As mentioned before, the current research aims at providing valuable insights into the chemical compositions of the asphalts and their impact on its physical properties. This information will be useful in generating adequate and efficient asphalt binders for road pavements. Also, this research provides new basic tools to study asphalt chemistry. However, to assess the importance of the findings obtained in a particular asphalt binder product are outside the scope of this work.
3.2. Thin-Layer Chromatography with Flame Ionization Detection (TLC-FID)
It should be noted that the asphalts have an exceptionally complex chemical composition. Solvent precipitation and chromatography are two of the techniques that have been efficient for fractionation of asphalts. For the current research, the TLC-FID technique is utilized.
Figure 1 demonstrates the classic TLC-FID chromatograms of asphalts. The four peaks visible in
Figure 1 depicts the SARA compositions (
saturates,
aromatics,
resins, and
asphaltenes).
Table 2 demonstrates the generic fractions of the asphalt material by normalizing the area. With the help of the standard composition, the colloidal instability index (I
C) is also estimated and presented in the table.
The difference between the chemical compositions of PA and NA is evident in
Table 2. It is observed that PA and NA have different values of SARA fractions, which results in varied colloidal instability index (I
C) values. When considering the group composition, the contents of aromatics hydrocarbon (18.97 wt %) are observed to be higher in PA. On the other hand, the contents of saturates (4.40 wt %) in PA are observed to be lower than NA.
When considering the stability aspect, it is observed that PA is slightly more stable than NA as it has a low I
C value. As seen in
Table 2, the values of saturates and asphaltenes are low in PA, which is why it has acquired higher stability. Also, the reason behind its stability is that PA has superior peptizability due to the smaller aggregation of asphaltenes’ micelle.
Otherwise, the saturates and asphaltenes are the lowest and highest polarity components of an asphalt, respectively, and the solubility of the asphaltenes in a colloidal system is enhanced by the presence of intermediate polarity species such as aromatics and resins. Thus, the higher the IC, the less stable the overall system is.
3.3. Matrix-Assisted Laser Desorption Ionization Time-of-Flight Mass Spectroscopy (MALDI-TOF-MS)
Molar mass is one attribute of a molecule that results in its solubility. With the increase in molar mass of molecules with a similar structure, their solubility decreases. When the asphalt fractions are separated, it is observed that those that are highly insoluble have higher molar mass. The classic mass spectra for both PA and NA are illustrated in
Figure 2. Due to the oligomeric nature of the asphalts, they are segregated into four regions: monomers from
m/
z 200 to 400, dimers from
m/
z 400 to 650, trimers from
m/
z 650 to 950, and tetramers from
m/
z 950 to 1600. The samples present significant signal intensity up to 48.38 ×10
4~63.16 ×10
4 a.u. It is revealed from the MALDI-TOF-MS analysis that the PA is comprised of molecules with higher molar mass (328 Da) than NA, which had 252 Da. These results are obtained from the comparison of PA and NA bitumens and are presented in
Figure 2. The reason behind these results is attributed to the higher heteroatom content and lower H/C ration in PA. On the other hand, when compared with the asphalt fractions, PA has high aromatics and low saturates.
3.4. FT-IR Analysis
In analytical chemistry, FT-IR is found to be one of the most flexible and resourceful techniques. It is efficient at providing valuable information about the chemical functional groups of asphalts in complex solids [
6].
Figure 3 demonstrates the infrared spectra of PA and NA. For the scenario of NA, the characteristics of IR spectra of clay minerals are clearly visible in
Figure 3, where Si‒OH and absorbed water on clay are illustrated through the band near 3650 cm
−1. In
Figure 3, the broad band around 1600 cm
−1 is assigned to the ring vibration of aromatic compounds. Whereas the band at 1083 cm
−1 represents the anti-symmetric unfolded vibration of Si‒O‒Si [
7], the other bands are detected within the range of 1000 and 650 cm
−1, which represent the characteristics of clay materials.
All of the asphaltic samples’ spectra exhibit similar stretching vibrations of classic CH2, which is demonstrated by the peaks within the range of 2854–2932 cm−1.
The peak due to carbonyl C=O group (i.e., carboxylates, ketones, and/or anhydrides) near 1700 cm−1 is prominent in the spectrum of NA and appears with no or a very weak intensity in the spectrum of PA; indicating that NA was much more severely oxidized than PA after a prolonged exposure to atmospheric air. This observation is fully in line with the results of elemental analysis (NA oxygen 1.38 wt %; PA oxygen 0.29 wt %).
