## 1 Introduction  
The inclusion of waste in materials for pavements and geotechnical engineering projects has become a focus of research. The complex influences of aggregate on soil mechanics [1, 2, 3, 4, 5] and excessive waste production create a challenge in investigating waste issues in different scenarios. Researchers across the world have used various waste materials [6, 7, 8], such as fibre and plastic, in geotechnical and pavement applications and investigated their performance.  
The filler used in asphalt mixes makes up a maximum of 5% of the total aggregate mass and can have a large impact on the asphalt's properties in accordance with Western Australian standards. Filler materials are defined as being finer than what would pass through a 0.075 mm sieve and play an important role in the performance of asphalt mixtures. The fillers affect the air voids in an asphalt mixture and the physical and chemical properties of bitumen.  
The addition of filler materials can greatly increase the efficiency of asphalt. Waste materials can be recycled or repurposed into an asphalt mix and maintain or improve its performance while also reducing landfill. The filler material used in an asphalt mixture affects the resulting pavement's performance. This is due to the high surface area of fine filler particles, which enhances absorption by the asphalt binder [9].  
The performance of asphalt mixes is significantly affected by the particle size distribution, shape, surface area, surface texture, void material and mineral composition of the aggregate and fillers used [10]. A portion of filler is absorbed by the bitumen while the rest reduces air voids. [11] According to [PERSON] et al. [10], when filler enters voids, it serves as additional contact points between aggregate particles, which provides a better stress distribution, and as micro-rollers, which allow greater aggregate packing. A rolling surface of contact is created when filler particles surround aggregates, resulting in increased compaction, density and pressure. An increased filler content improves the mixture's hardness;however, unnecessary filler degrades an asphalt mixture because more bitumen is required to bind the aggregate and filler [9; 12].  
Bitumen expansion occurs as a result of filler particles being absorbed, resulting in a thicker coating around the aggregates. Increased flexural stress dissipation and fatigue efficiency are possible with a thicker coating [9; 13]. Owing to the increased viscosity of the asphalt mastic, the stiffness of the bitumen is also increased by the adsorbed filler. Coarse and fine aggregates bind together more strongly as finer particles fill the voids in the asphalt mix matrix and enhance resistance to movement within it [12; 14]. [PERSON] [15] found that adding filler to a pavement improves its resistance to permanent deformation at the cost of stability and fatigue resistance. Additional filler, as discovered by [PERSON] [16], improves fatigue efficiency by extending the bitumen and reducing air voids. Furthermore, [PERSON] [17] found that although filler stiffens the mix, it also makes it more resistant to micro-cracking, which delays the onset of fatigue distress.  
The majority of previous research has concentrated on either improving the efficiency of asphalt pavements or the use of alternative filler materials. The aim of this study was to see how filler substitutes like GGBFS affect the rutting and fatigue efficiency of asphalt pavements. Mix designs with various percentages of GGBFS were employed to examine its effects on the rutting and fatigue efficiency of mixes compliant with the _Main Roads WA Specification 504 for Asphalt Wearing Course_. The aims of the study were to (1) assess the relationship between GGBFS proportion and the fatigue cracking and permanent deformation characteristics of hot mix asphalt pavement, and (2) determine the optimal GGBFS proportion.