Evaluation of the Influence of Phosphogypsum-Based Composite Filler on Performance of the SMA-13 Asphalt Mixture and Its Harmless Treatment

Abstract

Phosphogypsum is a waste from the phosphorus chemical industry which has certain environmental hazards. Using it as a substitute for building materials was thought to alleviate the problems of phosphogypsum pollution and natural mineral consumption. This study tried to develop an environmentally friendly phosphogypsum-based composite filler (PCF) that can be used as a filler in the SMA-13 asphalt mixture. The SMA-13 asphalt mixture was first designed, following which PCF containing phosphogypsum and steel slag powder was prepared. PCF’s composition and harmless treatment were determined based on the characterization of the overflowed concentration of fluoride ions, and the pH value of the PCF’s leaching solution was evaluated by ion chromatography and a pH meter. The effect of replacing the PCF content in the SMA-13 asphalt mixture was investigated according to high-temperature performance, moisture resistance, and low-temperature performance. Long-term overflowed harmful ion concentrations of PCF-based SMA-13 asphalt mixtures were also included. The results indicate that the steel slag powder content can reduce the overflowed fluoride ion concentration in phosphogypsum. The optimal composition of PCF was 65% phosphogypsum and 35% steel slag powder. The addition of PCF can enhance the SMA-13 asphalt mixture’s Marshall stability and dynamic stability when its content is over 20%. SMA-13 asphalt mixtures with PCF can meet the requirements of the specification, although their moisture resistance is reduced by PCF. PCF was proven to increase fracture toughness and energy in a semi-circular bending test at low temperatures, while 40% PCF showed the highest cracking resistance. Furthermore, PCF was able to reduce the long-term overflowed concentrations of fluoride ions and phosphate ions that could meet the environmental requirements. The results of this study provide academic support for the effective consumption of phosphogypsum in road engineering on a large scale.

1. Introduction

Road construction in China in recent decades has promoted the development of the state economy. The construction of the road system has thus consumed a huge amount of natural mineral materials. However, the overexploitation of natural minerals leads to natural environmental destruction. Recycling solid waste as a raw material in asphalt pavement construction was thought to be an effective way to lessen the pressure on environmental protection [1], which will benefit the sustainability of road engineering. The ordinary application of solid waste replaced aggregate in asphalt mixtures or was used as an additive such as fiber or a modifier of asphalt binder. Phosphogypsum [2,3] is a by-product of the production of wet-process phosphoric acid, whose particle size is generally 5–50 µm. Its main component is CaSO₄•2H₂O, while it also contains impurities like residual phosphoric acid, fluoride, acid-insoluble substances, and organic matter [4]. A large amount of phosphogypsum is produced globally every year, which seriously damages the ecological environment through polluting groundwater and wasting land resources. Gerrity [5] introduced a cement-stabilized phosphogypsum mixture whose performance was acceptable for use as a stabilized base material. Ding [6] used phosphogypsum to fabricate cold-bonded aggregates through granulation technology, which can be used as a lightweight aggregate, showing advantages such as low cost and difficulty. Li [7] used phosphogypsum as a modifier to fabricate phosphogypsum-modified asphalt. The results showed that its high-temperature performance was higher than that of neat asphalt according to rheological property enhancement. Dutta [8] developed a flyash-lime-phosphogypsum composite material for a base/subbase coarse material in road pavement whose strength and durability can satisfy the standard specifications.

However, there is no systematic study on the effect of phosphogypsum-based filler on the performance of the SMA asphalt mixture. Additionally, research on the harmless treatment of phosphogypsum and its application as a filler material has not been detailed, considering its environmentally harmful substances. It will surely threaten the application of phosphogypsum-based materials. Wu [9] reported a study on harmless treatment methods in which harmful and toxic components were solidified and stabilized by highly targeted solidification and stabilization technology. It suggested that calcium carbide slag and lime can be used as alkali-base neutralizers to lower acidity, reducing the overflow of harmful ions.

