This study examines the potential of adding electric arc furnace slag to bituminous mixtures to be heated by microwaves. The susceptibility of bituminous mixtures to microwave energy is limited and so, in order to improve the energy performance of the heating process, it is necessary to incorporate additives or components to the mixture so as to improve the capacity for microwave heating. The article presents the results of adding various components, (steel wool, scrap tire wire, silicon carbide, iron filings) and an alternative aggregate: electric arc furnace slag. According to the results obtained in the laboratory, slag addition of at least 5% by weight of the bituminous mixture represents the best option for both technical and economic reasons. The results may promote the valorization of this steel industry residue in bituminous mixtures by improving microwave heating response.
Electric arc furnace slag (EAFS) has suitable properties for use in construction materials, provided that the CaO and MgO content is controlled. These compounds can expand and cause disintegration of the material to which they are added (
EAFS can be incorporated into bituminous mixtures. In some cases this material has a CE marking as an aggregate for bituminous mixtures in accordance with EN 13043/AC (
Many of these advantages have been reported: Fernández et al. (
Regarding the general features of bituminous mixtures such as stability, water sensitivity, resilient modulus or fatigue life, these features are relatively unaffected and in certain cases improved by the incorporation of EAFS, as reported by Pasetto and Baldo (
This article gives close consideration to another attribute of bituminous mixtures that is definitely improved by EAFS; the ability of bituminous mixtures to be heated using microwave energy. It is useful to remember in this respect that several researchers have studied the application of microwave energy to heating bituminous mixtures. Some of the most interesting applications are the possibility of in-place repair of cracked surfaces, removal of damaged mixture during patching, removal of ice from frozen roads and the replacement of fuel or gas as a source of energy in asphalt plants.
Regarding in-place applications, Bosisio et al. (
Benedetto and Calvi (
However, all the researchers above coincided that there is limited capacity for heating bituminous mixtures by microwaves. It depends on the mineralogical characteristics of the aggregates among other factors. For example, basalt or limestone absorb more energy than silica when subjected to radiation with microwave energy (
In order to increase the susceptibility of the bituminous mixtures to microwave radiation, certain additives have been used. For example Wang et al. (
This article presents a more recent study on additives for improving the capacity of bituminous mixtures for heating with microwave energy. The experiments were carried out with steel wool, scrap tire wire, silicon carbide, iron filings and EAFS. From the results obtained and their market costs, as well as the simplicity of application at industrial level, the best option appears to be EAFS.
It is interesting to consider that an estimated 8.5 million tons of EAFS are produced every year by the principal European countries (
With this intention in mind, some companies committed to waste reuse have already obtained a CE marking of steel slag aggregates (
In order to reduce variability, the bituminous mixture used for this study was a mortar (sand + bitumen) made with limestone and conventional bitumen. Various additives were used with the objective of improving the susceptibility of the mortar to microwave energy, thereby decreasing the energy consumption necessary to increase its temperature. The following sections describe the characteristics of the bituminous mortar as well as of the additives.
The mortar was made with limestone sand. Calcium carbonate (CaCO3) from ground limestone was used as a filler. The resulting grading curve is presented in
Grading curve of the bituminous mortar
Sieve (mm) | 4 | 2 | 0.5 | 0.25 | 0.063 |
% Passing | 100.0 | 76.0 | 41.4 | 24.8 | 10.0 |
Grading curve of the bituminous mortar.
A 50/70 penetration bitumen (EN 12591) (
In order to determine the optimal binder content for the mixture, Marshall Test specimens were used (EN 12697-30) (
Marshall specimen of bituminous mortar split into two halves.
In principle, as indicated above, bituminous mixtures have a low susceptibility to microwave energy. However, greater energy efficiency can be achieved during the heating process by incorporating additives that are more susceptible to microwaves.
