This section details the results of the tests presented in the methodology, as well as the discussions about them. In this way, a series of partial conclusions are obtained that converge in the final conclusion.
3.1. Characterization of Raw Materials
As mentioned above, the characterization of the chemical composition and the physical or mechanical properties of the waste is essential for its correct use within the final material. Usually, this type of materials have a series of particularities, that even not being negative, if they must be taken into account for the conformation of a final material of quality.
Firstly, the discarded cellulose fibers from the papermaking industry were analyzed. These cellulose fibers, as indicated, are the additive used in the different families of bituminous mixtures to absorb a greater percentage of bitumen. They must therefore have a suitable chemical composition. To determine the chemical composition, the elemental analysis test was carried out, detecting the presence of carbon, nitrogen, hydrogen, and sulfur, because it is an organic material. The results of the elemental analysis test are detailed in
Table 3.
As can be seen, the elemental analysis test reflects the composition of an organic material. The percentage of carbon and hydrogen demonstrate such a composition. On the other hand, the low nitrogen and sulfur values reflect the material’s suitability for use in bituminous mixtures, since otherwise these pollutant elements could damage the bituminous mixture and produce polluting leachate. It should be noted that the sum of the elements analyzed does not correspond to 100%. This is due to the fact that the fibers have been washed with sodium hydroxide, so this chemical element has been retained in the matrix of the fibers without prejudice to the final material.
In turn, the cellulose fibers must be distributed homogeneously in the conformed bituminous mixtures to avoid the production of bleeding. In addition, they must be of an appropriate size for the correct formation of a mastic capable of resisting the traction loads of traffic. Therefore, the scanning electron microscope test was carried out to the aim to evaluate the size of the fibers. The images taken from this test are detailed in
Figure 2.
Figure 2 shows a millimeter size of cellulose fiber. This fiber size is ideal for use in bituminous mixtures, as larger sizes would make the mixing process more difficult. At the same time, it can be seen that there are no agglomerations of cellulose fibers that could be detrimental to the correct homogenization in bituminous mixtures. If this were not the case, bitumen bleeding and heterogeneities in the properties of the mixtures conformed could be produced. Therefore, the suitability of the fibers for the correct absorption of a higher percentage of bitumen into the bituminous mixtures was demonstrated through the above-mentioned tests.
After evaluating the cellulose fibers, the chemical composition of the ladle furnace slag was determined. To do this, the elemental analysis test was first carried out, reflecting the results shown in
Table 4.
The elemental analysis test showed the absence of harmful and polluting elements such as nitrogen and sulfur, and therefore their suitability for use in bituminous mixtures was confirmed. In turn, the percentage of carbon and hydrogen of the ladle furnace slag seems to correspond with high probability to the carbonation and hydration of different chemical compounds, not being a harmful fact but to be taken into account.
Given the mainly inorganic composition of the ladle furnace slag, the elemental composition was determined by means of the X-ray fluorescence test. This test reflected the results shown in
Table 5.
The X-ray fluorescence of the ladle furnace slag showed an elemental composition consisting mainly of the chemical elements calcium, magnesium, silicon, and aluminum. The presence of these elements is logical, since they come directly from the steel production process and consequently from the ladle furnace slag. In the refining stage of the metallurgical industry, lime or dolomites are added in order to eliminate impurities through the formation of this slag; therefore, the ladle furnace slag has the chemical composition of the added elements. On the other hand, the existence of iron in the sample is obvious, as the slag comes from a steel production process. The rest of the chemical elements are in low proportion and do not condition the characteristics of the final material. However, the leachate test was subsequently carried out to evaluate the concentration of these chemical elements and to determine whether they could produce environmental pollution. It should be noted that the X-ray fluorescence test provides an elemental composition. This elemental composition may be reflected in the form of oxides; however, it does not imply that these oxides actually exist in the sample.
For the determination of the main chemical compounds, the X-ray diffraction test was performed, reflecting the diffractogram shown in
Figure 3.
