**2. Materials**

#### *2.1. Steel-Slag Powder*

An experimental study was conducted on SSP of Baotou Iron and Steel Co., Ltd., Baotou, Inner Mongolia, China. The jaw crusher was used to grind the steel slag to make its particle size less than 0.075 mm. Its basic properties are shown in Table 1, and it can be seen that the indexes met the requirements of the test methods of aggregate for highway engineering. Meanwhile, XRF was used to detect the chemical composition as shown in Table 2. The main components of the SSP were Al, Si, Fe, Ca, Mg and Mn respectively.

**Table 1.** Basic properties of SSP.


**Table 2.** Chemical composition of SSP.


#### *2.2. Asphalt 2.2. Asphalt*

The AH-70 base asphalt was produced by Guochuang Co., Ltd., Wuhan, Hubei, China. The essential performance indexes of asphalt were tested in accordance with Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). The test results met the requirements as shown in Table 3. The AH-70 base asphalt was produced by Guochuang Co., Ltd., Wuhan, Hubei, China. The essential performance indexes of asphalt were tested in accordance with Standard Test Methods of Bitumen and Bituminous Mixtures for Highway Engineering (JTG E20-2011). The test results met the requirements as shown in Table 3.

**Compounds CaO Fe2O3 SiO2 MgO Al2O3 MnO TiO2 P2O5 Others** Steel slag 35.5 33.3 14.0 4.9 2.4 4.5 1.1 2.2 2.1

**Table 3.** Physical information of asphalt. **Table 3.** Physical information of asphalt.

**Table 2.** Chemical composition of SSP.

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#### *2.3. Antioxidant 2.3. Antioxidant*

tively.

The German BASF 1098 antioxidant used in this article was a nitrogen-containing hindered phenolic antioxidant with low volatility, resistance to extraction, non-toxicity, heatresistant oxidation, and other properties. The 1098 antioxidant in this paper was selected because it is a typical high temperature antioxidant with excellent refractory characteristics. The asphalt mixtures were mixed at 160 ◦C, so the antioxidant should have excellent high-temperature resistance. The photothermal aging resistance of polymer materials was effectively improved on account of its functions such as decomposing hydroperoxide, trapping free radicals, and trapping singlet oxygen. Its addition to asphalt was expected to trap thermal or photoinduction-free radicals based on the light-stabilization effect, thereby preventing further aging reactions of the asphalt. Its decomposition chemical equation and basic physical properties are presented in Figure 1 and Table 4, respectively. The German BASF 1098 antioxidant used in this article was a nitrogen-containing hindered phenolic antioxidant with low volatility, resistance to extraction, non-toxicity, heat-resistant oxidation, and other properties. The 1098 antioxidant in this paper was selected because it is a typical high temperature antioxidant with excellent refractory characteristics. The asphalt mixtures were mixed at 160 °C, so the antioxidant should have excellent high-temperature resistance. The photothermal aging resistance of polymer materials was effectively improved on account of its functions such as decomposing hydroperoxide, trapping free radicals, and trapping singlet oxygen. Its addition to asphalt was expected to trap thermal or photoinduction-free radicals based on the light-stabilization effect, thereby preventing further aging reactions of the asphalt. Its decomposition chemical equation and basic physical properties are presented in Figure 1 and Table 4, respec-

**Figure 1.** The decomposition chemical equation of antioxidant 1098. **Figure 1.** The decomposition chemical equation of antioxidant 1098.

**Table 4.** Basic physical properties of antioxidant 1098. **Table 4.** Basic physical properties of antioxidant 1098.


#### *2.4. Ultraviolet Absorbent*

The BASF UV-5411 used in this study was a hydroxyphenyl benzotriazole ultraviolet absorber, which is an ultraviolet absorber with high-efficiency light stabilization. The ultraviolet rays were converted into heat by UV-5411 through chemical action, thereby preventing further thermal-oxygen and ultraviolet aging of the asphalt. Its decomposition chemical equation and basic physical properties are shown in Figure 2 and Table 5, respectively. traviolet rays were converted into heat by UV-5411 through chemical action, thereby preventing further thermal-oxygen and ultraviolet aging of the asphalt. Its decomposition chemical equation and basic physical properties are shown in Figure 2 and Table 5, respectively. spectively.

The BASF UV-5411 used in this study was a hydroxyphenyl benzotriazole ultraviolet absorber, which is an ultraviolet absorber with high-efficiency light stabilization. The ul-

The BASF UV-5411 used in this study was a hydroxyphenyl benzotriazole ultraviolet absorber, which is an ultraviolet absorber with high-efficiency light stabilization. The ultraviolet rays were converted into heat by UV-5411 through chemical action, thereby preventing further thermal-oxygen and ultraviolet aging of the asphalt. Its decomposition chemical equation and basic physical properties are shown in Figure 2 and Table 5, re-

**Figure 2.** The decomposition chemical equation of UV-5411. **Figure 2.** The decomposition chemical equation of UV-5411. **Table 5.** Basic physical properties of UV-5411.

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**Table 5.** Basic physical properties of UV-5411. **Table 5.** Basic physical properties of UV-5411. **Information Unit Result**

*2.4. Ultraviolet Absorbent*

*2.4. Ultraviolet Absorbent*


#### *2.5. Preparation of Steel-Slag-Powder-Modified Asphalt Mortar* In this paper, asphalt mortar was prepared by using steel-slag powder instead of *2.5. Preparation of Steel-Slag-Powder-Modified Asphalt Mortar* In this paper, asphalt mortar was prepared by using steel-slag powder instead of

