## 4 The Role of Metallurgy Slags in Sustainable Construction
### Slags in Road Construction  
Slags have found extensive use in road construction, primarily as a base material for roads and as an aggregate for asphalt. Their rough texture and high density provide excellent mechanical properties, such as stability and durability, essential for heavy-duty applications [257]. Steel slag, for example, offers numerous benefits as a material for road construction, thanks to its advantageous physical and chemical properties [258]. Steel slag exhibits superior performance compared to natural aggregates, with abrasion values ranging from 15% to 25%, significantly lower than those of typical natural aggregates, which can exceed 30% [202]. Its crushing strength is higher, often above 200 MPa, compared to around 150 MPa for natural aggregates, and its soundness index is less than 12%, indicating better resistance to weathering [259]. In addition, its rough, angular texture provides excellent interlocking, improving skid resistance by up to 50% compared to conventional aggregates [260]. Steel slag also enhances resistance to moisture damage, with tensile strength ratios often exceeding 90%, compared to 70-80% for natural aggregates, and it shows reduced rut depth in asphalt mixtures, often measuring less than 2.5 mm in repeated load tests [257]. Studies have consistently shown that incorporating steel slag into road pavements significantly improves durability and fatigue resistance, with fatigue life increasing by up to 30% compared to mixes without slag [261]. Additionally, its versatility allows it to serve as both a coarse and fine aggregate, optimizing the performance of asphalt and concrete mixtures. Similarly, phosphorus slag has also been proven both theoretically and experimentally to be a feasible additive in asphalt concrete mixtures for road construction, enhancing the operational properties of road surfaces. [PERSON] et al. [262] have shown that the significant porosity of phosphorus slag improves asphalt concrete's structure, increasing the strength, density, and waterproofness by up to 20%, while reducing bitumen consumption by 15%. The authors also found that the slag's interaction with bitumen is similar to limestone, and it has comparable effects on bitumen aging. Additionally, the coarse texture of slag enhances the internal friction angle, stability, and resistance of asphalt concrete, particularly in high temperatures, making it an excellent alternative to conventional materials like limestone. Extending the exploration of alternative slag materials, [PERSON] et al. [263] successfully applied iron tailings slag in road subbase construction, demonstrating the material's potential when integrated with optimal additives. Their research found that a mixture containing 6:1 slag to fly ash ratio, 4% calcium oxide, 5% cement, and 0.02% water-resistant base stabilizer achieved an impressive 7-day unconfined compressive strength of 1.97 MPa and an elastic modulus of 286 MPa. Copper slag, another promising alternative material, has also been explored for its potential in road construction, especially as a substitute for fine aggregates in asphalt and structural fill applications. [PERSON] et al. [264] investigated the use of fine copper slag in road embankments, subgrades, and subbases, demonstrating its potential to enhance the mechanical properties of road layers. The study revealed that copper slag, due to its high specific gravity and angular particle shape, significantly improves the compaction characteristics and stability of road materials. The incorporation of 60% fine copper slag as a replacement for sand resulted in increased California Bearing Ratio (CBR, a parameter that measures the shear strength and load-bearing capacity of a soil or granular material) values, reaching up to 15%, compared to traditional soil-sand mixtures with CBR values of 8-10%.