Analysis of the diffractogram obtained from the X-ray diffraction test of ladle furnace slag shows the high content of amorphous or non-diffractant material. Calcite, olivine, and periclase are identified as the main phases. In turn, different silicates are also identified in smaller proportions, including magnesium silicate. This fact ensures that the slags studied do not have expansive characteristics, since magnesium oxide is not found in the sample. The elemental magnesium identified in the X-ray fluorescence test appears combined in the form of silicates, so that its structure is more stable and does not condition expansion problems.
Considering that ladle furnace slag will act as a filler in bituminous mixtures, a series of physical properties must be determined in order to ensure its quality. Firstly, the grading curve of the ladle furnace slag was analyzed as it comes from the production industry. This grading curve is detailed in
Figure 4.
As can be seen in
Figure 4, the particle size of ladle furnace slag as received from the production industry and unaltered has a high percentage of particles smaller than 0.063 mm. Therefore, its use as a filler in bituminous mixtures is suitable.
In addition, a series of physical properties were analyzed which are essential for evaluating the suitability of a material which performs the function of a filler in bituminous mixtures. These physical properties of the ladle furnace slag are detailed in
Table 6.
The particle density of the ladle furnace slag does not differ greatly from that of a commercial filler, 2650 kg/m3, so no volumetric corrections are necessary. In turn, the bulk density of the slag studied reflects an acceptable result, not being qualified as a powdery material. Finally, the null plasticity index of the ladle furnace slag shows the inexistence of particles that could cause subsequent expansion problems, such as clayey particles.
Based on the above, it can be established that the chemical composition and physical properties of the ladle furnace slag are suitable for use as filler in bituminous mixtures.
On the other hand, the electric arc furnace slags were analyzed chemically. For this purpose, the elemental analysis test was first carried out. The results of this test are shown in
Table 7.
The elemental analysis test reflected the chemical composition of an inorganic material, existing low percentages of carbon and hydrogen. At the same time, the inexistence of nitrogen and sulfur was reflected, demonstrating the non-production of polluting lixiviates of these chemical elements.
As it is an inorganic material, the X-ray fluorescence test was carried out in order to quantitatively determine the presence of the rest of the chemical elements not analyzed in the elemental analysis. The results of this test are shown in
Table 8.
The chemical composition of the electric arc furnace slag is directly derived from its production process. Therefore, there are high percentages of iron, as it is the main element of steel, as well as calcium, a chemical element present in the additive to the molten liquid for the formation of the slag. In turn, silicon and aluminum are common elements present in the scrap that is used to manufacture steel. Magnesium, manganese, and chromium are also common in the composition of steels. The other elements are found in such a reduced proportion that they do not produce modifications in the properties of the final material. However, the leachate test was subsequently carried out to verify that the concentration of the polluting chemical elements is below the limits set by the regulations.
Once the elemental composition had been determined, the main chemical compounds were identified through the X-ray diffraction test. The diffractogram derived from the X-ray diffraction test of the electric arc furnace slag is shown in
Figure 5.
The diffractogram of the electric arc furnace slag reflected the high content of amorphous or non-diffracting material. Iron oxide and mixed iron and manganese oxides were identified as the main phases. Wollastonite was also identified in a smaller proportion. Therefore, and unlike the ladle furnace slag, the results reflect the chemical composition of a much more stable material in which the chemical compounds present do not lead to variations in shape. In other words, the expansiveness of these electric arc furnace slags is non-existent.
Subsequently, after determining the chemical composition of the electric arc furnace slag, the physical properties of the slag proceeded to evaluate. The electric arc furnace slag performs the function of a coarse and fine aggregate in the bituminous mixture, which is why the tests carried out are those detailed in
Table 9.
Physical testing of the electric arc furnace slag showed that the density of the particles was higher than that of a conventional aggregate, considering as the norm that the density of a conventional aggregate is 2650 kg/m3. This fact, although it is not a problem, must be taken into account for the proportioning of the materials, so that volumetric corrections are necessary. If these corrections were not made, the properties of the final material could be damaged. At the same time, the sand equivalent test reflected the low proportion of colloidal particles, not producing subsequent problems of expansiveness. It is worth noting the excellent results obtained in the broken surfaces and flakiness index tests. These results reflect the singular shape of the electric arc furnace slag particles, with a multitude of sharp edges and similar dimensions in all three axes. They are therefore ideal for use in bituminous mixtures with discontinuous grading where the compressive loads of traffic are supported by internal friction of the particles.
