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Effects of circulating fluidized bed combustion (CFBC) fly ashes as filler on the performances of asphalt.

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ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
Published online 25 March 2009 in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/apj.267
Research Article
Effects of circulating fluidized bed combustion (CFBC) fly
ashes as filler on the performances of asphalt
Qin Li, Hui Xu, Xiaoru Fu, Chen Chen and Jianping Zhai*
State Key Laboratory of Pollution Control and Resource Reuse, and School of the Environment, Nanjing University, Nanjing 210093, P. R. China
Received 23 August 2008; Revised 15 December 2008; Accepted 15 December 2008
ABSTRACT: This work investigated the potential of utilizing circulating fluidized bed combustion (CFBC) fly ashes
(CFAs) as alternative filler, substituting mineral powders (MPs) that are widely used in asphalt concrete. Physicochemical characteristics of the CFAs and MPs, as well as effects of different mix designs of CFAs and asphalt on
asphalt performances were examined, including moisture susceptibility, viscosity, ductility, softening point, penetration,
and antiaging performances. The results of the study show that generally the CFAs have greater effects than the MPs
on improving the performances of asphalt, and that the specific surface area (SSA), free CaO (f-CaO), morphology,
and mineralogical phases of the CFAs are more favorable than those of the MPs respectively, while the alkaline
values, hydrophilic coefficients, particle size distributions (PSDs), and water contents of the two fillers are similar. It
is suggested that CFAs may be more suitable than MPs for the use as asphalt concrete filler.  2009 Curtin University
of Technology and John Wiley & Sons, Ltd.
KEYWORDS: CFBC fly ashes; filler; asphalt; mineral powders
INTRODUCTION
Asphalt is a widely available byproduct of the refining
process of crude oil in petroleum refineries. Among
a variety of applications of asphalt, a major one is
utilizing it as a binder for aggregates in public highway
and runway constructions,[1] where asphalt is consumed
at a very remarkable rate. Thus asphalt is the largest
volume of adhesive used in any applications.[2] With
remarkable advantages such as adequate mechanical
strength, appropriate elastic and plastic deformability,
high vibration damping, and low maintenance cost,
asphalt concrete has been prevailing in roadway surface
structures for decades of years.
However, a mass of filler, usually mineral powders
(MPs), must be used in the mixing process to improve
the compactness, strength, and durability of asphalt
concrete, as well as to enhance the adhesion between
asphalt and stone aggregates. Increasing demands for
the filler result in regular supply shortages and rising
prices of MPs, which are the most commonly used
filler for asphalt concrete in China. The production
of MPs is energy-consuming and usually causes a lot
of dust emissions. Moreover, the reserves of mineral
in nature are ultimately limited, so mass mining of
*Correspondence to: Jianping Zhai, School of the Environment,
Nanjing University, Nanjing 210093, P. R. China.
E-mail: jpzhai@nju.edu.cn
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
mineral conflicts with the social conception of sustainable development. Therefore, seeking for low-cost and
environment-friendly alternatives is worthwhile.
As one of the main promising solutions for a
clean, reliable, and economic combustion technology
for power generation, circulating fluidized bed combustion (CFBC) has exhibited quite a few advantages, such
as high combustion efficiency, low combustion temperature (ca. 850 ◦ C), large reduction in SO2 and NOX
emissions, wide fuel flexibility, and significant desulfurization rate.[3 – 5] Consequently, CFBC has gained wide
acceptance and become the most common fluidized
combustion design today.[6,7] However, the resultant
CFBC fly ashes (CFAs) distinctly differ from the conventional pulverized coal combustion fly ashes (PFAs)
in physical and chemical characteristics.[8,9] While the
majority of PFAs could be utilized in construction materials such as cement and concrete,[9,10] the disposal of
the increasing amount of CFAs still poses challenges
such as highly exothermic behavior on wetting, highpH leachate, and excessive expansion in landfill, etc.[11]
Generally, CFAs do not conform to either North American standards nor European ones as components or
additives to concretes.[12] In spite of the cementitious
properties of CFAs,[13] the use of CFAs in concretes
may result in structural damages and strength decreases
due to the available free lime contents, low SiO2 and
Al2 O3 contents, large specific surface area (SSA), high
water requirement, and harmful pores of CFAs.[12,14,15]
Asia-Pacific Journal of Chemical Engineering
EFFECTS OF CFBC FLY ASHES AS FILLER ON ASPHALT
Particularly, for the CFAs derived from cofiring of
coal and petroleum coke (PC) (i.e. the CFAs investigated in this study), the coarseness, unburnt carbon and
high loss on ignition (LOI) make the ashes especially
unsuitable for cement additives or mineral admixtures
in concretes.[16]
Although fly ashes have been intensively studied for
their present and potential applications, very limited
works referred to the use of fly ashes as asphalt concrete
filler,[17 – 20] and most of them focused on traditional
PFAs. Few reports have been found on utilizing CFAs
as filler for asphalt concrete.
