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Reuse of thermal power plant slag in hot bituminous mixes
Construction and Building Materials 49 (2013) 144–150
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Reuse of thermal power plant slag in hot bituminous mixes Fernando Moreno-Navarro a,1, Miguel Sol a, Mª. Carmen Rubio-Gámez a,⇑,2, Antonio Ramírez b a b
labIC. Laboratorio de Ingeniería de la Construcción, ETSICCP, University of Granada, C/Severo Ochoa s/n., 18071 Granada, Spain Sacyr, Paseo de la Castellana 83-85, 28046 Madrid, Spain
h i g h l i g h t s Reuse of thermal power plant slag in hot bituminous mixes (HMA). Substituting natural aggregate in the manufacture of hot mix asphalt. A test road section was built with HMA containing slags. The mechanical performance of the slag mix was found optimal. Boiler slag from thermal power plants can be used as a substitute for the fine aggregate.
a r t i c l e
i n f o
Article history: Received 6 May 2013 Received in revised form 11 July 2013 Accepted 20 July 2013 Available online 6 September 2013 Keywords: Thermal power plant slag Hot mix asphalt Roads Test section Waste recycling
a b s t r a c t The revalorization of industrial waste plays a key role in the solution of environmental and economic problems in the construction sector, thus actions contributing to the symbiosis between industries in the same region are of great benefit since they optimize resources in the area and also open the door to new business opportunities. In recent years, the electricity produced by thermal power stations has become a renewable energy source of great potential. Nevertheless, this type of energy is not without drawbacks because of the coal ash and boiler slag produced by coal combustion. Although the ash has different application as a construction material, boiler slag is more problematic since it must be deposited at landfills. The accumulation of this waste has become a serious economic and environmental problem. This paper presents the results of a research project, which analyzed the viability of reusing thermal power plant slag as a substitute for natural aggregate in the manufacture of hot mix asphalt (HMA). For this purpose, the waste was first characterized and tested in the laboratory with a view to its subsequent use in HMA. Based on the positive laboratory results, a test road section was built with this material so that its performance could be compared with that of the road section paved with a conventional HMA. The results obtained confirmed the aptness of thermal power plant slag for use in road construction. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, as a consequence of industrial development, the generation of electricity from fossil fuel combustion in thermal power plants has soared in countries all over the world. However the accumulation of waste generated by such energy production is now the focus of growing social concern [1,2]. Such wastes include bottom ash or boiler slag (material collected at the bottom of the boiler) and fly ash (finer material that rise with the flue gases from the furnace upwards, generally to be captured by particle
⇑ Corresponding author. Tel.: +34 958249445. E-mail addresses: [email protected] (F. Moreno-Navarro), [email protected] (Mª.C. Rubio-Gámez), [email protected] (A. Ramírez). 1 Tel.: +34 958249443. 2 www.labic.ugr.es. 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.090
filtration equipment) [3,4]. It is thus crucial to find ways to recycle these residues, and give them new uses that contribute to sustainable development. Because of the pozzolanic characteristics of fly ash, this waste has many potential applications in the construction sector which is an ideal scenario for the reuse an revalorization of industrial by-products. In this way, fly ash can be used to fill road embankments, to stabilize soils, or to manufacture concrete [5,6]. Indeed, the results of a wide range of research underline the fact that the construction sector and its activities are an ideal scenario for the reuse and revalorization of industrial by-products [7–9]. In contrast to fly ash, boiler slag has received considerably less attention [10] despite the fact that these materials are produced in similar quantities by thermal power plants [3]. As a solution for the socio-environmental problem of boiler slag, certain authors [10,11] propose the use of this waste as a substitute for part of the sand fraction in the manufacture of concrete. According to this research,
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Fig. 1. Comparison of thermal power plant slag (A) and sand (B).
