J. Cent. South Univ. (2021) 28: 2179-2192

DOI: https://doi.org/10.1007/s11771-021-4761-3

Performance and mechanism of solid waste coking sulfur paste modified asphalt mixture before and after curing

ZHAO Yong-le(赵永乐), LI Guo-qiang(李国强), LI Tao(李涛), WANG Hong-yu(王宏宇),ZHANG Shu-ting(张舒婷), ZHANG Yong-fa(张永发)

Key Laboratory of Coal Science and Technology, Ministry of Education and Shanxi Province,Taiyuan University of Technology, Taiyuan 030024, China

Central South University Press and Springer-Verlag GmbH Germany, part of Springer Nature 2021

Abstract: For the resource utilization of the solid waste coking sulfur paste and the improvement of performance of the asphalt mixture, a method for preparing modified asphalt mixture with coking sulfur paste modifier (CSPM) is herein proposed. Compared with the matrix asphalt mixture, the Marshall stability of the 30% CSPM modified asphalt mixture increased by 38.3%, the dynamic stability increased by nearly one time (reaching 1847.5 times/mm), the splitting strength ratio increased by 39.3% while the splitting tensile strength decreased by 11.7%. After curing, the performance of the CSPM modified asphalt mixture was further improved. The results show that CSPM improved the high temperature stability and water damage resistance of the asphalt mixture, and the low-temperature anti-cracking performance of that was slightly reduced. Chemical analysis of asphalt binders shows that a little sulfur reacted with asphalt to produce polysulfide compounds (R-S_x-R′), and a part of sulfur existed in the form of crystalline sulfur which was further increased after curing. The presence of crystalline sulfur as an inorganic filler is the key point for improving the high temperature stability and water resistance performance of modified asphalt mixture.

Key words: solid waste; coking sulfur paste modifier; asphalt mixture; performance; mechanism

Cite this article as: ZHAO Yong-le, LI Guo-qiang, LI Tao, WANG Hong-yu, ZHANG Shu-ting, ZHANG Yong-fa. Performance and mechanism of solid waste coking sulfur paste modified asphalt mixture before and after curing [J]. Journal of Central South University, 2021, 28(7): 2179-2192. DOI: https://doi.org/10.1007/s11771-021-4761-3.

1 Introduction

Wet oxidative desulfurization is the main desulfurization method used for coke oven gas. In this process, H₂S is mainly recovered in the form of coking sulfur paste (CSP) [1-3]. In addition to sulfur, CSP contains by-product salts such as thiocyanate and thiosulfate, organic substances, and other impurities [4]. The CSP is an industrial solid waste which is difficult to use and can cause serious pollution. The annual global coke production is about 4.8×10⁸ t, and about 9.6×10⁵ t CSP needs to be treated urgently. The current purification treatment methods for CSP can be divided into three major categories: melting method, gasification method, and solvent method. However, they are associated with widespread problems such as high energy consumption, large investment, complicated operation, flammable and explosive raw materials, easy secondary pollution, and poor product quality [5]. Therefore, the resource treatment and utilization of CSP is an important research topic in the coking industry.

At present, many studies have applied solid waste to the road engineering as an inorganic filler or modifier [6-8], which is one of the effective ways to deal with industrial solid waste. DAY [9] proposed a method of sulfur modified asphalt as early as 1866. Noted that the main component of CSP is sulfur. Sulfur-modified asphalt can improve the material quality and pavement performance, which maximizes the replacement of asphalt and improving the resource utilization efficiency [10-12]. NGUYEN et al [13] studied the effect of sulfur on the asphalt mixtures and pointed out that the performance of sulfur modified asphalt mixture is superior to the conventional asphalt mixture, improving the rutting resistance and fatigue resistance of the asphalt pavement. PAPIRER et al [14, 15] showed that sulfur helps in increasing the bonding properties of asphalt. It has been confirmed that sulfur can significantly enhance the mechanical properties of asphalt mixture such as elastic modulus and rutting resistance, and the asphalt mixture also exhibits excellent water resistance. GAWEL et al [16] concluded that the addition of sulfur could reduce the viscosity of asphalt binder and the mixing temperature of asphalt mixture, which is contributed to save energy. Also, sulfur can prevent asphalt with a high wax content from bleeding phenomenon. During the preparation and mixing process of the asphalt mixture, sulfur modifier can be added directly to the asphalt and uniformly mixed under the shearing action of the aggregate, and the operation is simple. The United States Federal Highway Administration has also demonstrated that adding sulfur could improve the performance of asphalt mixture and save construction costs. The United States and Canada have used sulfur-extended asphalt (SEA) technology to successfully complete 100 road projects. Therefore, CSP might be potentially applied in the road engineering.