natural mineral powder. The density of mineral powder and steel-slag powder is quite different. Taking limestone mineral powder as an example, its apparent relative density is 2.7, while the apparent relative density of steel-slag powder is 3.2. Therefore, the volume ratio was used to mix the asphalt mortar, and the volume percentage of asphalt: steel-slag powder was 1:0.3. In addition, Wei et al. [36] determined that the amount of asphalt and fillers was determined by the qualified asphalt concrete mix proportion, while the asphalt aggregate ratio was 4.9%. Therefore, the volume ratio of fillers to asphalt was determined as 0.3. The preparation process was shown in Figure 3 as follows: First, the steel-slag pow-In this paper, asphalt mortar was prepared by using steel-slag powder instead of natural mineral powder. The density of mineral powder and steel-slag powder is quite different. Taking limestone mineral powder as an example, its apparent relative density is 2.7, while the apparent relative density of steel-slag powder is 3.2. Therefore, the volume ratio was used to mix the asphalt mortar, and the volume percentage of asphalt: steel-slag powder was 1:0.3. In addition, Wei et al. [36] determined that the amount of asphalt and fillers was determined by the qualified asphalt concrete mix proportion, while the asphalt aggregate ratio was 4.9%. Therefore, the volume ratio of fillers to asphalt was determined as 0.3. natural mineral powder. The density of mineral powder and steel-slag powder is quite different. Taking limestone mineral powder as an example, its apparent relative density is 2.7, while the apparent relative density of steel-slag powder is 3.2. Therefore, the volume ratio was used to mix the asphalt mortar, and the volume percentage of asphalt: steel-slag powder was 1:0.3. In addition, Wei et al. [36] determined that the amount of asphalt and fillers was determined by the qualified asphalt concrete mix proportion, while the asphalt aggregate ratio was 4.9%. Therefore, the volume ratio of fillers to asphalt was determined as 0.3. The preparation process was shown in Figure 3 as follows: First, the steel-slag pow-

der and asphalt were placed in an oven to be heated (AH-70 asphalt at 135 °C and steelslag powder at 150 °C). Secondly, the asphalt was heated in an oil bath at 160 °C, and then the steel-slag powder (the volume ratio was 0.3) and modifiers (UV-5411 was 0–2.4 wt% and antioxidant 1098 was 0–4.8 wt%) were added with shearing at a low speed of 500 rpm for 15 min. Then, the machine was sheared at a high speed of 4000 rpm for 30 min to fully mix the steel-slag powder and modifiers with the asphalt. Finally, the prepared steel-slagpowder-modified asphalt mortar was transferred to a storage tank for use. The preparation process was shown in Figure 3 as follows: First, the steel-slag powder and asphalt were placed in an oven to be heated (AH-70 asphalt at 135 ◦C and steel-slag powder at 150 ◦C). Secondly, the asphalt was heated in an oil bath at 160 ◦C, and then the steel-slag powder (the volume ratio was 0.3) and modifiers (UV-5411 was 0–2.4 wt% and antioxidant 1098 was 0–4.8 wt%) were added with shearing at a low speed of 500 rpm for 15 min. Then, the machine was sheared at a high speed of 4000 rpm for 30 min to fully mix the steel-slag powder and modifiers with the asphalt. Finally, the prepared steel-slag-powder-modified asphalt mortar was transferred to a storage tank for use. der and asphalt were placed in an oven to be heated (AH-70 asphalt at 135 °C and steelslag powder at 150 °C). Secondly, the asphalt was heated in an oil bath at 160 °C, and then the steel-slag powder (the volume ratio was 0.3) and modifiers (UV-5411 was 0–2.4 wt% and antioxidant 1098 was 0–4.8 wt%) were added with shearing at a low speed of 500 rpm for 15 min. Then, the machine was sheared at a high speed of 4000 rpm for 30 min to fully mix the steel-slag powder and modifiers with the asphalt. Finally, the prepared steel-slagpowder-modified asphalt mortar was transferred to a storage tank for use.

**Figure 3. Figure 3.**  The preparation of steel-slag-powder-modified asphalt mortar. The preparation of steel-slag-powder-modified asphalt mortar.

Since the density of steel-slag powder was much higher than that of 70# base asphalt, the steel-slag powder would gradually segregate to the lower part of the asphalt mortar during the storage process, which resulted in poor uniformity. Therefore, the principle of current production was adopted during the use of steel-slag-powder-modified asphalt mortar in order to reduce the error caused by the experiment. current production was adopted during the use of steel-slag-powder-modified asphalt mortar in order to reduce the error caused by the experiment. **3. Experimental Methods** *3.1. Optimization Test of Dosage Simulation*

Since the density of steel-slag powder was much higher than that of 70# base asphalt, the steel-slag powder would gradually segregate to the lower part of the asphalt mortar during the storage process, which resulted in poor uniformity. Therefore, the principle of

#### **3. Experimental Methods** 3.1.1. Experimental Design Based on Opt LHD Sampling

#### *3.1. Optimization Test of Dosage Simulation* The basic principle of LHD sampling is to evenly divide each dimensional coordinate

#### 3.1.1. Experimental Design Based on Opt LHD Sampling interval [*x*kmin, *x*kmax], i∈[1, n] into m intervals in the n-dimensional space, and each small

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The basic principle of LHD sampling is to evenly divide each dimensional coordinate interval [*x*<sup>k</sup> min , *x*<sup>k</sup> max], <sup>i</sup> <sup>∈</sup> [1, n] into m intervals in the n-dimensional space, and each small interval is denoted as [*x*<sup>k</sup> i−1 , *x*<sup>k</sup> i ], i ∈ [1, m]. However, the uniformity of sampling points could not be considered in this sampling method, so the uniform space filling of sample points was not filled. Therefore, the Opt LHD sampling method was adopted to improve the uniformity of random LHD. The fit of factors and responses was more uniform and random within the design-space region, resulting in higher computational accuracy for the design of the experimental methods. In this paper, the dosage of antioxidant 1098 *x*<sup>1</sup> ∈ (0, 4.8) and the UV absorber 5411 *x*<sup>2</sup> ∈ (0, 2.4) were evenly divided into 25 sample spaces. The distribution of LHD and Opt LHD sampling designs are shown in Figure 4a,b, respectively. It can be seen that some regional sampling points were too concentrated in LHD sampling, while others were lost. However, the distribution of test points was more uniform in the Opt LHD. interval is denoted as [*x*ki−1, *x*ki ], i∈[1, m]. However, the uniformity of sampling points could not be considered in this sampling method, so the uniform space filling of sample points was not filled. Therefore, the Opt LHD sampling method was adopted to improve the uniformity of random LHD. The fit of factors and responses was more uniform and random within the design-space region, resulting in higher computational accuracy for the design of the experimental methods. In this paper, the dosage of antioxidant 1098 *x*1∈(0, 4.8) and the UV absorber 5411 *x*2∈(0, 2.4) were evenly divided into 25 sample spaces. The distribution of LHD and Opt LHD sampling designs are shown in Figure 4a,b, respectively. It can be seen that some regional sampling points were too concentrated in LHD sampling, while others were lost. However, the distribution of test points was more uniform in the Opt LHD.