Table 1 Physical properties of thermal power plant slag. Tests Grain size (UNE-EN 933-1) [17]
Sieves (mm)
4 2 0.5 0.25 0.063 Aggregate density in paraffin oil NLT-167/96 (g/cm3) [16] Sand equivalent (UNE-EN 933-8) [18] (%) Relative density and absorption Apparent density (g/ (UNE-EN 1097-6) [19] cm3) ADSS (g/cm3) Density after drying (g/ cm3) Water absorption after immersion (%)
Table 3 Grain size of the reference mix and the slag mix. Slag
AC 16 S mix
% material passing 100.0 50.0 32.0 26.0 4.2 1.58 79.0 1.73
Sieves (mm)
Reference
5% Slag
22.4 16 8 4 2 0.5 0.25 0.063
99.0 92.0 66.0 39.0 29.0 13.0 9.0 5.9
99.0 92.0 66.0 41.0 29.0 14.0 10.0 5.0
1.57 1.35 16.21
ADSS: Apparent relative density on a saturated surface-dry basis
boiler slag and fly ash have a similar composition, which means that they have the same pozzolanic nature. Other authors [3] have assessed the performance of bituminous mixes made from thermal power plant slag. The results showed that when part of the sand fraction of the mineral skeleton was replaced by slag (less than 15% of the aggregate), this did not cause a negative impact on the mix. In fact, the mix with slag was found to have an indirect tensile strength and resistance to rutting comparable to those of conventional asphalt mixes. In contrast, other research studies [12,13] claim that road surfaces paved with
bituminous mixes made of thermal power plant residues did not perform as well and deteriorated more rapidly under traffic loads. These contradictory results seem to indicate that the real problem with slag mixes lies in the fact that the characteristics of the slag depend on its origin [12,14]. Consequently, it is necessary to study the performance of this material in each case before using it in bituminous mixes. Based on these considerations, the revalorization of boiler slag as an alternative material needs to be explored in greater depth. Accordingly, the construction engineering laboratory of the University of Granada (Spain) and Sacyr, a Spanish construction company, carried out a joint research project that studied how to recycle boiler slag from a thermal power plant to manufacture HMA. A further objective of this research was to contribute to the symbiosis between industries in the same region so that the waste produced by one economic sector would become a resource for another, thus giving it added value and contributing to
Table 2 Aggregate characteristics. Tests
Particle grain size (UNE-EN 933-1) [17]
Sieves (mm) 22.4 16 8 4 2 0.5 0.25 0.063 Coarse aggregate shape. Flakiness index (UNE-EN 933-3) [20] Resistance to fragmentation (Los Angeles coefficient) (UNE-EN 1097-2) [21] Light-weight particles in aggregate (UNE-EN 1744-1) [22] Sand equivalent (UNE-EN 933-8) [18] (%) Relative density and absorption (UNE-EN 1097-6) [19] Apparent density (g/cm3) ADSS (g/cm3) Density after drying (g/cm3) Water absorption after immersion (%)
ADSS: Apparent relative density on a saturated surface-dry basis
12/18
6/12
0/6
Limestone
Limestone
Limestone
% Material passing 90 47 0 0 0 0 0 0.1 2.62 9.0 – – 2.87 2.86 2.86 0.18
% Material passing 100 100 44 1 1 0 0 0.4 18.76 23.0 0.05 – 2.88 2.86 2.85 1.70
% Material passing 100 100 100 72 50 16 7 0 19.86 23.7 0.05 75.2 2.75 2.74 2.73 1.9
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sustainable development. For this purpose, near the town of Vera (Almería, Spain), a road section was paved with an HMA made with slag from a thermal power plant in Carboneras (Almería) situated at about 40 km from the construction site.
The research project was carried out in three phases. In the first phase, the waste material was characterized and tested in the laboratory in order to evaluate whether it could be used as a substitute for natural aggregate in the design and manufacture of HMA
Table 4 Marshall Test results for the mixes manufactured in the laboratory. Mix
Specimen
Density (kg/m3)
Air voids in the mix (%)
Air voids in the aggregate (%)
Stability (kN)
Deformation (mm)
AC 16 S (Reference)
1 2 3 Mean
2546 2532 2537 2538
4.5 4.8 4.6 4.6
14.9 15.8 15.6 15.4
13.92 12.16 12.27 12.78
1.8 2.1 1.9 1.9
AC 16 S (5% Slag)
1 2 3 Mean
2525 2492 2513 2510
2.3 3.6 2.7 2.9
13.2 14.1 12.9 13.4
12.97 11.46 12.92 12.45
2.6 2.3 2.7 2.6
Table 5 Density and water sensitivity of the mixes manufactured in the laboratory. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
Density (kg/m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
2528 2517 2528 2524 2535 2500 2528 2521
2270.8 1986.5 1991.0 2082.8 1799.0 1498.0 1842.0 1713.0
82.2
2508.2 2511.9 2506.5 2508.9 2511.8 2506.3 2492.1 2503.4
2082.1 1916.2 1729.4 1909.2 1745.1 1736.5 1725.8 1735.8
90.9
Fig. 2. Manufacture of the slag mix at the asphalt plant.
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F. Moreno-Navarro et al. / Construction and Building Materials 49 (2013) 144–150 Table 6 Resistance to plastic deformations of the laboratory mixes.
Table 8 Resistance to plastic deformations of the mixes manufactured in the asphalt plant.