The research group used the ammonia-extraction method to effectively remove the water-soluble impurities in CSP. This method used CSP as the raw materials to prepare asphalt modifier. The pavement performance of the asphalt mixture is the direct basis for engineering applications. Although the sulfur content of the treated coking sulfur paste can reach more than 93%, it is still greatly different from pure sulfur in composition and properties. Therefore, the performance of the CSPM modified asphalt mixture needs to be investigated.

In this study, AC-20 was used as the aggregate gradation design to prepare CSPM modified asphalt mixture. CSPM modified asphalt mixture needs 15-20 d of mature period [17] to attain the stable performance. Therefore, the standard Marshall test, rutting test, freeze-thaw split test, water-immersed Marshall test, and low temperature split test were used to conduct systematic study of the pavement performances before curing and after curing for 20 d with CSPM modified asphalt mixture. The mechanism of CSPM modified asphalt mixture was analyzed by the chemical test of CSPM modified asphalt binder.

2 Experimental

2.1 Materials

2.1.1 Asphalt binders

Donghai brand 90# asphalt was used as the experimental material, referred to as DH for short. Its main properties are listed in Table 1.

Table 1 Properties of DH asphalt

2.1.2 Coking sulfur paste

The wet coking sulfur paste produced by the desulfurization section of Shanxi Coke Group Yixing Coking Co., Ltd. was used as the experimental raw material. The wet coking sulfur paste was dried in a vacuum drying oven for 8 h at 80 °C under the vacuum of 0.08 MPa, then ground into powder with particle size of about 0.2 mm. The above sample was labelled as CSP, which needed to be sealed and stored away from light. Table 2 lists the compositions of CSP [4].

CSP was thoroughly washed by the ammonia-extraction method [18] to effectively remove the water-soluble impurities and subjected to the sample treatment under the same conditions as CSP. The sulfur content of the obtained samples was 93.86%. Thereafter, heating and melting were conducted by adding a certain proportion of smoke　inhibitors to prepare coking sulfur paste modifier particles with particle size of about 2 mm. The above sample was labelled as CSPM.

Table 2 Composition of CSP (wt%)

2.1.3 Aggregates

The minerals were obtained from Jingyigou Stone Material Factory in Hejin city, China. The particle sizes of limestones were 15-23 mm, 10-15 mm, 5-10 mm, and 0-5 mm, respectively.

AC-20 graded asphalt mixture was selected as the target gradation in this test. The gradation design referred to JTG F40-2004 [19]. The gradation range requirements and gradation curve are shown in Figure 1.

Figure 1 Gradation range requirements and gradation curve

2.2 Preparation methods

Taking the substitution ratio of CSPM and the modification effect into full consideration [20], the additive amount of CSPM was determined to be 30% of the total mass of modified asphalt binder. The optimal asphalt content (OAC) of CSPM modified asphalt binder was calculated using the Marshall test method.

CSPM modified asphalt mixture was prepared by following JTG E20-2011 testing regulations for asphalt and asphalt mixtures for highway engineering [21]. The preparation process is involves heating and mixing. Heating process: The aggregate and mineral powder were separately maintained at (105±5) °C for 4 h, and then heated to 180 °C for future use. The asphalt and the mixing oven were heated to 130-140 °C for use. Mixing process: 　　1) Aggregate was stirred in the mixing oven for 30 s; 2) Asphalt and CSPM were added and kept stirring for 60 s; 3) Mineral powder was added and kept stirring for 90 s; 4) Temperature was maintained at 125 °C to mold the asphalt mixture test pieces. Finally, CSPM modified asphalt mixture and matrix asphalt mixture were labeled as CMAM and DH-AM, respectively.

CSPM modified asphalt binder was prepared with 70 wt% DH asphalt binder and 30 wt% CSPM. CSPM was added to DH asphalt binder at 135-140 °C. At the same time, CSPM and DH asphalt binder were thoroughly stirred using a mechanical mixer (2000 r/min) for 15 min. DH without additives was subjected to the same treatment of shearing and used as reference.

