**1. Introduction**

Construction of asphalt pavement usually consumes a large amount of asphalt, aggregate and filler [1]. Among the natural resources, aggregate constitutes the dominant part of the asphalt mixture [2]. However, great consumption of natural minerals for this purpose resulted in environmental and resource pressure. Therefore, solid wastes have been increasingly used to replace the aggregate and filler in order to reduce consumption of natural mineral resources. Phosphogypsum is a solid waste which is produced from phosphoric chemical industrial processes [3]. Its global annual production is about 280 million tons, which also leads to environmental pollution [4–8]. Therefore, consuming and recycling phosphogypsum as much as possible can benefit environment protection.

It has typically been used as a filler of pavement subgrade and to prepared modified asphalt [9–12]. Shen et al. [13] prepared phosphogypsum-steel slag powder-flyash as a solidified material of pavement subgrade. Results showed that its early strength and long-term strength were higher than those of cement stabilized granular materials. Rakesh et al. [14] also found the solidified material, which contained fly ash and 8% lime and 2% phosphogypsum, showed adequate unconfined compressive strength, split strength, slake durability criteria and California bearing ratio.

**Citation:** Wan, J.; Han, T.; Li, K.; Shu, S.; Hu, X.; Gan, W.; Chen, Z. Effect of Phosphogypsum Based Filler on the Performance of Asphalt Mortar and Mixture. *Materials* **2023**, *16*, 2486. https://doi.org/10.3390/ma16062486

Academic Editor: Giovanni Polacco

Received: 22 February 2023 Revised: 14 March 2023 Accepted: 17 March 2023 Published: 21 March 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

On the other hand, Amrani et al. [15] used 5 wt% phosphogypsum as modifier to prepare modified asphalt. It was found that the modified asphalt showed higher improvements in stiffness and deformation resistance than the values found with fly ash and phosphate sludge wastes. Cuadri et al. [16] used phosphogypsum coupled with sulfuric acid to prepare modified foamed asphalt. Results showed that foamed asphalt based on phosphogypsum has higher rutting resistance and foaming properties compared with the natural gypsum.

However, using phosphogypsum as filler of asphalt mixture has not been systematically studied. Additionally, how to alleviate its negative effect on moisture stability of asphalt material also requires further investigation, since this has hindered its application in road engineering [17]. Alkaline materials should neutralize acidic substances in phosphogypsum, which could improve its water stability and asphalt adhesion. Steel slag is a by-production of steel manufacture, and it has alkalinity and good mechanical properties [18–20]. Steel slag based mixtures showed adequate permanent deformation and durable performance [21]. In addition, the asphalt mixture with steel slag powder fillers showed better resistance to moisture damage, and better low-temperature crack resistance, than asphalt mixtures with limestone filler [22]. The steel slag powder is presumed to neutralize acidic substances in the phosphogypsum, thereby enhancing the moisture stability of asphalt mixtures in which phosphogypsum is employed as a filler.

The concept of overall desirability has been used to integrate optimization based on multiple objectives into a single objective [23]. This method facilitates the integration of indicators with different data ranges into a desirability measure, and can be used to calculated the composition for achieving the optimum performance [24]. Different performance values have been combined into overall desirability to determine the optimum content, with desirability values varying from 0 to 1. Hence, this method has been used to investigate the optimum PF content in this study.

The study aimed to develop a phosphogypsum based filler (PF) which contained steel slag powder, in order to put this waste product to good use. Firstly, the optimum dosage of steel slag powder as modifier was determined through analysis and characterization of PF based asphalt mortar (P-AM). Secondly, the optimized PF was used to partly replace limestone filler in asphalt mortar. Asphalt mortars containing mixed fillers of PF and limestone filler (PL-AM) at different filler-asphalt ratios were prepared and tested. Thirdly, penetration, softening point, ductility and penetration index were used to determine the PF content in replacing traditional limestone filler based on overall desirability. Finally, asphalt mixtures that employed the mixed filler containing PF were fabricated and tested to verify the feasibility of using phosphogypsum as filler after PF composition and content in the mixed filler were determined. The results of this study provide an original approach for consuming the phosphogypsum in asphalt mixture. This approach is positive for reducing phosphogypsum and steel slag, which is beneficial for environmental protection.