Steel slag [10,11], which is also a solid waste produced by the steel-making industry, has been widely used in the construction field due to its advantages like high-abrasion resistance and hardness. It has always been used as an aggregate, cement additive, and concrete admixture material [12]. Bosurgi [13] compared the environmental impact and mechanical properties of coarse basalt aggregate-based and coarse steel slag-based asphalt mixtures. The results suggest that a steel slag-based asphalt mixture shows a higher crushing resistance and a lower required asphalt content, which also has environmental benefits. Carvalho [14] evaluated basic oxygen furnace slag powders as supplementary cementitious materials. It was found to increase the eco-efficiency of cement-based composites in terms of mechanical performance. Chen [15] evaluated the effect of using steel slag powder as a filler in an asphalt mixture. It can be concluded that asphalt mixtures with SSP fillers showed better moisture resistance, permanent deformation resistance, and low-temperature performance than asphalt mixtures with limestone filler.

Additionally, using phosphogypsum as a filler directly reduces the mechanical performance of asphalt mixtures due to its acidity, which is thought to be one of the reasons why phosphogypsum is not used as a filler. Consequently, steel slag is supposed to neutralize the acidity of phosphogypsum to a certain extent owing to its alkalinity [16]. Shu [17] employed phosphogypsum and steel slag powder as a phosphogypsum-based composite filler. The overall desirability method and asphalt mortar were used to determine the composite filler’s composition and the replacement ratio of limestone filler. The results proved that the addition of steel slag powder can significantly reduce phosphogypsum-based composite filler (PCF)’s negative effect on the corresponding asphalt mixture’s moisture stability. However, the previous utilization amount of PCF in asphalt mixtures was generally low. How PCF affects the SMA asphalt mixture’s performance is still unclear, considering the high amount of PCF used in the mixture.

This study tried to develop a feasible PCF that can be used in the SMA-13 asphalt mixture without reducing its mechanical performance. An eco-friendly evaluation of PCF as well as PCF-based SMA-13 asphalt mixtures was also characterized by harmful ion concentration detection. Steel slag powder was used to mix with phosphogypsum to fabricate PCF, whose optimal composition was determined according to a harmful ion overflow experiment. The optimal composition of PCF is 65% phosphogypsum and 35% steel slag. Afterwards, how PCF’s replacement ratio of traditional limestone filler affected SMA-13 asphalt mixture’s mechanical performance was investigated. Lastly, long-term harmful ion overflow evaluation of the asphalt mixture by PCF content and overflow time dependencies was separately carried out. This study enhanced the utilization of phosphogypsum in asphalt mixtures and provided a theoretical basis for the application and environmental effects of phosphogypsum and steel slag-based material.

2. Materials and Methods

2.1. Materials

2.1.1. Asphalt

The asphalt binder used in this study was SBS modified asphalt provided by Guochuang Co., Ltd. (Hefei, China) Its property was tested according to the standard testing specification for asphalt and mixture testing (JTG E20-2011, in Chinese). The corresponding technical specification for the construction of highway asphalt pavements (JTG F40-2004, in Chinese) and test results are listed in

Table 1. Properties of SBS modified asphalt.

Property	Test Result	Technical Specifications	Standard
Penetration (25 °C)	58	40–60	JTG F40-2004
Softening point (R&B)	87	≥60
Ductility (10 °C)	32	≥20

.

2.1.2. Aggregate

The aggregate in the SMA-13 asphalt mixture was basalt which was collected from Zhangjiakou City.

Table 2. Property of basalt aggregate.

Testing Items	Technical Specifications	Test Results
Apparent relative density	≥2.90	2.983
Bulk volume relative density	—	2.960
Crushing value (%)	≤26	10.5
Water absorption rate (%)	≤3.0	0.39
Asphalt adhesion	—	Level 5

shows the properties of the basalt aggregates. Its properties were tested in accordance with standard for aggregate testing of highway engineering (JTG E42-2005, in Chinese). The results suggested that its properties can meet the specification of JTG F40-2004.