**Figure 4.** The distribution of LHD and Opt LHD sampling designs. (**a**) Random LHD sampling, (**b**) Opt LHD sampling. **Figure 4.** The distribution of LHD and Opt LHD sampling designs. (**a**) Random LHD sampling, (**b**) Opt LHD sampling.

#### 3.1.2. Experimental Design Based on Kriging Model

3.1.2. Experimental Design Based on Kriging Model Kriging, also known as spatial local interpolation, is an unbiased optimal estimation method for regional variables in a finite area based on variogram theory and structural analysis. The *x*<sup>1</sup> and *x*<sup>2</sup> were set as the inputs; G\*, δ, and the softening point were set as the Kriging, also known as spatial local interpolation, is an unbiased optimal estimation method for regional variables in a finite area based on variogram theory and structural analysis. The *x*<sup>1</sup> and *x*<sup>2</sup> were set as the inputs; G\*, δ, and the softening point were set as the outputs. A Kriging model was established for 25 sample data with its basic principle as follows:

outputs. A Kriging model was established for 25 sample data with its basic principle as follows: *x*<sup>0</sup> is set as the unobserved point that needs to be estimated, *x*1, *x*2, …, *x*N, are set as the observed points around it, and the observed values are correspondingly *y*(*x*1), *y*(*x*2), …, *x*<sup>0</sup> is set as the unobserved point that needs to be estimated, *x*1, *x*2, . . . , *x*N, are set as the observed points around it, and the observed values are correspondingly *y*(*x*1), *y*(*x*2), . . . , *y*(*x*N). The estimate of the unmeasured point is denoted as *ỹ*(*x*0), which is obtained by the weighted sum of the known observations of the adjacent observation points:

$$\widetilde{y}(\mathbf{x}\_0) = \sum\_{i=1}^{N} \lambda\_i y(\mathbf{x}\_i) \tag{1}$$

Among them, *λ<sup>i</sup>* is the undetermined weighting coefficient. The key to the Kriging interpolation is to calculate the weight coefficient *λ<sup>i</sup>* , which must satisfy two conditions:

1 The unbiased estimates: the true value of the evaluation point is set to *y*(*x*0). The *y*(*xi*), *<sup>y</sup>*e(*x*0) and *<sup>y</sup>*(*x*0) can be regarded as a random variable.

$$E[\tilde{y}(\mathbf{x}\_0) - y(\mathbf{x}\_0)] = \mathbf{0}, \sum\_{i=1}^{N} \lambda\_i = 1 \tag{2}$$

2 The variance of the difference between the true value *y*(*x*0) and the estimated value is minimal.

$$D[\tilde{y}(\mathbf{x}\_0) - y(\mathbf{x}\_0)] = \min \tag{3}$$

$$D[\tilde{y}(\mathbf{x}\_0) - y(\mathbf{x}\_0)] = -\sum\_{i=1}^{N} \sum\_{j=1}^{N} \lambda\_i \lambda\_j \gamma(\mathbf{x}\_i, \mathbf{x}\_j) + 2\sum\_{i=1}^{N} \lambda\_i \gamma(\mathbf{x}\_i, \mathbf{x}\_0) \tag{4}$$

where, *γ*(*x<sup>i</sup>* , *x<sup>j</sup>* ) represents the semivariance value of the parameter when the distance between the two points *x<sup>i</sup>* and *x<sup>j</sup>* is used as the distance h. The *γ*(*x<sup>i</sup>* , *x*0) is the semivariance value of the parameter when the distance between the two points *x<sup>i</sup>* and *x*<sup>0</sup> is used as the distance h.

#### 3.1.3. The Multi-Objective Optimization Based on NSGA-II

The simultaneous optimization of multiple sub-objectives is a multi-objectives optimization problem. In most cases, it is generally impossible to concurrently achieve the optimum for multiple sub-goals. The ultimate goal of solving multi-objectives optimization problems is to harmonize various objectives, hence each sub-objective is as optimal as possible. In the non-dominated ranking, NSGA-II was selected to approach the individual of the Pareto front, which enhanced its ability to advance. In the Pareto optimal relationship, the individuals in the group were compared according to their target function vector and divided into multiple frontier layers that were controlled in sequence. NSGA-II is widely used due to its advantages of solving the Pareto solution set with good accuracy and dispersion. NSGA-II was used to optimize the Kriging approximation model in this research. The constraints, optimization objectives, lower and upper bounds were set according to the established mathematical model to obtain a set of solutions that all satisfy the conditions. The main process was in Figure 5 as follows:


**Figure 5.** Flowchart for solving the Pareto solution set. **Figure 5.** Flowchart for solving the Pareto solution set.

3.1.4. Optimize Experimental Design 3.1.4. Optimize Experimental Design

The sampling points were established based on Opt LHD, as shown in Table 6. The steel-slag-powder-modified asphalt mortar (SSP-MAM) with different contents of antioxidant and UV absorber was prepared to undergo TFOT short-term aging and seven-day UV aging. The complex shear modulus (G\*), softening point and phase angle (δ) of the aged SSP-MAM were recorded by observation. The sampling points were established based on Opt LHD, as shown in Table 6. The steel-slag-powder-modified asphalt mortar (SSP-MAM) with different contents of antioxidant and UV absorber was prepared to undergo TFOT short-term aging and seven-day UV aging. The complex shear modulus (G\*), softening point and phase angle (δ) of the aged SSP-MAM were recorded by observation.


