified fly ash

The finite element models were developed to assess the structural capacity of pavements with

and without solidified fly ash. As illustrated in Figure 1a, the control pavement consisted of 15-cm

hot mix asphalt (HMA), 25-cm unbound base, and 1.5-m subgrade. For the pavement containing

solidified fly ash, a 5-cm solidified fly ash layer was placed between unbound base course and

subgrade. A 565 kPa traffic load was uniformly applied in the center of pavements. The

corresponding critical responses (i.e., stress/strain at critical locations) were computed to evaluate the

long-term performance (i.e., rutting and fatigue cracking). Thereby, the benefits/disbenefits of

solidified fly ash as pavement material were quantified. The finite element analysis also took into

account the influence of freeze-thaw cycles on the long-term pavement performance. Figure 1b

presents one example of meshed axisymmetric finite element model. Fine mesh was used in the

vicinity of the load. The HMA, base course and subgrade were represented as 8-node biquadratic

homogeneous solid elements with reduced integration. Table 1 shows the materials properties used in

the finite element models. The Young’s modulus of solidified fly ash was calculated based on an

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empirical equation, as shown in Equation 5.

3445 1000ccE f= + × (5)

where E is the Young’s modulus of solidified fly ash (unit: MPa); and ccf is the unconfined

compressive strength (unit: MPa).

Fig. 1

Table 1

Leaching test method for heavy metal

The crushed samples collected after the compressive strength test were oven dried at 105°C for

24 h. Then the dried samples were further crushed manually until the particle size was less than 9.5

mm. Aliquots of the 50 g of the crush samples were put into a polyethylene bottle and leached using

an extraction buffer of acetic acid and sodium hydroxide (pH 2.88 ± 0.05) at a liquid/solid radio of

20 : 1, and vibrated at 30 rpm and 25°C for 18 h. The extract was filtered into a 100 mL flask by a

0.45µm pore size syringe filter, and then acidified by nitric acid to pH < 2. Subsequently, the

concentrations of targeted potentially toxic metals in the resulting filtrate were measured using

Atomic Absorption Spectroscopy (AAS) (TAS-990, Persee General, China). All the samples were

prepared in duplicate.

Results and discussion

Characterization of the materials

The physical and chemical properties were listed in Table 2. The natural moisture content of the

fly ash is 2.5%. The maximum and minimum density are 0.78 and 0.51 g/cm3, respectively. The

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plastic limit and liquid limit of fly ash are 54.42% and 85.36%, respectively. The swell index is 4.00

mL/2g-solid, which represents the non-swelling characteristic. The fly ash disposal exhibits an

alkaline feature, which has a pH value of 12.59. The EC value of fly ash is 78.3 mS/cm, which

demonstrates that a large amount of soluble salt exists in fly ash and thus increases the difficulty in

immobilization. The hydraulic conductivity of fly ash is 8.92 × 10−7 m/s. The diverse hydraulic

conductivities can be attributed to the difference in the particle size. The water retention capacity is

128.7%. Particle size distribution curve of four different fly ashes is shown in Fig. 2. Compared with

the three other fly ashes (S1, S2, S3) used by Zhai et al. (2016), the particle size of the fly ash used in

this study is finer, which is due to the different composition of waste material.

Concerning the N2-BET adsorption tests, the correlation coefficient was 0.999, indicating that

the results obtained were reliable. The specific surface area of the fly ash used in this study is 8.008

m2/g. The average pore size is 84.81 Å, and the micropore volume is 1.812 × 10-2 mL/g. The

quantitative analysis of the fly ash samples, obtained by the X-ray fluorescence (XRF) spectrometer,

is shown in Table 3, together with the results for a 100% cement sample. Note that the major

elements of the fly ash were SiO2 (2.66%), Al2O3 (1.12%), CaO (45.81%), MgO (1.00%), SO3

(5.36%), Na2O (3.35%) and K2O (3.84%), while the major elements of the cement were SiO2

(25.40%), Al2O3 (8.89%), Fe2O3 (2.02%), CaO (53.80%), MgO (4.56%) and SO3 (3.68%),

respectively. Comparing the presented results with previous studies on the composition of fly ash, a

complete agreement was observed concerning the types of oxides that were present in the fly ash (He

et al. 2016; Anastasiadou et al. 2012; Sobiecka et al. 2014; Luna et al. 2011). It can be seen that the

sum of chemical compositions listed in Table 3 is not 100%. There may be two important reasons for

this phenomenon: on the one hand, X-ray fluorescence (XRF) can only be used to determine the

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element content, not to quantify the mineral composition. And the test results of XRF are expressed

in the form of oxides. Fly ash is mainly composed of metal oxide and nonmetal oxide. However, the

same or similar components can have different phase structure (Karlfeldt and Steenari 2007). Take

calcium for an example, in XRF, it appears in the form of CaO. In X-ray diffraction (XRD), as

shown in Fig. 3, it can be seen that the element calcium exists in a variety of forms, such as CaCO3,

CaClOH, Ca(OH)2, Ca(OH)2•4H2O and so on. So, the use of calcium oxide to represent the content

of calcium, the total number of elements will be reduced. Except for calcium, other elements and

their compounds have similar phenomena. On the other hand, the detection range of the majority of

modern laboratory XRF instruments is between sodium (Na) and uranium (U) (Shackley 2012).

Unfortunately, as shown in Fig. 3, the fly ash contains crystal water, which cannot be detected. The

highest content of CaO in fly ash was due to its lime pretreatment for acid gas scrubbing (Mu et al.