2.1.3. Filler and Phosphogypsum

Limestone powder collected from Huangshi City was usually used as a filler of the asphalt mixture, which was employed as a control group for the PCF of the SMA-13 asphalt mixture in this study. The phosphogypsum used was a pale gray powder as presented in Figure 1, which originated from the chemical plant in Xiaogan City and was used to replace the limestone powder. Figure 2 shows SEM images of phosphogypsum at 200× and 5000× magnification. Steel slag powder was also used as an additive of PCF due to its possible neutralizing effect on the acidity of phosphogypsum. The densities of the limestone powder, steel slag powder, and phosphogypsum were 2.751 g/cm³, 3.498 g/cm³, and 1.818 g/cm³. The element composition of phosphogypsum was characterized by the X-ray fluorescence (XRF) given by its corresponding oxides, the results of which are shown in

Table 3. Element composition characterization of phosphogypsum through XRF.

Composition	Content (%)
F	1.05
Na2O	0.12
Al2O3	0.86
SiO2	6.28
P2O5	0.98
SO3	44.08
K2O	0.66
CaO	33.19
TiO2	0.12
Fe2O3	0.6
MgO	0.07
BaO	0.2
Loss on ignition	11.72
Other	11.79

.

2.2. SMA-13 Asphalt Mixture Gradation Design

The SMA-13 asphalt mixture, namely a stone mastic asphalt of framework dense structure, was selected as the designed gradation. Figure 3 illustrates the mineral gradation of the SMA-13 asphalt mixture used in this study. The asphalt–mineral weight ratio was determined as 5.7% after trial and error, regardless of the filler type.

2.3. Experimental Method

Figure 4 shows the outline summary of this study. Phosphogypsum and steel slag powder were used to prepare PCF, partly replacing ordinary limestone filler in this study. Firstly, phosphogypsum and steel slag powder were uniformly mixed according to different compositions. A harmful ion overflow experiment on the powder mixture, based on ion chromatography, was conducted to evaluate steel slag’s digestion effect on harmful ions of phosphogypsum. The optimal PCF composition that showed the best harmful ion digestion effect was consequently determined. Afterwards, SMA-13 asphalt mixtures with filler containing different PCF proportions were prepared. The high-temperature stability, moisture stability, and low-temperature performance of corresponding the SMA-13 asphalt mixtures were characterized and analyzed. How PCF’s replacement ratio of limestone filler affects its mechanical performance was thus concluded. Lastly, an evaluation of the long-term harmful ion overflow in the SMA-13 asphalt mixture depending on the PCF replacement ratio and overflow time was conducted. The overflow characteristics of harmful ions from phosphogypsum during the SMA-13 asphalt mixture’s service life can be investigated.

PCF content refers to its volume replacement ratio to traditional limestone filler in the SMA-13 asphalt mixture. The composition of filler for the SMA-13 asphalt mixture is shown in

Table 4. Different composition of filler for the SMA-13 asphalt mixture.

Experimental Group	Content of Phosphogypsum and PCF	Content of Limestone Powder
P50	Phosphogypsum 50%	50%
PCF0	PCF 0%	100%
PCF20	PCF 20%	80%
PCF40	PCF 40%	60%
PCF60	PCF 60%	40%
PCF80	PCF 80%	20%
PCF100	PCF 100%	0%

. The PCF contents were designed as 20%, 40%, 60%, 80% and 100%. In addition, the P50 group with 50% pure phosphogypsum was also involved in a following study to investigate phosphogypsum’s long-term harmful ion overflow in an asphalt mixture without harmless treatment.

2.4. High-Temperature Stability

The high-temperature stability of the SMA-13 asphalt mixture was evaluated depending on the Marshall stability and dynamic stability test according to JTG F20-2011. Compacted Marshall specimens of SMA-13 asphalt should be tested on a Marshall stability apparatus after being kept in a 60 °C water bath for 30 min. The temperature of the dynamic stability apparatus should be applied at 60 °C, during which the heat preservation time of the track plate specimens should be more than 5 h. Dynamic stability can be calculated by Equation (1).

$$DS={\displaystyle \frac{{(t}_2-t_1)\times 42}{d_2-d_1}}\times c_1\times c_2$$

(1)

where $DS$ = the dynamic stability of the asphalt mixture, (cycle/mm); $t_1$, $t_2$ = the test time, usually 45 min and 60 min; $d_1$, $d_2$ = the deformation of the specimen surface corresponding to the test specimens $t_1$ and $t_2$, (mm). $c_1$, $c_2$ = the correction factor of the testing machine or specimen, dimensionless.