The first additive used was a metallic fiber. The variable electromagnetic field of the microwaves induces a current that heats the fibers by the Joule effect and the rest of the mixture by direct contact. Granular additives made up of materials containing molecules that are excited by microwaves were also tested. These additives go through an immediate temperature increase and heat the rest of the bituminous mixture by contact. Granular additives have a significant operational advantage over metallic fibers, which tend to form clusters during the preparation of the bituminous mixture. The granular material is distributed homogeneously through the entire mass of the mixture as if it were simply another fraction of the aggregate.
As for the fibers considered in the study, two kinds of steel wool were used: the first type was a “coarse steel wool”, with 0.10-0.12 mm thick wires, the second type was a “medium steel wool”, with a thickness of 0.04-0.06 mm. Steel wool is sold in coils and the fibers were cut into pieces of approximately 10 mm in length.
As previously observed, both of these fibers have a tendency to form clusters due to the fact that they bend with ease and tangle with one another. Steel wire from shredded end-of-life tires was also used. In this case, the pieces were approximately 10 mm in length. All of these materials were incorporated in quantities of 0.2% and 0.4% by weight of the bituminous mixture.
Silicon carbide was one of the granular additives used in the study because of its sensitivity to microwaves. It was also incorporated in proportions of 0.2% and 0.4% by weight of the mass of the mixture. Iron filings from an industrial lathe were also used in the same percentages. The fraction size of both additives used was 0.063/0.25 mm.
Finally, slag from an electric arc furnace, fraction size 0/2 mm, was used to improve the susceptibility of the bituminous mixture due to the high content of metallic compounds. This material was incorporated in quantities of 2%, 5%, 10% and 20% by weight of the mass of the bituminous mixture since this is an alternative aggregate rather than an addition and its price is similar to natural aggregates.
EASF fractions after sieving.
Due to the fact that higher percentages of the slag were added compared to the use of other materials, slag was substituted for aggregates of the same size, as mere addition would have caused a significant change to the grading curve of the mortar.
Original chemical composition of slag (before hydration)
Chemical Composition | (%) |
---|---|
Al2O3 | 8.81 |
CaO | 24.28 |
Fe2O3 | 40.49 |
MgO | 3.02 |
MnO | 4.72 |
SiO2 | 12.60 |
P2O5 | 0.36 |
Other substances | 5.72 |
First of all, the bituminous mortar test specimens were prepared by including the corresponding additives. These test specimens were then subjected to heating in a microwave oven.
The microwave oven used to heat the bituminous mixture in this study had an output of 1200 W and a 230 V, 50 Hz power supply. The oven produced microwave heat of up to 800 W, with a frequency of 2.45 GHz.
Each bituminous mortar specimen weighed approximately 1000 g. These specimens were placed separately in the microwave oven and heated for periods of 60 s. Between each two consecutive periods, the oven was opened and the surface temperature was measured using an infrared gun. The same procedure was carried out with the specimens containing additives and to the specimens without additives used as a reference.
An electricity meter was used to measure the energy consumption of the oven during the heating of the specimens. It was determined that the power consumed by the microwave oven was not dependent on the additive used. However, the temperature of the specimen was shown to be dependent on the type and proportion of the additive in the mixture.
The results for the control mixture and the mixture with additives are shown in the following charts (
Energy consumed – temperature (steel wool).
Energy consumed – temperature (scrap tire wire).
Energy consumed – temperature (silicon carbide).
Energy consumed – temperature (iron filings).
Energy consumed – temperature (slag).
The figures show that the temperature change of the test specimen was roughly linear. Therefore, the energy temperature points approached a straight regression line, which show correlation coefficients of almost 1 in each case.
In the light of these charts, the different additives and their proportions affected the gradients of the regression lines of the temperature reached against the energy consumed. In order to visualize the results as a whole,
Slope of the temperature / energy consumption graphs.