The objective of this work is, therefore, to investigate
the potential of making use of CFAs as alternative filler
for asphalt concrete. Effects of CFAs and MPs on the
performances of asphalt, such as moisture susceptibility, viscosity, ductility, softening point, penetration, and
antiaging performances, were studied. Physico-chemical
characteristics of the CFAs and MPs were also examined, including alkaline value, particle size distribution
(PSD), hydrophilic coefficient, specific gravity, SSA,
morphology, as well as mineralogical phase.
MATERIALS AND METHODS
Materials
The asphalt (AH-70 heavy traffic petroleum asphalt)
employed in this study was provided by China
Petroleum and Chemical Corporation (SINOPEC) under
the brand of Donghai.
The CFAs, originated from cofiring of coal (70%,
cal.) and high-sulfur PC (30%, cal.), were collected
from the first of four rows of electrostatic precipitator
(ESP) ash-hoppers of No. 5 CFBC boiler unit (Alhstrom, Finland) at Jingling Petrochemical Power Plant,
SINOPEC (Nanjing, China).
The MPs were obtained from Asphalt Concrete Plant,
Nanjing Tongli Road and Bridge Engineering Co., Ltd
(Nanjing, China).
Sample preparation
The samples of the CFAs and MPs were prepared at
State Key Laboratory of Pollution Control and Resource
Reuse, Nanjing University. Prior to any analyses except
the PSD determinations, the CFAs and MPs were
screened at 20 meshes (0.9 mm) to exclude abnormal
coarse particles. A portion of the screened CFAs and
MPs were subsequently milled to particle size below
0.075 mm for the determinations of alkaline values and
hydrophilic coefficients.
Characterization of asphalt, CFAs, and MPs
The key properties of the asphalt, including penetration,
ductility, softening point, solubility, flash point, density,
and wax content, were determined according to Chinese
National Standards and Chinese Industrial Standard.
The results of the tests are summarized in Table 1.
The physico-chemical characteristics of the prepared
CFAs and MPs samples, such as chemical composition,
alkaline value, hydrophilic coefficient, PSD, SSA, specific gravity, water content, mineralogical phase, and
morphology, were analyzed at Center of Modern Analysis (CMA) and State Key Laboratory of Pollution
Control and Resource Reuse, Nanjing University.
The chemical compositions of the samples, including major oxide contents of the CFAs and MPs, as
well as heavy metal and radioactive element contents
of the CFAs, were examined on an ARL 9800 XP+
X-ray fluorescence spectrometer (XRF) (Thermo Electron, Switzerland). The specific radioactivities of the
radioactive elements in the CFAs were analyzed according to Chinese National Standard GB/T 11 743 (Gamma
spectrometry method of analyzing radionuclides in soil).
The results of the determination and the analysis are
presented in Table 2.
The alkaline values of the CFAs and MPs were tested
using a comparative method, taking calcium carbonate (analytical grade reagent) as the criterion (alkaline
value = 1). Samples (size <0.075 mm) of 2 g of CFAs,
Table 1. Key properties of Donghai AH-70 asphalta .
Experimental data
National Standard
Mensuration
Penetration
(25 ◦ C, 0.1 mm)
Ductility
(15 ◦ C, cm)
Softening
point (◦ C)
Solubility
(%)
Wax
content (%)
Flash
point (◦ C)
Density
(25 ◦ C, g/cm3 )
74
60–80
GB/T 4509
>150
≥100
GB/T 4508
46.4
44–54
GB/T 4507
99.78
≥99.0
GB/T 11 148
2.16
≤3.0
SH/T 0425
>240
≥230
GB/T 267
1.02
–
GB/T 8928
a
GB/T 4509: Chinese National Standard for determination of asphalt penetration.
GB/T 4508: Chinese National Standard for determination of asphalt ductility.
GB/T 4507: Chinese National Standard for determination of softening point of asphalt.
GB/T 11 148: Chinese National Standard for determination of asphalt solubility.
SH/T 0425: Chinese Industrial Standard for determination of wax content of asphalt.
GB/T 267: Chinese National Standard for determination of flash and ignition points of petroleum products.
GB/T 8928: Chinese National Standard for determination of specific gravity and density of asphalt.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
DOI: 10.1002/apj
227
Q. LI ET AL.
Asia-Pacific Journal of Chemical Engineering
Table 2. Physico-chemical characteristics of CFBC fly ashes and mineral powders.