**Table 6.** Opt LHD experimental design and results. **Table 6.** Opt LHD experimental design and results.

25 1.4 1.9 17.193 81.3 64.3 Based on the experimental basis and the literature of the laboratory, the content of antioxidant 1098 was in the range of 0–4.8%, represented by *x*1; the content of UV-5411 was in the range of 0–2.4%, represented by *x*2.

## *3.2. Properties of SSP*

Empyrean XRD (Panaco, The Netherlands) and Axios XRF (Almelo, The Netherlands) were used to characterize the physicochemical properties of the SSP.

#### *3.3. Aging-Test Method*

#### 3.3.1. Short-Term Thermal-Oxygen-Aging Test

The 85-type asphalt film oven was used for the TFOT aging test to simulate the shortterm thermal-oxygen-aging behavior of SSP-MAM in the mixing, paving, and compaction processes. The test temperature was controlled at 163 ± 1 ◦C, the rotating speed of the turntable was set at 5.5 ± 1 r/min, the tilt angle of the turntable and the horizontal plane was not more than 3◦ , and the aging time was 5 h.

#### 3.3.2. Accelerated-UV-Aging Test

The LHX-205 type intelligent numerical-control ultraviolet-aging test box was used to simulate accelerated-ultraviolet-aging experiments. The SSP-MAM sample after short-term thermal-oxygen aging was placed on the horizontal turntable of the ultraviolet-aging box. The ultraviolet radiation intensity on the surface of the asphalt sample was 30 W/cm<sup>2</sup> by adjusting the height of the horizontal turntable. The wavelength of ultraviolet light was 365 nm, the experiment temperature was set to 60 ◦C, and the experiment time was seven days.

#### *3.4. Asphalt Mortar Characteristic Analysis Method*

#### 3.4.1. Functional Group Characteristics of Asphalt Mortar

In the FTIR, the molecular structure was observed by measuring the vibrational and rotational spectra of molecules. A Nexus intelligent FTIR was adopted to analysis the functional groups of SSP-MAM. First, the asphalt/carbon disulfide solution with a mass concentration of 5% was configured. Then the asphalt sample was dissolved in the carbon-disulfide solution, and the prepared solution was dropped onto the KBr wafer. The SSP-MAM sample film could be obtained after the complete volatilization of the carbon-disulfide solution. The scanning beam range of the infrared spectrometer was 4000–400 cm−<sup>1</sup> , and the number of scans was 64.

#### 3.4.2. Rheological Properties of Asphalt Mortar

The dynamic shear rheometer (DSR) is an instrument applied to analyze the rheological properties of polymer materials. It has been widely used due to its good experimental accuracy and repeatability, since SHRP recommended using it to test the rheological properties of asphalt in 1993. The rheological properties of SSP-MAM samples were tested by MCR-102 DSR (Anton Paar, Graz, Austria). The rheological parameters at different temperatures were scanned under the strain-control mode. The scanning frequency was controlled as 10 rad/s, the temperature scanning range was 30–80 ◦C, the upper and lower plate diameter was 25 mm, the heating rate was 2 ◦C/min, and the plate spacing (asphalt film thickness) was 1 mm.

#### *3.5. Leaching Behavior of Asphalt Mortar*

TCLP developed by the US Environmental Protection Agency (EPA) was adopted for the toxicity-extraction experiment of steel slag in this research. The leaching agent was used to adjust the pH of the solid waste for tumble-extraction experiments in order to implement the Resource Conservation and Regeneration Act (RCRA) for the management of hazardous and solid waste. Simultaneously, the leaching content of inorganic components in solid waste was simulated by judging its contamination capacity.

In this experiment, the leaching behavior of heavy-metal ions from SSP and SSP-MAM was investigated. SSP-MAM was specially prepared into particles smaller than 9.5 mm to meet the requirements of the test samples in TCLP. The specific experimental steps in Figure 6 were as follows: First, 50.0 g of the particle was weighed and placed in a 2 L

polyethylene bottle for later use. Secondly, the acetate buffer solution with pH value of 2.88 ± 0.05 was prepared, and the corresponding amount of acetate buffer solution was added according to the liquid- solid ratio of 20:1 (L/kg). Then, the polyethylene bottle was placed on the inversion shaker, the rotation speed of the shaker was set to 30 rpm, the shaking time was 18 ± 2 h, and the temperature was 23 ± 2 ◦C. Finally, the mixture was allowed to stand after shaking, and the clarified filtrate was collected in a centrifuge tube with filter paper for next step of testing. The leaching concentration of heavy-metal elements was determined by atomic absorption spectrometry. polyethylene bottle for later use. Secondly, the acetate buffer solution with pH value of 2.88 ± 0.05 was prepared, and the corresponding amount of acetate buffer solution was added according to the liquid- solid ratio of 20:1 (L/kg). Then, the polyethylene bottle was placed on the inversion shaker, the rotation speed of the shaker was set to 30 rpm, the shaking time was 18 ± 2 h, and the temperature was 23 ± 2 °C. Finally, the mixture was allowed to stand after shaking, and the clarified filtrate was collected in a centrifuge tube with filter paper for next step of testing. The leaching concentration of heavy-metal elements was determined by atomic absorption spectrometry. placed on the inversion shaker, the rotation speed of the shaker was set to 30 rpm, the shaking time was 18 ± 2 h, and the temperature was 23 ± 2 °C. Finally, the mixture was allowed to stand after shaking, and the clarified filtrate was collected in a centrifuge tube with filter paper for next step of testing. The leaching concentration of heavy-metal elements was determined by atomic absorption spectrometry.

of hazardous and solid waste. Simultaneously, the leaching content of inorganic compo-

nents in solid waste was simulated by judging its contamination capacity.

