2.2.1. Characterization of Raw Materials

The waste and commercial materials were treated as detailed before in order to be able to carry out physical and chemical characterization tests, as well as for use in subsequent tests.

Firstly, the electric arc furnace slag, ladle furnace slag and cellulose fibers were analyzed by elemental analysis, determining the percentage of carbon, nitrogen, hydrogen and sulfur in the samples. In turn, the slags were subjected to the X-ray fluorescence test, as they are inorganic materials, unlike cellulose fibers from the papermaking industry.

Once their chemical composition had been determined and the presence of contaminating elements that could prejudice the final bituminous mixture analyzed, a series of physical tests were carried out on the different wastes according to the function that each one plays within the bituminous mixture.

The cellulose fibers from the papermaking industry were evaluated with a scanning electron microscope (Carl Zeiss, Oberkochen, Germany) at different magnifications and after metallization with carbon. The size of the fibers obtained after pre-treatment and the existence of agglomerations that could impair the homogeneous distribution of the fibers in the bituminous mixture were thus observed.

The ladle furnace slag was subjected to particle density tests (standard UNE-EN 1097-7) [50], to evaluate the possible volumetric corrections required; bulk density tests (standard UNE-EN 1097-3) [51], to determine whether it is a powdery material that is detrimental to its proportioning; and plasticity index tests (standards UNE 103103 and UNE 103104) [52,53], to evaluate the possible existence of clayey particles that could create expanding problems in the final mix.

The electric arc furnace slag was subjected to particle density tests (standard UNE-EN 1097-7) [50], to determine whether volumetric corrections were necessary; a sand equivalent test (standard UNE-EN 933-8) [54], to evaluate the percentage of colloidal particles that could damage the final mixture; percentage of crushed surface tests (standard UNE-EN 933-5) [55]; and flakiness index tests (standard UNE-EN 933-3) [56], for the qualification of the aggregate, since the SMA mixture resists the loads of traffic on the mineral skeleton, and therefore the particles must have certain shapes; resistance to fragmentation tests (standard UNE-EN 1097-2) [57], to qualify the hardness of the material and its suitability for high traffic; resistance to freezing and thawing cycles tests (standard UNE-EN 1367-1) [58], to evaluate the aggregate's resistance to thermal fatigue; and determination of the value of polished stone (standard UNE-EN 1097-8) [59], to quantify the effect on the aggregate of the continuous tire friction of the with the pavement and, consequently, its durability through time.