2.3 Test methods

2.3.1 Pavement performance test of asphalt mixtures

Standard Marshall test (GB/T 0709-2011), rutting test (GB/T 0719-2011), freeze-thaw split test (GB/T 0729-2000), water-immersed Marshall test (GB/T 0709-2011), and low-temperature split test (GB/T 0716-2011) were all carried out following the Chinese standard JTG E20-2011 [21].

2.3.2 Chemical test of asphalt binder

The four components test (GB/T 0618-1993) separated asphalt into saturate, aromatic, resin and asphaltene (SARA) by using the alumina column chromatography according to the Chinese standard JTG E20-2011[21].

The change of functional groups in CSPM modified asphalt binder was analyzed by a type VERTEX 70 Fourier transform infrared spectroscopy (FTIR) spectrometer from Bruker. The range of wave number was from 4000 to 400 cm^-1. About 1-2 mg sample was added to 100 mg KBr for grinding until it was well mixed. Then, the above powder mixture was pressed into sheet for FTIR scanning.

Gel permeation chromatography (GPC) test was performed with an Agilent 1260 instrument　when the mobile phase was tetrahydrofuran. Divide GPC chromatograms into 13 slices and analyze the change of the molecular weight distribution of CSPM modified asphalt binders before and after curing. The 1-5 slices, 6-9 slices, and 10-13 slices are defined as large molecular size (LMS), medium molecular size (MMS), and small molecular size (SMS), respectively [22].

Differential scanning calorimetry (DSC) was performed to measure the content of crystalline sulfur in CSPM modified asphalt binders using Netzsch STA449 F5 from Germany. CSPM modified asphalt binder sample (about 5 mg) was heated from 50 to 150 °C at a heating rate of 5 K/min in N₂ atmosphere.

Optical microscopy (OM) test was applied to observe the crystalline sulfur morphology of CSPM modified asphalt binders using an optical microscope from Phenix, Jiangxi, China. The modified asphalt sample was heated to 120 °C, which was dropped in the centre of a slide (about 5-10 mg). Then, the slide was placed horizontally in an oven at 135 °C for 10 min. The above specimen needed to be cooled slowly at room temperature.

3 Results and discussion

3.1 Optimal CSPM modified asphalt binder content

CSPM and asphalt are regarded as an integrated binder. According to the predetermined ratio, the conventional Marshall method design is used to determine the optimum content of the whole binder. Then the actual dosage of CSPM and asphalt is determined according to the proportion of 30% CSPM. Five kinds of CSPM modified asphalt binder contents (4.1%, 4.5%, 4.9%, 5.3% and 5.7%) were used to prepare the mixture.

According to the Marshall method design, the optimum CSPM modified asphalt binder content was determined. Figure 2 shows the relationship between the content of CSPM modified asphalt binder and the physical-mechanical index of CSPM modified asphalt mixture.

The optimum CSPM modified asphalt binder content is calculated using the following formula [15]:

OAC₁=(a₁+a₂+a₃+a₄)/4=(4.78%+4.96%+4.57%+4.58%)/4=4.72%                            (1)

where OAC₁ is the average value of a₁, a₂, a₃ and a₄. In formula (1), a₁, a₂, a₃, and a₄ represent the CSPM modified asphalt binder contents, which correspond to the maximum stability, the largest bulk density, the mid value of volume of air voids (VV), and the mid value of voids filled with asphalt, respectively. The mid values of volume of air voids and VFA follow the Marshall test standard of the dense grade asphalt mixture. OAC₂ is the average of OAC_max and OAC_min. In formula (2), OAC_max and OAC_min represent the CSPM modified asphalt binder content, which respectively correspond to the maximum and minimum VFA in accordance with the Marshall test standard of dense grade asphalt mixture.

OAC₂=(OAC_min+OAC_max)/2=(4.34%+4.98%)/2=4.66%                            (2)

OAC is the average of OAC₁ and OAC₂.

