# Waste Silt as Filler in Hot Mix Asphalt: A Laboratory Characterization
## 5 Discussion  
For the current study, silt was used in two forms, namely, untreated and thermally treated, as a filler to produce mastic specimens. According to our previous study [22], sharp peaks of calcite (CaCO\({}_{3}\)) were observed from the semiquantitative X-ray powder diffraction (XRD) analysis of the waste silt. The data also indicated that the laboratory calcination of the waste silt at 750 \({}^{\circ}\)C led to the total collapse of the calcite peak, indicating its complete degradation into calcium oxide (CaO), which is also present in limestone filler [25; 26; 27]. According to the master curves, the addition of normal waste silt to bitumen produced a mastic with lower stiffness compared with M-C and M-SC. This was observable at all tested frequencies and/or temperatures. The higher potential for rutting was highlighted by the Jnr values, where M-SN was found to have the highest values amongst all mastics regardless of the load level. However, mastic produced with calcined silt demonstrated a similar rutting performance to M-C, which was produced with normal limestone filler. The Jnr values also indicated the best rutting performance for M-SC followed closely by M-C. Meanwhile, in terms of percent recovery, the M-C samples showed the best elastic behavior. The stiffness of the samples containing calcined silt could have been due to the higher CaO content compared with the M-SN samples [28]. The literature indicates that CaO content was shown to have a good correlation with rutting potential, meaning that higher CaO content could increase the rutting resistance of the samples. The binder used for all mastic samples was 50/70 unmodified bitumen. Thus, the percent recovery for all mastics were reported as zero. Moreover, this could also explain the fact that none of the mastics showed reduced stiffness at low temperatures, making them prone to low-temperature fatigue cracking. The mastic produced with normal silt had the lowest G* value at high-frequency values, indicating its better performance in terms of fatigue cracking. The lower stiffness of M-SN could be related to the lower CaO content. Moreover, studies suggested that the presence of Al\({}_{2}\)O\({}_{3}\) and SiO\({}_{2}\) in filler could result in a stiffer binder mastic. Calcination usually increases the reactive aluminosilicate elements within fillers. Therefore, mastic produced with calcined silt had higher stiffness when compared with M-SN. In addition to the mastic samples, HMA specimens were produced. The indirect tensile strength test can be considered an indicator of the cohesion properties between mastic and aggregates. The results demonstrated an insignificant difference in terms of performance between the mixtures. Thus, the inclusion of untreated or thermally treated silt in the mixture did not affect the cohesion properties of the samples when compared with the control mix [29]. However, the sample produced with untreated silt had the lowest strength, whereas HMA-SC performed similarly to the control samples produced with limestone as filler. The behavior of the calcined silt could have been due to its similarity to the limestone filler in terms of chemical composition [28]. Laboratory specimens containing thermally treated waste silt showed high resistance against moisture damage, as shown by the ITSR values. Water-insoluble minerals present in fillers, such as calcite, portlandite and dolomite, can produce a strong filler-binder bond [30; 31] and could have been responsible for the high moisture resistance of samples produced with calcined silt. The stiffness sensitivity of specimens to temperature was examined using an advanced dynamic test (ITSM). All three samples showed similar mechanical behavior toward temperature sensitivity and no significant difference was observed. To sum up, the rheological and mechanical properties of the produced HMA specimens were in line with the Italian specifications, where the untreated silt and calcined silt showed similar performances to the control samples produced with limestone as filler.