2017). In the acid gas removal reactor, there is a special device where the quick lime reacts with

water to produce calcium hydroxide and subsequently the calcium hydroxide is mixed with a large

amount of fly ash. The surface moisture of fly ash enhances the sorption of acid gases to promote the

neutralization reaction. Therefore, fly ash mixed with the neutralizer residue and the content of

calcium increased to 45.81% (Su et al. 2015). According to X-ray diffraction (XRD) analysis, the

investigated fly ash sample had a highly complex mineralogy. As shown in Fig. 3, the main

crystalline compounds were calcite (CaCO3), halite (NaCl) and sylvite (KCl). In addition, the

presence of crystal structures, such as calcium chlorite hydroxide (CaClOH), silicon chloride (SiCl4),

calcium chloride hydrate (Ca(ClO)2•4H2O), iron sulfide (FeS) and portlandite (Ca(OH)2) were also

verified. According to the diffraction pattern obtained by XRD, the main components of this sample

include calcium chloride hydroxide, halite, silver aluminum sulfide, silicon chloride, stannite and

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sylvite, which confirms the results of XRF. The amorphous phase was also abundant in fly ash (Alba

et al. 1997). Fly ash can be regarded as essentially consisting of three types of components:

crystalline minerals, unburnt carbon particles, and non-crystalline aluminosilicate glass (Ward and

French 2006). According to Kenworthy et al. (1980), amorphous phase was found to account for

about 40% of the total amount of fly ash by EPMA (Electron Probe Micro Analysis). Fig. 3, XRD

diffraction pattern, shows many small, corrugated peaks, which means fly ash contains large amounts

of amorphous glass material (Liang et al. 2014). Glass is amorphous or non-crystalline, and does not

have regular arrays of atoms that produce definitive peaks in XRD studies. Broad patterns or

“humps” may nevertheless be seen in X-ray diffractograms derived from different types of glassy

materials. Along with an overall reduction in the peak intensities derived from the crystalline

components, due to dilution and/or mass absorption effects, these humps provide an indication that

glass is present in fly ashes and other samples subjected to XRD analysis (Ward and French 2006).

The amorphous phase cannot be measured by XRD, however, its content and chemical composition

can be estimated from chemical and XRD data (Ward and French 2006; Ward et al. 2001). Expected

heavy metal rich mineral phases (such as PbCl2 or K2ZnCl2) have not been identified. In most cases,

the heavy metals in fly ash are enriched in the amorphous phase, and the distribution in the crystal

phase is very small. Different authors suggest separation methods (magnetic and density separation)

to concentrate over 2% on heavy metal crystalline phases (Eighmy et al. 1995; Kirby et al. 1993).

Table 2

Table 3

Fig. 2

Fig. 3

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Compressive strength test results

Fig. 4 (a) shows the compressive strength of cement-based solidification samples, pretreated

with chelating agent (3%, 6% and 9%), and under ordinary curing conditions for 7 to 28 days. It was

found that the strength decreased as the percentage of cement loading reduced. As shown in Fig. 4

(a), based on 3% chelating agent mixed, the compressive strength of cement-based matrices

(cement/ash = 10%, 20%, 30% and 40%) after 7 days of solidification are 0.38 MPa, 1.18 MPa, 1.64

MPa and 1.65 MPa respectively. The products of cement hydration are lapped and bound by the

multiple gravitation. This is the reason why cement blocks have a high compressive strength.

However, the residues in fly ash that are not completely burnt can influence the compressive strength

of the solidified matrix. Due to the presence of fly ash, the high levels of residues in fly ash not only

reduce the concentration of products from cement hydration, which have connection effect, but also

destroy the structure of products of hydration (Liu et al. 2008). Meanwhile, the residues also have a

certain influence on the delay of the setting time. The reason for this is that the residues are porous

and hydrophobic. So the cement clinker can easily be wrapped in the inner of the carbon hole of the

residues to form a protective film, which hinders the full contact of the water with the cement clinker,

so as to disturb the hydration reaction and delay the setting time of the cement.

As can be observed in Fig. 4 (a), the compressive strength of the solidified matrix increases with

curing time. Compared to 7 days of curing with 3% chelating agent, the compressive strength of

mixtures with 10%, 20%, 30% and 40% cement content for 28 days increases by 915%, 166%, 147%

and 207%, respectively. This is because the hydration reaction is more sufficient when the curing

time is longer. Two processes are involved during the period of cement hydration reaction. One is

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through-solution hydration, which involves several factors including dissolution of anhydrous

compounds into their ionic constituents, formation of hydrates in the solution, and eventual

precipitation of hydrates from the supersaturated solution. The other is solid-state hydration of

cement and the reactions occurred directly at the surface of the anhydrous cement compounds.

During this period, no compounds dissolve into solution. In the early stage of cement hydration,

through-solution hydration is dominant. At the post-stage of cement hydration, the hydration of

residual cement particles may occur by the solid-state reactions and the migration of ions in the

solution becomes restricted (Mehta and Monteiro 2014).

Chelating agent content has a certain effect on the strength of the solidified ash. As shown in

Fig. 4(a), in the early stage (7-14 days), when the cement content is less than 20%, the unconfined

compressive strength of the mixtures decreases with the increase of the chelating agent content;

when the cement content is higher than 20%, the strength of the mixtures has no significant change.

The compressive strength is relatively low in the early stage, which demonstrates that the presence of

heavy metals such as Zn2+ and Cu2+ has a negative effect on the hydration reaction of cement and

hinders the hydration reaction of cement. When the cement content is at a low level, the heavy metals

Al3+, Pb2+ are mainly affected by the stabilizing effect of the chelating agent. On one hand, the

chelating agent consumes cement and a large amount of SiO2, Al2O3, CaO in fly ash, which results in

the reduction of the materials that contribute to the hydration and pozzolanic reaction. Thus, the

macroscopic exhibition is that the hydration and pozzolanic reaction is inhibited. On the other hand,

the free heavy metal ions in fly ash decrease significantly by the chelation, which simultaneously

promotes the hydration of cement. Therefore, the macro performance shows a slight decrease in

strength. In the late stage, when the chelating agent content increases from 3% to 6%, the

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compressive strength of the mixtures significantly increases. While the compressive strength of the

mixtures decreases slightly when the content of chelating agent continued to increase to 9%.