OAC=(OAC₁+OAC₂)/2=(4.72%+4.66%)/2=4.69%                             (3)

The optimum CSPM modified asphalt binder content was 4.69%. In this situation, the asphalt-aggregate ratio was 4.92%. The mass ratio of CSPM to asphalt was 30:70 in this experiment. Therefore, the CSPM content and the asphalt content were 1.48% and 3.44%, respectively. As per engineering experience, the optimum asphalt-aggregate ratio of the matrix asphalt mixture is 4.5%. Compared with the matrix DH asphalt mixture, CSPM modified asphalt mixture can reduce the dosage of asphalt by 1.06%. Therefore, the replacement ratio of CSPM to asphalt was 1.48: 1.06 (approximately 1.4:1). It will significantly reduce the paving costs. In this work, the pavement performance of CMAM and DH-AM was compared at the optimal asphalt-aggregate ratio.

3.2 High-temperature stability

3.2.1 Marshall stability

Figure 3 shows the physical contrast of the DH-AM and CMAM Marshall test pieces. As shown in Figure 4(b), CMAM is lighter in color, denser on surface, and less cracked. Table 3 shows the results of the Marshall test. The Marshall stability (MS) of CMAM before and after curing meets the requirements of technical indicators. Before curing, the stability of DH-AM was 6.03 kN and the flow value was 3.48 mm, while the stability of CMAM was 8.34 kN, and the flow value was 2.52 mm. Compared to DH-AM, the stability increased by 38.3% and the flow value reduced by 27.6%. After 20 d of curing, the stability of DH-AM was 6.34 kN and the flow value was 3.18 mm, the stability of CMAM was 9.21 kN and the flow value was 2.31 mm. The improvement effect of the strength of the mixture was more obvious. The Marshall test results show that the addition of CSPM improved the pressure resistance of the asphalt mixture under vertical load.

Figure 2 Marshall stability versus binder content (a), bulk density versus binder content (b), volume fraction of air voids (VV) versus binder content (c), voids filled with asphalt (VFA) versus binder content (d), flow value versus binder content (e), and voids in mineral aggregate (VMA) versus binder content (f)

3.2.2 Dynamic stability and rut depth

Figure 4 shows the physical contrast of the rutting test. From the exterior, it can be observed that the color of the asphalt mixture changes from black to grayish black. The reduction in the amount of asphalt made the mixture drier and the surface denser, which is consistent with the appearance of the standard Marshall test piece. The CMAM rolling marks were shallow and had no clear deformations. Figure 5 shows the rut depth data from the rutting test. The rutting depths of the CMAM before and after curing were 4.557 and 4.714 mm, which were lower than those of the DH-AM, respectively. The rutting depths of the DH-AM before and after curing were 8.378 and 6.959 mm, respectively. Table 4 lists the results of the rutting test. Before curing, the dynamic stability of DH-AM was 909.2 times/mm, and the dynamic stability of CMAM was 1847.5 times/mm, which was 1.03 times higher than that of DH-AM and was much higher than the requirement (800 times/mm) in “Technical Specifications for Construction of Highway Asphalt Pavement” [19]. After 20 d of curing, the dynamic stability of CMAM increased by nearly 10%, reaching to 2032.2 times/mm compared with that before curing. The rutting test results show that the addition of CSPM enhanced the permanent deformation resistance of asphalt mixture at high temperatures.

Figure 3 Standard Marshall specimens:

Table 3 Results of Marshall test

Figure 4 Photos of wheel tracking:

Figure 5 Rut depth data from rutting test

Table 4 Results of wheel tracking test

3.3 Moisture susceptibility

3.3.1 Freeze-thaw splitting test

Table 5 shows the results of freeze-thaw split test. The freeze-thaw splitting strength ratio (TSR) was defined as the ratio of the tensile strength after the freeze-thaw cycle (R_T2) and the tensile strength without the freeze-thaw cycle (R_T1). The results show that the residual stability before and after CMAM curing is higher than the requirements in Ref. [19]. Before and after the freeze-thaw cycle, the change in tensile strength of CMAM was more clear, mainly because of curing CMAM under the low temperature conditions during the freeze-thaw split test. Before curing, the freeze-thaw splitting strength ratio of DH-AM was 86.2% and that of CMAM was 38.4% higher than DH-AM, reaching 120.1%. After 20 d of curing, the freeze-thaw splitting strength ratio of CMAM decreased to 95.5%, which was still higher than that of DH-AM. These results show that the addition of CSPM effectively improved the water damage resistance of the asphalt mixture.