At the same time, it should be noted that bituminous mixtures with discontinuous grading have a series of advantages over mixtures with continuous grading, mainly a higher void content. However, this discontinuous grading makes it necessary for the coarse aggregate to be of high quality, since it is the main responsible for supporting the compressive loads of traffic. Therefore, the coarse aggregate must have adequate shape and mechanical characteristics. The shape characteristics of the particles of the electric arc furnace slag have already been defined previously, so it is necessary to corroborate the mechanical resistance.
This mechanical resistance has been evaluated through the test of resistance to fragmentation and resistance to freezing–thawing cycles, reflecting values of 13 ± 1% and 0.551 ± 0.016% respectively. At the same time, it is essential that the aggregates provide an adequate macrotexture and microtexture for a correct friction between the tire and the pavement. This property is evaluated with the determination test of the polished stone. This test reflected a value of 58 ± 1, demonstrating the correct resistance in time of the aggregates and ensuring the roughness of the pavement during its working life.
Finally, and to corroborate that high concentrations of chemical pollutants are not leached, the leachates from the electric arc furnace and ladle furnace slags were analyzed in accordance with the UNE-EN 12457-3 standard. The results of the leachings of the different chemical elements are detailed in
Table 10.
The concentration of the chemical elements in the leaching of electric arc furnace slag and ladle furnace slag does not exceed the limits established by Spanish regulations, since the heavy metals analyzed are in low proportion in both industrial by-products. Therefore, it can be assured that there will be no subsequent pollution problems from the use of the slags and that their use for bituminous mixtures is acceptable.
3.2. Conforming of Bituminous Mixtures and Tests
Once the physical properties and chemical composition of the waste had been determined, as well as their suitability for conforming to bituminous mixtures, the different families of mixtures detailed in
Table 2 proceeded to conform with increasing percentages of bitumen. The procedure for forming and compacting is the detailed in the methodology.
The different families of bituminous mixtures were tested to determine their physical properties. Firstly, the bulk density was calculated, showing the results detailed in
Figure 6.
The bulk density is the density of the bituminous mix considering the volume of voids. Therefore, it can be seen that the HC family conformed with conventional materials has a lower bulk density than the SC and SS families. The latter families have very similar bulk densities, because only the filler employed varies.
In turn, the maximum density is another of the essential physical properties for evaluating a bituminous mixture and obtaining the void content. The maximum density of the different families of mixtures with different percentages of bitumen is shown in
Figure 7.
The maximum density is the density of the bituminous mix without consideration of the voids in the mix. Therefore, and since electric arc furnace slag has a higher density than hornfels aggregate, the SC and SS mix families have a higher maximum density than the HC family. In turn, it can be seen that the increase in the percentage of binder causes the maximum density for all families to decrease, a fact that can be expected by the low density of the bitumen.
On the other side, the determination of the void content in the mix from the maximum density and the bulk density is essential. The void content is reflected in
Figure 8 for all families of bituminous mixtures.
The void content in the mix of the three families of bituminous mixtures is very similar, with small variations in the highest percentages of bitumen between the HC mix and the SC and SS mixes. However, these variations are negligible. It should be noted that the content of voids in the mix is one of the properties that most influences the quality of the bituminous mix, since it conditions its drainability, its roughness for the friction of the tire with the pavement, and even the absorption of noise. The values obtained for all the families of bituminous mixtures are acceptable according to the detailed Spanish regulations and are usual in mixtures with discontinuous grading.
The Marshall test was used to evaluate the resistance of the bituminous mixtures. This test evaluates whether the percentage of bitumen is inadequate, creating excessive plastic deformations due to an excess of bitumen, or low resistance due to insufficient bitumen. The Marshall results of the families of conformed mixtures are detailed in
Figure 9.
The Marshall test of the bituminous mix families showed that the SS family, composed of electric arc furnace slag as an aggregate and ladle furnace slag as a filler, had the best resistance. Next, the SC family conformed with calcareous filler has a lower resistance, and lastly, the family of bituminous mixtures conformed with commercial materials HC. This variation in the Marshall resistance of the different families of bituminous mixtures confirms, on the one hand, that the use of electric arc furnace slag allows acceptable results to be obtained, and, on the other hand, that the incorporation of ladle furnace slag as filler makes it possible to obtain resistances that are much higher than those of calcareous filler. It should be noted that the high percentages of bitumen obtained in the three families of bituminous mixtures are due to the addition of discarded cellulose fibers from the papermaking industry. Obtaining adequate resistances and high percentages of bitumen corroborate the suitability of cellulose fibers as an additive for this purpose.
