# Waste Silt as Filler in Hot Mix Asphalt: A Laboratory Characterization
## 1 Introduction  
Aggregate plants and quarries are providing a daily supply of aggregates for use in civil engineering projects, which require the extraction and production of approximately 3000 million tons of non-renewable natural aggregates each year [1]. During this process, various waste powders are generated that have hidden value and could serve as a potential substitute for traditional aggregate and fillers in construction materials. The type of by-product produced depends on the stone being processed and the extraction and production methods used at the plants. As a result, the asphalt pavement sector is facing new challenges that require the improvement of pavement performance to meet user needs and provide more sustainable solutions. Waste and recycled materials were successfully added to asphalt concrete to enhance their properties and provide additional capabilities and functionalities [2; 3; 4; 5; 6]. For example, replacing some mineral aggregates with crumb rubber from tires can reduce the stiffness of asphalt layers and make the environment safer for pedestrians [7]. Other approaches require the total or partial substitution of hot mix asphalt (HMA) fillers with waste mineral fillers [8; 9; 10; 11; 12]. Fillers occupy between 5 and 12% of asphalt mixture and are very fine materials that mostly pass sieve sizes 0.063 mm (EN 13043) or 0.075 mm (ASTM D242) depending on the standard used. In contrast to its little amountin a bituminous mixture, its contribution to the physical and chemical properties of the material is significant. For instance, fillers complete and satisfy gradation curves and can alter the strength and volumetric properties of mixtures [13]. A study evaluated the effect of different filler types and ratios on various HMA mechanical properties. It was concluded that an increase in filler content led to a decrease in the optimal binder content. Furthermore, a drop in the moisture damage resistant value was observed [14]. A similar study indicated that the amount and type of filler dramatically affected the indirect tensile strength (ITS) of HMA specimens. The authors showed that the addition of hydrated lime or cement bypass dust could improve the ITS values of pavements and were superior to common limestone dust [15]. According to the literature, fillers could influence the thermal sensitivity, aging and thermal performance of asphalt mixtures [12]. In addition, HMA samples composed of calcium carbonate and hydrated lime fillers exhibit higher ITS values compared with limestone fillers [16]. Excessive usage of common filler could produce several environmental risks, such as the depletion of natural resources. For this reason, finding an affordable and sustainable alternative to replace common fillers in asphalt pavements seems necessary. Various waste materials were used as fillers to produce sustainable asphalt pavements. For instance, the possibility of using fly ash, brick powder, and construction and demolition (C&D) waste in asphalt pavement as a filler was investigated [8]. Limestone filler was used as the reference filler and the results were compared with the control samples. The authors claimed that the waste fillers showed strong water damage resistance. Moreover, the fly ash presented acceptable binding with the bitumen, whereas the C&D waste had high CaO content, which is one of the main attributes for using limestone filer. Ceramic waste and marble waste dust were partially introduced into HMA, whereas limestone dust was used as the control sample [9]. The recommended substitution rates of ceramic waste and marble dust with the limestone filler were reported as 35 and 15%, respectively. In a similar approach, ceramic waste was applied as the filler in asphalt mixtures [11]. The waste powder was replaced with limestone filler from 25 to 100%. In all cases, the samples containing waste ceramic had better Marshall stability.