Table 5 Results of freeze-thaw split test

3.3.2 Immersion Marshall test

Table 6 lists the results of water-immersed Marshall test. The immersion residual stability (MS₀) was the ratio of the Marshall stability after immersion in water for 48 h (MS₁) and the Marshall stability without being immersed in water (MS). Before curing, the immersion residual stability of DH-AM was 91.5% while that of CMAM was 87.2%. Compared with DH-AM, the immersion residual stability of CMAM decreased but still met the technical index of immersion residual stability of ≥85% [19]. After 20 d curing, the immersion residual stability of CMAM and DH-AM was at the same level, reaching 90.1%. In the case of the same immersion residual stability, higher stability indicates that the stronger the cohesive force between asphalt and aggregate is, the more favorable it is for the asphalt mixture to resist water damage. As observed from the stabilities after immersion, the immersion residual stability of DH-AM generally did not change before and after curing, which was 5.52 kN before curing and 5.71 kN after curing. After curing, the immersion residual stability of CMAM became 8.34 kN, which was 46.0% higher than the stability of DH-AM after curing. Therefore, the results of the water-immersed Marshall test show that CSPM could effectively improve the water damage resistance of the mixture after curing.

Table 6 Results of immersion Marshall test

3.4 Low-temperature splitting test

Table 7 lists the comparison results of low-temperature splitting test. The results show that the deformations at failure of the two types of asphalt mixtures before and after curing were basically the same. However, the splitting tensile strengths were different. Before curing, the splitting tensile strength of DH-AM was 2.82 MPa, and reduced to 2.50 MPa after curing. The splitting tensile strength of DH-AM varied greatly before and after curing. The splitting tensile strength of CMAM before curing was 2.49 kN, which was lower than that of DH-AM before curing, a reduction of 11.7%. The splitting tensile strength of CMAM after curing was 2.40 kN, which is lower than that of DH-AM after curing. The main reason for this phenomenon is that the temperature sensitivity of CSPM was lower. Asphalt played a decisive role in the low temperature anti-cracking performance. Lower asphalt content in CMAM led to the reduction of CMAM splitting tensile strength.

Table 7 Results of low-temperature splitting test

3.5 Chemical analysis of asphalt binder

For the chemical analysis of the asphalt mixture, there are few suitable detection methods. Therefore, the four components test, FTIR, GPC, DSC, and OM analysis were carried out on 30% CSPM modified asphalt binder to further analyze the essential reasons for the performance change of CSPM modified asphalt mixture [23].

3.5.1 SARA analysis

Figure 6(a) shows the comparison diagram of the separated four components. The colors of saturate, aromatic, resin and asphaltene were colorless, dark brown, black brown, and black, respectively. The solvent in the four components was recycled and then dried in a vacuum drying oven, as shown in Figure 6(b). Note that the crystalline sulfur was abundant in the saturate fraction.

Figure 6 Four components of CSPM modified asphalt binder:

Table 8 lists the four components contents of CSPM modified asphalt binder before and after curing. The curing slightly impacted the four components contents of CSPM modified asphalt binder. As the result of adding CSPM, the contents of asphaltene and saturate increased, while the contents of aromatic and resin decreased. The saturate content increased by about 16%. As seen in Figure 6(b), the crystalline sulfur was abundant in the saturate fraction. The result indicates that sulfur dissolved in the saturate fraction is one of the reasons for the increase of saturate content. The asphalt with less than 10% asphaltene is a sol type asphalt [24]. The asphaltene content of CSPM modified asphalt binder was 7.51%, higher than that of DH asphalt binder. Therefore, CSPM modified asphalt and DH asphalt both are the sol type asphalt. The addition of CSPM reduced the proportion of aromatic and resin in modified asphalt. The specific reasons for the increase of saturate and asphaltene would be further studied in Section 3.5.2 using FTIR.