Major oxide contents of CFBC fly ashes and mineral powders (wt%)
CFAs
MPs
SiO2
40.37
0.56
Al2 O3
23.22
0.13
Fe2 O3
3.02
0.05
TiO2
0.84
0.01
CaO
18.41
31.23
MgO
0.53
21.48
K2 O
0.48
0.03
Na2 O
0.31
–
P2 O5
0.11
0.13
f-CaO
7.03
–
LOIa
9.91
46.30
SO3
2.06
–
Heavy metal and hazardous element contents (ppm)
As
9.1
Experimental data for CFAs
in soil of pH < 6.5
GB/T 8173
in soil of pH > 6.5
Cr
71.0
100
100
Cu
58.3
250
500
250
500
Cd
1.0
Se
16.0
Mo
9.8
5
10
15
15
10
10
Pb
33.8
Ni
134.9
300
1000
200
400
Mass concentration and specific radioactivity (Bq/Kg)b of radioactive nuclide in CFBC fly ashes
238
U
6.0 × 10−6
a
b
232
Th
18.6 × 10−6
226
Ra
2.0 × 10−12
40
K
0.47 × 10−6
CTh
75.52
CRa
73.40
CK
120.08
External irradiation
Conformed
Internal irradiation
Conformed
LOI, loss on ignition.
According to GB 6566, Chinese National Standard for radioactive nuclide limit for building materials.
Table 3. Hydrophilic coefficient, SSA, specific gravity, water content and alkaline value of CFBC fly ashes and
mineral powders.
CFAs
MPs
a
Hydrophilic coefficient
SSA (m2 /g)
Specific gravity
Water content (%)
Alkaline valuea
0.89
0.81
0.74
0.30
2.19
2.82
<1
<1
0.89
1.07
Taking calcium carbonate as the criterion (alkaline value = 1).
2 g of MPs, and 2 g of calcium carbonate were loaded
into three flasks, respectively, followed by an addition
of 100 ml of 0.5 N sulfuric acid solution into each flask.
The flasks were then transferred into an oil-bath cauldron for 30 min of reflux boiling at 130 ◦ C. Then the
samples were cooled to ambient temperature, and the
H+ concentration in the supernatant fluid in each flask
was finally determined. The alkaline value of the samples was calculated according to the following formula:
H+ quantity consumed
by sample
alkaline value = +
H quantity consumed
by calcium carbonate
(1)
The hydrophilic coefficients of the CFAs and MPs
were tested according to the following procedure. A
sample of 5 g of filler (CFAs or MPs, size <0.075 mm)
was submerged in water in a measuring cylinder, while
another sample of 5 g of the same filler was submerged
in kerosene in another measuring cylinder. The volumes of the two samples were determined after 24 h of
submergence, and the hydrophilic coefficient was calculated by the following formula:
hydrophilic coefficient =
VW
VO
where VW represents the volume of the sample submerged in water, and VO indicates the volume of the
sample submerged in kerosene.
The alkaline values, hydrophilic coefficients, SSAs,
specific gravities, and water contents of the CFAs and
MPs are listed in Table 3.
The PSDs of the CFAs and MPs were measured by
a Mastersizer 2000 (Malvern, UK) and the results are
shown in Fig. 1.
Determination of the mineralogical phases was conducted on an ARL X’TRA high-performance powder
(2)
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Cumulative undersize (%)
228
100
90
80
70
60
50
40
30
20
10
0
0.1
CFBC fly ashes
Mineral powders
1
10
100
1000
Particle size (µm)
Figure 1. Particle size distributions of CFBC fly ashes
and mineral powders.
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECTS OF CFBC FLY ASHES AS FILLER ON ASPHALT
Figure 2. XRD pattern of the CFAs showing the presence
of Q (quartz), A (anhydrite), C (calcite), L (lime), and P
(portlandite).
Figure 4. SEM photomicrograph of mineral powders.
Figure 3. SEM photomicrograph of CFBC fly ashes.
X-ray diffractometer (XRD) (Thermo Electron, Switzerland) with Cu Kα radiation. Step scans were performed
over the range of 15–60◦ 2θ with stepping intervals of
0.04◦ and a counting time of 0.12 s (scanning rate 10.00
◦
/min, 50 kV, 40 mA) and the results are presented in
Fig. 2.
The morphologies of the CFAs and MPs were
observed
by
an
X650
scanning
electron
microscope (SEM) (Hitachi, Japan) and polarized optical microscopy (POM). The SEM photomicrographs of
the CFAs and MPs are shown in Figs 3 and 4.
Performance tests for asphalt
Effects of the CFAs and MPs fillers on the performances
of asphalt, including moisture susceptibility, viscosity, ductility, softening point, penetration, and antiaging
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
performances, were investigated at State Key Laboratory of Pollution Control and Resource Reuse, Nanjing
University.