It is well-documented that the compressive strength and failure strain of cement-based samples

can reflect directly the deterioration degree caused by freeze-thaw attacks. Figs. 4(b) and 4(c) show

the compressive strength of cement-based mixtures under different freeze-thaw cycles. Table

4 shows the variation of applied pressure versus the strain of cement-based samples subjected to 5

and 10 freeze-thaw cycles. It can be seen that the cement content has a significant influence on the

compressive strength and failure strain of mixtures, which are exposed to freeze-thaw cycles. With

the increase of cement content from 10% to 40%, the compressive strength of the specimens

significantly increases at 14 days. However, the increase of compressive strength is relatively flat at

7 days and 28 days. As shown in Table 4, under the condition of 5 freeze-thaw cycles, 3% chelating

agent and 28 curing days, the failure strains of the mixtures with 10%, 20%, 30% and 40% cement

content are 15.421%, 12.521%, 11.843% and 7.464%, respectively.

In the process of freezing and thawing water firstly enters the biggest pores of hardened

cement paste and subsequently reaches the smaller pores. According to Everett (1961), ice crystals in

the pores tend to grow continuously and penetrate deeper into smaller pores of the hardened cement

paste and thus fill capillary pores with water or freezing solution. When water or freezing solution

freezes, i.e. the volume increases by 9%, the hardened cement paste generates internal stresses, and

finally crumbles (Everett and Haynes 1965). According to Kalliopi (2006), the destructive effect

during freezing depends on the content of water in hardened cement paste. Based on Table 2, the

water retention capacity and specific surface area of the fly ash is 128.7% and 7.944 m2/g,

respectively, which indicates that the more fly ash is added, the more water is attracted. Thereby, the

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compressive strength of fly ash mixtures decreases accordingly.

As shown in Fig. 4 (b) (c), the unconfined strength of the mixtures decreases by increasing the

number of freeze-thaw cycles at 7 and 14 days. During freezing phase, ice crystals are formed and

then, during thawing phase, these crystals start to melt and free water appears in the sample. The free

water moves to lower parts of the sample due to gravity force. Consequently, upper parts of the

specimen lose their moisture and the adhesion between fly ash and hydrate particles becomes weaker.

Therefore, the cement-based samples in thawing phase cannot return to its initial situation before

freezing. This phenomenon also leads to the increase of volume of sample. The structures of samples

deteriorate when such freeze-thaw action is repeated (Ghazavi and Roustaie 2010). However, at the

curing time of 28 days, the compressive strength of the samples subjected to 10 cycles is greater than

the samples experienced 5 cycle attacks. According to Chang et al.(2014), the mechanical properties

of studied soil are affected considerably by the number of freezing-thawing cycles, and after 7-9

freeze-thaw cycles, the compressive strength reaches the minimum value. According to Su et al.

(2008), the physico-mechanical characters of the investigated soils become stable after 10

freezing-thawing cycles，such as water content，density，cohesion，and internal friction angle.

Generally, the cohesion of the samples increases with the repeated freezing-thawing cycles.

In general, the loss of weight and/or the decrease of secant modulus is used to evaluate the

degradation in freeze-thaw tests. As shown in Table 4, it can be observed that weight loss of

cement-based samples decreases as the cement/fly ash ratio increases. Under the condition of 5

freeze-thaw cycles and 28 curing days, the weight losses of different mixtures (cement/ash = 10%,

20%, 30% and 40%) are 1.36%, 1.04%, 0.08% and 0% with 3% chelating agent; 1.68%, 3.07%,

1.79% and 0.33% with 6% chelating agent; and 7.11%, 4.78%, 4.56% and 3.35% with 9% chelating

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agent. Meanwhile, it can be clearly seen that the chelating agent content affect the resistance of

mixture to the freeze-thawing negatively. The weight loss of samples indicates that the bonding of

aggregates and gelled materials are damaged, thereby causes aggregates to break away from the

specimen and results in mass reduction (Chen et al. 2014). According to Wu (1979), the pores can be

classified into four grades: harmless pores, less harmful pores, harmful pores and serious damage

pores. Harmless pores, which have a diameter of less than 20 nm, and with pore grade that is not

damaged in terms of mechanical strength, permeability, and durability of the specimens; less harmful

pores, with pore diameter greater than 20 nm and less than 50 nm, and with pore grade that has a

minor damage for the performance of the specimens; harmful pores, with pore diameter more than

50 nm and less than 200 nm, and with pore grade that has damage for the performance of the

specimens; and serious damage pores, with pore diameter greater than 200 nm, and with pore grade

that has serious damage for the performance of the specimens (Wu 1979; Wang and Niu 2016). The

porosity and pore grade of the specimens change with the prolonged period of positive and negative

temperature alternation. The volume percentage of the harmless pores in the specimens reduces, and

the porosity of the pores whose diameter is more than 20 nm increases. The volume percentage of

each pore grade is obviously transformed when the number of freezing and thawing cycles increases.

The content of the pores with diameter less than 20 nm is low. Nevertheless, the content of the three

other pore grades and their porosity significantly increases. Therefore, the microstructure of the

specimens is incompact. The relative dynamic elasticity modulus and the relative mechanical

properties are small, and the weight loss rate is great. (Wang and Niu 2016)

Fig. 5 shows the secant modulus of cubic samples under freeze-thaw cycles. As illustrated,

when the chelating agent is 3%, the secant modulus ranges of samples with different mix ratios are

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3.44 - 69.99 MPa (10%), 8.43 - 70.60 MPa (20%), 17.41 - 66.56 MPa (30%) and 7.57 - 120 MPa

(40%), respectively. In the case of the chelating agent with a mixing amount of 6%, the secant

modulus ranges are 9.74 - 54.39 MPa, 8.07 - 74.16 MPa, 17.55 - 101.24 MPa and 4.33 - 37.79 MPa,

respectively. With the chelating agent content is 9%, the secant modulus ranges are 4.97 - 31.21 MPa,

10.37 - 46.88 MPa, 10.07 - 55.11 MPa and 7.98 - 26.36 MPa, respectively. It can be observed that

curves of secant modulus rises at the initial alternative cement/ash ratios (10% - 30%). This

demonstrates that larger cement/ash ratio leads to a larger resistance to freeze-thaw attacks. Generally,

the cement-based samples with smaller weight loss in the freeze-thaw cycles have considerably

denser microstructure, and thus, exhibit larger value of relative secant modulus (He et al. 2016).