Table 8 Four components contents of CSPM modified asphalt binder before and after curing

3.5.2 FTIR analysis of SARA

Figure 7 shows the four components FTIR spectra of CSPM modified asphalt binder before and after curing. Compared with DH asphalt binder, the saturate FTIR spectra of CSPM modified asphalt binder exhibited absorption peaks at 471 cm^-1 in Figure 7(a). This peak corresponded to the stretching vibration absorption of S—S bond. The result demonstrates that the saturation fraction of asphalt tends to react with sulfur to generate polysulfide compounds (R-S_x-R′), which is another reason for the increase of saturate fraction in CSPM modified asphalt binder. After 20 d of curing, the absorption peak intensity of S—S bond decreased, which indicated that polysulfide was partially decomposed to form polysulfide with a lower degree of polymerization and free sulfur [17]. In addition, it is found that the four components FTIR spectra of CSPM modified asphalt binder were similar to DH asphalt binder regardless of curing. The functional groups of the aromatic, resin and asphaltene fraction were unchanged in Figures 7(b)-(d). The result shows that the aromatic, resin and asphaltene fraction did not react with CSPM to produce new substances. Therefore, the behavior of the increase of asphaltene may be attributed to the fact that CSPM promoted the oxidation condensation of some resin fraction to transform asphaltene. Table 9 provides the FTIR peak assignment of the four components [25], where υ, δ, γ and ρ represent stretching vibration, scissoring vibration and out-of-plane bending vibration, and rocking vibration, and subscripts s and as are for symmetrical and asymmetrical, respectively.

Figure 7 Four components FTIR spectra of CSPM modified asphalt binder before and after curing:

Table 9 FTIR peak assignment of asphalt four components

3.5.3 GPC analysis

The GPC chromatograms and the molecular size distributions of CSPM modified asphalt binder before and after curing are presented in Figures 8 and 9, respectively. The LMS of CSPM modified asphalt binder was 24.56%, which was 5.84% greater than that of DH asphalt binder. After 20 d of curing, the LMS of CSPM modified asphalt binder dropped to 19.95% and remained above DH asphalt binder. Combined with the four compounds FTIR analysis, this was mainly caused by the formation and decomposition of polysulfide compounds. Before curing, the MMS and SMS of CSPM modified asphalt binder were lower than those of DH asphalt binder. After curing, the MMS of CSPM modified asphalt binder had a smaller variance, and the SMS of CSPM modified asphalt binder is basically the same as that of DH asphalt binder. Therefore, there was a little influence of curing on MMS. Table 10 lists the number average molecular weight (M_n) and average molecular weight (M_w) of CSPM modified asphalt binder before and after curing. The results show that the M_n and M_w of CSPM modified asphalt binder were increased by 7.97% and 31.94% respectively. After curing, M_n and M_w decreased and were slightly higher than those of DH asphalt binder. Besides, the dispersion of CSPM modified asphalt binder increased significantly regardless of curing. This suggests that the addition of CSPM increases the gap of asphalt molecular size [26]. A small part of CSPM reacted with the saturate fraction, and the remaining CSPM might exist in the form of dissolved and crystallized sulfur.

Figure 8 GPC chromatograms of CSPM modified asphalt binder before and after curing

Figure 9 Molecular size distributions of CSPM modified asphalt binder before and after curing

Table 10 Number average molecular weight (M_n) and average molecular weight (M_w) of CSPM modified asphalt binder before and after curing

3.5.4 DSC analysis

Figure 10 shows the DSC curves of CSPM modified asphalt binder before and after curing. The crystalline sulfur phase exhibited the melting peak at 119.42 °C in the DSC curve, which was used to calculate the content of crystalline sulfur in CSPM modified asphalt binder [27]. The heat absorbed during the melting of the crystalline sulfur is 36.69 J/g. The heat absorbed of CSPM modified asphalt binder before curing at 115.17 °C was 2.897 J/g and the content of crystalline sulfur was 7.896%. After curing, the heat absorbed was 4.119 J/g at 116.55 °C and the content of crystalline sulfur was 11.226%. The content of crystalline sulfur increased by 3.33% after curing. The crystalline sulfur acted as inorganic filler to enhance high temperature deformation resistance of asphalt mixture.

