Various researchers are developing efforts to integrate waste and by-products as alternative materials in road construction and maintenance, reducing environmental impacts and promoting a circular economy. Among the alternative materials that several authors have studied regarding their use as partial or total substitutes for natural aggregates in the asphalt paving industry, the steel slag aggregate (SSA) and recycled concrete aggregate (RCA) from construction demolition waste (CDW) stand out. This paper reviews and discusses the characteristics and performance of these materials when used as aggregates in asphalt mixtures. Based on the various studies analyzed, it was possible to conclude that incorporating SSA or RCA in asphalt mixtures for road pavements has functional, mechanical, and environmental advantages. However, it is essential to consider some possible drawbacks of these aggregates that are discussed in this paper, to define the acceptable uses of SSA and RCA as sustainable feedstocks for road paving works.

Steel slag is a byproduct generated as waste during the steelmaking process and can be considered a cost-effective and environmentally acceptable alternative to replace natural aggregates. Using steel slag aggregates (SSA) to produce asphalt mixtures promotes sustainability and circular economy principles by using an industrial byproduct as a raw material. Thus, this work mainly aims to design more sustainable asphalt mixtures with high amounts of SSA that fit the circular economy expectations. This work developed two asphalt mixtures with SSA for surface (AC 14 surf) and binder/base (AC 20 bin/base) courses. Initially, the excellent wearing and polishing resistance of SSA and their good affinity with bitumen demonstrated the potential of this byproduct to be used in asphalt mixtures. Then, when analyzing the influence of using two different SSA incorporation rates (50% and a percentage close to 100%) in both asphalt mixtures, it was concluded that the use of SSA should be limited to 75% to avoid excessive air void contents and durability problems. The importance of considering the different particle densities of SSA and natural aggregates was highlighted during the mix design by defining a relationship between an effective and equivalent binder content. Finally, the mechanical performance of AC 14 and AC 20 with 75% SSA incorporation was compared to identical conventional mixtures produced with natural granite aggregates. The results obtained showed that the asphalt mixtures with 75% SSA have some workability problems due to the rough and porous surface of SSA. However, they present an excellent water sensitivity and permanent deformation resistance, surpassing the performance of the conventional asphalt mixtures.

Using aggregates from alternative sources has been considerably encouraged in recent decades. Reducing the consumption of natural aggregates from quarries (which have a substantial economic, visual, and environmental impact) is increasingly a concern. These needs have led to the broader use of more sustainable aggregates, increasing the incorporation percentages and extending their use to more demanding pavement layers (e.g., surface). In order to prove the efficiency of recycled concrete aggregates (RCAs) under such conditions, the “CirMat” project was developed. Among other works and tests, an asphalt concrete (AC) incorporating 52.3% RCA was characterized mechanically and environmentally. Empirical properties were evaluated, including the Marshall test (S = 20.2 kN; F = 2.9 mm) and resistance to permanent deformation (WTS = 0.10 mm/10³ cycles), as well as a life cycle assessment (LCA), which confirmed that nine indicators were improved (from 1% to 93%). The test samples were taken from mixtures produced in the laboratory and at a plant (after which they were applied on a construction site). Comparing the results with those obtained in a reference AC (with natural aggregates), it was possible to conclude that the performance of the AC with RCAs was very similar. Therefore, the use of these aggregates, at high rates, does not represent additional risks for asphalt mixtures and has lower environmental impacts in most categories.

Road paving recycling has been acquiring more relevance in society, especially within the paradigm of a circular economy. The addition of waste materials in asphalt mixtures is an excellent solution to face the gradual emergence of a great diversity of waste materials and reduce the production costs. This study aims to evaluate the addition of commercial and laboratory-produced polymer modified binders as rejuvenators in recycled asphalt mixtures with high contents of reclaimed asphalt pavement material (RAP). A commercial polymer modified binder (PMB) and a conventional bitumen modified with 5% of styrene-butadiene-styrene (SBS) or 4% of Regefalt were added to RAP aged bitumen and compared with a rejuvenated binder. Fatigue, permanent deformation and water sensitivity tests carried out on recycled mixtures produced with those binders showed that polymer modified binders could be used as rejuvenators to improve their performance significantly. The recycled asphalt mixture produced with the commercial PMB presented the best mechanical performance. The polymer-modified binders revealed an ageing resistance equivalent to that of the control rejuvenated binder, or slightly better in the case of the final binder with SBS polymer.

Self-healing in asphalt mixtures is a property that can be enhanced by external heating, which causes a thermal expansion that increases the flow of bitumen with reduced viscosity through the cracks. Therefore, this study aims to evaluate the effects of microwave heating on the self-healing performance of three asphalt mixtures: (1) conventional, (2) with steel wool fibers (SWF), and (3) with steel slag aggregates (SSA) and SWF. After evaluating the microwave heating capacity of the three asphalt mixtures with a thermographic camera, their self-healing performance was determined with fracture or fatigue tests and microwave heating recovery cycles. The results demonstrated that the mixtures with SSA and SWF promoted higher heating temperatures and presented the best self-healing capacity during the semicircular bending test and heating cycles, with significant strength recovery after a total fracture. In contrast, the mixtures without SSA presented inferior fracture results. Both the conventional mixture and that containing SSA and SWF presented high healing indexes after the four-point bending fatigue test and heating cycles, with a fatigue life recovery of around 150% after applying two healing cycles. Therefore, the conclusion is that SSA greatly influences the self-healing performance of asphalt mixtures after microwave radiation heating.