Meanwhile, these three figures present that there is a significant decrease in the secant modulus of

the samples when the cement content increases from 30% to 40%, which indicates that there is a

threshold value for cement content. When the cement content is higher than the threshold, the secant

modulus of samples will decrease. This may be due to two reasons. On one hand, there is plenty of

water needed during the process of hydration reaction and the excessive cement content can lead to

the lack of water, then, result in the incomplete hydration reaction. Using different length scales, four

composite media could be identified for cement pastes, including the C-S-H matrix, the hydration

products, the skeleton of cement paste and the cement pastes. And the physical properties of them are

different, so the secant modulus is not the same. On the other hand, there is a competition

phenomenon between the cement and the fly ash concerning the water absorption. When the cement

absorbs too much water, its bonding strength may not be as strong as the bonding strength of fly ash

in a certain moisture content. At a low water content level, the fly ash has the potential to form a

relatively dispersed particle, which cannot form a good-quality aggregate. Combined with the above

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reasons, the secant moduli of the samples do not increase with the increase of cement content, but

show a downward trend.

Fig. 4

Fig. 5

Table 4

Finite element analysis

The Mechanistic-Empirical Pavement Design Guide (MEPDG) developed a fatigue cracking

model for asphalt pavements, which is shown in Equation 6.

2 3

1

1 1k k

f

t

N CkEε

=

(6)

where fN is the fatigue life of asphalt pavements, C is the laboratory to field adjustment factor,

1k , 2k , and 3k are model coefficients, tε is the tensile strain at the bottom of HMA layer, and E

is the stiffness of HMA. It is seen that the tensile strain at the bottom of HMA is a critical factor

affecting the fatigue life of pavements (Gu et al. 2016b). Figure 6 shows the comparison of tensile

strains at the bottom of HMA among the pavements with and without solidified fly ash. It is seen that

placing a solidified fly ash layer between unbound base course and subgrade reduces the tensile

strain at the bottom of HMA by 10% approximately. This indicates that the application of solidified

fly ash to asphalt pavement is effective in prolonging the fatigue life of pavements. Figure 1 also

shows that the number of freeze-thaw cycles has negligible influence on the tensile strain at the

bottom of asphalt pavements. This demonstrates that the solidified fly ash layer can prevent the

freeze-thaw induced cracking damage to asphalt pavements.

The MEPDG also pointed out that the rutting damage of subgrade significantly contributes to

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the total rutting of asphalt pavements. The rutting damage of subgrade is mainly affected by the

compressive strain distribution in subgrade (Gu et al. 2015 and 2016a). Figure 7 compares the

compressive strain distributions in subgrade among the pavements with and without solidified fly ash.

It is shown that the pavements containing solidified fly ash have much less compressive strains in

subgrade than the control one (i.e., pavement without fly ash). This indicates that the solidified fly

ash layer is capable of reducing the rutting damage of subgrade, and thereby reducing the total

rutting distress of asphalt pavements. In addition, the compressive strain distribution of pavements

with solidified fly ash is not sensitive to the number of freeze-thaw cycles. This demonstrates that the

solidified fly ash layer also diminishes the freeze-thawing-induced rutting damage of asphalt

pavements. Therefore, it is concluded that placing the solidified fly ash layer between unbound base

course and subgrade is beneficial to prolong the fatigue life and reduce the rutting distress of asphalt

pavements.

Fig. 6

Fig. 7

Leaching test results

Suitability of application of cement-chelated solidified fly ash in regards to environmental

issues can be evaluated through leachate analysis. In view of the negative effects of leaching in most

applications, the allowable limit of leaching should be determined. Accordingly, the toxicity

characteristic leaching procedure (TCLP) from US Environmental Protection Agency (EPA) was

conducted to investigate the leaching behavior of heavy metals and the corresponding criteria.

According to Reijnders (2005), the leaching tests that are commonly used may be a poor indicator of

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what happens in real life, and they may either overestimate or underestimate the actual leaching

amount. As for the allowable limit of leaching, Inyang et al. (2003) had brought forward a method to

calculate the maximum leachability that is acceptable for waste-amended construction materials on

the basis of a risk criterion. As expected, the quality of the maximum allowable concentration of

contaminant in the concrete increases as the concentration of the target contaminant specified in

drinking water standard agency increases. The TCLP leachates of the untreated fly ash contain high

concentration of Cd (6.4 mg/L) and Pb (15.5 mg/L), and certain amount of Cr, Cu, Zn and Ni. Fig. 8

presents the TCLP leaching test results of solidified fly ash samples after 7, 14 and 28 days of curing.

In Table 5, regulatory thresholds for filling materials in US and European countries are observed

(Saikia et al. 2008; Haugsten and Gustavson 2000). With the addition of 9% chelating agent and 30%

cement, the increase of the curing time to 28 day reduces the leaching concentration of Cr, Cu, Pb,

Zn, Ni and Cd to 0.05 mg/L, 0.09 mg/L, 0.1 mg/L, 0.21 mg/L, 0.1 mg/L and 0.02 mg/L, respectively.

Apparently, the concentration of selected elements in the leachates of cement-chelated solidified

MSWI fly ash meets the requirement from German, French, Austrian, Switzerland, Belgium and US.