The band at 1034 cm
−1 may result from the sulfoxide S=O stretching vibrations. Also, the peaks at 872 cm
−1, 810 cm
−1, and 748 cm
−1 may relate to the out-of-plane bending vibrations of C‒H in phenyl [
8]. The sharp peak located at 722 cm
−1 in the PA spectrum can be associated with long, straight-chain methylene. Overall, the NA contains polar groups such as carbonyl and some clay minerals as observed from the IR spectrum. It is also constituted of minor complex components, which have distinct chemical functional groups.
3.5. Raman Spectroscopy (RS)
One of the most influential and efficient methods for characterizing asphaltic materials is Raman spectroscopy [
9,
10]. This method is, therefore, applied here to analyze the characteristics of PA and compare it with NA. The same has been demonstrated in
Figure 4, which presents the Raman spectrum within the Raman shift range of 200~3600 cm
−1. This spectrum demonstrated two first-order characteristic bands of all the asphaltic materials. The first is the D (defect) band at 1344.98~1348.79 cm
−1 and the second is the G (graphite) band at 1577.90~1607.50 cm
−1 of graphitic carbon. Whereas the emergence of the D band is due to disordered structures or defects in the carbon, the G band originates from the tangential stretching vibrations of the aromatic C‒C bonds. It is observed from
Figure 4 that, in NA, the D and G bands are feebler than those of PA. It is inferred from this revelation that NA has worse order than PA. The analysis results of TLC-FID and elemental analysis data showed that NA has a high concentration of saturates (14.17 wt %) and about 1.38 wt % oxygen, where the higher amounts of oxygen atoms are bound to aliphatic carbon. Therefore, as is evident from these analyses, the higher oxygen content and aliphatic structure have resulted in higher disorder in the structure of NA as compared to PA.
There are various bands that have emerged in the second-order Raman spectra of asphaltic materials at ~2450, ~2695, ~2735, ~2950, and ~3248 cm
−1. Such bands are allocated to both overtone scattering and combination scattering.
Figure 4 shows the same, where all of the asphaltic materials display a weak Raman band within the range of 2500~2800 cm
−1; this attribute agrees to the overtone of the D band. D band is originally referred to as the G' band as it is symmetric and is apparent in the second-order Raman spectra of crystalline graphite. It is also referred as the 2D or D* band by several researchers [
11]. The subtle band of 383 cm
−1 reveals the amorphous sp
3-bonded carbon.
3.6. Nuclear Magnetic Resonance Spectroscopy (1H-NMR)
Figure 5 gives the proton NMR spectra of varied asphalts. Also,
Table 4 and Figure 6 present the positions of the chemical shifts [
12]. The distinct types of protons in the asphalt samples are analyzed in terms of their relative proportions through the integration of the spectra. The same are presented in
Table 3.
PA and NA are found to exhibit similar spectra, as is evident in the studies of
1H-NMR (
Figure 5). However, the
1H-NMR data in
Table 3 shows the highly complex and rich nature of NA as compared to PA.
The deuterated chloroform (CDCl3) is presented by the peak of 1H-NMR spectra at 7.3 ppm. To obtain this result, each of the asphaltic samples was dissolved for the NMR measurements.
When considering the
1H-NMR spectra, which is donated by protons, the aromatic peaks are easily determined (H
ar; δ = 6.5~9.5 ppm) from the aliphatic ones (H
al; δ = 0.5~4.5 ppm). Also, the spectra were rendered more dependable and consistent. Furthermore, the aliphatic peaks were segregated into three types of protons (α, β, and γ) as per their respective positions in the aromatic core: H
α, δ = 2.0~4.5 ppm; H
β, δ = 1.0~2.0 ppm; and H
γ, δ = 0.5~1.0 ppm. However, the borders defined here are approximate as the impact of heteroatoms and metals on peak shifting is ignored. Irrespective of this, the domains described in
Table 4 are often utilized.
Table 3 gives the data about the aliphatic hydrogen concentration, which is found to be higher in NA than PA. This revelation shows that NA is composed of molecules that are highly branched. It is worth mentioning that aromatic conjugated compounds not bearing hydrogen atoms cannot be detected by
1H-NMR.
An illustration of the types of hydrogen is displayed in
Figure 6 below:
Description and chemical shift ranges in
1H-NMR is given above (
Table 4).
Har2: 1; Har1: 2; HF: 3, HA: 4; Hα1: 5a, b, c; Hβ2: 6; Hβ1: 7; and Hγ: 8.
Hydrogen in α-methyl group: 5a; Hydrogen in α-methylene in alkyl side chain: 5b; and
Hydrogen in α-methylene in hydroaromatic ring (5c).
3.7. Ultraviolet and Visible Spectroscopy (UV-VIS)
Ultraviolet and visible spectroscopy is utilized to examine the structure of the aromatic component in the asphalts.