2.5. Moisture Stability

An evaluation of the SMA-13 asphalt mixture’s moisture stability was conducted on the basis of an immersion Marshall test and freeze-thaw splitting test according to JTG E20-2011. The immersion Marshall stability ratio (MSR), as a result of the immersion Marshall test, was the residual Marshall stability after Marshall specimens were soaked in a water bath at 60 °C for 48 h. The tensile strength ratio (TSR) was used to imply the residual tensile strength after a freeze–thaw process, during which Marshall specimens were frozen at −18 °C for 16 h and then kept in a 60 °C water bath for 24 h in a saturated state. The MSR and TSR of the asphalt mixture were characterized. The MSR and TSR were calculated by Equations (2) and (3), respectively:

$${MSR}_0={\displaystyle \frac{{MSR}_1}{\mathrm{M}\mathrm{S}\mathrm{R}}}\times 100$$

(2)

where

• MSR: the average stability of specimen in moisture at 60 °C for 30 min (kN);
• MSR₁: the average stability of specimen in moisture at 60 °C for 48 h (kN);
• MSR₀: the average residual stability of specimen in moisture.

$$TSR={\displaystyle \frac{R_{\mathrm{T}2}}{R_{T1}}}\times 100$$

(3)

where

• TSR: the average strength ratio of a freeze–thaw splitting test;
• R_T1: the splitting tensile strength of the specimens without a freeze–thaw cycle;
• R_T2: the splitting tensile strength of specimens after a freeze–thaw cycle.

2.6. Low Temperature Crack Resistance

A semi-circular bending test (SCB) at a low temperature was chosen to illustrate the SMA-13 asphalt mixture’s low temperature crack resistance in this study according to AASHTO TP 105-2020. The fracture toughness and fracture energy of an induced fracture on a notch of the SCB specimens were introduced. The diameter of each SCB specimen’s size was 101.5, while its thickness was 50 mm. Each SCB specimen needed to be notched from the center of a semicircle whose notch depth was around 6 mm. A universal mechanical testing equipment was introduced to apply the vertical loading, whose loading rate on the principal axis was 0.05 mm/min, and it would stop when the SCB specimen fractured. The SCB specimens needed to be frozen at −10 °C for 3 ± 0.5 h in a temperature control box before testing. The fracture toughness and fracture energy were calculated according to Equations (4)–(7):

$$K_{IC}=Y_I{\sigma }_0\sqrt{\pi a}$$

(4)

$${\sigma }_0={\displaystyle \frac{P_C}{2rt}}$$

(5)

where

• $K_{IC}$ = fracture toughness (MPa × m0.5);
• $Y_I$= the normalized stress intensity factor (dimensionless);
• $P_C$= the critical load (N);
• $r$= radius of specimens (m);
• $t$= the specimen thickness (m);
• $a$ = the notch length (m).

$$G_F={\displaystyle \frac{W_F}{A_{lig}}}$$

(6)

$$W_f=\sum _i^n\left(u_{i+1}-u_i\right)\ast P_i+{\displaystyle \frac{1}{2}}\ast \left(u_{i+1}-u_i\right)\ast (P_{i+1}-P_i)$$

(7)

where

• $G_F$= fracture energy (J/m²)
• $W_f$ = fracture energy (J);
• $A_{lig}$ = ligament area (m²);
• $P$ = applied load (N);
• $u$ = load line displacement (m);
• $P_i$ = applied load (N) at the i load step application;
• $P_{i+1}$ = applied load (N) at the i + 1 load step application;
• $u_i$ = load line displacement (m) at the i step;
• $u_{i+1}$ = load line displacement (m) at the i + 1 step.

