Evaluation of Rutting and Fatigue Behaviors of Asphalt Binders Modified with Calcium Lignosulfonate

Asphalt mixes encounter different distress during the life cycle of asphalt pavements, among which rutting and fatigue failure are prominent. Moreover, the addition of environmentally friendly modifiers into the asphalt binder to increase the performance of asphalt mixes has been a challenging phenomenon for researchers. Calcium lignosulfonate (CL) is a waste material that is a byproduct of the wood industry. In this paper, the impact of the calcium lignosulfonate on the rutting and fatigue performance of the asphalt binder was investigated. For this purpose, the Dynamic Shear Rheometer (DSR) was utilized to run the Multiple Stress Creep Recovery (MSCR) test and evaluate G∗/sinδ and G∗·sinδ indices for the asphalt binder containing different percentages of CL. *e elastic recovery test was also conducted on the asphalt binder. In order to analyze the thermal storage stability of CLmodified asphalt binders, the storage stability test was considered. *e Field Emission Scanning Electron Microscope test showed that using CL as a binder modifier makes the texture of the asphalt binder spongy and porous. *e conventional test results indicated that increasing the CL amount in the asphalt binder led to an increase in the stiffness of the asphalt binder. *e rheological test results showed that the rutting resistance of the asphalt binder improved by adding up to 15% of the CL powder; however, the fatigue performance and the elasticity of the asphalt binder declined by increasing the CL content in the asphalt binder. *e storage stability test revealed that the dispersion of CL in the asphalt binder was uniform; moreover, the presence of CL in the asphalt binder could not adversely affect the thermal storage stability of the modified asphalt binder.


Introduction
Calcium lignosulfonate (CL) is an amorphous yellow powder, which is made of wood lignin. Wood lignin is a byproduct of the wood industry that consists of phenylpropane units [1]. Every year, a large amount of lignin waste is produced and disposed of in the world, which causes environmental problems [2]. erefore, finding a way to reuse this material in the industry is a major concern to the researchers. To this aim, this material is used as an additive in concrete and asphalt mixes. It is shown that the use of this material in the asphalt industry improves some of the performance characteristics of asphalt binder and mixes. In addition, it provides environmental benefits [2,3]. For example, previous findings showed that the use of biomass materials in the construction process of asphalt mixes lessens the need for harsh chemical materials, which leads to minimizing health risks of humans and reducing chemical pollutions of aquatic environments located near the asphalt pavements [4,5].
In 2013, wood lignin was used as an asphalt binder modifier. For this purpose, 5% and 10% of wood lignin were added to two different types of neat and polymer-modified asphalt binder. e rheological properties of the modified asphalt binder were investigated. It was shown that using wood lignin can improve the rutting resistance of the asphalt binder. e improvement was more significant compared with the neat asphalt binder. ere was also no significant change in the low-temperature performance of the asphalt binder when modified with the wood lignin compare with the virgin asphalt binder [6]. e antiaging performance of the lignin-modified asphalt binder was investigated in 2017 [7]. Moreover, it was shown that the wood lignin has the potential to substitute a portion of the asphalt binder. Lignin modification improved the aging and rutting resistance of the asphalt binder. However, it had an adverse effect on the fatigue performance of the asphalt binder. Another study in 2020 investigated the performance of asphalt binder modified with different percentages of wood lignin [8]. It was shown that using up to 8% of wood lignin improved the rutting and aging resistance of the asphalt binder, while it did not have any substantial impact on the fatigue performance. An improvement in rutting, aging resistance, and a reduction in the fatigue performance of the asphalt binder containing lignin was reported in another study in 2020 [9]. Arafat et al. showed that the original source of lignin was influential on the physical, chemical, and rheological behavior of the supplied lignin; as an illustration, lignins achieved from different plant materials had various properties of phenylpropanoids [2]. Batista et al. used the kraft lignin supplied from the pulp and paper industries to modify the asphalt binder [10]. e modified specimens exhibited higher rutting resistance at high temperatures. Moreover, lignin could increase the toughness of asphalt binder against thermal cracking at low temperatures. Just like in other studies, McCready and Williams revealed that the presence of lignin within the asphalt binder improved its performance at high temperatures, whereas low-temperature performance degraded slightly [11]. Besides, they found that the dispersion of lignin in the asphalt binder increased its flow activation energy.
It can be seen from the literature that wood lignin as a waste material has a positive effect on the rutting behavior of asphalt binder. ere are various types of lignin, namely, organosolv, kraft, klason, enzymatic hydrolyzed, sulfonated, and calcium lignosulfonate (CL) lignin [2]. However, there exists little information about the effect of derivatives of this substance, such as calcium lignosulfonate, on the performance of asphalt binders. Meanwhile, CL has enough potential to be utilized as a plant-based asphalt binder modifier because, on the one hand, CL chemical composition consists of reactive phenolic, hydroxyl, and aromatic groups, which can be used for modification of petroleum chemical products [12]. On the other hand, CL chemical structure has similarities with petroleum bitumen [13]. erefore, this study was performed to gain a comprehensive understanding of the influence of CL on the rutting and fatigue properties of the asphalt binders.