The Marshall test also makes it possible to obtain the deformation or displacement of bituminous mixtures at the time of their breakage. The Marshall deformation of all the families of bituminous mixtures conformed is detailed in
Figure 10.
The Marshall deformations of the different families of samples reflect how the filler family of ladle furnace slags, SS, presents a lower deformation. This fact corroborates the rigidity of this mixture and its strength, as well as the cementitious properties of the ladle furnace slag. Next, the family conformed of electric arc furnace slag and calcareous filler SC has intermediate deformations between the two families, also showing acceptable results. Finally, the HC family of samples conformed to conventional materials obtains values of over 3 mm of deformation in the highest percentages of bitumen. These values are higher than those regulated by the standards and therefore limited for use.
3.3. Determination of Optimal Material Combinations and Comparison of Results
Once the physical and mechanical properties of the different families of bituminous mixtures with increasing binder percentages had been determined, the optimum bitumen percentage was chosen. This percentage was selected on the basis of the results of the Marshall test, choosing the percentage of bitumen that developed the greatest resistance. The choice of this test to obtain the optimum combination of materials is mainly based on the fact that this is the test that most conditions the quality of the bituminous mix, since the high percentages of bitumen in the mixes are prone to the formation of plastic deformations. However, the choice of the percentage of bitumen for each family as the maximum of the Marshall stability was decided as long as the other properties of the conformed mixtures were acceptable.
As discussed,
Table 11 shows the results of the optimum bitumen percentages for each family, as well as the physical and mechanical properties obtained for each combination. These properties were obtained after conforming and testing various samples with the optimum combination of materials for each family.
The percentages of optimum bitumen for the three families of bituminous mixtures are very similar; however, the percentage is slightly higher for the SS family. This is due to the greater specific surface area of the ladle furnace slag which allows a higher percentage of bitumen to be absorbed, as mixtures conformed with electric furnace slag and hornfels aggregate obtain similar percentages. The bulk density and maximum density are higher for mixtures conformed with electric arc furnace slag due to the higher density of this material; however, the void content is similar in all three families and acceptable according to the regulations.
It can be seen how the Marshall stability is far superior in the mix conformed with electric arc furnace and ladle furnace slags, demonstrating the excellent properties of ladle furnace slags as fillers in hot mix asphalt. This fact is corroborated with a lower Marshall deformation, showing a material of high rigidity and resistance. In turn, the Marshall stability of the samples conformed with electric arc furnace slag is superior to that of hornfels aggregate, as it reflects higher values and lower deformations. The goodness of electric arc furnace slag and ladle furnace slag in hot mix asphalt is therefore demonstrated.
Finally, and with the aim of corroborating the durability of the families of bituminous mixtures over time, the wheel-tracking test was carried out. This test measures the deformation produced in the bituminous mix by the continuous passage of a standardized wheel with load during thousands of cycles. The results of this test are shown in
Figure 11.
The wheel-tracking test reflects how the bituminous mix conformed with electric arc furnace slag and ladle furnace slag has less deformation after the test, demonstrating the rigidity, strength, and durability previously determined by the Marshall test. Electric arc furnace slag also provides durable bituminous mixes as shown in
Figure 11. The HC bituminous mix, even having a higher deformation, develops acceptable results, corresponding to a quality bituminous mix.
In turn, the binder drainage test determined that no binder bleeding occurred in any of the three families of bituminous mixtures formed with the optimal combination of materials. Therefore, the effectiveness of discarded cellulose fibers from the papermaking industry as an additive to absorb a higher percentage of bitumen was confirmed.
In short, it can be said that the results reflect the quality of bituminous mixtures for use as a wearing course on roads with high traffic, reflecting even better results the family of bituminous mixtures conformed with electric arc furnace slag, ladle furnace slag, and cellulose fibers.