2.7. Overflowed Harmful Ion Detection

Characterization of the overflowed concentration of harmful ions such as F- was carried out when the PCF was prepared, which was explained in Figure 5 according to the solid waste-determination of the fluoride-ion selective electrode method (GB/T 15555.11-1995). Phosphogypsum and steel slag powder should be mixed and previously insulated at 150 °C, simulating the actual production conditions of an asphalt mixture. In the first stage, 100 g PCF sample and 1000 mL deionized water were put in a specific container for oscillation. These containers were then placed on a reciprocating horizontal oscillation device with an oscillation frequency of 110 ± 10 times/min and an amplitude of 40 mm. The oscillation time for each test was 8 h. Oscillated containers were taken from the device and stewed for 16 h as Stage 3 illustrates. The leaching solution containing spilt ions was filtrated by using a vacuum filter to remove any suspended solids. Afterwards, as shown in Stage 5, the filtered solution was acknowledged. At the final stage, the concentration characterization of overflowed ions was characterized depending on ion chromatography [18].

Additionally, a compacted Marshall specimen instead of the PCF samples described in Figure 5 was used to detect objects in evaluating the long-term harmful ion overflow. The volume of corresponding deionized water was ten times the actual mass of PCF used in each compacted SMA-13 sample. For instance, the volume of deionized water must be 2000 mL if 200 g PCF is used in a specific SMA-13 Marshall specimen. It should be noted that all other experimental steps were consistent with those showed in Figure 5.

Table 5. Different compositions of PCF.

Composition (Volume Ratio)	Phosphogypsum	Steel Slag Powder
P100	100%	0%
P80-S20	80%	20%
P75-S25	75%	25%
P70-S30	70%	30%
P65-S35	65%	35%

lists the different compositions of PCF prepared to determine the optimal composition according to the harmful ion overflow evaluation. In addition, this study also detected the pH value of the leachate based on a pH meter.

2.8. Long-Term Overflowed Harmful Ion Concentration

The long-term overflowed harmful ion concentration also depends on the overflowed harmful ion detection method that was explained in Figure 5. Compacted Marshall specimens of the SMA-13 asphalt mixture were firstly maintained in a water bath temperature control box of 60 °C for 24, 48, 72, and 96 h, respectively. This operation was to stimulate the harmful ions stored in asphalt mixtures, making them more prone to overflow. The longer the immersion time, the easier it is to overflow, which can reflect the long-term harmful ion overflow conditions during the SMA-13 asphalt mixture’s service life. Afterwards, the harmful ion concentration of the solution should be tested by ion chromatography according to the overflowed harmful ion detection method described above.

3. Results and Discussion

3.1. Effect of PCF Composition on Fluoride Ion Overflow Evaluation

Steel slag powder was used as a modifier in PCF that improved the corresponding asphalt mixture’s mechanical properties according to a previous study [17]. The results of penetration, softening point, ductility, and the penetration index of asphalt mortar suggested that steel slag powder’s volume proportion in PCF should be 23%. Furthermore, this study tried to figure out how steel slag powder’s volume proportion affects the fluoride ion overflow level.

The ion chromatography results which illustrated fluoride ion overflow concentration are partly shown in Figure 6 and Figure 7. Table 3 suggests that fluoride is probably the dominant hazardous substance which limits the use of phosphogypsum material. It proves that the PCF of P65-S35 showed much lower soluble chloride, phosphate, and fluoride ions concentrations than the PCF of P100. Only fluoride ions were detectable in the P65-S35 sample owing to the harmless treatment of steel slag powder. Figure 8 presents the fluoride ion overflow concentration of PCF at different compositions. It was found that P100, namely the pure phosphogypsum, overflowed fluoride ions of nearly 100 mg/L, which was far over the upper limit of fluoride ion concentration (≤20 mg/L) specified by the integrated wastewater discharge standard (GB 8978-1996, in Chinese).

Additionally, a significant decrease in fluoride ion overflow concentration can be observed after partially replacing phosphogypsum with steel slag powder according to the results of the other samples. It proved that the addition of steel slag powder could inhibit the overflow of fluoride ions from phosphogypsum. The content of steel slag powder in PCF showed no clear effect on the overflowed fluoride ion concentration, while only the corresponding result of P65-S35 at 12.15 mg/L can meet the specification of GB 8978-1996. Figure 9 presents the pH value of PCF at different compositions, in which it was found that the pH value of the mixture of phosphogypsum and steel slag powder was positively correlated to the steel slag powder’s content. It was specified that the pH value of sewage should be between 6 and 9 according to GB 8978-1996, which suggested that only P65-S35’s leaching solution, whose pH value was 6.13, was compliant with the specified requirements.