In China, for the safe use of heavy metal-amended materials, relevant environmental quality

assessment standards were established, such as "Environmental quality standard for surface water"

(GB 3838-2002) and "Soil environmental quality standard" (GB 15618-1995). Meanwhile, based on

the characteristics of highway engineering, the "Specifications for environmental impact assessment

of highways" (JTG B03.2006) was developed. The surface water evaluation in highway engineering

of the latter standard is the same as the former two. Taking into account the heavy metals in the

solidified fly ash will eventually enter the soil and water, so "Soil environmental quality standard"

and "Environmental quality standard for surface water" are adopted to evaluate the acceptable

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leaching of the treated fly ash as a roadbed material. Limit values of soil environmental quality and

surface water environmental quality are presented in Tables 6 and 7, respectively. With the addition

of 9% chelating agent and 40% cement, the increase of the curing time to 28 day reduces the

leaching concentration of Cr, Cu, Pb, Zn, Ni and Cd to 0.03 mg/L, 0.07 mg/L, 0.03 mg/L, 0.09 mg/L,

0.1 mg/L and 0.02 mg/L, respectively. It is obvious that the leaching concentration of heavy metals is

much smaller than the limit value of soil environmental, and all conform to the soil environment

quality standard. Meanwhile, the analysis results indicated that the concentrations of other metals

except Cd can achieve the water standard level Ⅴ, which is suitable for the agricultural water and

general landscape water. After the modification of fly ash, the influence of leaching concentration of

heavy metal on soil is almost negligible, which may have little negative effect on surface water.

Assessment of pollutant release cannot only be conducted by standard short-term leaching tests

but also long-term release studies (Ferreira et al. 2003; Reijnders 2005). Barna et al. (2000) proposed

a methodology for the modelling of long-term release from a water storage reservoir for fire control,

the bottom of which was constructed with MSW fly ash mixed with hydraulic binders and water.

Although the scenario cannot be considered as a common application, the developed methodology

can be used in other applications. Triano and Frantz (1992) evaluated the leaching behavior of

concrete containing fly ash. They reported limited amounts of heavy metals in the leachate, below

the toxicity limits according to the EPTOX toxicity procedure. According to Ferreira et al. (2003),

application as lightweight aggregates is not expected to pose environmental problems, since heavy

metal leaching is not significant when concrete is used under sheltered conditions. According to

Chen (2012), the degree of compaction also influences the leaching behavior of heavy metals. With

the increase of compactness, the leaching concentration of heavy metals decreases, which shows the

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importance of ensuring a proper degree of compaction during construction.

As shown in Fig. 8, after 7-day curing, the leaching amounts of heavy metals decrease by

99.34% (Pb), 99.0% (Cd), and 86.41% (Ni) when compared to the untreated fly ash. The leaching

amounts of Cr, Cu and Zn reduce by the ranges of 86.36% - 91.71%, 50.95% - 64.94% and 53.82% -

92.73%, respectively. After 28 days of curing, the leaching amounts of heavy metals reduce by

99.37% (Pb), 99.07% (Cd), and 86.43% (Ni), respectively. The leaching amounts of Cr, Cu and Zu

decrease by the ranges of 86.49% - 93.54%, 51.35% - 68.42% and 64.65% - 95.43%, respectively.

Compared against the reduction of leaching amount of cement-based solidification utilized in Tang et

al. (2016), the reduction of leaching amount in this study is much higher. This indicates that the

solidification/stabilization of fly ash by cement and chelating agent has better fixation capacity.

According to Pesonen et al. (2016), cement-based solidifications were stable and the heavy metals

were immobilized well in the sample mortars during the long-term diffusion test, at least by the

physical encapsulation mechanism. In stabilization, the contaminants of a waste material are

converted into less soluble, mobile, or toxic forms through chemical processes, but the physical

characteristics of the waste are not changed by stabilization. In solidification, the waste material is

encapsulated in a solid monolithic block. The contaminants do not necessarily interact chemically

with the solidifying reagent. Instead, they are embedded in the solidified matrix by physical

processes. Solidification does not necessarily involve a chemical interaction between the wastes and

the solidifying reagents, but may mechanically bind the waste into the monolith. Contaminant

migration is restricted by vastly decreasing the surface area exposed to leaching and/or by isolating

the wastes within an impervious capsule (Conner and Hoeffner 1998). The distinction between

chemical and physical immobilization is frequently not clear, and both mechanisms operate

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simultaneously. Briefly, chemical immobilization occurs at an atomic scale, whereas physical

immobilization occurs at a micron scale (Glasser 1997).

As shown in Fig. 8, with the increase of cement/ash ratio, the leaching concentration of heavy

metals decreases. When the curing time reaches 7 d and the addition of chelating agent is 9%, the

leaching concentration ranges of Cd, Pb, Zn and Cu are 0.06 - 0.04 mg/L, 0.12 - 0.09 mg/L, 0.6 - 0.4

mg/L and 0.08 - 0.07 mg/L respectively, which significantly reduces with the cement addition

increasing from 10% to 40%. Such a phenomenon is attributed to the following cement curing

mechanisms. (1) Physical Cementation and Adsorption. Calcium silicate hydrate (C-S-H), the

primary cementing and hardening chemical formed by hydration, is generated at the surface, which

acts as a barrier between the immobilized solid wastes and the surrounding liquids. The C-S-H

controls the potential adsorption of ions in the liquids onto the surface of the solid wastes as well as

the immobilization of heavy metals in the mixture. It has large surface area (10 - 50 m2/g), which

attribute to the massive adsorption with heavy metal ions. Thus, the leaching of pollutants accords

with environmental suitability (Sobiecka et al. 2014). (2) Double Decomposition Precipitation

Reaction. Based on the high alkaline environment, due to the cement hydration reaction, heavy metal

ions (such as Cd2+, Pb2+, Cr3+) were prevented from leaching in the form of low solubility of

hydroxide precipitation. (4) Isomorphous Substitution. Ettringite (Aft), another cement hydration

product, also makes a great contribution to the prevention of heavy metal leaching. Heavy metal

cations can be firmly bounded in the cement structure by substituting the cations in the ettringite

crystal structure. For example, Zn2+, Cd2+, Pb2+, Ni2+ and Al3+ will substitute Ca2+ in the ettringite

crystal structure, and Al3+ in ettringite crystal structure can be replaced by Cr3+, Mn3+, Ti3+, Ti4+, Si4+.