Figure 7 illustrates the UV signal data (absorptivity vs. wavelength) within the range of 240~600 nm by utilizing the UV instrument. The motive behind selecting this UV range is that it has a specific sensitivity to fractions with high contents of aromatic molecules. As per the literature [
13], it was observed that the onset wavelength of absorption peak at 270, 320, 380, 470, and 580 nm were of benzene, naphthalene, anthracene, tetracene, and pentacene, respectively. Near 260 nm of the region, an absorption band with an extended wavelength tail is highly significant, as presented in
Figure 7. This signifies that the components equipped with benzene rings are dominant for all asphaltic materials. While comparing the spectrum of PA with NA, it was observed that the PA spectrum exhibits weaker absorption band at 261.5 nm. The same is depicted in
Figure 7. The findings suggest that NA is equipped with a higher concentration of conjugated systems and/or larger chromophores as compared to PA. To conclude, the UV-Vis spectra have indicated a similar composition of aromatic rings in the aromatic nuclei of both the asphalts.
3.8. X-ray Diffraction (XRD)
Figure 8 and
Figure 9 demonstrate the X-ray diffraction patterns of PA and NA, respectively. The wide peaks at 18°~26° and 42° in
Figure 8 represent the X-ray diffractogram of PA. The γ-peak appears at around 2θ = 18.90° because of the aliphatic chains or condensed saturated rings. The peak settled at approximately 2θ = 23.20° is known as the graphene band or (002)-band. The formation of the graphene band is emerged by staging the aromatic molecules existing in the asphaltenic structure. At the 2θ value (42.39°), a weak band is formed, which is due to the influence of first (100) nearest neighbors in the ring structure [
14,
15].
The NA material is rendered sticky due to the presence of organic mineral substances. The features of resilience and toughness of asphalts is obtained with the help of organic substances present in it, as it provides the characteristics of binding. H
2O
2 digestion was deployed to evaluate the organic fraction of NA. It was observed to be 91.17 wt %.
Figure 9 demonstrates the XRD pattern of NA. This figure elucidates the major crystalline components composing the bulk sample of NA. After kaolinite and illite, quartz is found to be the clay material that is present in the greatest abundance. Lower amounts of smectite, calcite, montmorillonite, fluorapatite, and schertelite were also detected. As per this analysis, the mineral composition of the NA is assessed to be over 95% quartz (SiO
2) and a few percent Feldspars (K-component; KAlSi
3O
8). On the other hand, the XRD pattern of PA did not show peaks symbolizing the crystalline structure.
Compared to the FT-IR technique, X-ray diffraction is a powerful tool in the identification and characterization of minerals in NA. The bulk of the clay fraction of many natural asphalts is crystalline, but clay particles are too small for optical crystallographic methods to be applied. Therefore, XRD has long been a mainstay in the identification of clay-sized minerals in natural asphalts. It is interesting to note that fine mineral matter is expected to strengthen the bitumen in the NA and impart hardiness and enhance pavement surface properties (better tire–road surface interaction), if is added as a modifier in paving-grade asphalt cements.
3.10. Thermogravimetric Analysis (TGA)
To acquire and select the best attributes and measurements for asphalt products, it is imperative to gauge the thermal stability of the asphalt materials pertaining to their weight loss due to volatilization. This is an essential property that needs consideration when generating high-performing asphalts for particular applications.
Thermogravimetric analysis was performed to comprehend the thermal behavior of the asphaltic materials.
Figure 11 and
Figure 12 represent the TGA (Thermogravimetric analysis) and DTA (Differential thermogravimetric) curves acquired at the heating rate of 10 °C/min, respectively.
Table 5 presents the onset and the offset temperature of thermal degradation (T
onset, T
offset), yield of carbonaceous residue, and the maximum decomposition temperature (T
max) at 700 °C.
When the temperature is subjected to an increase, the tendencies of thermal decomposition of the two asphaltic materials are observed to be similar. Up to at least 180 °C (TGA curve), the samples are found to be stable, after which mass loss occurs. The analytical findings reveal that both PA and NA exhibit different weight loss behavior within a temperature range of 25 °C to 700 °C for 1 h and 13 min.
After subjecting the samples to high temperatures, it was observed that NA lost large amounts of weight as compared to PA. This indicates that NA has low thermal stability among its molecules. This is in accordance with the elemental analysis and TLC-FID data (
Table 1 and
Table 2). Also, when considering that H
2 and CH
4 evolution is the reason behind higher weight loss during heating [
16], the asphalt with the higher hydrogen content (i.e., NA) has a larger weight loss.