In this work, the water-boiling test was employed
to determine the moisture susceptibility of asphalt,
as prescribed by Chinese Industrial Standard JTJ 052
(Specifications and test methods of asphalt and asphalt
mixtures for highway engineering). Samples of filled
asphalt with mass ratios of MPs to asphalt at 1 : 1, of
CFAs to asphalt at 1 : 1 (CFAs equal to MPs in mass),
0.8 : 1 (CFAs equal to MPs in volume), and 0.4 : 1 (CFAs
equal to MPs in SSA) were examined. The raw asphalt
was also tested as reference. Each sample was agitated
adequately at 140 ◦ C before two coarse aggregates (size
>13.2 mm), alkaline limestone and acidic granite, were
added to fulfill the test.
The viscosity was determined using two experimental
methods, the rotary viscometry and the standard viscometry, at 95 ◦ C.
The ductility was tested by measuring the length (cm)
of set samples when they were stretched to break at
certain speed and temperature.
The softening point was examined using the ring and
ball method.
The results of the tests of moisture susceptibility,
viscosity, ductility, and softening point are summarized
in Table 4.
The penetration was tested at the temperatures of 15,
25 and 30 ◦ C, and the results are shown in Table 5.
The antiaging performances of the filled asphalt were
assessed according to the rolling thin-film oven test
(RTFOT). Samples with various mix designs of CFAs
or MPs and asphalt were taken into vials, heated at
163 ◦ C, and circumvolved into thin films at the rotating
speed of 15 ± 0.2 rpm. After 75 min of aeration with
hot air, the weight loss, residual ratio of penetration,
and ductility decrease of each sample were determined.
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
DOI: 10.1002/apj
229
230
Q. LI ET AL.
Asia-Pacific Journal of Chemical Engineering
Table 4. Effect of CFBC fly ashes on the ductility, softening point, moisture susceptibility and viscosity of asphalt.
Flaked asphalt (area %)
Filler
Filler to asphalt
(mass ratio)
Ductility (15 ◦ C, cm)
Softening point (◦ C)
Limestone
Granite
None
MPs
CFAs
CFAs
CFAs
0:1
1:1
1:1
0.8 : 1
0.4 : 1
>150
120
109
130
136
46.4
50.6
67.1
65.3
56.8
9.33
5.53
4.93
4.10
4.37
86.3
75.3
34.7
27.7
34.3
None
MPs
CFAs
CFAs
CFAs
CFAs
CFAs
a
Standard viscometry (s)a
Rotary viscometry (Pa·S)a
18
40
325
307
251
206
174
0.63
1.46
13.18
10.34
8.27
4.78
2.49
0:1
1:1
1:1
0.8 : 1
0.7 : 1
0.6 : 1
0.4 : 1
Tested at 95 ◦ C.
Table 5. Effect of CFBC fly ashes on the penetration and anti-aging performances of asphalt.
Penetration (0.1 mm)
Filler
Filler to asphalt
(mass ratio)
15 ◦ C
25 ◦ C
30 ◦ C
A
B
Ra
PI
T800 (◦ C)
T1.2 (◦ C)
MPs
CFAs
CFAs
CFAs
1:1
1:1
0.8 : 1
0.4 : 1
20.0
23.4
25.0
26.1
48.8
50.1
52.0
57.3
75.0
69.5
79.0
88.7
0.0383
0.0317
0.0331
0.0352
0.727
0.896
0.898
0.886
0.9999
0.9993
0.9993
0.9996
0.29
1.61
1.30
0.87
56.82
63.32
60.58
57.30
−16.91
−25.77
−24.74
−22.92
Penetration (0.1 mm) (25 ◦ C)
None
MPs
CFAs
CFAs
CFAs
a
0:1
1:1
1:1
0.8 : 1
0.4 : 1
Ductility (cm) (15 ◦ C)
Weight loss after
calefaction (%)
After
calefaction
Residual penetration
(%)
After calefaction
Decrease
0.56
0.27
0.03
0.09
0.12
46.0
29.5
71.9
31.9
72.0
62
60
77
63
83
73.0
47.7
73.4
103.6
105.1
77.0
72.3
25.6
26.4
30.9
R, correlation coefficient.
The results of the antiaging performance tests are also
listed in Table 5.
RESULTS AND DISCUSSIONS
Chemical composition and alkalinity of CFAs
and MPs
Table 2 shows the chemical compositions, heavy metal
and hazardous element contents, as well as radioactive
characteristics of the CFAs.
According to Chinese National Standard GB 8173
(Control standards of pollutants in fly ash for agricultural use), most of the heavy metal and hazardous
element contents of the CFAs are lower than the concentration limits, with that of Se (16 ppm) as the only
exception.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Since the Se concentration is only 1 ppm higher than
15 ppm prescribed by GB 8173, and the restrictions
of heavy metals and hazardous elements in roadway
constructions are generally not taken for as severe as in
agricultural applications, the safety of using the CFAs as
filler for paving asphalt is considered to be acceptable.
The radioactive element contents of the CFAs are
also regarded as being safe in accordance with Chinese
National Standard GB 6566 (Radioactive nuclide limit
for building materials).