In this experiment, the leaching behavior of heavy-metal ions from SSP and SSP-MAM was investigated. SSP-MAM was specially prepared into particles smaller than 9.5 mm to meet the requirements of the test samples in TCLP. The specific experimental steps in Figure 6 were as follows: First, 50.0 g of the particle was weighed and placed in a 2 L

of hazardous and solid waste. Simultaneously, the leaching content of inorganic compo-

In this experiment, the leaching behavior of heavy-metal ions from SSP and SSP-MAM was investigated. SSP-MAM was specially prepared into particles smaller than 9.5 mm to meet the requirements of the test samples in TCLP. The specific experimental steps in Figure 6 were as follows: First, 50.0 g of the particle was weighed and placed in a 2 L polyethylene bottle for later use. Secondly, the acetate buffer solution with pH value of 2.88 ± 0.05 was prepared, and the corresponding amount of acetate buffer solution was added according to the liquid- solid ratio of 20:1 (L/kg). Then, the polyethylene bottle was

nents in solid waste was simulated by judging its contamination capacity.

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**Figure 6.** Procedures of TCLP. **Figure 6.** Procedures of TCLP.

**Figure 6.** Procedures of TCLP.

#### *3.6. Experiment Plan 3.6. Experiment Plan 3.6. Experiment Plan*

The experimental plan was shown in Figure 7. The experimental plan was shown in Figure 7. The experimental plan was shown in Figure 7.

The partial-approximation model, contour plot, and R-Squared accuracy of G\* and **Figure 7.** Experiment plan. **Figure 7.** Experiment plan.

#### *x*1, *x*<sup>2</sup> are presented in Figure 8a–c, and so on for phase angle and softening point. From **4. Results and Discussions**

Figure 8c,f,i, the values of R2 can be obtained as 0.96652, 0.95828, and 0.95732, respectively, **4. Results and Discussions** *4.1. Optimization Test of Dosage Simulation*

where the blue line represents the actual average value, indicating that the fitting accuracy and reliability were high. The fit of anisotropy and exponential functions were selected as *4.1. Optimization Test of Dosage Simulation* The partial-approximation model, contour plot, and R-Squared accuracy of G\* and *x*1, *x*<sup>2</sup> are presented in Figure 8a–c, and so on for phase angle and softening point. From Figure 8c,f,i, the values of R2 can be obtained as 0.96652, 0.95828, and 0.95732, respectively, where the blue line represents the actual average value, indicating that the fitting accuracy and reliability were high. The fit of anisotropy and exponential functions were selected as The partial-approximation model, contour plot, and R-Squared accuracy of G\* and *x*1, *x*<sup>2</sup> are presented in Figure 8a–c, and so on for phase angle and softening point. From Figure 8c,f,i, the values of R<sup>2</sup> can be obtained as 0.96652, 0.95828, and 0.95732, respectively, where the blue line represents the actual average value, indicating that the fitting accuracy and reliability were high. The fit of anisotropy and exponential functions were selected as the Kriging model by comparing the output R<sup>2</sup> values. Figure 8b,e,h are contour maps of the projections of Figure 8a,d,g on the horizontal plane, respectively. The denser the contour

lines and the greater the slope of the fitted surface, the more significant the influence of this factor was. The contour lines in the figure were dense and steep, indicating that there was significant correlation between the amount of modifier and G\*, δ, and the softening point. When the first peak appeared, for G\*, δ, and the softening point, the corresponding dosages of *x*<sup>1</sup> were 2.15%, 1.0%, and 1.1%, respectively. It showed that the effect of antioxidants on δ was more significant, that is, less antioxidant could change δ. Meanwhile, the corresponding dosages of *x*<sup>2</sup> were 0.25%, 0.76%, and 0.38%, respectively. It was clarified that the effect of the UV absorber on G\* was more significant, that is, less UV absorber could change G\*. of this factor was. The contour lines in the figure were dense and steep, indicating that there was significant correlation between the amount of modifier and G\*, δ, and the softening point. When the first peak appeared, for G\*, δ, and the softening point, the corresponding dosages of *x*<sup>1</sup> were 2.15%, 1.0%, and 1.1%, respectively. It showed that the effect of antioxidants on δ was more significant, that is, less antioxidant could change δ. Meanwhile, the corresponding dosages of *x*<sup>2</sup> were 0.25%, 0.76%, and 0.38%, respectively. It was clarified that the effect of the UV absorber on G\* was more significant, that is, less UV absorber could change G\*.

the Kriging model by comparing the output R2 values. Figure 8b,e,h are contour maps of the projections of Figure 8a,d,g on the horizontal plane, respectively. The denser the contour lines and the greater the slope of the fitted surface, the more significant the influence

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**Figure 8.** Partial-approximation model, contour plot, and R-squared accuracy of output and input. (**a**) Partial-approximation model of G\*, (**b**) Contour plot of G\*, (**c**) R-Squared accuracy of G\*, (**d**) Partial-approximation model of δ, (**e**) Contour plot of δ, (**f**) R-Squared accuracy of δ, (**g**) Partialapproximation model of softening point, (**h**) Contour plot of softening point, (**i**) R-Squared accuracy of softening point. **Figure 8.** Partial-approximation model, contour plot, and R-squared accuracy of output and input. (**a**) Partial-approximation model of G\*, (**b**) Contour plot of G\*, (**c**) R-Squared accuracy of G\*, (**d**) Partialapproximation model of δ, (**e**) Contour plot of δ, (**f**) R-Squared accuracy of δ, (**g**) Partial-approximation model of softening point, (**h**) Contour plot of softening point, (**i**) R-Squared accuracy of softening point.

In this paper, the softening point and phase angle were selected as research objects as examples. The optimized Pareto front is shown in Figure 9. The parameters of the NSGA-II algorithm were set as: population size was 100, evolutionary generation was 100, hybridization-distribution coefficient was 20, hybridization probability was 0.9, mutationdistribution coefficient was 100, and mutation probability was 0.9. All the points in Figure 9 were qualified non-inferior solutions. It was necessary to select the appropriate solution in order to determine the final optimization scheme. The individuals with larger crowding distances had greater advantages in accordance with the optimization strategy of the In this paper, the softening point and phase angle were selected as research objects as examples. The optimized Pareto front is shown in Figure 9. The parameters of the NSGA-II algorithm were set as: population size was 100, evolutionary generation was 100, hybridization-distribution coefficient was 20, hybridization probability was 0.9, mutationdistribution coefficient was 100, and mutation probability was 0.9. All the points in Figure 9 were qualified non-inferior solutions. It was necessary to select the appropriate solution in order to determine the final optimization scheme. The individuals with larger crowding distances had greater advantages in accordance with the optimization strategy of the NSGA-II algorithm. The optimal value of the modifier dosage *x*<sup>1</sup> was 1.2%, and *x*<sup>2</sup> was 0.5% after weighing by the NSGA-II algorithm.