As can be seen in
Figure 11, the linear superposition of the component reactions in asphalt corresponds to the TGA curves with significant peaks and shoulders. As mentioned before, the asphalt components are segregated into four categories of SARA composition:
saturates,
aromatics,
resins, and
asphaltenes [
17]. Each of these components has different combustion activity (S + A > R > A) [
18], thus each component reaction may correspond to a single mass loss peak.
For PA and NA, at temperatures of 333.3 °C, 441.62 °C, and 546.62 °C; and 286.63 °C, 433.30 °C, and 489.96 °C, respectively, the three major exothermic events are easily differentiated. The existence of these three events is due to the susceptibility of present organic material to igniting in the atmosphere. After the completion of the thermal treatment process, the NA sample had a charred residue of 8.83 wt %, while PA had 0.99 wt % of residue. Such a revelation indicates that NA is constituted of mineral materials that are transformed into ash during combustion.
The three pronounced regions of weight loss, i.e., (
i), (
ii), and (
iii), are evident in
Figure 11.
Region (
i) is formed due to the radical polymerization reaction, whereby the oil content comprising primarily saturates and aromatics is subjected to distillation and combustion [
19]. As the oil content has low molecular weight and pyrolysis temperature, they are removed at a low temperature.
In region (ii), the peripheral functional groups and heteroatom bonds are fragmented, the resins are oxidized and dehydrogenated, and then asphaltenes and char are generated in the cracking/polymerization reactions. The weight loss of asphalts in this region is due to these reactions. The second peak (405~500 °C) is attributed to the release of secondary volatiles and char combustion.
The last region (
iii) is reflected in the second mass loss peak in the TGA peak, within the temperature range of 530~680 °C. In this stage, the aromatic molecules show a steady increment, asphaltenes decompose [
20], and char is formed due to the condensation of free radical molecules. The char, which is found to be comparatively stable, is eventually burned.
3.11. Differential Scanning Calorimetry (DSC)
The asphalts are composed of a combination of distinct hydrocarbons with varied chemical structures and molecular weights. Therefore, it is crucial to comprehend their compositions and their impact on determining the complete thermal behavior of asphalts.
Figure 13 demonstrates the DSC scans of both PA and NA.
In the temperature range of −40 °C to 0 °C, asphalts exhibit a broad glass transition. When asphalts are subjected to the DSC scans, different melting peaks or shifts within the peak positions due to variant temperatures are observed, which correspond to the varied shapes of asphalts. There are two distinct glass transition temperatures exposed through the DSC curves, namely, the upper glass transition (T
g2) and the lower glass transition (T
g1). Whereas T
g2 is obtained from the maltene–asphaltene interphase region, T
g1 is obtained from the maltene phase [
21]. When NA is cooled or heated, the crystallites melt and generate a small endothermal peak (T = +44.15 °C) next to the glass transition (T
g2 = −11.45 °C).
Both PA and NA have Tg (onset) near −36.5 °C. The low molecular weights of the materials in the asphaltic fractions are the facilitator of the onset of Tg. Nonetheless, the differences between their major glass transition temperatures are dissimilar. When comparing PA with NA, PA has a lower, weaker glass transition (Tg1 = −31.16 °C) than NA, which has a high glass transition (Tg1 = −28.12 °C). The reason behind this characteristic is the differences in their chemical structure, molecular weight, and distribution. NA has a high concentration of saturates (14.17 wt %) and other sulfur-containing compounds with an average molecular weight of 252 Da, which makes it a black sticky material.
In the case of PA, it has approximately 328 Da of molecular weight along with aromatics in multi-ring structures, 4.40 wt % paraffin, and 18.97 wt % naphthene aromatics. The high molecular weight and inflexible nature of the multi-ring molecular structures in PA result in its greater Tg transition.
Tg is defined as the temperature at which all the molecular translational motion of the asphalt freezes, which leads to the rigidness of the material at or below this temperature. The Tg is related to the low-temperature performance of asphalts. PA is found to have a more brittle nature in low temperature (approximately between −40 and 0 °C) as compared to NA.
The endothermic peaks are more evident in PA than NA. Two major melting peaks are detected in the PA’ DSC thermogram that relate to the dissolution of crystallized fractions in the hydrocarbon matrix. Also, there is a small peak emerging between these two endothermic peaks.
When heating the asphalts to the wax dissolution, Lesueur (2009) [
22] explained the wide endothermic effects occurring between −30 °C and 100 °C in several naturally wax- (or saturate-) encompassing binders. Hence, the highly visible broad endothermic band up to 100 °C in all the asphaltic samples relates to the melting of diverse naturally occurring waxes existing in the asphalt [
22,
23].