Mix
Specimen
WTS (mm/103 load cycles)
Final deformation (mm)
Mix
Specimen
WTS (mm/103 load cycles)
Final deformation (mm)
AC 16 S (Reference)
1 2 Mean
0.040 0.048 0.044
0.553 0.715 0.634
AC 16 S (Reference)
1 2 Mean
0.102 0.088 0.095
1.732 1.478 1.605
AC 16 S (5% Slag)
1 2 Mean
0.195 0.139 0.167
4.998 3.679 4.339
AC 16 S (5% Slag)
1 2 Mean
0.222 0.234 0.228
4.003 5.159 4.581
and also to verify its compliance with the PG-3 General Technical Specifications for Roads and Bridge Works in Spain [15]. In the second phase, the reproducibility of this slag mix in an operating asphalt plant was evaluated as well as its placing on a test road section that was built for this purpose. Finally, the third and last phase, which is still in progress, will study the long-term performance of the test section paved with the slag mix. The following sections describe the research methodology used. (a) Laboratory studies: Analysis and characterization of thermal power plant slag Analysis and design of slag mixes (b) Test road section. On-site application and study: Manufacture in plant of slag mixes. Reproducibility Spreading and compacting Quality control 2. Laboratory studies 2.1. Materials The boiler slag came from a combined cycle electrical power plant, based on coal combustion. The main waste by-products from the combustion process were fly ash (which was already being recycled in a cement factory near the power station) and the coal slag to be used in this research as a substitute for natural aggregate in HMA. This avoided the deposit and accumulation of this waste at landfills and at the same time contributed to sustainable development in the region. The size of the slag particles was 0–31.5 mm and their apparent density was 1.58 g/cm3, as determined in paraffin oil, according to NLT standard 167/96 [16]. Because of the low mechanical strength of the coarser slag fractions, certain authors [3,14] recommend that thermal power plant slag should be used as a substitute for the sand fraction so that the mechanical performance of the mixes will not be negatively affected. For this reason, the slag was sieved to eliminate oversized particles and retain only those with a size of 0–3 mm (see Fig. 1). Table 1 lists the physical properties of the resulting material. As can be observed, all of its characteristics comply with Spanish PG-3 specifications for the use of slag as a substitute for the sand fraction in bituminous mixes. The bituminous mixes were manufactured with limestone aggregate for both the coarse and fine fractions. The aggregate in this project came from a quarry located in Antas (Almería) about 15 km from the road construction site. Table 2 shows the characteristics of the aggregate. The filler used in the mixes was general-purpose cement CEM II/B-L 32.5 N (UNE-EN 197-1) [23], 95% of which had a particle size smaller than 0.063 mm and an apparent density of 0.7 g/cm3 (NLT-176) [24]. The binder was conventional
B 50/70 bitumen with a penetration value of 68 mm (UNE-EN 1426) [25], a softening point of 50.4 °C (UNE-EN 1427) [26], and a Fraas breaking point of 8 °C (UNEEN 12593) [27].
2.2. Analysis and design of slag mixes After the properties of the material had been characterized, an AC 16 S (UNE-EN 13108-1) [28] bituminous mix was designed, in which 5% of the sand fraction was replaced with thermal power plant slag. The Marshall Test was used to design the HMA and determine the optimal blend and gradation of aggregates (NLT-159/00) [29]. The slag percentage was based on previous research [3,14], and the grain size was designed so that the mineral skeleton complied with Spanish PG-3 regulations [15]. At the same time, an AC 16 S reference mix was designed, which did not have any waste fraction and which only contained natural aggregate. Both mixes were elaborated at a temperature of 160 °C, using the same manufacturing process, given that the slag was used as part of the aggregate. Mix performance was then evaluated with the water sensitivity test (UNE-EN 12697-12) [30] and the wheel-tracking test (UNE-EN 12697-22) [31]. Table 3 shows the mineral skeletons of the reference mix and the slag mix. The grain-size distribution of the slag mix was performed by volume (instead of by weight) since this waste has a lower density than the natural aggregate. The optimal bitumen content for each mix was set at 4.3% (of the mix weight), based on the Marshall Test results (NLT 159/00) [29]. Table 4 shows the mean density (g/cm3), air voids in the aggregate (%), air voids in the mix (%), stability (kN) and deformation (mm) values for the reference mix and for the slag mix. The mixes were manufactured in the laboratory with the optimal binder content. Despite the fact that the density of both mixes was similar, the slag mix had fewer air voids because of the addition of the waste material. The test results for the strength parameters showed that the stability of the slag was almost the same as that of the reference mix. However, its deformation value was somewhat higher, which meant that its resistance to plastic deformation might not be as good. The results of the water sensitivity test (UNE-EN 12697-12) [30] showed that the slag mix performed better in terms of retained strength, which could signify an improvement in the cohesiveness of the mastic (Table 5). This coincides with the results of other research [14,32] on thermal power plant slag mixes and their resistance to water. Our study found that the mix with slag (as a substitute for the sand fraction) was less sensitive to water than the reference mix with natural aggregate. Furthermore, the indirect tensile strength values of the slag mix were very similar to those of the reference mix, which meant that the strength of the mix was not affected by the addition of the waste in the sizes and percentages used. To complete the laboratory study of the mechanical performance of the mixes, the wheel-tracking test was performed (UNE-EN 12697-22) [31] to evaluate the influence of the waste fraction on mix resistance to plastic deformations. Table 6 shows the values of the mean deformation slope for two test specimens of each mix as well as the mean value of this final deformation.
