4TH INTERNATIONAL CONFERENCE
BITUMINOUS MIXTURES AND PAVEMENTS
THESSALONIKI, GREECE, 19-20 APRIL 2007
THE INFLUENCE OF DIFFERENT MINERALS ON THE MECHANICAL
RESISTANCE OF ASPHALT MIXTURES
Roberto Carlos Ribeiro
Centro de Tecnologia Mineral, [email protected]
Universidade Federal do Rio de Janeiro
Julio César G. Correia
Centro de Tecnologia Mineral, [email protected]
Peter R. Seidl
Centro de Tecnologia Mineral, [email protected]
ABSTRACT
The adsorption of five asphalts and their asphaltene and maltene constituents, dissolved in toluene
onto quartz, feldspar and biotite and their effect on the mechanical properties of their respective
asphalt cements have been investigated. The adsorption of asphalts A and C was greater than that
of the all the other samples, maxima being observed between 3.5 and 4.0 mg.g-1 for gneiss and
quartz and increasing to 5 and 6 mg.g-1 for feldspar and biotite. The eletrophoretic mobility of
quartz did not change on adsorption of different asphaltenes, in contrast to feldspar and biotite,
indicating that the sites responsible for the surface charge of these minerals were affected by the
presence of the adsorbed organic species. This fact was confirmed by FTIR. With regard to the
mechanical resistance of asphalt mixtures, only the asphalts A and C gave values acceptable to
Brazilian National Department of Terrestrial Infra-structure (DNIT). These results indicate that the
chemical interaction among minerals and asphalts affected the mechanical resistance of the asphalt
mixture.
KEY WORDS: adsorption, asphalt, asphaltenes, maltenes and minerals aggregates.
1. INTRODUCTION
An asphalt pavement can be defined as a mixture of approximately 5% of asphalt and 95% of
mineral aggregate [1]. Asphalt is responsible for binding mineral aggregates and is constituted by
asphaltenes and maltenes. Asphaltenes correspond to the polar constituents of asphalt and can be
precipitated by the addition of low molecular weight alkanes [2], being defined [3, 4, 5] according
to their solubility in non-polar solvents and not by their chemical structure. The common definition
is that they correspond to the crude oil components insoluble in n-heptane and soluble in toluene or
benzene [6, 7, 8].
Mineral aggregates represent the largest portion, in weight, of the pavement and are mainly
responsible in for supporting the weight of traffic. Minerals more commonly utilized in paving are
basalt, gneiss and granite; feldspar, quartz and iron may be found in the mineralogical composition
of these rocks [9,10]
Currently norms used for evaluation of the quality of asphalt pavements, only take into account
mechanical tests [11, 12, 13, 14, 15, 16] without any concern for the chemical interaction among
their constituents.
The interactions among asphaltenes, maltenes, and basalt rock having observed that asphaltene
adsorption was greater than that of maltenes. Based on this observation, the objective of this work
is to show the relationship between the chemical and mineralogical interaction of the constituents of
the asphalt mixture and the mechanical resistance of the asphalt pavement [17].
2. EXPERIMENTAL
2.1 Isolation of asphaltenes and maltenes
Asphaltenes and maltenes were isolated from samples of 5 crude oils from Brazil. The experimental
procedure used was the ASTM 2007.
2.2 Mineralogical analysis
The gneiss rock was submitted a mineralogical analyzed in the Coordination of Mineral Analysis by
Center of Mineral Technology – CETEM.
2.3 Adsorption
The adsorption of asphaltenes and asphalts by gneiss, quartz, feldspar and biotite was determined by
measuring the decrease in concentration of toluene solutions of these compounds after contact with
the minerals. The concentration was measured spectrophotometriccaly at 402 nm. At this
wavelength, asphaltenes and maltenes show a relatively high absorption although a small shoulder
rather than a well-defined maximum is observed. Although the method has limitations, it has been
used successfully to measure the adsorption of these fractions on solid substrates [18].