NSGA-II algorithm. The optimal value of the modifier dosage *x*<sup>1</sup> was 1.2%, and *x*<sup>2</sup> was

NSGA-II algorithm. The optimal value of the modifier dosage *x*<sup>1</sup> was 1.2%, and *x*<sup>2</sup> was

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0.5% after weighing by the NSGA-II algorithm.

0.5% after weighing by the NSGA-II algorithm.

**Figure 9.** Pareto front. **Figure 9.** Pareto front. *4.2. Properties of SSP*

#### *4.2. Properties of SSP 4.2. Properties of SSP* The mineral phase of SSP was characterized by XRD as shown in Figure 10. It can be

The mineral phase of SSP was characterized by XRD as shown in Figure 10. It can be seen from the diffraction pattern that there were a variety of diffraction peaks with serious overlap, which indicates that the types of minerals contained in the steel slag were quite complex. The strongest diffraction peaks and three strong peaks appearing at 2θ of 32.961 and 33.067 belonged to silicate minerals, indicating that C3S and C2S were the main mineral phases of steel slag. The mineral phase of SSP was characterized by XRD as shown in Figure 10. It can be seen from the diffraction pattern that there were a variety of diffraction peaks with serious overlap, which indicates that the types of minerals contained in the steel slag were quite complex. The strongest diffraction peaks and three strong peaks appearing at 2θ of 32.961 and 33.067 belonged to silicate minerals, indicating that C3S and C2S were the main mineral phases of steel slag. seen from the diffraction pattern that there were a variety of diffraction peaks with serious overlap, which indicates that the types of minerals contained in the steel slag were quite complex. The strongest diffraction peaks and three strong peaks appearing at 2θ of 32.961 and 33.067 belonged to silicate minerals, indicating that C3S and C2S were the main mineral phases of steel slag.

**Figure 10.** The XRD of SSP. **Figure 10.** The XRD of SSP.

#### *4.3. Functional-Group Characteristics of Asphalt Mortar* aging. The formula to calculate the index of the characteristic functional group carbonyl

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*4.3. Functional-Group Characteristics of Asphalt Mortar*

Infrared spectroscopy tests were carried out on modified asphalt mortar (MAM) and unmodified asphalt mortar (UAM) to investigate the change characteristics of functional groups during thermal-oxygen and ultraviolet aging. The characteristic peaks of oxygencontaining functional groups were more distinct due to the absorption of oxygen during aging. The formula to calculate the index of the characteristic functional group carbonyl (C=O) and sulfoxide (S=O) of asphalt were as follows: (C=O) and sulfoxide (S=O) of asphalt were as follows: *<sup>I</sup>*C=O <sup>=</sup> <sup>S</sup> <sup>1700</sup> cm−<sup>1</sup> S 2000~600 cm−<sup>1</sup> ×100% (5) *<sup>I</sup>*S=O <sup>=</sup> <sup>S</sup> <sup>1030</sup> cm−<sup>1</sup>

$$I\_{\text{C=O}} = \frac{\text{S}\_{1700 \text{ cm}^{-1}}}{\text{S}\_{2000 \text{ }\sim 600 \text{ cm}^{-1}}} \times 100\% \tag{5}$$

Infrared spectroscopy tests were carried out on modified asphalt mortar (MAM) and unmodified asphalt mortar (UAM) to investigate the change characteristics of functional groups during thermal-oxygen and ultraviolet aging. The characteristic peaks of oxygencontaining functional groups were more distinct due to the absorption of oxygen during

*I*S=<sup>O</sup> = S1030 cm−<sup>1</sup> <sup>S</sup>2000∼600 cm−<sup>1</sup> × 100% (6) where S1700 cm<sup>1</sup> is the area of the carbonyl peak band centered around 1700 cm−1, S1030 cm<sup>1</sup> is the area of the sulfoxide peak band centered around 1030 cm−1, and S2000~600 cm<sup>1</sup> is the area of the spectral bands between 2000 and 600 cm−1.

where <sup>S</sup>1700 cm−<sup>1</sup> is the area of the carbonyl peak band centered around 1700 cm−<sup>1</sup> , S1030 cm−<sup>1</sup> is the area of the sulfoxide peak band centered around 1030 cm−<sup>1</sup> , and <sup>S</sup>2000∼600 cm−<sup>1</sup> is the area of the spectral bands between 2000 and 600 cm−<sup>1</sup> . In the UAM, the value of *I*S=O and *I*C=O were 1.56% and 1.3%, respectively, while the values of *I*S=O and *I*C=O in the MAM were 0.9% and 1.1%, respectively. It revealed that the

In the UAM, the value of *I*S=O and *I*C=O were 1.56% and 1.3%, respectively, while the values of *I*S=O and *I*C=O in the MAM were 0.9% and 1.1%, respectively. It revealed that the content of oxygen-containing functional groups and the degree of photo-oxidation of the MAM were declined. Therefore, the thermal-oxygen- and ultraviolet-aging resistance of the MAM was improved. content of oxygen-containing functional groups and the degree of photo-oxidation of the MAM were declined. Therefore, the thermal-oxygen- and ultraviolet-aging resistance of the MAM was improved. In the Figure 11, the characteristic absorption peaks of *I*S=O and *I*C=O in UAM appeared

In the Figure 11, the characteristic absorption peaks of *I*S=O and *I*C=O in UAM appeared at 1033 cm−<sup>1</sup> and 1692 cm−<sup>1</sup> , respectively, while in MAM they appeared at 1676 cm−<sup>1</sup> and 998 cm−<sup>1</sup> , respectively. The reason for the different degrees of bathochromic shift of the two characteristic absorption peaks was that the antioxidants and UV absorbers contained auxiliary color groups such as non-bonded-electron heteroatom-saturated groups. Simultaneously, the absorption peaks being shifted to long wavelengths under the combined action of conjugation and auxochrome groups was attributed to the presence of aromatic heterocycles in antioxidants and UV absorbers. at 1033 cm−1 and 1692 cm−1, respectively, while in MAM they appeared at 1676 cm−1 and 998 cm−1, respectively. The reason for the different degrees of bathochromic shift of the two characteristic absorption peaks was that the antioxidants and UV absorbers contained auxiliary color groups such as non-bonded-electron heteroatom-saturated groups. Simultaneously, the absorption peaks being shifted to long wavelengths under the combined action of conjugation and auxochrome groups was attributed to the presence of aromatic heterocycles in antioxidants and UV absorbers.