A previous study concluded that the appropriate content of steel slag powder should be 23% [17] in accordance with the mechanical properties of the corresponding asphalt mortar. However, 23% steel slag powder in PCF is inevitably unreasonable considering that the harmful ion overflow concentration will exceed the standard when the steel slag powder is lower than 35%. Consequently, the PCF composition in the following study was determined as P65-S35 for maximizing the utilization of phosphogypsum.

3.2. Effect of PCF Content on Asphalt Mixture Mechanical Property

3.2.1. High-Temperature Performance

The Marshall stability, which was tested at 60 °C, can reflect an asphalt mixture’s high-temperature strength. Figure 10 illustrates the Marshall stability results of different compositions. The results implied that the SMA-13 asphalt mixture with PCF generally showed a higher Marshall stability than those of the P50 and PCF0 asphalt mixtures, in which the PCF80 samples illustrated the highest value. This indicated that the addition of PCF was able to enhance the strength of the SMA asphalt mixture at high temperatures. In addition, Figure 11 presents the dynamic stability of the asphalt mixture, which was positively correlated to their rutting resistance. All samples’ dynamic stabilities were over 3000, which is the minimum value specified by JTG F40-2004. The P50 sample was found to have the lowest value, proving that the addition of phosphogypsum might lower the corresponding rutting resistance. On the other hand, the dynamic stabilities of the PCF samples were also found to be higher than those of the PCF0 samples when its content was over 20%. Therefore, PCF was supposed to improve the rutting resistance of the SMA-13 asphalt mixture, which was consistent with the result of Marshall stability, so that a high temperature can evidently be developed.

3.2.2. Moisture Resistance

The moisture resistance of the PCF-based SMA-13 asphalt mixture should be cautiously detected due to the potential poor adhesion between aggregates and binders caused by the acidity of phosphogypsum. Figure 12 presents the immersion Marshall stability ratio results. The MSR0 values of the PCF-based SMA-13 asphalt mixture and P50 sample were lower than that of control group, suggesting that the addition of phosphogypsum showed a possible negative effect on the SMA asphalt mixture’s resistance to immersion. Additionally, the MSR0 values of all asphalt mixtures can meet the requirement of the JTG F40-2004 standard, which specifies that the MSR0 should be over 80%. The Marshall stability before and after the immersion of PCF-based samples was found to be higher than that of samples without PCF, which implied that the PCF-based SMA asphalt mixture still maintained sufficient strength after immersion.

On the other hand, Figure 13 presents the tensile strength ratio result of the samples. The TSR value of all samples was over 80%, and the PCF content did not show an explicit effect. PCF-based samples showed lower splitting tensile strengths than that of the control group, regardless of being before and after the freeze–thaw process. This indicated that the PCF-based SMA-13 asphalt mixture was still of adequate moisture resistance, even though using phosphogypsum to replace limestone filler probably reduced the SMA-13 asphalt mixture’s moisture resistance.

3.2.3. Low-Temperature Performance

The SCB test results that indicate samples’ resistance to cracking at −10 °C are presented in Figure 14 and Figure 15, reflecting the effect of PCF content and phosphogypsum on the SMA-13 asphalt mixture’s low-temperature performance. Fracture energy and toughness, which refer to the required energy consumption for cracking fracture and the peak strength upon cracking fracture, were employed as corresponding indicators. Both the fracture energy and toughness of the PCF0 sample were the lowest, which suggests that using a phosphogypsum-based filler in the SMA asphalt mixture can help to increase the low-temperature cracking resistance. Samples containing PCF also showed a higher fracture energy and toughness than that of the P50 sample, proving that the addition of steel slag powder in phosphogypsum-based filler was positive for low-temperature performance. Furthermore, both the fracture energy and toughness were found to reach the highest value when the PCF content was 40%. Therefore, the appropriate use of PCF was expected to improve the low-temperature crack resistance of the SMA-13 asphalt mixture compared to traditional fillers.