Table 7 Density and water sensitivity of mixes manufactured in the asphalt plant. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
2431 2427 2406 2421 2434 2400 2403 2412
2079.0 1873.0 1909.0 1953.7 2048.0 1964.0 1451.0 1821.0
93.2
2477 2492 2466 2478 2471 2468 2491 2477
2833.0 2689.8 2220.0 2580.9 2537.0 2369.0 2365.0 2423.7
93.9
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These results show that the addition of thermal power plant slag to the AC 16 S mix increased the plastic deformations recording during the wheel tracking test. Similarly, the mean deformation slope between 5000 and 10,000 load cycles increased in the slag mix, which indicates a lower resistance to permanent deformations. This also coincides with the higher deformation values obtained during the Marshall Test. The resistance to plastic deformation of the slag mix was found to be slightly lower than the limit specified in Spanish regulations for roads with the heaviest traffic load levels. However, because of resistance to fracturing and to water action showed by the mix, it was decided to use it to pave a test road section located on a service road with light vehicle traffic. Accordingly, after completing the mix design phase, our objective was to evaluate the possible use of this waste in HMA for city streets and service roads since this would reduce the consumption of natural aggregate.
3. On-site testing 3.1. Reproducibility of the mix in an asphalt plant After the laboratory phase, which evaluated the recycling of thermal power plant slag in HMA, it was decided to manufacture
the mix and try to reproduce its mechanical properties in an actual asphalt mixing plant. For this purpose, the same laboratory materials and dosages were used to elaborate both the slag mix and the reference mix. The process used to manufacture both mixes was also the same. There was thus no need to use any auxiliary machinery or to modify the asphalt plant in any way, which greatly facilitated the reuse of this waste. Accordingly, the slag was fed into the plant as though it were sand. It was then blended in with the rest of the aggregate and added to the mixture with the binder and filler. Fig. 2 shows the manufacturing process of the slag mix: (A) discontinuous asphalt mixing plant where the HMA were manufactured; (B) aggregate feed hoppers; (C) slag dosage on the conveyor belt; (D) mix exit point. The manufacturing temperature for the mixes was the same as in the laboratory (160 °C). To monitor the reproducibility of the mechanical performance of the mixes manufactured in the asphalt plant, samples were taken and subsequently compacted in the laboratory. These test specimens were subjected to the same tests as the previous
Fig. 3. Spreading and compaction of the AC 16 S slag mix.
Table 9 Density, stiffness, and water sensitivity of the mixes used to pave the road sections. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/m3)
Indirect tensile strength (kPa)
Indirect tensile strength ratio (%)
Stiffness at 15 °C (kPa)
Density (kg/m3)
Indirect tensile strength (kPa)
Indirect tensile strength ratio (%)
Stiffness at 15 °C (kPa)
2373 2364 2389 2375 2380 2361 2369 2370
1419.5 1357.6 1538.9 1438.7 1369.1 1211.4 1280.5 1287.0
89.5
6687.5
2334 2364 2351 2350 2305 2371 2355 2344
1062.0 1281.0 1176.0 1173.0 811.9 1166.0 982.6 986.9
84.1
5801.7
Table 10 Density and water sensitivity of the mixes after 6 months from the construction of the road section. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
2325 2335 2319 2326 2333 2366 2295 2331
1090 1530 1920 1520 1530 1430 1280 1410
93.1
2401 2413 2383 2399 2409 2388 2378 2392
1680 1900 1720 1760 1750 1810 1670 1740
98.9
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laboratory test specimens. Tables 7 and 8 show the test results obtained for both mixes. As can be observed, the reference mix improved its resistance to water though its indirect tensile strength values remained the same as in the laboratory. On the other hand, the slag mix showed better indirect tensile strength values than in the laboratory as well as a slightly higher retained strength. Once again, this highlights the good mechanical performance of the slag mix. It also confirms the improved cohesiveness of the bituminous mastic, due to the pozzolanic properties of slag, as described in other research [33]. In the wheel-tracking test (Table 8), permanent deformations slightly increased in the slag mix as well as in the reference mix. Nevertheless, the mean deformation slope values of the mixes showed that both were apt for use in the construction of service roads, which are generally not subjected to heavy traffic loads. 3.2. Spreading and compacting As the final step in the assessment of the viability of reusing thermal power plant slag in HMA, a test section of a service road was built near the power plant that produced the slag. This section was a two-way road, five meters wide. The slag mix was placed on a subsection with a length of 500 m and the reference mix, on a contiguous subsection of identical length. This made it possible to compare the response of both mixes to the same traffic loads. The placing of the slag mix was performed with the same machinery as the reference mix: a paver, vibratory road roller, and a pneumatic road roller. The compaction temperature was monitored with a thermometer so that it coincided with the laboratory compaction temperature (155 °C). The same compaction energy was thus employed for the slag mix and the reference mix. Fig. 3 shows the phases of the construction process of the test road section: (A) spreading; (B) compaction; (C) newly constructed road; (D) temperature monitoring from a truck before spreading; (E) temperature monitoring during compaction; (F and G) extraction of pavement cores. 3.3. Results of the test road section and quality control In order to monitor the performance of the test road section, pavement cores were extracted at the construction site after each mix was compacted. The water sensitivity (UNE-EN 12697-12) [30] and stiffness (UNE-EN 12697-26 at 15 °C) [34] of the samples were measured to control the quality of the mixes used to pave the road surface. Table 9 shows the apparent density and the stiffness of the mixes (manufactured in the asphalt plant, compacted at the site, and then extracted as samples). It also shows the results of the water sensitivity test. Although the retained strength values for the two mixes were slightly lower than the values obtained for laboratory mixes, these mixes were still apt for the pavement of service roads. These lower retained strength values could be due to the lower density of the mixes compacted at the road construction site, which also contributed to the decrease in indirect tensile strength. Furthermore, the stiffness values of the cores show that the addition of slag to the AC 16 S mix reduced the stiffness modulus. This might be related to the fact that in comparison to natural aggregate, thermal slag has a lower resistance to mechanical forces. Even so, the stiffness values of both mixes are acceptable since they have sufficient bearing capacity for the vehicle traffic level typically associated with service roads. Six months after the construction of the test road section, and with the mixes under traffic loads, new cores were extracted in order to analyze its evolution. The results obtained in terms of density and water sensitivity have been shown in Table 10. As it can be
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observed, after several months of service life, the density of the cores has not change significantly, but the indirect tensile strength has been increased in both cases (reference mix, and slag mix), due to a possible improvement of the cohesion of the mix caused by action of the traffic loads. Furthermore, the retained strength has been also increased, proving that the performance of slag mixtures during its service life and after the winter is good (the effect of the traffic and environmental agents does not affect the mechanical properties of the mix).
