## 2 Materials and Methods
### Methodology
#### 2.2.1 Characterization of Biomass Bottom Ash  
Firstly, it is essential to chemically and physically characterize the biomass bottom ash in order to analyze, on the one hand, the elements and chemical compounds present in it and, on the other hand, the physical properties that corroborate its suitability as a filler in bituminous mixtures. The biomass bottom ash samples for the subsequent tests were prepared as described in the materials section. The first of the tests performed is the X-ray fluorescence test, because the biomass bottom ash comes from a combustion process, and it is assumed that the nature of the sample will be mainly inorganic. Therefore, the X-ray fluorescence test was performed with the commercial ADVANT'XP+ (AD-VANT'XP+, Thermo Fisher, Waltham, MA, USA) to detect the presence of the higher atomic weight chemical elements. It should be noted that the X-ray fluorescence test is an elemental analysis, i.e., it provides the chemical elements present in the sample as well as the proportion in which they are found. However, the chemical elements have greater or lesser activity depending on the chemical compound in which they are combined. Therefore, it is essential to determine the main chemical compounds present in the sample. These chemical compounds are determined in this research through the X-ray diffraction test. Powder X-ray diffraction is a very powerful technique capable of evaluating (identifying, quantifying, and studying) the crystalline phases present in any type of material, whether natural or synthetic, as well as determining the crystalline structure of solids. This test was carried out with the equipment model X'Pert PRO of the commercial brand PANalytical (X'Pert PRO, PANalytical, Malvern, UK). In turn, the utilization of an industrial waste must avoid environmental pollution caused by its disposal in landfills. Therefore, in order to evaluate that this fact occurs, in this research, the biomass bottom ash leachate is analyzed according to the UNE-EN 12457-3 standard [48]. In this way, it is possible to confirm that the concentration of contaminating elements in the biomass bottom ash leachate are lower than the limits established by the regulations, and consequently, the use of this waste is viable for bituminous mixtures. For this purpose, the dry mass used was 0.175 kg, and the following two steps were carried out. First, a quantity of lixiviant (deionized water) was added to obtain a ratio L/S = 2 L/kg, and then stirred for 6 h at a speed of between 5 and 10 revolutions/minute. Finally, the sample was filtered through a 0.45 um filter. Secondly, deionized water was added on top of the previous sample until a L/S ratio = 10 L/kg was achieved. The sample was then shaken for 18 h and finally filtered through 0.45 um filters. The sample obtained was analyzed for the determination of the concentrations of the pollutants. For the analysis of the leachate, the commercial equipment Agilent 7900 (7900, Agilent, Santa Clara, CA, USA) was used. After analyzing the chemical composition of the biomass bottom ash and evaluating its chemical suitability for use as a filler in bituminous mixtures, the general physical properties of the ash were determined to corroborate its suitability. The first of the grading tests was the determination of the grading curve, since the ashes will be used as filler in bituminous mixtures and must have a small particle size. This test, using the laser particle size test technique, can detect particles between 1 and 2000 micrometers in size. The Mastersizer 2000 LF (Mastersizer 2000 LF, Malvern Instruments, Malvern, UK) was used for this test. Subsequently, in order to obtain qualitative information on the surface of the particles, a scanning electron microscope test was performed on the biomass bottom ash. In this way, the surface of the ashes could be observed, and thanks to the high magnifications of this instrument, the presence of micropores that significantly influence the specific surface of the biomass bottom ashes could be detected.