3.3. Long-Term Overflowed Harmful Ion Detection

Long-term overflowed harmful ion concentrations of P50, PCF40, and PCF80 samples were detected. Long-term detection is different, with Figure 16, Figure 17, Figure 18 and Figure 19 presenting the ion chromatography result of the P50 sample after being immersed in 60 °C water for 24 h, 48 h, 72 h, and 96 h. It showed that the main overflowed ions were the fluoride ion, chloride ion, sulfate ion, phosphate ion, and nitrate ion. These ions were thought to be from phosphogypsum in accordance with the result of Figure 6, which refers to the overflow results of untreated phosphogypsum. It was analyzed that long-term immersion in 60 °C water may lead to possible chemical reactions, which reduced the harmless effect of steel slag powder. Additionally, chloride ions, sulfate ions, and nitrate ions widely exist in nature and their contents met environmental protection requirements, so they were not discussed in the following study.

Figure 20 and Figure 21 showed the concentration of overflowed fluoride ions and phosphate by different immersion times, respectively. The results suggested that all concentrations of overflowed fluoride ions were below 1.2 mg/L, which was far lower than the specified upper limit of fluoride ions (≤20 mg/L, GB 8978-1996). In addition, the concentration of fluoride ions increased along with the immersion time, proving that overflowed fluoride ions would gradually increase in the SMA-13 asphalt mixture’s service time. The fluoride ion concentration of the P50 sample was generally higher than that of the PCF-based sample, while PCF80’s fluoride ion concentration was higher than that of PCF40. It suggested that steel slag powder could reduce the overflow concentration of fluoride ions, which could be developed by increasing the PCF content. On the other hand, the phosphate ion concentration of the P50 sample was found to be higher than that of the PCF40 and PCF80 samples. It also proved that steel slag powder can play a role in dissolving harmful substances in PCF. The immersion time showed no explicit effect on the overflowed phosphate ion concentration, while those of the PCF40- and PCF80-based samples were generally lower than 1 mg/L. It can fundamentally meet the requirement of sewage that can be discharged into IV and V waters on the basis of GB 8978-1996. In a word, PCF was proved to be favorable for reducing the overflowed concentration of fluoride ions and phosphate ions, and their long-term overflowed concentrations can meet the environmental requirement.

4. Conclusions

The utilization of phosphogypsum as a filler of asphalt mixtures was thought to be a sufficient way to consume existing phosphogypsum. However, its application in the SMA asphalt mixture and a corresponding study on harmful substance inhibition has not been reported in detail yet. This study tried to combine phosphogypsum and steel slag powder as a phosphogypsum-based composite filler (PCF), which was used to partly replace the filler of an SMA-13 asphalt mixture. The overflowed fluoride ion concentration and pH value detection determined the optimal composition and harmless treatment evaluation of PCF. The effect of the PCF content on the asphalt mixture’s mechanical properties was characterized based on the high-temperature performance, moisture resistance, and low-temperature performance. Lastly, the long-term harmful ion overflow detection was also included in this study to show the long-term harmless effect of PCF in the SMA-13 asphalt mixture’s service time.

(1) The dominant harmful substance in phosphogypsum throughout this study was fluoride ions, whose concentration cannot meet the specification of environmental standards. The overflowed fluoride ion concentration showed a generally decreasing tendency as the steel slag powder content increased in PCF. The fluoride ion concentration of the PCF’s leaching solution containing 65% phosphogypsum and 35% steel slag powder was 12.15 mg/L, while its pH value was 6.13, which can meet the specification of wastewater discharge standards. Therefore, the optimal composition of PCF was determined as 65% phosphogypsum + 35% steel slag powder.

(2) The addition of PCF can generally help to enhance the SMA-13 asphalt mixture’s high-temperature performance due to its Marshall stability and dynamic stability which was developed when the PCF content was over 20%. PCF reduced the immersion Marshall stability ratio but showed no explicit effect on the tensile strength ratio, while the PCF-based SMA-13 asphalt mixture can meet the requirement of the specification. The PCF-based sample showed a higher fracture toughness and energy in semi-circular bending tests, in which the SMA-13 asphalt mixture with 40% PCF showed the highest cracking resistance in low temperatures. Furthermore, PCF was able to reduce the long-term overflowed concentration of fluoride ions and phosphate ions, and their long-term overflowed concentrations can meet the environmental requirement.