It should be mentioned that the SO3 content (2.06
wt%) in the CFAs is much lower than expected, possibly
due to the formation of high-sulphur fouling and bottom ashes (circulating fluidized bed combustion bottom
ashes[CBAs]) in CFBC boilers. It was observed that the
fouling deposited inside the boiler was composed predominantly of CaSO4 ,[21,22] and that the CBAs might
also contain high SO3 contents up to 17.60 wt%.[23]
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECTS OF CFBC FLY ASHES AS FILLER ON ASPHALT
The alkalinity of filler may have an influence on the
adhesion between asphalt and aggregates with stronger
alkalinity being beneficial. As shown in Table 2, while
the MPs primarily consist of alkaline components such
as CaO and MgO, the CFAs mainly contain acidic
components such as SiO2 .
In the case of CFAs, a reduction of adhesions between
aggregates and asphalt might occur from the high
alkaline components point of view. However, the use
of large amounts of calcium sorbent (limestone or
dolomite) for in situ sulphur capture may usually result
in high CaO contents in CFAs.[11,24,25] It is shown in
Table 2 that the free CaO (f-CaO) content in the CFAs
is as high as 7% (wt%), and that there exist a certain
amount of Na2 O and K2 O components in the CFAs,
which would lead to an increase in the alkalinity of the
CFAs.
As for the MPs, the CaO component exists mainly
in the form of calcium carbonate (calcite) crystalloid,
exhibiting chemical inertness under most circumstances.
Similar to the case of CaO, the MgO content of MPs
is chemically inert too, although the content is significantly higher (21.48%) than that of the CFAs (0.53%).
Therefore, the alkalinity of the CFAs and MPs could
not be judged simply by their chemical compositions. In
this study, the alkalinity was determined using a comparative method, which takes pure calcium carbonate
as the criterion (alkaline value = 1). The testing results
shown in Table 3 indicate that the alkaline value of the
MPs is 1.07 and that of the CFAs is 0.89. Because
the molar mass of Mg (24) is less than that of Ca
(40), the molar quantity of MgCO3 is larger than that
of CaCO3 while equal in weight. Accordingly, the H+
consumed by MgCO3 is more than that consumed by
CaCO3 at the same weight. Thus, the alkaline value of
the MPs is higher than 1 due to their high MgO content
(21.48%, Table 2). Although there are some remarkable
differences between the chemical compositions of CFAs
and MPs (Table 2), the alkaline value of the CFAs still
reaches 0.89, only 17% less than that of the MPs. Due
to the higher content of active f-CaO, alkaline Na2 O
and K2 O, the CFAs present an alkalinity similar to that
of the MPs, suggesting that, from the alkalinity point of
view, the CFAs could be a suitable substituent for the
MPs used as asphalt concrete filler.
PSD of CFAs and MPs
Figure 1 shows that the particle size of the CFAs is
smaller than that of the MPs on the whole, and that the
PSD of the CFAs is similar to that of the MPs. Both
the PSDs are continuous. According to the prescriptions
listed in Table 6, both the CFAs and the MPs meet the
requirements prescribed by JTJ 032 (Chinese Industrial
Standard for standard specification for construction and
acceptance of highway asphalt pavements).
Hydrophilic coefficient, SSA, and specific
gravity of CFAs and MPs
Hydrophilic coefficient represents the ratio of the affinity of filler and water to that of filler and asphalt, which
indicates the mechanical strength and stability of watersaturated asphalt concrete. No filler with hydrophilic
coefficient ≥1 should be blended into asphalt concrete.
Otherwise, fatal damages to the surface structure of
asphalt concrete would occur due to the volume expansion caused by water infiltration.
As shown in Table 3, the hydrophilic coefficient of
the CFAs is appreciably larger than that of the MPs.
However, the hydrophilic coefficient of the CFAs is
still less than 1, suggesting that the CFAs are qualified
for asphalt concrete filler in terms of hydrophilic
coefficient.
Table 3 also shows that the SSA of the CFAs is
almost 2.5 times of that of the MPs. As shown in Figs 3
and 4, while the surfaces of the MPs are smooth and flat,
those of the CFAs contain a lot of micropores, which
results in larger SSA.
The specific gravity of the CFAs is 2.19, appreciably
less than 2.55 prescribed by the Chinese National
Standard for the specific gravity of MPs filler in asphalt
concrete.
Morphology and mineralogical phase of CFAs
and MPs
As shown in Figs 3 and 4, the CFAs basically consist
of irregular slag like particles of different sizes with
micropores on the surfaces, whereas the MPs contain
Table 6. Fineness of CFBC fly ashes and mineral powders.