Objectives and Scope of Work
In this paper, first, the physical properties of CL powder and virgin asphalt binder were determined. Meanwhile, chemical components and surface morphology of CL were investigated using XRF and FESEM techniques, respectively. Second, different blends were prepared by dispersion CL into the virgin asphalt binder in the proportions of 5%, 10%, 15%, and 20% by mass of the asphalt binder. ird, conventional binder tests were performed on CL-modified samples, including penetration, softening point, ductility, and kinematic viscosity tests. Finally, the asphalt binder's rutting, fatigue, and elasticity potential were evaluated to study the impact of CL on the asphalt binder's behavior using the Multiple Stress Creep Recovery (MSCR) test and G * /sinδ and G * ·sinδ indices. Figure 1 depicts the flow chart of experiments performed in this study.

Asphalt Binder.
e neat asphalt binder used in this study was 60/70 penetration grade. Some conventional properties of this asphalt binder, such as penetration, viscosity at 135°C, softening point, and specific gravity, were determined as listed in Table 1. It can be concluded that the asphalt binder physical test results were within the specification limits.

Calcium Lignosulfonate.
e purified calcium lignosulfonate used in this study was supplied from Ligno Tech South Africa Co., Ltd. is product is obtained from sulfite pulping of wood and consists of three alcohols, namely, coniferyl alcohol, p-coumaryl alcohol, and sinapyl alcohol [14]. e chemical components of CL were determined by X-ray fluorescence (XRF) analysis. e mentioned analysis was carried out by the ARL PERFORM'X spectrometer. Table 2 expresses the weight percentage of main oxides based on bulk chemical analysis technique. Moreover, the physical properties of this substance are summarized in Table 3.

Sample Preparation.
To prepare the specimens, 5, 10, 15, and 20 percent of the CL by weight of total asphalt binder were dissolved in virgin asphalt binder (60/70 penetration grade). For this aim, the CL powder was gradually added to the asphalt binder, and the mix was blended using a highshear mixer with the rotational speed of 3000 rpm for 30 minutes at 160°C. Field Emission Scanning Electron Microscope (FESEM) images were then taken to show the morphology and the texture of the asphalt binder after modification [15]. e test was carried out at the ambient temperature and a scale of 200 µm. e microscopic pictures of the asphalt binder are presented in Figure 2. As shown in Figure 2, it is clearly visible that the CL influenced the asphalt binder texture after the mixing process. e neat asphalt binder has a smooth surface texture with little distortion. e CL modification has made the asphalt binder texture spongy and porous.

Conventional Asphalt Binder Tests.
Conventional tests present valuable outputs from the physical properties of the asphalt binder, which can be attributed to some performance behavior of asphalt binder [16]. In this research, the effects of CL content on the penetration (AASHTO T49), softening point (AASHTO T 53), ductility (ASTM D113), and kinematic viscosity at 135°C (AASHTO T201) were investigated. e viscosity was also measured using the rotational viscosity in accordance with AASHTO T316 at 135°C and 165°C as compaction and mixing temperatures, respectively. e flow activation energy, which shows the sensitivity of asphalt binder to temperature and its resistance to pumping and    Advances in Civil Engineering 3 mixing with aggregates, is calculated using the Arrhenius Equation as follows [17,18]: where η is the asphalt binder viscosity (Pa·s), Ef is the activation energy (kJ·mol −1 ), R is the universal gas constant, which is equal to 8.314E − 3 kJ·mol −1 ·K −1 , T is the temperature (K), and A is the constant of the equation. For calculating the Ef parameter, at first, the plot of ln η versus 1/Tshould be depicted. en, a linear equation should be fitted into the data. Afterward, the slope of the equation will be equal to Ef/R. erefore, Ef will be calculated by multiplying the slope of the equation to the R-value.

