Coal tar obtained by coal coking of Erdenet Mining Corporation in Mongolia was used in this study. This tar was distilled to 360˚C, and the residue or coal tar pitch was employed as a modifier for bitumen. The basic material used in the study was BND90/130 bitumen from Russia.

The chemical composition of the coal tar was determined by gas chromatograph/mass spectrometer (Agilent 5973N). The tar was distilled into fractions of IBP-180˚C (gasoline-like fuel), 180˚C - 360˚C (diesel-like fuel), and >360˚C (residue or coal tar pitch) [30] . Functional groups of the hydrocarbons of each fraction were investigated by FT-IR (Alpha II, Bruker, Germany).

The physical and mechanical properties of pristine bitumen and modified bitumen with various amounts of coal tar pitch were analyzed. MNS 5109-2001, MNS 5211-2002, and MNS 5211-2002 standards were used to assess penetration, softening point, and ductility, respectively. The composition of the hydrocarbon groups of bitumen and modified bitumen was determined by the SARA method [31] , which involved precipitating asphaltene with n-heptane in a volume 40 times that of the research sample. Then the maltenes separated from the asphaltene were passed into the Soxhlet apparatus with activated silica gel, and saturated hydrocarbons were extracted by n-heptane, aromatic hydrocarbons by toluene, and resin components by 1:1 toluene-ethanol solution.

The chemical compositions of coal tar obtained by coal coking are summarized in Table 1.

As shown in Table 1, coal tar contains considerable amounts of aromatic hydrocarbons, especially benzene and naphthalene derivatives in a total content of 30.74%. The majority of the chemical composition of coal tar consists of high molecular aromatic and heterocyclic hydrocarbons [32] [33] . However, the composition of coal tar can’t be explained completely yet these days. Coal tar includes 20% - 30% phenolic compounds [34] and the main representatives are phenol, (o-, m-, p-) cresol, dimethyl phenol, ethylphenol, methyl ethyl phenol, and trimethyl phenol [35] [36] . The tar analyzed in this study contains about 13.96% of them and nitrogen-containing compounds which are mostly indoles, carbazoles, pyridines, and quinolines [37] and was about 1.66%. The yield of distilled fractions of coal tar is shown in Table 2.

The coal tar analyzed in our study has a higher boiling point than 98˚C. Some researchers found out that the IBP of the tar obtained by catalytic hydroprocessing was 118˚C [30] and 159˚C [38] . Furthermore, Maloletnev A et al. [39] and Bai Z et al. [40] reported that the IBP of the coal tar obtained by distillation was 137˚C and 70˚C, respectively. The IBP of coal tar obtained in different ways is 70˚C - 159˚C in earlier studies, which is near to our results. It can be concluded that tar has less of the compounds which evaporate at low temperatures due to coal tar obtained by the thermal treatment of coal at relatively high temperatures. As shown in Table 2, diesel-like fuel represents more than 50% of distilled fractions of coal tar, which is suitable for the processing of basic components in diesel fuel. Some researchers such as Jipeng Meng et al. [41] , Tao Kan et al. [30] and Dong Li et al. [42] reported that the yield of diesel fraction is 43% - 64% in coal tar, which is close to our data. All of the fractions obtained from coal tar are valuable and useful, and many researchers have recently focused on the tar residue > 360˚C, which has a yield of 50% - 60% [43] .

Figure 2 presents the FT-IR results of distilled fractions obtained by coal tar.