In addition, the characteristic peaks of C-H stretching vibration of N-H and aromatic

**Figure 11.** Infrared analysis of asphalt mortar. **Figure 11.** Infrared analysis of asphalt mortar.

In addition, the characteristic peaks of C-H stretching vibration of N-H and aromatic hydrocarbon benzene ring in MAM were present at 3644 cm−<sup>1</sup> and 680 cm−<sup>1</sup> , respectively, but they did not appear in UAM. This illustrated that antioxidants and UV absorbers had chemical reactions with asphalt to generate new functional groups during the aging process of asphalt, rather than simple physical blending [37]. sition of asphalt from a highly elastic state at low temperature to a viscous fluid state at high temperature due to the increase in the free volume of asphalt with increasing temperature. Therefore, the decrease in the maximum shear stress and the increase in the maximum shear strain of the asphalt led to the decrease in G\*. Meanwhile, the G\* of the UAM was larger, which was due to the oxidation reaction of the asphalt film in contact

chemical reactions with asphalt to generate new functional groups during the aging pro-

Figure 12 showed that G\* decreased with the increase in temperature, indicating that the rheological properties of SSP-MAM had obvious temperature dependence. The tran-

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cess of asphalt, rather than simple physical blending [37].

#### *4.4. Rheological Properties of Asphalt Mortar* with the oxygen in the air at high temperature. The content of aromatics in asphalt was

*4.4. Rheological Properties of Asphalt Mortar*

Figure 12 showed that G\* decreased with the increase in temperature, indicating that the rheological properties of SSP-MAM had obvious temperature dependence. The transition of asphalt from a highly elastic state at low temperature to a viscous fluid state at high temperature due to the increase in the free volume of asphalt with increasing temperature. Therefore, the decrease in the maximum shear stress and the increase in the maximum shear strain of the asphalt led to the decrease in G\*. Meanwhile, the G\* of the UAM was larger, which was due to the oxidation reaction of the asphalt film in contact with the oxygen in the air at high temperature. The content of aromatics in asphalt was decreased, while pectin and asphaltene was increased. Therefore, a molecular weight migration from small to large occurred between the asphalt components. The asphalt was hardened by the reduction in plastic flow deformability based on the increase in the overall molecular weight of the asphalt. The G\* in MAM was smaller on account of the modifier chemically reacting with oxygen, thereby preventing the contact of the bitumen film with oxygen. decreased, while pectin and asphaltene was increased. Therefore, a molecular weight migration from small to large occurred between the asphalt components. The asphalt was hardened by the reduction in plastic flow deformability based on the increase in the overall molecular weight of the asphalt. The G\* in MAM was smaller on account of the modifier chemically reacting with oxygen, thereby preventing the contact of the bitumen film with oxygen. The anti-aging ability of bitumen was enhanced with the increase in phase-angle ratio and the decrease in viscosity loss during the aging process. It can be observed that the phase angle increased with the increase in temperature, which was larger in MAM than in UAM. It illustrated that the free radicals generated by the asphalt monomer due to thermal oxygen and light were captured by antioxidants, while the ultraviolet rays were converted into heat by ultraviolet absorbent through chemical action, thus preventing further thermal-oxygen and ultraviolet aging of the asphalt.

**Figure 12.** Rheological properties of asphalt mortar. **Figure 12.** Rheological properties of asphalt mortar.

*4.5. Leaching Behavior of Asphalt Mortar* Figure 13 presents the leaching results of heavy-metal ions. It can be clearly discerned that the leaching concentrations of As, Cr6+ and Cu were higher, followed by Zn, Pb, Ni, and finally Cd. Meanwhile, the leaching concentration of heavy-metal ions in the SSP was the largest, followed by the aged SSP-MAM, and finally the unaged SSP-MAM. A possible cause was that the SSP was coated with chemically stable bitumen to pre-The anti-aging ability of bitumen was enhanced with the increase in phase-angle ratio and the decrease in viscosity loss during the aging process. It can be observed that the phase angle increased with the increase in temperature, which was larger in MAM than in UAM. It illustrated that the free radicals generated by the asphalt monomer due to thermal oxygen and light were captured by antioxidants, while the ultraviolet rays were converted into heat by ultraviolet absorbent through chemical action, thus preventing further thermal-oxygen and ultraviolet aging of the asphalt.

#### vent it from reacting with the acidic extractant. On the other hand, the high-polarity water *4.5. Leaching Behavior of Asphalt Mortar*

could not fully contact the SSP on account of the low polarity of the asphalt. Therefore, asphalt had a certain physical solid-sealing effect on the release of heavy-metal ions in the SSP, and further reduced the leaching heavy-metal-ion content by reducing the chance of contact between the SSP and the extraction solution. In addition, the encapsulation ability Figure 13 presents the leaching results of heavy-metal ions. It can be clearly discerned that the leaching concentrations of As, Cr6+ and Cu were higher, followed by Zn, Pb, Ni, and finally Cd. Meanwhile, the leaching concentration of heavy-metal ions in the SSP was the largest, followed by the aged SSP-MAM, and finally the unaged SSP-MAM.

of the asphalt to the heavy-metal ions in the SSP was also imparity. The leaching amount A possible cause was that the SSP was coated with chemically stable bitumen to prevent it from reacting with the acidic extractant. On the other hand, the high-polarity

water could not fully contact the SSP on account of the low polarity of the asphalt. Therefore, asphalt had a certain physical solid-sealing effect on the release of heavy-metal ions in the SSP, and further reduced the leaching heavy-metal-ion content by reducing the chance of contact between the SSP and the extraction solution. In addition, the encapsulation ability of the asphalt to the heavy-metal ions in the SSP was also imparity. The leaching amount of As and Pb decreased the most, which were 47.8% and 46.4%, respectively, followed by Cr6+ (22.6%) and Cd (21.2%), Zn (10.6%), and finally, Ni (7.4%) and Cu (5.2%). *Materials* **2022**, *15*, x FOR PEER REVIEW 15 of 18 of As and Pb decreased the most, which were 47.8% and 46.4%, respectively, followed by Cr6+ (22.6%) and Cd (21.2%), Zn (10.6%), and finally, Ni (7.4%) and Cu (5.2%).