The distinction between chemical and physical immobilization is frequently not clear, and

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aforementioned curing mechanisms operate simultaneously. According to Yousuf et al. (1992), zinc

retards the hydration and setting of cement by the surface precipitation of an amorphous layer of zinc

hydroxide on the anhydrous clinker grains, and that calcium hydroxide reacts with the zinc

hydroxide to form CaZn2(OH)6•2H2O. According to Lin et al. (1994), the major cement component,

dicalcium silicate (C2S) was used as a model solidification agent to solidify/stabilize the copper

oxide. And the mechanisms in which CuO is confined in the C2S paste system can be explained as

physical encapsulation and chemical stabilization. Furthermore, the formation of a solid solution of

Cu-Ca-Si compounds is also a major solidification/stabilization mechanism (Lin et al. 1994).

Obviously, reduction of heavy metals leaching potential is observed as a function of the

stabilization time. As shown in Fig. 8, based on 9% chelating agent and 40% cement mixed, after 7,

14 and 28 days of solidification of fly ash, the leaching values of Pb are 0.09 mg/L, 0.07 mg/L and

0.03 mg/L respectively, the leaching concentrations of Cd are 0.05 mg/L, 0.04 mg/L and 0.03 mg/L

respectively, and the leaching values of Cr are 0.08 mg/L, 0.05 mg/L and 0.03 mg/L respectively. The

leaching concentration gradually decreases with the increasing curing time. The sustained hydration

reaction of cement provides a high alkaline environment, in which the low solubility of hydroxide

precipitation (Pb(OH)2, Cd(OH)2, Cr(OH)3) can be formed. Meanwhile, this phenomenon shows the

influence of pH value on the stability of solidified heavy metals. The increase of pH value results in

a higher immobilization of the heavy metals. According to Thevenin and Pera (1999), lead fixation is

due to a double phenomenon: first the precipitation of a metallic hydroxide and then encapsulation of

this compound in the C-S-H phase. It is confirmed that Pb immobilization in calcium silicate hydrate

(C-S-H) occurs as soon as C3S begins to hydrolyze. The “Pb-C-S-H” is referred as a C-S-H of

regular stochiometry where lead replaces calcium. According to Dimitris and Meng (2003), in the

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presence of chromate ions and during ettringite formation, chromate will substitute sulfate in the

ettringite crystal structure to form what is known as Cr-substituted ettringite.

In addition, chemical solidifying agents also shows a superior solidification performance for

heavy metals. As shown in Fig. 8, the leaching concentration ranges of Pb with the addition of 3%,

6% and 9% chelating agent are 0.10 - 0.14 mg/L, 0.09 - 0.11 mg/L and 0.06 - 0.10 mg/L respectively,

which indicates that the chelating agent used in this study exhibit good stability. The chelating agent

contains two dithiocarboxy chelating groups, and thereby has an extraordinary binding capacity and

stability. The chelating equation of this chemical solidifying agent with the heavy metals is illustrated

in Fig. 9. It is illustrated that the coordination complex for this chelating agent is a straight-line

structure for its two thiolate groups, known as coordination polymerization precipitation in

Coordination Chemistry (Wang et al. 2015).

Fig. 8

Fig. 9

Table 5

Table 6

Table 7

Conclusions

This study demonstrated the influence of cement content, chelating agent content, curing time

and the number of freeze-thaw cycles on the strength characteristics and heavy metal toxicity of

solidified fly ash. The major findings were summarized as follows.

(1) According to the compressive strength test results, the compressive strength of the mixtures

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grew with the increase of cement/fly ash ratio and curing time. In the early stage (7 - 14 days), based

on less than 20% chelating agent, chelating agent content affects compressive strength negatively.

With the increase of freeze-thaw cycles, the compressive strength of cement-stabilized fly ash

decreased from 7 to 14 days, and then increased from 14 to 28 days.

(2) The loss of weight and/or the decrease of secant modulus were measured to evaluate the

degradation in freeze-thaw tests. According to results, with the increase of cement content, the

weight loss of the samples decreased and the secant modulus first decreased (cement/ash: 10% - 30%)

and then increased (cement/ash: 30% - 40%). On the contrary, chelating agent content is proportional

to the weight loss and affects the resistance of mixture to the freeze-thawing negatively.

(3) Placing the solidified fly ash layer between unbound base course and subgrade was

beneficial to prolong the fatigue life and reduce the rutting distress of asphalt pavements. The

number of freeze-thaw cycles has negligible influence on the performance (e.g., fatigue cracking and

rutting) of pavements containing the solidified fly ash layer.

(4) Compared with cement-based solidification, the cement-chelated solidification is of more

environmental suitability. For the leaching test results, it was shown that the leaching concentration

of heavy metals decreased as the cement/ash ratio, curing time and chelating agent content increased.

Competing interests

The authors declare that they have no competing interests.

Acknowledgments

The research presented herein is supported by the National Nature Science Foundation of China

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(50879023, 41630633), China Postdoctoral Science Foundation funded project (2016M591756),

Jiangsu Planned Projects for Postdoctoral Research Funds (1601175C), Natural Science Foundation

of Jiangsu Province (BK20140979), and project from Jiangsu Provincial Department of Housing and

Urban-Rural Development (2016ZD18). The research is also supported by Jiangsu Provincial

Transport Bureau (2016T05) and Bureau of Housing and Urban-Rural Development of Suzhou.