4. Conclusions This article describes a research study on the viability of reusing thermal power plant slag in bituminous mixes for road surfaces. The use of these wastes in the same region can help to create symbiosis between regional industries, reduce the consumption of natural aggregate, and lower the costs of manufacturing asphalt mixes. The phases of this research project included the characterization and design of the mix in the laboratory, its on-site placing on a test road section, and the subsequent monitoring and evaluation of its performance. Of the two AC 16 S mixes in the study, one was manufactured with 5% slag whereas the reference mix was manufactured only with natural aggregate. The main conclusions that can be derived from this research are the following: Boiler slag from thermal power plants can be used as a substitute for the fine aggregate fraction in hot bituminous mixes since its properties comply with the official requirements for this type of material. The mechanical performance of the slag mix was found to be comparable to that of the reference mix. In fact, the slag mix had a better resistance to water though it had a slight increase in permanent deformations. Despite its lower resistance to plastic deformations, its mechanical performance was still apt for the pavement surfaces of roads that are not subjected to heavy traffic loads. This research has demonstrated the viability of manufacturing and placing thermal power plant slag mixes with the same machinery used for conventional bituminous mixes. This is an important advantage and heightens the valorization of this waste material since its use does not entail extra production costs. This research has shown that the reuse of thermal power plant slag in bituminous mixes is a promising alternative to natural quarry aggregate. This waste can be considered cost-effective when it is recycled in the same geographic region and applied to city streets and service roads subjected to light vehicle traffic loads (especially in rehabilitation and maintenance work). Not only does the recycling of slag in HBMs reduce the environmental impact caused by the consumption of natural resources, but it also mitigates the problems associated with the generation, deposit, and accumulation of waste material at landfills.
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[21] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 10972: Tests for mechanical and physical properties aggregates – Part 2. Methods for the determination of resistance to fragmentation. Madrid; 2010. [22] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 17441: Tests for chemical properties of aggregates – Part 1. Chemical analysis. Madrid; 2013. [23] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 197-1: Cement – Part 1. Composition, specifications and conformity criteria for common cements. Madrid; 2011. [24] Dirección General de Carreteras. NLT-176/92: Densidad aparente del polvo mineral en touleno. Ensayos de carreteras. Ministerio de Obras Públicas y Transportes (MOPT), Madrid; 1992. [25] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 1426: Bitumen and bituminous binders – Determination of needle penetration. Madrid; 2007. [26] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 1427: Bitumen and bituminous binders – Determination of the softening point – Ring and Ball method. Madrid; 2007. [27] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 12593: Bitumen and bituminous binders – Determination of the Fraass breaking point. Madrid; 2007. [28] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 131081: Bituminous mixtures – Material specifications – Part 1: Asphalt Concrete. Madrid; 2008. [29] Dirección General de Carreteras. NLT-159/00: Resistencia a la deformación plástica de mezclas bituminosas empleando el aparato Marshall. Ensayos de carreteras. Ministerio de Obras Públicas y Transportes (MOPT), Madrid; 2000. [30] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 1269712: Bituminous mixtures – Test methods for hot mix asphalt – Part 12. Determination of the water sensitivity of bituminous specimens. Madrid; 2009. [31] AENOR, Asociación Española de Normalización y Certificación. UNE-EN 1269712: Bituminous mixtures – Test methods for hot mix asphalt – Part 22. Wheel tracking. Madrid; 2008. [32] Asi I, Assa’ad A, Ramme BW. Effect of jordanian oil shale fly ash on asphalt mixes. J Mater Civil Eng 2005;17:553–9. [33] Chindaprasirt P, Jaturapitakkul C, Chalee W, Rattanasak U. Comparative study on the characteristics of fly ash and bottom ash geopolymers. Waste Manage 2009;29:539–43. [34] AENOR, Asociación Española de Normalización y Certificación, UNE-EN 1269712: Bituminous mixtures – Test methods for hot mix asphalt – Part 26, Stiffness. Madrid; 2012.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Reuse of thermal power plant slag in hot bituminous mixes Fernando Moreno-Navarro a,1, Miguel Sol a, Mª. Carmen Rubio-Gámez a,⇑,2, Antonio Ramírez b a b
labIC. Laboratorio de Ingeniería de la Construcción, ETSICCP, University of Granada, C/Severo Ochoa s/n., 18071 Granada, Spain Sacyr, Paseo de la Castellana 83-85, 28046 Madrid, Spain
h i g h l i g h t s Reuse of thermal power plant slag in hot bituminous mixes (HMA). Substituting natural aggregate in the manufacture of hot mix asphalt. A test road section was built with HMA containing slags. The mechanical performance of the slag mix was found optimal. Boiler slag from thermal power plants can be used as a substitute for the fine aggregate.