Figure 10 DSC curve of CSPM modified asphalt binder before and after curing

3.5.5 Microstructures by OM analysis

Figure 11 shows the morphology of DH asphalt binder and CSPM modified asphalt binder before curing. In Figure 11(a), the surface of DH asphalt binder evenly distributed exhibiting “bee like structure” (black bee like) [28]. Compared with DH asphalt binder, the number of “bee like structure” on the surface of CSPM modified asphalt binder increased obviously, as shown in Figure 11(b), indicating that CSPM can promote the formation of “bee like structure”. The micro crystalline sulfur (white spot) was uniformly dispersed in the CSPM modified asphalt binder, which played a role of aggregate in modified asphalt as filler or structural agent. After curing for 20 d, the sulfur unit structure similar to network structure appeared in the CSPM modified asphalt binder, as shown in Figure 12(a). Regions I and II in Figure 12(a) are enlarged as shown in Figures 12(b) and (c). As shown in Figure 12(c), the amount of “bee like structure” in the sulfur unit structure decreased and the image of “bee like structure” was not clear and white. This might be attributable to the formation of a discontinuous micro crystalline sulfur film on the surface of the sulfur unit structure, covering part of the “bee like structure”. Figure 12(c) shows the tubercles (black bulge and aggregated white bright spot) in the sulfur unit structure. The tubercles might be composed of small sulfur particles formed by slow crystallization of dissolved sulfur in CSPM modified asphalt binder. In addition, the micro crystalline sulfur outside the sulfur unit structure (Region I) increased significantly after curing, as shown in Figure 12(b). During the curing process, the dissolved sulfur in CSPM modified asphalt binder crystallized slowly and the micro crystalline sulfur migrated to form a large sulfur unit structure, which allowed sulfur to fill the remaining voids and bring higher cohesion.

Figure 11 Morphology of DH asphalt binder and CSPM modified asphalt binder before curing:

Figure 12 Morphology of CSPM modified asphalt binder after curing:

4 Mechanism

Asphalt binder and the chemical components on the surface of the minerals are rearranged, forming a layer of diffusion structure film with the thickness of δ₀ on the surface of the minerals (as shown in Figure 13(a)). The asphalt within the film layer is called as the structural asphalt, and asphalt outside this layer thickness is called free asphalt. Based on the analysis in Section 3.5, CSPM has the combined characteristics of inorganic filler modifier (crystalline sulfur) and chemical modifier (chemically bound sulfur). The modification mechanism is shown in Figure 13.

During the mixing of CSPM and asphalt, CSPM increased the contents of asphaltene and saturate and decreased the contents of aromatic and resin. However, CSPM modified asphalt was still a sol type asphalt (see Figure 13(b)), as the same as DH asphalt (see Figure 13(a)). In addition, the chemical reaction occurred between a small amount of sulfur and asphalt to form polysulfide compounds (R-S_x-R′). For CSPM modified asphalt binder, the generation of the polysulfide compounds and the change of asphaltene increased the number average molecular weight (M_n) and average molecular weight (M_w) of the asphalt. This behaviour can enhance the elasticity and thixotropic properties of the asphalt binder, increasing the pressure resistance of the asphalt mixture and high temperature resistance to deformation.

Before curing, the content of crystalline sulfur was 7.896%. The crystalline sulfur was uniformly dispersed in the asphalt as a structural agent (see Figure 13(b)), which could reduce the fluidity of asphalt binder and effectively improve the Marshall stability and dynamic stability of CMAM. The freeze-thaw splitting strength ratio was higher than 100% before curing because the freeze-thaw cycle accelerated the precipitation of crystalline sulfur in the modified asphalt mixture, which significantly increased the splitting strength after freezing and thawing.

After 20 d of curing, the content of crystalline sulfur was 11.226%. The high-temperature stability and water resistance performance of CMAM were further improved because of the conversion of dissolved sulfur to crystalline sulfur. The crystalline sulfur is a sulfur eight-ring structure (see Figure 13(c)), which has similar structural characteristics to the aromatic component in the asphalt [29, 30]. It is easily adsorbed on the surface of the asphalt during crystallization and migrated into a larger sulfur unit structure to form a strong cohesion. Meanwhile, the slow precipitation of crystalline sulfur filled a part of gap between the mineral and asphalt (see Figure 13(c)) to form the structure with greater compactness and less porosity, effectively preventing the relative slip between the minerals. This further improved the bearing capacity of the joint surface of the minerals and asphalt. Besides, the main reason for improving the water damage resistance of the mixture was that the precipitated crystalline sulfur reduced the porosity of the mixture. The crystalline sulfur was insoluble in water and did not easily absorb water. The saturated water content in the mixture was made to be reduced; thereby, reducing the water pressure inside the pores and alleviating the damage of the mixture due to water.