Standard specification by JTJ 032a (wt %)
Size range (mm)
<0.600
<0.150
<0.075
a
Experimental data (wt %)
Highway of grade 1
Highway of other grades
CFAs
MPs
100
90–100
75–100
100
90–100
70–100
100
100
85.7
100
90
86.0
JTJ 032, Chinese Industrial Standard for standard specification for construction and acceptance of highway asphalt pavements.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
DOI: 10.1002/apj
231
232
Q. LI ET AL.
mainly physical grinding minerals in irregular fibriform,
prism or rectangle with smooth and flat surfaces.
The filler may have a selective adsorption on asphalt
due to the micropores on its surface. In this case,
the active asphalt content in the sample is adsorbed
on the filler surface, the pitch content is adsorbed in
the micropores, and the oil content is adsorbed along
the capillaries into the inner of the filler. Therefore,
the pitch and oil contents on the surface of the filler
decrease, resulting in a relative increase in the asphalt
content, and leading to the changes in asphalt properties
such as increases in cohesive force and thickness. Also,
the heat resistance and water stability of the filled
asphalt are improved to certain extents. From this point
of view, it is suggested that the CFAs, with a lot of
micropores on the surface, are more suitable than the
MPs for use as asphalt concrete filler.
The XRD pattern of the CFAs (Fig. 2) shows that the
main mineralogical phases of the CFAs are α-quartz
(α-SiO2 ), anhydrite (CaSO4 ), lime (CaO), portlandite
[Ca(OH)2 ] and calcite (CaCO3 ), generally in the form
of polyphasic conglomeration. Namely, the glass phase
and crystalline phase coexist in various ash granules.
When used as the filler, the CFAs may have a function
of adhesive aid because of their high quick lime (f-CaO)
content, which is beneficial to improving the strength
and durability of the asphalt concrete. On the contrary,
the MPs have no such function due to their major
mineralogical phases being holocrystalline.
Effect of CFAs on the moisture susceptibility of
asphalt
The asphalt films on the surfaces of the aggregates may
be flaked off by water to some extent at a certain temperature. Therefore, the moisture susceptibility indicates
the strength and water stability of the surface structure
of asphalt pavement.
According to Chinese Industrial Standard JTJ 052,
the moisture susceptibility of asphalt is determined
using two methods, the water-boiling test (also termed
static water flaking-off test) for aggregates with size
>13.2 mm and the water-soaking test for aggregates
with size ≤13.2 mm; in the case of aggregates with
both size >13.2 mm and size ≤13.2 mm; the waterboiling test with aggregates of size >13.2 mm is taken
as the criteria.
The moisture susceptibilities of the filled asphalt
samples as determined by the water-boiling test are
listed in Table 4. As low percentage of flaked area of
asphalt film reflects high moisture susceptibility, it can
be concluded that either the CFAs or the MPs have a
favorable effect on the moisture susceptibility of the
asphalt. When the mass ratios of the two fillers to
asphalt are equal at 1 : 1, the CFAs show stronger effect
than the MPs on improving the moisture susceptibility.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
Asia-Pacific Journal of Chemical Engineering
Among all the tested samples that are filled with the
CFAs, the one with the mass ratio of filler to asphalt at
0.8 : 1 presents the best moisture susceptibility. Besides,
the samples filled with alkaline limestone aggregates
show superior moisture susceptibility to those filled
with acidic granite aggregates.
Two factors may be responsible for the CFAs’ effect
on the moisture susceptibility of asphalt. One is the
considerable increases in the viscosity of the filled
asphalt (Table 4) caused by the use of the CFAs, the
other is the CFAs’ function of adhesive aid resulted
from the high f-CaO content (Table 2).
Effect of CFAs on asphalt viscosity
Viscosity at 60 ◦ C is employed as grading criterion for
asphalt in USA, Australia, etc. due to its significance
in the quality of asphalt. High viscosity is believed
beneficial to enhancing the bonding strength of asphalt
concrete.
To ensure the precision of the experimental data, the
authors did not adopt 60 ◦ C as the testing temperature,
although it is the common temperature used for the test
of asphalt viscosity. In this work, the asphalt samples
were found to be too thick at 60 ◦ C after being filled
with the CFAs or MPs. Therefore, the authors chose
95 ◦ C instead.
As shown in Table 4, according to the results of the
standard viscometry, while the viscosity of the raw
asphalt is only 18 s, that of the asphalt sample filled
with the MPs at the mixing ratio of 1 : 1 reaches 40 s.
Furthermore, those of the samples at different CFAsto-asphalt ratios (1 : 1, 0.8 : 1, 0.7 : 1, 0.6 : 1, and 0.4 : 1)
are even higher in the range of 174–325 s. It can be
concluded from the data that the use of the two fillers
can effectively improve the viscosity of the asphalt,
and that the CFAs have greater effect than the MPs on
enhancing the asphalt viscosity. These conclusions are
also supported by the results of the rotary viscometry
listed in Table 4.