a r t i c l e
i n f o
Article history: Received 6 May 2013 Received in revised form 11 July 2013 Accepted 20 July 2013 Available online 6 September 2013 Keywords: Thermal power plant slag Hot mix asphalt Roads Test section Waste recycling
a b s t r a c t The revalorization of industrial waste plays a key role in the solution of environmental and economic problems in the construction sector, thus actions contributing to the symbiosis between industries in the same region are of great benefit since they optimize resources in the area and also open the door to new business opportunities. In recent years, the electricity produced by thermal power stations has become a renewable energy source of great potential. Nevertheless, this type of energy is not without drawbacks because of the coal ash and boiler slag produced by coal combustion. Although the ash has different application as a construction material, boiler slag is more problematic since it must be deposited at landfills. The accumulation of this waste has become a serious economic and environmental problem. This paper presents the results of a research project, which analyzed the viability of reusing thermal power plant slag as a substitute for natural aggregate in the manufacture of hot mix asphalt (HMA). For this purpose, the waste was first characterized and tested in the laboratory with a view to its subsequent use in HMA. Based on the positive laboratory results, a test road section was built with this material so that its performance could be compared with that of the road section paved with a conventional HMA. The results obtained confirmed the aptness of thermal power plant slag for use in road construction. Ó 2013 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, as a consequence of industrial development, the generation of electricity from fossil fuel combustion in thermal power plants has soared in countries all over the world. However the accumulation of waste generated by such energy production is now the focus of growing social concern [1,2]. Such wastes include bottom ash or boiler slag (material collected at the bottom of the boiler) and fly ash (finer material that rise with the flue gases from the furnace upwards, generally to be captured by particle
⇑ Corresponding author. Tel.: +34 958249445. E-mail addresses: [email protected] (F. Moreno-Navarro), [email protected] (Mª.C. Rubio-Gámez), [email protected] (A. Ramírez). 1 Tel.: +34 958249443. 2 www.labic.ugr.es. 0950-0618/$ - see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.07.090
filtration equipment) [3,4]. It is thus crucial to find ways to recycle these residues, and give them new uses that contribute to sustainable development. Because of the pozzolanic characteristics of fly ash, this waste has many potential applications in the construction sector which is an ideal scenario for the reuse an revalorization of industrial by-products. In this way, fly ash can be used to fill road embankments, to stabilize soils, or to manufacture concrete [5,6]. Indeed, the results of a wide range of research underline the fact that the construction sector and its activities are an ideal scenario for the reuse and revalorization of industrial by-products [7–9]. In contrast to fly ash, boiler slag has received considerably less attention [10] despite the fact that these materials are produced in similar quantities by thermal power plants [3]. As a solution for the socio-environmental problem of boiler slag, certain authors [10,11] propose the use of this waste as a substitute for part of the sand fraction in the manufacture of concrete. According to this research,
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Fig. 1. Comparison of thermal power plant slag (A) and sand (B).