In the IBP-180˚C and 180˚C - 360˚C fractions showed the deformation variation of −OH groups in the 3400 - 3200 (3325 cm⁻¹) absorption ranges, and the alkane (−CH₂, −CH₃) in the absorption ranges of 2959 cm⁻¹ and 2856 cm⁻¹, aromatic C=C valence fluctuations at 1600 cm⁻¹, 1457 cm⁻¹, aromatic C−H deformational fluctuations at 812 сm⁻¹, 752 сm⁻¹, 690 сm⁻¹ and, C−O deformational fluctuations at 1243 cm⁻¹ 1236 cm⁻¹ [44] [45] . As seen in Figure 2, three different fractions are complex and with many overlapping bands. The intensity of peaks 3400 - 3200 range (3325 cm⁻¹) corresponding to −OH groups in the above 360˚C fraction significantly decreased compared to other fractions. Here, phenol and its derivatives can be increased in the middle fraction. The main influence of

light and medium fractions to heavy residues is that the refined absorption of aromatic C=C valence is stronger in the region of 1600 cm⁻¹ and 1454 cm⁻¹, which can be explained by the high contents of polycyclic aromatic hydrocarbons in this fraction.

The characteristics of bitumen alter depending on the doped amount of coal tar pitch. Table 3 lists the physical and mechanical parameters of BND 90/130 pristine bitumen and coal tar pitch.

Coal tar pitch obtained by distillation of coal tar was a brittle powdery and black solid material at standard conditions. As shown in Table 3 that the bitumen used in this study has high elasticity, softening point, and penetration, which are within the BND90/130 bitumen standard. Modified bitumen prepared with various amounts of coal tar pitch doped into the bitumen (BND90/130), then determined the basic properties of ductility, penetration, and the softening point. In Figure 3, we can see the influence of coal tar pitch on the ductility and penetration properties of the bitumen.

Our results revealed the addition of 3% coal tar pitch was the highest results than other ones, which could be improved bitumen ductility by 12.4% or 97.9 cm to 110 cm. Bitumen ductility is one of the crucial parameters of bitumen and increases in ductility decrease road break. The addition of tar pitch to bitumen produced favorable effects due to enhancing force of intermolecular adhesion of the imported bitumen. Figure 3 shows the influence of the addition of coal tar pitch on the ductility and penetration of imported bitumen. Results from our study revealed that the appropriate percentage of coal tar pitch was 3%.

As seen in Figure 3, the bitumen penetration reduces as the amount of coal tar pitch increases. The coal tar pitch has a high softening point and solid state at standard conditions, and can greatly improve the hardness abilities of the bitumen system. The softening point evaluates bitumen hardness characteristics, and

Figure 4 shows the relation between the rate of coal tar pitch and softening point.

Figure 4 shows that the percentage of coal tar pitch is directly proportional to the softening point. As the softening point rises, road construction with that bitumen has the advantage of avoiding the formation of the road. The ability to ductility and the softening point of bitumen is directly related to the content of resin, asphaltene, and oil in the bitumen composition. The penetration is a key factor influencing bitumen quality and is disproportional to the value of softening point as can be seen in Figure 5.

Figure 5 shows that by increasing the amount of the coal tar pitch, the penetration decreases while the softening point increases and that these two parameters are inversely related. Physico-mechanical determination is insufficient to evaluate the characteristics of bitumen. Therefore, chemical groups of the bitumen composition and the modified bitumen were determined by the SARA method [31] . The modified bitumen was named modified bitumen 1, 2, and 3 depending on the percentage of the coal tar pitch, respectively, and the results are listed in Table 3.

Table 4 shows that the studied pristine bitumen has low asphaltene contents in its composition. Coal tar pitch contains high concentrations of resin and asphaltene compounds, which can significantly improve bitumen quality. As a result, the addition of coal tar pitch to bitumen enhances adhesion while decreasing weathering [1] . Table 4 shows that the content of resin and asphaltene for the modified bitumen-2 has increased by 1% - 2% compared to the pristine bitumen, however, the content of saturated and aromatic hydrocarbons decreased. Lowering the saturated hydrocarbons in the bitumen system can be decreased its rheological properties or persistent damage caused by environmental and climate impacts, inversely, with increasing asphaltenes in bitumen leads to the improvement of the above mentioned properties and problems [46] . The chemical

composition of bitumen is the most important factor in the influence of road deformation. Asphalt-rich bitumen is valuable for that.