The improvements caused by the CFAs on the asphalt
viscosity may be largely contributed by the numerous
micropores on the surface of the asphalt (Fig. 3),
which may have a sort of adsorption effect on the oil
components in the asphalt.
Moreover, the increasing extent of the asphalt viscosity goes up further as the mixing ratio of CFAs to
asphalt rises. For instance, it goes from 9.67 up to 18.06
times when the mixing ratio rises from 0.4 : 1 to 1 : 1.
Effect of CFAs on asphalt ductility
The asphalt ductility reflects the antidistortion performance of the asphalt, with higher ductility representing
better antidistortion performance in general.
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
DOI: 10.1002/apj
Asia-Pacific Journal of Chemical Engineering
EFFECTS OF CFBC FLY ASHES AS FILLER ON ASPHALT
It can be concluded from the ductility data in Table 4
that, either the CFAs or the MPs has an unfavorable
effect on the asphalt ductility, and that such effect
of the CFAs is greater than that of the MPs at the
same mixing ratio of filler to asphalt (1 : 1). Furthermore, Table 4 shows that, when the mixing ratios of
CFAs to asphalt are 0.4 : 1, 0.8 : 1, and 1 : 1, the asphalt
ductility is 136, 130, and 109, respectively, indicating a decreasing tendency in the asphalt ductility as
the mixing ratio of the CFAs to asphalt increases. The
lowest ductility (109 cm) occurs when the mixing ratio
of CFAs to asphalt is 1 : 1. However, even the lowest
one is still higher than the minimum ductility (100 cm)
prescribed by Chinese National Standard GB/T 4508
(Chinese National Standard for determination of asphalt
ductility).
The main reason for the negative correlation between
the ductility and the mixing ratio may lie in the fact
that the asphalt viscosity goes higher when more CFAs
are filled into the asphalt, as shown in Table 4.
Effect of CFAs on the softening point of
asphalt
To a certain extent, the softening point indicates the
temperature stability of asphalt. Higher softening point
means better high-temperature stability, lower temperature susceptibility, and stronger antidistortion strength.
As shown in Table 4, at the fillers-to-asphalt ratios
of 1 : 1, while the use of the MPs filler heightens the
softening point by less than 10% (from 46.4 to 50.6 ◦ C),
the fill of the CFAs upgrade the softening point by
almost 45% (from 46.4 to 67.1 ◦ C). Hence it can be
concluded that, when used as the filler, the CFAs have
a greater improving influence than the MPs on the
softening point of asphalt.
Effect of CFAs on asphalt penetration
Penetration is one of the key parameters of asphalt,
which is expressed by the deepness (1/10 mm) vertically penetrated by a standard needle under 100 g
load for 5 s at a certain temperature. Linear regression
analysis was applied according to formula 3, and formulas 4–6 were employed to calculate the penetration
index (PI), equivalent weight softening point (T800) and
equivalent weight brittle point (T1.2) of the asphalt. The
formulas 3–6 are shown as follows:
lgP = A × T + B
(20 − 500 × A)
PI =
(1 + 50 × A)
2.9031 − B
lg800 − B
=
T800 =
A
A
(3)
(4)
(5)
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
T1.2 =
0.0792 − B
lg1.2 − B
=
A
A
(6)
where P represents penetration; T indicates temperature; A and B are regression coefficients in formula 3.
As shown in Table 5, the penetrations of the samples
filled with the CFAs are generally appreciably greater
than that of the samples filled with the MPs. At 25 ◦ C
and the mixing ratio of 1 : 1, the penetration value of the
sample filled with the CFAs is 50.1, while that of the
sample filled with the MPs is 48.8. Compared with the
penetration value of the raw asphalt (74, Table 1), the
use of the CFAs and the MPs decrease the penetration
value by 32 and 33%, respectively, indicating that the
CFAs and MPs have similar effects on the penetration
of asphalt.
In formulas 3–6, the value of A reflects the temperature susceptibility of asphalt with higher A value
suggesting higher temperature susceptibility and poorer
temperature stability. The T800 value represents the
high-temperature stability, which indicates the rutting
resistance of asphalt, with higher T800 value meaning
better high-temperature stability. Also, it is shown in
Table 5 that when the mixing ratios of the two fillers
to asphalt are equal at 1 : 1, the A and T1.2 values of
asphalt filled with the CFAs are lower than those of
the asphalt filled with the MPs, respectively; the T800
values and especially the PI values of asphalt filled
with the CFAs are higher than those of the asphalt
filled with the MPs, respectively. All the A, PI, T800
and T1.2 values indicate that the temperature susceptibility, high-temperature stability and low-temperature
crack resistance of the asphalt filled with the CFAs are
superior to those of the asphalt filled with the MPs.