2.4 Eletrophoretic mobility
The eletrophoretic mobilities of the mineral samples were measured using an Electrophoresis
Apparatus. A stock suspension containing 200 mg of + 325 mesh mineral in 200 mL of 5 . 10-3 mols
. dm3- KNO3 solution was prepared using an ultrasonic bath. For each measurement the pH of the
suspension was modified and maintained at a particular value for five minutes by the addition of
HNO3 and NaOH. The pH value was measured again after another period of five minutes and this
value was considered the equilibrium pH. Ten measurements of the velocity of the particles were
taken for each pH and averaged to calculate the eletrophoretic mobility. All measurements were
made in a temperature-controlled room at 25 ºC.
The sample was dispersed in electrolyte solutions of pH 2 and maintained at this pH for at least 30
minutes. Subsequently the eletrophoretic mobilities were measured always increasing the pH for
successive measurements [18].
2.5 Fourier transform infrared spectroscopy
Fourier transform infrared (FTIR) spectra of gneiss in the presence and absences of asphalt A, were
recorded using a Bomem spectrometer, model MB 102. The samples were analyzed as KBr pellets
[19, 20].
2.6 Mechanical resistance of asphalt mixture
For evaluation of the mechanical resistance 3 probes of asphalt mixture (asphalt + gneiss) were
used. The first probe was evaluated for resistance to traction for diametrical compression without
any type of conditioning. The other two were subject to a conditioning process specified in the
AASHTO T 283/89 method.
3. RESULTS
Adsorption (mg/g)
3.1 Mineralogical analysis
The mineralogical analysis indicates the presence of 62% of feldspar, 25% of quartz and 13% of
biotite. These results are considered plausible, since they are in agreement with the results found in
the literature for that type of rock [21,22].
3.2 Adsorption
The adsorption of asphalt on gneiss is shown in figure 1. A maximum adsorption at 3.5 mg.g-1 is
observed for asphalt A and C.
4
3
2
1
0
0
5
10
15
20
Asphalt concentration (mg/L)
A
B
C
D
E
Figure 1: Adsorption of asphalts on gneiss.
Adsorption of asphaltenes on quartz, feldspar and biotite are presented in figures 2, 3 and 4. The
adsorption of the asphaltenes on feldspar and biotite surfaces is more significant arriving at a
maximum value of adsorption around 7 and 8 mg/g (figures 3 and 4), higher that those found for
gneiss (4 mg/g) (figure 1). Such results can be related to the ease with which the asphaltene, free
from maltenes, associates with surface of the minerals, as had already been verified [17].
In relation to the process of adsorption to the surface of the quartz that corresponds to 25% of the
mineralogical composition, figure 2, a low response is observed for all of the asphaltene under
study, a maximum of value around 4.5 mg.g-1 being arrived at. Such results indicate that the
presence of quartz in the composition of gneiss seems to be unfavorable for the interaction with
asphaltene, corresponding the to smallest adsorption of asphalt on gneiss.
The low values of adsorption of asphaltenes on the surface of quartz are probably related to the
structure of quartz (SiO2), since it does not contain aluminum. These results, indicate that aluminum
has an important role in the structure of the mineral aggregates adsorb on asphaltenes and,
consequently, with better association with the asphalt mixture. However, the presence of the quartz
is very important, since the mechanical resistance of some rocks is related to its silicon content.
Adsorption (mg/g)
5
4
3
2
1
0
0
5
10
15
20
Asphaltene concentration (mg/L)
A
B
C
D
E
Adsorption (mg/g)
Figure 2: Adsorption of asphaltenes on quartz.
8
6
4
2
0
0
5
10
15
20
Asphaltene concentration (mg/L)
A
B
C
D
E
Adsorption (mg/g)
Figure 3: Adsorption of asphaltenes on feldspar.
8
6
4
2
0
0
5
10
15
20
Asphaltene concentration (mg/L)
A
B
C
D
E
Figure 4: Adsorption of asphaltenes on biotite.