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Figure Captions

Fig. 1 Finite element modeling of pavements with/without solidified fly ash: (a) Illustration of

Pavements with and without Solidified Fly Ash; (b) Example of Meshed Finite Element Model of

Pavement

Fig. 2. Grain size distribution curve for fly ash.

Fig. 3. XRD analysis of fly ash.

Fig. 4 compressive strength of cement-based samples: (a) Standard curing condition; (b) 5

freeze-thaw cycles; (c) 10 freeze-thaw cycles.

Fig. 5 Secant modulus loss of cement-based samples: (a) 3% chelating agent; (b) 6% chelating agent;

(c) 9% chelating agent.

Fig. 6 Comparison of tensile strains at the bottom of HMA

Fig. 7 Comparison of compressive strain distributions in subgrade

Fig. 8 Heavy metals leachability: (a) 7-day curing; (b) 14-day curing; (c) 28-day curing.

Fig. 9 The bonding sites of chelating agent for heavy metal S/S

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Table 1 Material properties inputted in finite element models

Material Type Young’s Modulus (MPa) Poisson’s Ratio

HMA 2100 0.35

Unbound Base 200 0.4

Subgrade 70 0.4

Fly Asha 7335 0.4

Fly Ash – 5 Freeze-Thaw Cycles 8255 0.4

Fly Ash – 10 Freeze-Thaw Cycles 8415 0.4

a Fly Ash with 10% cement and 3% chelating agent that has 28 curing days

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Table 2 Properties of fly ash

Property Units Standard Values

Maximum density g/cm3 JGS 0162 0.78

Minimum density g/cm3 JIS A 1224 0.51

Plastic limit % GB/T50123-1999 54.42

Liquid limit % GB/T50123-1999 85.36

Swell index mL/2g-solid ASTM D 5890-06 4.00

Moisture content % JIS A 1203 2.5

Hydraulic conductivity m/s JIS A 1218 8.92×10-7

pH - JGS 0211 12.9

EC mS/cm JGS 0212 78.3

Water retention capacity % JGS 0151 128.7

Grain size distribution GB/T 50123-1999

Clay fraction (≤ 0.005 mm) % 12.59

Silt fraction (0.075-0.005 mm) % 9.01

Sand fraction (2-0.075 mm) % 78.40

Uniformity coefficient - 4.99

Coefficient of curvature - 0.73

N2-BET

Correlation coefficient (R2) - 0.999

Specific surface area m2/g 8.008

Average pore size Å 84.81

Micropore volume

(Diameter< 102.12 nm) mL/g 1.812×10-2

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Table 3 Chemical compositions of fly ash and cement

Compound Unit Fly ash Porland cement

SiO2 % 2.66 25.40

Al2O3 % 1.12 8.89

Fe2O3 % 0.74 2.02

CaO % 45.81 53.80

MgO % 1.00 4.56

SO3 % 5.36 3.68

Na2O % 3.35 0.24

K2O % 3.84

TiO2 % 0.11 0.31

ZnO % 0.56

PbO % 0.19

CdO % 0.01

MnO % 0.03

CuO % 0.06

Cr2O3 % 0.01

Cl

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Table 4 Failure strain and weight loss of cement-based samples under freeze-thaw cycles

Chelating

agent

(%)

Cycles

Cement

content

(%)

Curing time (days)

7 14 28

εf (%) Mass

Loss (%) εf (%)

Mass

Loss (%) εf (%)

Mass

Loss (%)

3

5

10 13.20 1.51 10.29 0 15.42 1.36

20 13.14 2.6 12.51 0.68 12.52 1.04

30 6.06 0.6 10.82 -0.51 11.84 0.08

40 7.96 0.16 6.35 0.82 7.46 0

10

10 11.00 -5.92 12.09 -0.18 11.51 2.88

20 15.25 -0.34 12.85 -0.17 12.32 0.77

30 7.39 0 13.46 -0.51 14.49 0.5

40 9.73 -0.17 9.50 0.66 10.58 1.33

6

5

10 13.17 1.13 9.50 0.93 13.30 1.68

20 16.39 1.2 10.13 -0.35 12.66 3.07

30 11.83 0.93 5.78 0.34 12.62 1.79

40 9.93 0.9 11.30 0.5 10.86 0.33

10

10 11.22 -0.17 10.13 1.28 11.57 0.69

20 12.13 -0.35 11.89 0.52 15.44 2.27

30 10.09 -0.17 9.98 -0.84 11.24 0

40 10.91 0.33 9.58 -0.99 10.87 -0.08

9

5

10 12.57 1.84 14.29 5 18.63 7.11

20 14.12 0.68 12.00 1.38 8.67 4.78

30 11.92 0.99 12.15 0.93 10.36 4.56

40 9.56 0.08 7.03 1.57 10.67 3.35

10

10 9.96 2.46 12.41 1.66 10.65 9.54

20 11.21 1.02 13.98 0.86 11.45 6.04

30 10.21 0.67 11.86 1.01 9.97 2.74

40 7.23 1.47 11.40 0.65 11.95 2.01

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Table 5 Regulatory thresholds for filling materials in US and European countries

Country Germany Austrian France Switzerland Belgium US

Class Class Ⅰ

Inert

Class A

Inert

Industrial

waste Inert

Construction

material

EPA

hazardous

Unit mg/kg mg/kg mg/kg mg/kg mg/kg mg/kg

Pb 2 1 100 1.3 100

Cd 0.5 0.05 50 0.03 20

Cr 0.05 0.02 10 0.5

Cu 10 2 - 2 0.5

Ni 2 1 100 2 0.75

Zn 5 10 500 10 2.8

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Table 6 Standard values of soil environmental quality (mg/L)

Heavy metals GradeⅠa GradeⅡb Grade Ⅲc

Natural pH < 6.5 pH 6.5-7.5 pH > 7.5 pH > 6.5

Cr (paddy field) 90 250 300 350 400

Cr (dry land) 90 150 200 250 300

Cu (farmland) 35 50 100 100 400

Cu (orchard) - 150 200 200 400

Pb 35 250 300 350 400

Zn 100 200 250 300 500

Ni 40 40 50 60 200

Cd 0.2 0.3 0.3 0.6 1.0

a Limit values for protecting natural ecology and maintaining the soil quality in the natural

setting.

b Limit values for protecting agricultural productions and human health.

c Limit values for protecting the normal growth of plants.