Storage Stability Test.
e storage stability test was conducted according to ASTM D5892 to evaluate the storage stability of the polymerized asphalt binder at high temperatures. For this aim, the modified asphalt binder was filled into a tube (25 mm in diameter and 140 mm in height) and kept at the temperature of 163°C for 48 h. e specimens were then kept in a freezer at a temperature of −7°C for 4 h. Afterward, the softening point tests were carried out on the asphalt binder specimens taken from the top and bottom of the tube, and the differences between the softening point of the top and bottom of the specimen were reported (Figure 3).

Multiple Stress Creep Recovery (MSCR) Test.
Rutting is one of the most prevalent distresses of asphalt mixes [21,22]. Multiple Stress Creep Recovery (MSCR) test is one of the most popular tests to evaluate the potential of the asphalt binder to resist the rutting phenomenon, especially when the asphalt binder is modified with polymers [23,24]. e MSCR test is conducted at temperatures of 50, 60, and 70°C based on ASTM D7405 to evaluate the high-temperature performance of the asphalt binder [25,26]. For this purpose, the asphalt binder specimens were subjected to an oscillatory stress-controlled loading of 100 Pa for 10 cycles, followed by another 10 cycles of 3200 Pa loading with a loading time of one second and a recovery time of nine seconds. e average nonrecoverable creep compliance (J nr ) and the average percentage of recoverable strain (R) obtained from both stages of the MSCR test are reported as the rutting parameters of the binder, which are calculated using the following equations [27]: 4.4. G * /sinδ and G * ·sinδ Indices. Fatigue and rutting are prominent among the occurring distress of asphalt mixes   Advances in Civil Engineering located in the moderate-and high-temperature areas [28,29]. e G * /sinδ and G * ·sinδ indices are usually considered to evaluate the asphalt binder's fatigue and rutting potential. In this research, the Dynamic Shear Rheometer (DSR) was utilized to evaluate the fatigue and rutting performance of the asphalt binder at moderate and high temperatures in accordance with AASHTO T315 [13]. e complex shear modulus (G * ) and phase angle (δ), which are the parameters describing the rheological performance of the asphalt binder, are calculated during this evaluation [30]. e rutting performance parameter (G * /sinδ) and the fatigue resistance parameter (G * ·sinδ) were then calculated and reported. A higher value of G * /sin(δ) and a lower value of G * ·sin(δ) are favorable in terms of rutting and fatigue performance, respectively. e Superpave specifications specify a minimum value of 1.0 kPa for the G * /sinδ of asphalt binders (unaged samples) at the high-performance grade temperature; moreover, a maximum value of 5000 kPa is considered for G * ·sinδ of asphalt binders at the lowperformance grade temperature [31].

Elastic Recovery Test.
e elastic recovery test is usually carried out to recognize how different polymers affect the elasticity of the polymer-modified asphalt binder [32]. In this research, in order to evaluate the effect of CL on the elasticity of the asphalt binder, the elastic recovery test was carried out on the binder containing different percentages of the modifier according to AASHTO T301 [33]. For that test, the asphalt binder samples were stretched at a speed of 5 cm/ min in a water bath; meanwhile, the water bath temperature was constant at 25°C (Figure 4). Samples were cut from the middle when specimens were stretched to 10 cm. en, the length of the samples was measured after 30 min and 60 min, respectively. e elastic recovery was determined using the following equation: where L (cm) presents the measured length of the sample (30 and 60 min after cutting that in half ).