#### **2. Materials and Methods**

#### *2.1. Raw Material*

#### 2.1.1. Phosphogypsum and Steel Slag Powder

Figure 1 show the appearance of phosphogypsum and steel slag powder. Table 1 shows the main chemical components of phosphogypsum and steel slag powder; the elements are given in form of their oxides. According to XRF analysis, the main component of phosphogypsum is CaSO4·2H2O and its density is 2.371 g/cm<sup>3</sup> . The physically adsorbed moisture of phosphogypsum will be removed at 100 ◦C heating. Studies reported that phosphogypsum will then turn into hemihydrate phosphogypsum (CaSO4·0.5H2O) when the temperature reaches about 130 ◦C. Hemihydrate phosphogypsum will turn into anhydrous hard phosphogypsum (CaSO4) when the temperature exceeds 1200 ◦C.

Aggregate

**Figure 1.** (**a**) Phosphogypsum powder, (**b**) steel slag powder. **Figure 1.** (**a**) Phosphogypsum powder, (**b**) steel slag powder.

**Table 1.** The chemical composition of phosphogypsum and steel slag powder. **Table 1.** The chemical composition of phosphogypsum and steel slag powder.


2.1.2. Asphalt Table 2 shows the neat asphalt with penetration range of 60–80 used in this study. Mechanical properties of the asphalt are characterized according to standard testing specification for asphalt and mixture testing (JTG E20-2011, Beijing, in Chinese) and technical Through XRF analysis, the main component of steel slag is Al2O3, and its density is 3.498 g/cm<sup>3</sup> . The steel slag powder had been exposed in the air for over 1 year. It was found that the free CaO (f-CaO) of steel slag powder was 2.1%, which was below the upper limit of f-CaO. This indicated that steel slag powder will not lead to volume instability.

the temperature reaches about 130 °C. Hemihydrate phosphogypsum will turn into anhy-

Through XRF analysis, the main component of steel slag is Al2O3, and its density is 3.498 g/cm3. The steel slag powder had been exposed in the air for over 1 year. It was found that the free CaO (f-CaO) of steel slag powder was 2.1%, which was below the upper limit of f-CaO. This indicated that steel slag powder will not lead to volume instability.

drous hard phosphogypsum (CaSO4) when the temperature exceeds 1200 °C.

#### specification for construction of highway asphalt pavements (JTG F40-2004, Beijing, in Chinese). 2.1.2. Asphalt

**Table 2.** Mechanical properties of asphalt. **Technical Index Test Results Requirements**  Penetration (25 °C, 0.1 mm) 70.7 60–80 Table 2 shows the neat asphalt with penetration range of 60–80 used in this study. Mechanical properties of the asphalt are characterized according to standard testing specification for asphalt and mixture testing (JTG E20-2011, Beijing, in Chinese) and technical specification for construction of highway asphalt pavements (JTG F40-2004, Beijing, in Chinese).

Softening point (°C) 49.0 ≥46 **Table 2.** Mechanical properties of asphalt.


#### ance with the standards for aggregate testing of highway engineering (JTG E42-2005, Bei-2.1.3. Aggregate and Filler

jing, in Chinese). The results indicated that limestone filler can meet the specification requirements. **Table 3.** The basic properties of the aggregate and filler. Table 3 shows the properties of the aggregate and filler, which were tested in accordance with the standards for aggregate testing of highway engineering (JTG E42- 2005, Beijing, in Chinese). The results indicated that limestone filler can meet the specification requirements.

Apparent specific density 2.851 ≥2.5 Crush value (%) 20.7 ≤28 Water absorption (%) 0.8 ≤3.0

**Technical Index Test Results Requirements** 

Filler


**Table 3.** The basic properties of the aggregate and filler.

*Materials* **2023**, *16*, x FOR PEER REVIEW 4 of 21

#### *2.2. Experimental Methods* Figure 2 illustrates the outline of this study. Firstly, this study introduces the prepa-