Table 1 Physical properties of thermal power plant slag. Tests Grain size (UNE-EN 933-1) [17]
Sieves (mm)
4 2 0.5 0.25 0.063 Aggregate density in paraffin oil NLT-167/96 (g/cm3) [16] Sand equivalent (UNE-EN 933-8) [18] (%) Relative density and absorption Apparent density (g/ (UNE-EN 1097-6) [19] cm3) ADSS (g/cm3) Density after drying (g/ cm3) Water absorption after immersion (%)
Table 3 Grain size of the reference mix and the slag mix. Slag
AC 16 S mix
% material passing 100.0 50.0 32.0 26.0 4.2 1.58 79.0 1.73
Sieves (mm)
Reference
5% Slag
22.4 16 8 4 2 0.5 0.25 0.063
99.0 92.0 66.0 39.0 29.0 13.0 9.0 5.9
99.0 92.0 66.0 41.0 29.0 14.0 10.0 5.0
1.57 1.35 16.21
ADSS: Apparent relative density on a saturated surface-dry basis
boiler slag and fly ash have a similar composition, which means that they have the same pozzolanic nature. Other authors [3] have assessed the performance of bituminous mixes made from thermal power plant slag. The results showed that when part of the sand fraction of the mineral skeleton was replaced by slag (less than 15% of the aggregate), this did not cause a negative impact on the mix. In fact, the mix with slag was found to have an indirect tensile strength and resistance to rutting comparable to those of conventional asphalt mixes. In contrast, other research studies [12,13] claim that road surfaces paved with
bituminous mixes made of thermal power plant residues did not perform as well and deteriorated more rapidly under traffic loads. These contradictory results seem to indicate that the real problem with slag mixes lies in the fact that the characteristics of the slag depend on its origin [12,14]. Consequently, it is necessary to study the performance of this material in each case before using it in bituminous mixes. Based on these considerations, the revalorization of boiler slag as an alternative material needs to be explored in greater depth. Accordingly, the construction engineering laboratory of the University of Granada (Spain) and Sacyr, a Spanish construction company, carried out a joint research project that studied how to recycle boiler slag from a thermal power plant to manufacture HMA. A further objective of this research was to contribute to the symbiosis between industries in the same region so that the waste produced by one economic sector would become a resource for another, thus giving it added value and contributing to
Table 2 Aggregate characteristics. Tests
Particle grain size (UNE-EN 933-1) [17]
Sieves (mm) 22.4 16 8 4 2 0.5 0.25 0.063 Coarse aggregate shape. Flakiness index (UNE-EN 933-3) [20] Resistance to fragmentation (Los Angeles coefficient) (UNE-EN 1097-2) [21] Light-weight particles in aggregate (UNE-EN 1744-1) [22] Sand equivalent (UNE-EN 933-8) [18] (%) Relative density and absorption (UNE-EN 1097-6) [19] Apparent density (g/cm3) ADSS (g/cm3) Density after drying (g/cm3) Water absorption after immersion (%)
ADSS: Apparent relative density on a saturated surface-dry basis
12/18
6/12
0/6
Limestone
Limestone
Limestone
% Material passing 90 47 0 0 0 0 0 0.1 2.62 9.0 – – 2.87 2.86 2.86 0.18
% Material passing 100 100 44 1 1 0 0 0.4 18.76 23.0 0.05 – 2.88 2.86 2.85 1.70
% Material passing 100 100 100 72 50 16 7 0 19.86 23.7 0.05 75.2 2.75 2.74 2.73 1.9
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sustainable development. For this purpose, near the town of Vera (Almería, Spain), a road section was paved with an HMA made with slag from a thermal power plant in Carboneras (Almería) situated at about 40 km from the construction site.
The research project was carried out in three phases. In the first phase, the waste material was characterized and tested in the laboratory in order to evaluate whether it could be used as a substitute for natural aggregate in the design and manufacture of HMA
Table 4 Marshall Test results for the mixes manufactured in the laboratory. Mix
Specimen
Density (kg/m3)
Air voids in the mix (%)
Air voids in the aggregate (%)
Stability (kN)
Deformation (mm)
AC 16 S (Reference)
1 2 3 Mean
2546 2532 2537 2538
4.5 4.8 4.6 4.6
14.9 15.8 15.6 15.4
13.92 12.16 12.27 12.78
1.8 2.1 1.9 1.9
AC 16 S (5% Slag)
1 2 3 Mean
2525 2492 2513 2510
2.3 3.6 2.7 2.9
13.2 14.1 12.9 13.4
12.97 11.46 12.92 12.45
2.6 2.3 2.7 2.6
Table 5 Density and water sensitivity of the mixes manufactured in the laboratory. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
Density (kg/m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
2528 2517 2528 2524 2535 2500 2528 2521
2270.8 1986.5 1991.0 2082.8 1799.0 1498.0 1842.0 1713.0
82.2
2508.2 2511.9 2506.5 2508.9 2511.8 2506.3 2492.1 2503.4
2082.1 1916.2 1729.4 1909.2 1745.1 1736.5 1725.8 1735.8
90.9
Fig. 2. Manufacture of the slag mix at the asphalt plant.
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F. Moreno-Navarro et al. / Construction and Building Materials 49 (2013) 144–150 Table 6 Resistance to plastic deformations of the laboratory mixes.
Table 8 Resistance to plastic deformations of the mixes manufactured in the asphalt plant.