Conventional Asphalt Binder Tests
Result. e results of the penetration, softening point, kinematic viscosity, and ductility are summarized in Table 4. For each sample, the reported value was obtained by averaging the values of three replicates. As can be seen in Table 4, the softening point increases by increasing the CL content in the asphalt binder, which shows that the temperature sensitivity of the asphalt binder decreases by increasing the CL content. Increasing the CL content also reduces the penetration values, and the stiffness of the asphalt binder increases by adding the CL material into the asphalt binder. e ductility, which is the adhesion parameter, reduces by adding CL powder into the asphalt binder. According to previous studies, the kraft lignin also dwindled the penetration degree and increased the modified asphalt binder's hardness [13]. e viscosity of the asphalt binder increases by almost 50% when the CL content is increased. e increase of viscosity may result in a dramatic increase in the mixing and compaction temperature of the asphalt mixes. In the present study, the viscosity of the asphalt binder containing different amounts of the CL material at 165°C and 135°C was measured using the rotational viscosity method to investigate the pumpability, mixability, and workability of the CL-modified asphalt binder.
e results of the rotational viscosity are presented in Figure 5. As can be observed, the viscosity of all modified asphalt binders experiences an upward trend by increasing the CL content, which will be challenging for pumpability and workability during the asphalt construction process.
is is the same result as Xu et al. and Gao et al. reported about the effect of lignin on the asphalt binder's viscosity [7,8]. Moreover, to analyze the significance of the presence of CL within the asphalt binder on the rotational viscosity, a one-way analysis of variance (ANOVA) was conducted. e results of one-way ANOVA performed using Minitab software are presented in Table 5. e null hypothesis that was all means are equal was assumed for the statistical analysis. It can be found that the p value was less than 0.05  Advances in Civil Engineering for both temperatures; therefore, the null hypothesis was rejected. Meanwhile, the Tukey method with a 95% confidence level was considered for ranking the asphalt binder's viscosity values at two temperatures. Based on Table 6, it can be concluded that the specimens containing 20% CL had the highest values of rotational viscosity at both temperatures. Table 7 illustrates the Arrhenius equation of five different types of samples on the basis of two different temperatures (135°C and 165°C). e results of flow activation energy are also calculated and depicted in Figure 6. As can be seen in the figure, the activation energy decreases dramatically when 5% of the CL material is added to the asphalt binder. However, by increasing the CL content, the activation energy goes up until it becomes just higher than the activation energy of the virgin asphalt binder. In other words, the temperature susceptibility of the binder increases when 5% of the CL material is added to the binder. is phenomenon may be due to the puffy structure of the binder resulting from adding the CL material. However, adding more CL     Advances in Civil Engineering compensates for this defect, and the activation energy becomes almost as high as the neat binder when 15% to 20% of the CL material is added. Various studies have indicated that using different types of lignin as an asphalt binder modifier enhanced the flow activation energy; however, there have been limited studies on the effect of CL on the asphalt binder's flow activation energy [8,13].

Storage Stability Test Result.
e results of the storage stability test are presented in Table 8. If the differences between the softening point of the top and down of the specimen become more than 2.2°C, the modified asphalt cannot be stored, and its usage will encounter many problems [34]. It can be seen from the results that the storage stability values are less than the threshold as mentioned above; thus using 20% of the CL does not have any substantial adverse effect on the storage stability of the asphalt binder. Yu et al., in their study, revealed that the dispersion of soda lignin powder in the asphalt binder was comparatively uniform and stable, which led to excellent thermal storage stability of modified specimens [13].

Multiple Stress Creep Recovery (MSCR) Test Result.
e results of the MSCR test are presented in the bar charts of Figures 7-10. As can be seen, the Jnr value declines by increasing the amount of CL at both stress levels of 0.1 and 3.2 kPa. Hence, it can be concluded that the rutting resistance of the asphalt binder improves when the amount of CL increases in the binder.
It is also indicated that adding CL leads to an incremental increase in the recovery percentage of the asphalt binder, especially at a stress level of 3.2 kPa. is performance shows that using the CL in the asphalt binder brings an increase in the elasticity of the modified samples and subsequently can improve the rutting performance. Comparing the results at different temperatures shows that the same trend has happened for all three studied temperatures. e present results in this section are compatible with the study of Arafat et al., which indicated that kraft lignin improved the rutting performance of asphalt binders [2]. A similar finding was presented by Yu et al., which illustrated that using soda lignin powder for asphalt binder modification increased the restored recovery values [13]. As   Advances in Civil Engineering illustrated in this study, it is worth mentioning that the utilization of CL as an asphalt binder modifier can further improve the rutting resistance and elastic recovery of asphalt binders.