In China, pavement performances are graded into four
levels, A, B, C and D, on the basis of the A and PI
values, as shown in Table 7. Compared with the levels
in Table 7, the data in Table 5 indicate that both the
high-temperature stability and low-temperature crack
resistance of the asphalt filled with either the CFAs or
the MPs have reached the best level (level A).
Effect of CFAs on the antiaging performances
of asphalt
The gradual changes of asphalt, such as volatilization,
oxidation, polymerization, and inner structural variations, under the effects of natural factors like heat,
oxygen, light and water, are termed asphalt aging,
which may result in deteriorations in performances and
longevity of asphalt pavement. Two testing methods,
thin-film oven test (TFOT) [The American Society for
Testing and Materials (ASTM) D1754, Standard test
method for effect of heat and air on asphaltic materials] and RTFOT (ASTM D2872, Standard test method
for effect of heat and air on a moving film of asphalt),
Asia-Pac. J. Chem. Eng. 2009; 4: 226–235
DOI: 10.1002/apj
233
234
Q. LI ET AL.
Asia-Pacific Journal of Chemical Engineering
Table 7. Performance level of asphalt pavement.
Requirement for
high-temperature stability
Performance level
◦
Temperature ( C)
Maximum A value
Minimum PI value
Requirement for low-temperature
cracking resistance
A
B
C
A
B
C
D
> 30
0.0467
−1.0
20–30
0.0482
−1.2
< 20
0.0489
−1.4
< − 37.0
0.0467
−1.0
−21.5 to −37.0
0.0482
−1.2
−9.0 to −21.5
0.0489
−1.4
> − 9.0
0.0514
−1.6
are widely used in laboratories for testing short-term
asphalt aging.[26] While the former uses rotating pans
filled with a thin asphalt film, the latter employs rolling
cylindrical asphalt containers. Although the two methods are much similar to each other, several researchers
concluded that they are not interchangeable and that
RTFOT appears to be more severe than TFOT on the
majority of binders.[27]
Therefore, this study adopted RTFOT for evaluating
the antiaging performances of the filled asphalt samples.
The weight loss after calefaction, residual ratio of
penetration, and ductility decrease of the samples were
determined according to the RTFOT.
As shown in Table 5, in all the asphalt samples, raw
or filled, occurred aging phenomena like weight loss,
penetration reduction, and ductility decrease after the
RTFOT. Table 5 also shows that the weight loss and
ductility decrease of the asphalt samples filled with the
MPs are less than those of the raw asphalt; furthermore,
those of the asphalt filled with the CFAs are even
far less. While the residual ratio of penetration of the
asphalt filled with the MPs is a little less, those of the
asphalt filled with the CFAs are clearly more than that
of the raw asphalt. Thus it may be concluded that the
antiaging properties of the asphalt can be improved by
using either the CFAs or the MPs as the filler, and that
the CFAs may be more beneficial than the MPs to the
antiaging properties.
5. The use of CFAs is beneficial in improving the
strength and durability of the asphalt concrete due to
the adhesive aid caused by the high content of quick
lime (f-CaO), which is one of the main mineralogical
phases of the CFAs, whereas the MPs have no such
benefit with their major mineralogical phases being
holocrystalline.
6. The f-CaO content, SSA, morphology, and mineralogical phases of the CFAs are more favorable than
those of the MPs respectively for being used as
asphalt concrete filler, while the alkaline value, PSD,
hydrophilic coefficient, specific gravity, and water
content of the CFAs are similar to those of the MPs
respectively.
7. It is recommended that, when used as asphalt concrete filler, the MPs should be substituted by the
CFAs at the substitution ratio of 1 : 0.8 (MPs-toCFAs, mass ratio), where the volume of the CFAs
equals to that of the MPs.
Although further studies are needed, the results of the
present work suggest that CFAs may be more suitable
than MPs when used as asphalt concrete filler.
NOMENCLATURE
GB, GB/T Chinese National Standard.
JTJ, SH/T Chinese Industrial Standard.
CONCLUSIONS
On the basis of the experimental results, the following
conclusions can be drawn.
Acknowledgements
1. The CFAs have generally more favorable effects than
the MPs on most of the asphalt performances, with
ductility as the only exception.
2. Although the CFAs lead to more decrease in asphalt
ductility than the MPs, all the samples filled with
the CFAs conform to the minimum ductility limit
(100 cm) prescribed by GB/T 4508.
3. The CFAs show similar alkalinity to that of the MPs
due to their high f-CaO content and active Na2 O and
K2 O components.
4. On the basis of their microporous structure, the CFAs
may have an effect of selective adsorption on asphalt,
whereas the MPs do not.
The authors gratefully acknowledge the supports provided by SINOPEC Jingling Petrochemical Power Plant
and the Center of Modern Analysis, Nanjing University.
 2009 Curtin University of Technology and John Wiley & Sons, Ltd.
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