Potencial Zeta (mV)
3.2 Eletrophoretic mobility
The eletrophoretic mobility – pH diagram obtained for the gneiss with asphalts is presented in
figure 5. The results for gneiss indicate that the surface charge of this rock is negative, and its
isoeletric point is observed between pH 2 and 3, in agreement with the data reported in the literature
[23]. The results after interaction with asphalt indicate that the adsorption does not modify the sites
responsible for the charge on the rock because the isoeletric point was not altered.
0
-20
0
5
10
15
Gneiss
Gneiss+ CAP A
-40
Gneiss + CAP B
-60
Gneiss + CAP C
Gneiss + CAP D
-80
Gneiss + CAP E
-100
pH
Figure 5: Eletrophoretic mobility of gneiss without and with the presence of asphalts.
Zeta Potential (mV)
Figures 6, 7 and 8 present the results of eletrophoretic mobility of quartz, feldspar and biotite in
presence of asphaltenes and maltenes (resins).
The results for quartz indicate that surface charge of this mineral is negative, and its isoeletric point
is the same as the ones obtained for gneiss. The results after interaction with asphaltene or resin
indicate that the adsorption does not modify the sites responsible for the charge on the mineral.
0
-20
0
10
20
Q uartz
-40
Q uartz +
A sphaltens
-60
Q uartz +
resins
pH
Figure 6: Eletrophoretic mobility of quartz without and with the presence of asphaltenes or resins.
In the case of feldspar and biotite, the eletrophoretic behavior was influenced by the adsorption of
asphaltenes, corroborating the strong influence of the asphaltene on this mineral surface. The
behavior of those asphaltenes zeta potential was investigated by Barraza [24] in agreement with
this study.
Zeta Potential (mV)
20
Feldspar
0
-20 0
5
10
15
Feldspar +
Asphaltens
-40
-60
Feldspar + resins
-80
-100
pH
Figure 7: Eletrophoretic mobility of feldspar with and without the presence of asphaltenes or resins.
Zeta Potential (mV)
0
-10 0
5
10
-20
15
-30
Biotite
Biotite + Asphaltens
-40
Biotite + resins
-50
-60
pH
Figure 8: Eletrophoretic mobility of biotite without and with the presence of asphaltenes or resins.
3.3 Fourier transform infrared spectroscopy
Figures 9 and 10 shows the FTIR spectra of gneiss in the absence and presence of asphalt,
respectively. From figure 9, it can be verified that gneiss has absorptions at 1820 cm-1, 1500 cm-1,
1000 cm-1, 770 cm-1 and 595 cm-1, characterizing the presence of quartz, feldspar and biotite in the
gneiss composition, according to the mineralogical analysis [25].
Figure 10, reveals that asphalt is adsorbed on the surface of gneiss. The most intense bands at 2900
cm-1 and 2890 cm-1 are due to –CH3 and –CH2 stretching of alkyl chains present in asphalt structure
[4] results are in agreement with the studies of Blanco [26].
Figure 9: spectra of gneiss.
Figure 10: spectra of gneiss + asphalt.
3.4 Mechanical resistance of asphalt mixture
The ratios of mechanical resistance (%) of the asphalt mixtures are: 130 for A, 66,78 for B, 98 for
C, 77 for D and 64 for E. The limit of SUPERPAVE specifications is of 80%. In this case, the
asphalt mixture produced with asphalts A and C led to values within specifications.
Those results are in good agreement with the physiochemical interactions that are proposed, as
indicated by the adsorption and zeta potential measurements, indicating a strong connection
between the chemical interaction and the mechanical resistance of the asphalt mixture.
4. CONCLUSIONS
It can be concluded that the mechanical resistance of asphalt pavements is related directly to the
chemical interactions of their constituents, asphalts and mineral aggregates.
It can also be verified, that asphaltenes are responsible for the interaction with the surface of the
rocks used as aggregates and that feldspar and biotite minerals, present in these rocks, are
responsible for promoting such adsorptions.
In relation of the quartz, this mineral is the responsible for the resistance and hardness of the rock,
however it does not contribute to adsorption of asphalts. This observation can be related to the
absence of aluminum in their structure.
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