G * /sinδ and G * ·sinδ Indices.
e results of the rutting performance at moderate and high temperatures are depicted in Figures 11 and 12. Figure 11 shows the calculated values of the G * /sin(δ) parameter at temperatures of 50, 60, and 70°C. It is worth mentioning that the temperature ranging from 50°C to 70°C was considered to simulate the high-temperature region [35]. It can be seen that using up to 15% of the CL additive can improve the rutting performance of the asphalt binder incrementally. Almost 70% improvement in the rutting parameter is observed when 15% of the CL material is added to the neat asphalt binder.
On the other hand, the rutting parameter falls by almost 15% when the CL content increases from 15% to 20%; however, it is still higher than the rutting parameter of the neat asphalt binder. e same trend is seen at all studied temperatures. Comparing the results of the rutting parameter at different temperatures show that the rutting    Advances in Civil Engineering parameter declines dramatically when the temperature increases by 10°C from 50°C to 60°C. Moreover, the sensitivity of all asphalt binder samples to temperature is almost equal. e fatigue analysis was performed based on the lowmoderate temperature region (ranging from 24°C to 30°C) [35]. Figure 12 shows the results of G * ·sin(δ) at 24°C and 30°C. G * ·sin(δ) is the parameter for the fatigue resistance of the asphalt binder at intermediate temperatures [36]. It can be observed that increasing the amount of the CL to 15% in the asphalt binder leads to an almost 25% increase in the G * ·sin(δ) parameter at both studied temperatures. is performance indicates that the fatigue resistance of the asphalt binder declines slightly by increasing the CL content. is performance can be due to the stiffening effect of the CL on the asphalt binder, which can make it brittle and less resistant to fatigue. A similar trend can be observed at both studied temperatures. Norgbey et al., in their study, revealed that the addition of lignin powder into the virgin asphalt binder enhanced the rutting resistance properties of modified samples, which is consistent with the results of this study [9]. Also, Gao et al. indicated that the fatigue strength of the asphalt binder degraded as a result of adding lignin [8]. e results of G * ·sin(δ) in this study bring the same results about the effect of CL on the asphalt binder fatigue life.

Elastic Recovery Test Result.
e results of the elastic recovery test are presented in Figure 13. e recovery percentage is calculated after 30 and 60 minutes of cutting the asphalt binder in a 25°C water bath. As can be seen, the main proportion of the recovery occurs in the first thirty minutes, and from the thirtieth to the sixtieth minute after cutting the sample, a small amount of the sample's length is recovered. e recovery percentage declines substantially by adding CL. erefore, the elasticity of the binder decreases when the CL is increased, and adding 10% and 20% of the CL leads to a 40% and 87% reduction in the elastic recovery, respectively.
is performance can provide an adverse effect on the fatigue and cracking performance of the asphalt binder.

Conclusions
In this study, the effect of calcium lignosulfonate as a waste by-product of the wood industry on the performance properties of asphalt binder was investigated through the MSCR, and elastic recovery tests. In addition, the values of G * /sin(δ) and G * ·sin(δ) were determined. e study conclusions are as follows: (i) e FESEM test results indicated that the CL modification formed the asphalt binder texture spongy and porous, which positively affected the elasticity performance of CL-modified asphalt binders. (ii) e conventional test results showed that the viscosity and softening point of the asphalt binder increased, and the penetration declined by increasing the CL content, which emanated from the stiffening effect of the CL powder on the asphalt binder. (iii) e rotational viscosity test showed that adding CL into the binder will be challenging for mixing and compacting the asphalt mixes. (iv) e dispersion of CL into the asphalt binder cannot adversely affect the thermal storage stability of the modified asphalt binder. en, CL-modified   asphalt binders can be considered uniform and stable for storage at high-temperature conditions. (v) e MSCR results showed that the percent recovery and the nonrecoverable creep compliance improved by increasing the CL content in the asphalt binder at high temperatures, which led to rutting resistance enhancement for CL-modified asphalt binders. (vi) e G * /sin(δ) parameter approved the results of the MSCR test and showed that the presence of CL within the asphalt binder improved the rutting resistance of asphalt binders. e rutting performance improved with CL content until it reached the most at 15%. More CL content affected the rutting performance decline. However, in comparison to the rutting performance of the control sample, it was enhanced at all CL contents used in this study. (vii) Because of the stiffening effect of the CL on the asphalt binder, the G * ·sin(δ) parameter increased slightly by increasing the CL content in the asphalt binder, which showed an increase in brittleness of asphalt binders leading to the fatigue performance degradation. (viii) e elastic recovery test results showed that adding CL powder led to a significant decrease in the elasticity of the asphalt binder, which led to fatigue life degradation. Among the four types of asphalt binders prepared by 5%, 10%, 15%, and 20% of CL, the asphalt binder with 20% CL was more prone to cracking because it had the least elastic recovery values. e high sensitivity to cracking can lead to reflecting cracks propagation through asphalt pavement layers.

Data Availability
At present, the data used to substantiate and corroborate the conclusions of this research are under prohibition because of restrictions and privacy rules. ree months after the publication of this research, the corresponding author will be able to consider the requests for outputs.