In conclusion, the crystalline sulfur acted as an inorganic filler to hold the overall structure of the asphalt mixture, which enhanced the structural strength [27, 31]. Therefore, the crystalline sulfur plays a key role in improving the high-temperature stability and water resistance performance of CMAM.

5 Conclusions

1) The performance and mechanism of CSPM modified asphalt mixture were systematically studied. The optimum content of CSPM modified asphalt binder is 4.69% and the asphalt-aggregate ratio is 4.92%. The amount of CSPM is 1.48%, the amount of asphalt is 3.44%, and the substitution ratio of CSPM to asphalt is 1.4: 1.

2) Adding CSPM could effectively improve the high-temperature performance and water damage resistance, slightly decreasing the low-temperature crack resistance of the modified asphalt mixture. This is because the lower asphalt content resulted in a decrease in the CMAM splitting tensile strength. Therefore, the low-temperature crack resistance of CMAM needs to be further improved. In future research, the addition of plasticizers and other substances can be used as one of the directions to improve the low-temperature performance of CMAM.

Figure 13 Mechanism of CSPM modified asphalt mixture:

3) CSPM modified asphalt is a sol type asphalt and CSPM could promote the increase of the asphaltene fraction. Besides, a few sulfur reacts with asphalt to produce polysulfide compounds (R-S_x-R′). These behaviors result in the increase of the number average molecular weight and average molecular weight.

4) The crystalline sulfur is uniformly dispersed in CSPM modified asphalt binder. After curing, the content of crystalline sulfur increases by 3.33%, forming the sulfur unit structure composed of tiny crystalline sulfur and sulfur tubercles. These behaviors are the critical factor to improve the road performance of CSPM modified asphalt mixture. Using CSPM in road engineering can extend the service life of the road, replace part of the asphalt, reduce the paving cost, and at the same time realize the resource utilization of solid waste.

Contributors

The overarching research goals were developed by ZHAO Yong-le, LI Guo-qiang, and ZHANG Yong-fa. ZHAO Yong-le and LI Tao provided the measured asphalt mixture performance data, and analyzed the measured data. ZHAO Yong-le, WANG Hong-yu and ZHANG Shu-ting provided the characterization analysis of asphalt binders and proposed the mechanism of modification. The initial draft of the manuscript was written by ZHAO Yong-le, LI Guo-qiang, LI Tao, and ZHANG Yong-fa. All authors replied to reviewers’ comments and revised the final version.

Conflict of interest

ZHAO Yong-le, LI Guo-qiang, LI Tao, WANG Hong-yu, ZHANG Shu-ting, and ZHANG Yong-fa declare that they have no conflict of interest.

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(Edited by FANG Jing-hua)

中文导读

固体废弃物焦化硫膏改性沥青混合料养护前后路用性能及机理

摘要：为了资源化利用焦炉煤气净化脱硫过程中产生的固体废弃物焦化硫膏，并提高沥青混合料的路用性能，提出了以焦化硫膏为原料制备焦化硫膏沥青改性剂(CSPM)的方法。与基质沥青混合料相比，添加30%的CSPM制备改性沥青混合料的马歇尔稳定度提高了38.3%，动稳定度提高了近1倍(达到1847.5次/mm)，劈裂强度比提高了39.3%，劈裂抗拉强度降低了11.7%。养护后，CSPM改性沥青混合料性能较养护前得到进一步提高。结果表明，添加30%的CSPM提高了沥青混合料的高温稳定性和抗水损害性能，但其低温抗裂性能略有下降。沥青胶结料的化学分析结果表明，少量的硫与沥青反应生成多硫化物(R-S_x-R′)，部分硫以结晶硫的形式存在，并在养护后进一步增多。结晶硫作为无机填料是提高改性沥青混合料高温稳定性和耐水性的关键。

关键词：固体废弃物；CSPM；沥青混合料；性能；机理

Foundation item: Project(201703D321006) supported by the Shanxi Provincial Key Research and Development Project (Social Development), China

Received date: 2020-05-06; Accepted date: 2020-09-16

Corresponding author: LI Guo-qiang, PhD, Lecturer; Tel: +86-351-6018676; E-mail: liguoqiang01@tyut.edu.cn; ORCID: https://orcid. org/0000-0003-1539-9854