It may be observed that the most susceptible mixtures to microwave energy were those with EAFS (5–20% by weight of mass of the mixture), followed by the metal additives: steel wool (0.2–0.4%) and scrap tire wire (0.2–1.6%). Nevertheless, neither the silicon carbide (0.2–0.4%) nor the iron filings (0.2–0.4%) had any effect on the heating of the mixture when using microwave energy and with the percentages of the additives used.
When considering the inclusion of these additives in bituminous mixtures on a real scale and for large projects, two fundamental aspects should be taken into consideration: the cost and the viability of including the additives in a homogenous manner into the bituminous mixture manufactured on an industrial scale.
From an economic point of view, the inclusion of EAFS by replacing part of the aggregates does not imply an additional cost because at present the cost per ton of EAFS is similar to the prices of natural aggregates (
Market prices of the products used and effect on the price of the bituminous mixture (compiled by authors)
MATERIAL | PRICE (€/t) | Additive (%) over mass of bituminous mixture | PRICE (€/t bituminous mixture) |
---|---|---|---|
Conventional aggregate | 4.60 | 0 | 42.185 |
Slag (steel works aggregate) | 4.50 | 2 | 42.182 |
5 | 42.178 | ||
10 | 42.171 | ||
20 | 42.157 | ||
Silicon carbide | 1350.00 | 0.2 | 45.952 |
0.4 | 49.719 | ||
Steel Wool | 3060.00 | 0.2 | 50.740 |
0.4 | 59.295 | ||
Iron filings | n/a |
- | - |
Scrap tire wire | n/a |
- | - |
Not commonly found on the market.
From a construction point of view, the steel wool tends to get tangled up during the mixing process forming clusters inside the mixture and so uniformity during industrial manufacturing is uncertain. However, EAFS being a granular product similar to other aggregates can be mixed perfectly in an asphalt plant and requires no installation modifications. Treatment is exactly the same as for natural aggregates normally used in these plants.
In light of the results obtained, several conclusions can be drawn. Firstly, bituminous mixtures can be successfully heated using microwave energy. However, their susceptibility to microwave energy is limited and therefore require certain additives in order to make the heating process more energy efficient.
This study considered various additives that increase susceptibility to microwave heating: steel wool, metallic wires, iron filings, silicon carbide and electric arc furnace slag. To compare the effect of these additives, experimental bituminous mixtures were heated using microwaves in the laboratory. A microwave oven was used with an output of 1200 W and a 230 V, 50 Hz power supply. The oven can produce microwaves of up to 800 W, with a frequency of 2.45 GHz. A mass of material (1000 g) was heated in the microwave oven resulting in an increase in temperature and energy consumed during the process was measured.
Under the laboratory conditions of this study, the bituminous mixtures heated up much faster by incorporating electric arc furnace slag (5-20% over the weight of the mixture), steel wool (0.2-0.4%) or scrap tire wires (0.2-1.6%).
Scrap tire wires are not often found on the market and steel wool has a tendency to form clusters during mixing, which causes a heterogeneous mixture.
In contrast, due to its granular shape, slag is distributed homogeneously throughout the bituminous mortar, making it possible for conventional asphalt plants to make use of it on an industrial scale without difficulty. Several companies that process slag have already obtained a CE marking for the slag as an aggregate for bituminous mixtures. Furthermore, the price of the slag is similar to that of other aggregates, meaning that the economic viability of the mixture is not compromised.
Furthermore, if we consider the economic and environmental criteria, electric arc furnace slag can be considered an optimal component for the production of bituminous mixtures susceptible to microwave heating, used on a percentage base of at least 5% by weight of the mixture.
This research was developed within the TRAINER Project – “Development of new technology for autonomous, intelligent self-healing of materials” – financed by the Centre for the Development of Industrial Technology (CDTI) of the Spanish Government within the program supporting the National Strategic Consortia for Technical Investigation (CENIT).
The authors wish to acknowledge Raquel Casado, Ana F. Pereira and Carlos Martín-Portugués from Acciona for their contribution to this study.