**Figure 13.** Heavy-metal leaching from asphalt mortar. **Figure 13.** Heavy-metal leaching from asphalt mortar.

Obviously, the aged SSP-MAM was larger than the unaged SSP-MAM in terms of ionic leaching concentration. The reason was that the asphalt structure was changed under the synergistic effect of oxygen and ultraviolet rays. The flexibility and ductility of the asphalt became depraved with the decrease in light components and the increase in asphaltene. At this point, the risk of leaching heavy-metal ions increased with the inferior asphalt coating performance on SSP. Obviously, the aged SSP-MAM was larger than the unaged SSP-MAM in terms of ionic leaching concentration. The reason was that the asphalt structure was changed under the synergistic effect of oxygen and ultraviolet rays. The flexibility and ductility of the asphalt became depraved with the decrease in light components and the increase in asphaltene. At this point, the risk of leaching heavy-metal ions increased with the inferior asphalt coating performance on SSP.

#### **5. Conclusions 5. Conclusions**

rithm.

SSP was used to replace natural mineral to prepare asphalt mortar in this research, and the thermal-oxidation- and ultraviolet-aging resistance of SSP-MAM was evaluated. The multi-objective optimization analysis of antioxidants and UV absorbers was carried out using the NSGA-II. A chemically reactive composite-modified asphalt mortar with good comprehensive properties was prepared. The optimization model was verified according to the experimental results. Based on the above research content, the main con-SSP was used to replace natural mineral to prepare asphalt mortar in this research, and the thermal-oxidation- and ultraviolet-aging resistance of SSP-MAM was evaluated. The multi-objective optimization analysis of antioxidants and UV absorbers was carried out using the NSGA-II. A chemically reactive composite-modified asphalt mortar with good comprehensive properties was prepared. The optimization model was verified according to the experimental results. Based on the above research content, the main conclusions of this paper were as follows:

clusions of this paper were as follows: 1. The denser the contour lines and the greater the slope of the fitted surface, the more significant the influence of this factor was. The contour lines were dense and steep, indicating that there was significant correlation between the dosage of modifier and G\*, δ, and the softening point. When the first peak appeared, for G\*, δ, and the softening point, the corresponding dosages of *x*<sup>1</sup> were 2.15%, 1.0%, and 1.1%, respectively, while the corresponding dosages of *x*<sup>2</sup> were 0.25%, 0.76%, and 0.38%, respectively. All the points in the Pareto front were qualified non-inferior solutions. The 1. The denser the contour lines and the greater the slope of the fitted surface, the more significant the influence of this factor was. The contour lines were dense and steep, indicating that there was significant correlation between the dosage of modifier and G\*, δ, and the softening point. When the first peak appeared, for G\*, δ, and the softening point, the corresponding dosages of *x*<sup>1</sup> were 2.15%, 1.0%, and 1.1%, respectively, while the corresponding dosages of *x*<sup>2</sup> were 0.25%, 0.76%, and 0.38%, respectively. All the points in the Pareto front were qualified non-inferior solutions. The individuals with larger crowding distances had greater advantages in accordance with the optimization

individuals with larger crowding distances had greater advantages in accordance with the optimization strategy of the NSGA-II algorithm. The optimal value of the modifier dosage *x*<sup>1</sup> was 1.2%, and *x*<sup>2</sup> was 0.5% after weighing by the NSGA-II algo-

values of *I*S=O and *I*C=O in the MAM were 0.9% and 1.1%, respectively. This revealed

strategy of the NSGA-II algorithm. The optimal value of the modifier dosage *x*<sup>1</sup> was 1.2%, and *x*<sup>2</sup> was 0.5% after weighing by the NSGA-II algorithm.


The UV-5411 and antioxidant 1098 increased the thermal oxygen and UV-aging resistance of SSP-MAM. Therefore, the mixing performance of the asphalt mixture was improved and its service life was prolonged. The solid-sealing effect of asphalt on SSP reduced the risk of leaching of heavy-metal ions, thus improving the application potential of SSP instead of mineral powder. However, due to the high economic cost of UV absorbers and antioxidants, their large-scale applications are limited. Therefore, future research can begin with reducing the dosage of modifiers, finding cheap replacement materials with excellent performance, or introducing new materials to reduce the dosage of UV absorbers and antioxidants.

**Author Contributions:** Conceptualization, Y.L., S.A., H.X., Y.Z. and X.Y.; data curation: S.W., S.A., H.X. and Y.Z.; methodology, Y.L., P.C., S.A. and H.X.; formal analysis, Y.L., P.C., Y.Z. and X.Y.; investigation, Y.L., S.A. and X.Y.; writing—original draft, Y.L.; writing—review and editing, P.C.; supervision, S.W.; project administration, S.W.; funding acquisition, S.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** The work presented in this paper is financially supported by the National Key R&D Program of China (No. 2018YFB1600200), National Natural Science Foundation of China (No. 51978547), Technological Innovation Major Project of Hubei Province (2019AEE023) and Key R&D Program of Hubei Province (2020BCB064).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study were available on request from the corresponding author.

**Acknowledgments:** The authors acknowledge Wuhan University of Technology for materials and experimental instrument support sincerely.

**Conflicts of Interest:** The authors declare no conflict of interest.