Figure 2 illustrates the outline of this study. Firstly, this study introduces the preparation of PF. The content of steel slag powder as modifier of PF was set as 0%, 20%, 40%, 60%, 80% and 100%, respectively. The optimum composition of steel slag powder and phosphogypsum was then investigated through characterization of PF based asphalt mortar (P-AM). Secondly, the optimized PF was used to partly replace limestone powder, which formed mixed filler containing PF and limestone powder. PF-L-AM (PL-AM) with filler-asphalt mass ratios of 0.8, 1.0 and 1.2 were prepared. Optimum PF content of the mixed filler was calculated by overall desirability based on the mechanical properties of PL-AM. How filler-asphalt ratio affect PL-AM's rheological properties was also considered. Finally, asphalt mixtures that employed the mixed filler containing PF were fabricated and tested. The feasibility of using phosphogypsum as filler was then verified after the PF composition and content in mixed filler were determined. ration of PF. The content of steel slag powder as modifier of PF was set as 0%, 20%, 40%, 60%, 80% and 100%, respectively. The optimum composition of steel slag powder and phosphogypsum was then investigated through characterization of PF based asphalt mortar (P-AM). Secondly, the optimized PF was used to partly replace limestone powder, which formed mixed filler containing PF and limestone powder. PF-L-AM (PL-AM) with filler-asphalt mass ratios of 0.8, 1.0 and 1.2 were prepared. Optimum PF content of the mixed filler was calculated by overall desirability based on the mechanical properties of PL-AM. How filler-asphalt ratio affect PL-AM's rheological properties was also considered. Finally, asphalt mixtures that employed the mixed filler containing PF were fabricated and tested. The feasibility of using phosphogypsum as filler was then verified after the PF composition and content in mixed filler were determined.

**Figure 2.** Outline of study. **Figure 2.** Outline of study.

#### 2.2.1. Preparation of PF 2.2.1. Preparation of PF

This study employed steel slag powder as the modifier in PF since phosphogypsum was acidic and highly hydrophilic, which could lead to poor moisture stability when using phosphogypsum as filler of asphalt mixture alone. Steel slag powder is generally al-This study employed steel slag powder as the modifier in PF since phosphogypsum was acidic and highly hydrophilic, which could lead to poor moisture stability when using phosphogypsum as filler of asphalt mixture alone. Steel slag powder is generally alkaline which should neutralize the acidity of phosphogypsum in a certain degree, benefiting the

der were first crushed and screened. Both had a particle size less than 0.075 mm.

in PF should be determined according to the mechanical characterization of P-AM.

kaline which should neutralize the acidity of phosphogypsum in a certain degree, benefiting the adhesion between asphalt and aggregate. Phosphogypsum and steel slag pow-

phosphogypsum (CaSO4·2H2O) was thus turned into hemihydrate phosphogypsum (CaSO4·0.5H2O) as pretreatment before preparing PF. The dosage of steel slag powder in PF was designed as 0%, 20%, 40%, 60%, 80% and 100%, respectively, based on preliminary tests. Therefore, 6 kinds of PF were prepared. The optimum content of steel slag powder adhesion between asphalt and aggregate. Phosphogypsum and steel slag powder were first crushed and screened. Both had a particle size less than 0.075 mm.

Both phosphogypsum and steel slag powder were first heated at 135 ◦C for 5 h. The phosphogypsum (CaSO4·2H2O) was thus turned into hemihydrate phosphogypsum (CaSO4·0.5H2O) as pretreatment before preparing PF. The dosage of steel slag powder in PF was designed as 0%, 20%, 40%, 60%, 80% and 100%, respectively, based on preliminary tests. Therefore, 6 kinds of PF were prepared. The optimum content of steel slag powder in PF should be determined according to the mechanical characterization of P-AM.

#### 2.2.2. Preparation of Asphalt Mortar

A high-speed mixing device was used to prepare asphalt mortar, and the mixing temperature was 150 ◦C. The mixing time was 45 min to ensure the uniformity of materials, and during this time, the rotational speed was varied. The initial rotational speed and time were set as 800 r/min for 15 min after PF or limestone filler were put in with the asphalt. Then, the rotational speed was increased to 1800 r/min for another 15 min. The rotational speed was increased to 3600 r/min for the final 15 min.

PFs of different composition were used to prepare P-AM; their volume was adjusted with that of the limestone filler to avoid the effect of volume difference. The filler-asphalt mass ratios of P-AM were 1:1. Additionally, limestone filler was used to prepare a control group. Thus, by replacing limestone filler at different proportions in equal volume, six kinds of P-AM were prepared to investigate the optimum dosage of steel slag powder based on their mechanical properties.

The optimum PF content in replacing limestone filler was then investigated. The PF content in the mixed filler was 0%, 25%, 50%, 75% and 100%. PL-AM was thus introduced. PL-AM's filler-asphalt mass ratios were 0.8, 1.0 and 1.2, respectively. The total volume of the mixed filler containing PF and limestone filler was unchanged when preparing PL-AM. Through characterization and analysis of properties, the optimum replacing proportion of PF for limestone filler can be ascertained. Limestone filler asphalt mortar (L-AM) was also prepared as control group.