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Table 7 Standard values of surface water environmental quality (mg/L)

Heavy metals Class Ⅰa Class Ⅱb Class Ⅲc Class Ⅳd Class Ⅴe

Cr ≤ 0.01 ≤ 0.05 ≤ 0.05 ≤ 0.05 ≤ 0.1

Cu ≤ 0.01 ≤ 1.0 ≤ 1.0 ≤ 1.0 ≤ 1.0

Pb ≤ 0.01 ≤ 0.01 ≤ 0.005 ≤ 0.005 ≤ 0.1

Zn ≤ 0.05 ≤ 1.0 ≤ 1.0 ≤ 2.0 ≤ 2.0

Cd ≤ 0.001 ≤ 0.005 ≤ 0.005 ≤ 0.005 ≤ 0.01

Ni

a Limit values for the source water and the national nature reserve.

b Limit values for the first-grade surface sources protection zones for domestic and drinking

water.

c Limit values for the secondary -grade surface sources protection zones for domestic and

drinking water.

d Limit values for the common process water and the human body non-direct contact

entertainment water .

e Limit values for agricultural water and general landscape water.

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(a)

Control Pavement

Pavement with Solidified Fly Ash

(b)

Fig. 1 Finite element modeling of pavements with/without solidified fly ash: (a) Illustration of

Pavements with and without Solidified Fly Ash; (b) Example of Meshed Finite Element Model of

Pavement

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Fig. 2. Grain size distribution curve for fly ash.

0 20 40 60 80 100

C1

C2C3

S4

C1

S4C3

B

S4

S4 BB

C1C1

C2: Calcite

S2: Silicon Chloride

S4

H:Halite

S4: Sylvite

Intensity (CPS)

2Theta (deg.)

C1

C1: Calcium Chloride Hydroxide

S2

H

B

B:Beryllium Palladium

C2

I

I:Iron Sulfide

S3

S3:StanniteA

A:Amesite-2H1

S1S1:Silver Aluminum Sulfide

C3:Calcium Chlorite Hydrate

L

L:Lead Bismuth Arsenate

P

P:Portlandite

Fig. 3. XRD analysis of fly ash.

1 0.1 0.010

20

40

60

80

100

Cumulative percentage (%)

Particle size (mm)

Fly ash in this study

S1

S2

S3

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(a)

(b)

0

2

4

6

8

1028 days14 days7 days

9633 6396

Cement content: 10%

Cement content: 20%

Cement content: 30%

Cement content: 40%

Compressive strength (MPa)

Chelating agent content (%)

Freeze-thaw cycles: 5

9

(c)

0

2

4

6

8

10

96396396

28 days14 days7 days

Freeze-thaw cycles: 10 Cement content: 10%

Cement content: 20%

Cement content: 30%

Cement content: 40%



3

Fig. 4 compressive strength of cement-based samples: (a) Standard curing condition; (b) 5

freeze-thaw cycles; (c) 10 freeze-thaw cycles.

0

2

4

6

8

10

969 3 3 6 96

28 days14 days



Cement content: 10%

Cement content: 20%

Cement content: 30%

Cement content: 40%

Standard curing condition

7 days

3

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(a)

(b)

(c)

Fig. 5 Secant modulus loss of cement-based samples: (a) 3% chelating agent; (b) 6% chelating

agent; (c) 9% chelating agent.

10 20 30 400

25

50

75

100

125Chelating agent: 3%

Secant modulus (MPa)

Cement content (%)

7 days, 5 cycles

14 days, 5 cycles

28 days, 5 cycles

7 days, 10 cycles

14 days, 10 cycles

28 days, 10 cycles

10 20 30 400

25

50

75

100



Cement content (%)

7 days, 5 cycles

14 days, 5 cycles

28 days, 5 cycles

7 days, 10 cycles

14 days, 10 cycles

28 days, 10 cycles

10 20 30 400

25

50

75

100



Cement content (%)

7 days, 5 cycles

14 days, 5 cycles

28 days, 5 cycles

7 days, 10 cycles

14 days, 10 cycles

28 days, 10 cycles

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220

240

260

280

300

5 cycles 10 cycles0 cycle

Tensile strain (µε)

Without fly ash

Fly ash under freeze-thaw cycles

Control

Fig. 6 Comparison of tensile strains at the bottom of HMA

Fig. 7 Comparison of compressive strain distributions in subgrade

500450400350300200

Subgrade Depth (cm)

Without fly ash

Fly ash - 0 cycle

Fly ash - 5 cycles

Fly ash - 10 cycles

250

Compressive Strain Distribution (µε)

0

5

10

15

20

25

30

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(a)

(b)

(c)

Fig. 8 Heavy metals leachability: (a) 7-day curing; (b) 14-day curing; (c) 28-day curing.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

96396396396396396

CdNiZnPbCu

Leachability (mg/L)


Cement content: 10%

Cement content: 20%

Cement content: 30%

Cement content: 40%

7 daysCr

3

0.0

0.2

0.4

0.6

0.8

1.0

1.2CdNiZnPbCuCr

96396396396396396

Cement content: 10%

Cement content: 20%

Cement content: 30%

Cement content: 40%

Leachability (mg/L)


14 days

3

0.0

0.2

0.4

0.6

0.8

1.0

1.2

3 9639639639639639

CdNiZnPbCuCr

Cement content: 10%

Cement content: 20%

Cement content: 30%

Cement content: 40%

Leachability (mg/L)


28 days

6

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Fig. 9 The bonding sites of chelating agent for heavy metal S/S

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