Mix
Specimen
WTS (mm/103 load cycles)
Final deformation (mm)
Mix
Specimen
WTS (mm/103 load cycles)
Final deformation (mm)
AC 16 S (Reference)
1 2 Mean
0.040 0.048 0.044
0.553 0.715 0.634
AC 16 S (Reference)
1 2 Mean
0.102 0.088 0.095
1.732 1.478 1.605
AC 16 S (5% Slag)
1 2 Mean
0.195 0.139 0.167
4.998 3.679 4.339
AC 16 S (5% Slag)
1 2 Mean
0.222 0.234 0.228
4.003 5.159 4.581
and also to verify its compliance with the PG-3 General Technical Specifications for Roads and Bridge Works in Spain [15]. In the second phase, the reproducibility of this slag mix in an operating asphalt plant was evaluated as well as its placing on a test road section that was built for this purpose. Finally, the third and last phase, which is still in progress, will study the long-term performance of the test section paved with the slag mix. The following sections describe the research methodology used. (a) Laboratory studies: Analysis and characterization of thermal power plant slag Analysis and design of slag mixes (b) Test road section. On-site application and study: Manufacture in plant of slag mixes. Reproducibility Spreading and compacting Quality control 2. Laboratory studies 2.1. Materials The boiler slag came from a combined cycle electrical power plant, based on coal combustion. The main waste by-products from the combustion process were fly ash (which was already being recycled in a cement factory near the power station) and the coal slag to be used in this research as a substitute for natural aggregate in HMA. This avoided the deposit and accumulation of this waste at landfills and at the same time contributed to sustainable development in the region. The size of the slag particles was 0–31.5 mm and their apparent density was 1.58 g/cm3, as determined in paraffin oil, according to NLT standard 167/96 [16]. Because of the low mechanical strength of the coarser slag fractions, certain authors [3,14] recommend that thermal power plant slag should be used as a substitute for the sand fraction so that the mechanical performance of the mixes will not be negatively affected. For this reason, the slag was sieved to eliminate oversized particles and retain only those with a size of 0–3 mm (see Fig. 1). Table 1 lists the physical properties of the resulting material. As can be observed, all of its characteristics comply with Spanish PG-3 specifications for the use of slag as a substitute for the sand fraction in bituminous mixes. The bituminous mixes were manufactured with limestone aggregate for both the coarse and fine fractions. The aggregate in this project came from a quarry located in Antas (Almería) about 15 km from the road construction site. Table 2 shows the characteristics of the aggregate. The filler used in the mixes was general-purpose cement CEM II/B-L 32.5 N (UNE-EN 197-1) [23], 95% of which had a particle size smaller than 0.063 mm and an apparent density of 0.7 g/cm3 (NLT-176) [24]. The binder was conventional
B 50/70 bitumen with a penetration value of 68 mm (UNE-EN 1426) [25], a softening point of 50.4 °C (UNE-EN 1427) [26], and a Fraas breaking point of 8 °C (UNEEN 12593) [27].
2.2. Analysis and design of slag mixes After the properties of the material had been characterized, an AC 16 S (UNE-EN 13108-1) [28] bituminous mix was designed, in which 5% of the sand fraction was replaced with thermal power plant slag. The Marshall Test was used to design the HMA and determine the optimal blend and gradation of aggregates (NLT-159/00) [29]. The slag percentage was based on previous research [3,14], and the grain size was designed so that the mineral skeleton complied with Spanish PG-3 regulations [15]. At the same time, an AC 16 S reference mix was designed, which did not have any waste fraction and which only contained natural aggregate. Both mixes were elaborated at a temperature of 160 °C, using the same manufacturing process, given that the slag was used as part of the aggregate. Mix performance was then evaluated with the water sensitivity test (UNE-EN 12697-12) [30] and the wheel-tracking test (UNE-EN 12697-22) [31]. Table 3 shows the mineral skeletons of the reference mix and the slag mix. The grain-size distribution of the slag mix was performed by volume (instead of by weight) since this waste has a lower density than the natural aggregate. The optimal bitumen content for each mix was set at 4.3% (of the mix weight), based on the Marshall Test results (NLT 159/00) [29]. Table 4 shows the mean density (g/cm3), air voids in the aggregate (%), air voids in the mix (%), stability (kN) and deformation (mm) values for the reference mix and for the slag mix. The mixes were manufactured in the laboratory with the optimal binder content. Despite the fact that the density of both mixes was similar, the slag mix had fewer air voids because of the addition of the waste material. The test results for the strength parameters showed that the stability of the slag was almost the same as that of the reference mix. However, its deformation value was somewhat higher, which meant that its resistance to plastic deformation might not be as good. The results of the water sensitivity test (UNE-EN 12697-12) [30] showed that the slag mix performed better in terms of retained strength, which could signify an improvement in the cohesiveness of the mastic (Table 5). This coincides with the results of other research [14,32] on thermal power plant slag mixes and their resistance to water. Our study found that the mix with slag (as a substitute for the sand fraction) was less sensitive to water than the reference mix with natural aggregate. Furthermore, the indirect tensile strength values of the slag mix were very similar to those of the reference mix, which meant that the strength of the mix was not affected by the addition of the waste in the sizes and percentages used. To complete the laboratory study of the mechanical performance of the mixes, the wheel-tracking test was performed (UNE-EN 12697-22) [31] to evaluate the influence of the waste fraction on mix resistance to plastic deformations. Table 6 shows the values of the mean deformation slope for two test specimens of each mix as well as the mean value of this final deformation.
Table 7 Density and water sensitivity of mixes manufactured in the asphalt plant. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
2431 2427 2406 2421 2434 2400 2403 2412
2079.0 1873.0 1909.0 1953.7 2048.0 1964.0 1451.0 1821.0
93.2
2477 2492 2466 2478 2471 2468 2491 2477
2833.0 2689.8 2220.0 2580.9 2537.0 2369.0 2365.0 2423.7
93.9
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These results show that the addition of thermal power plant slag to the AC 16 S mix increased the plastic deformations recording during the wheel tracking test. Similarly, the mean deformation slope between 5000 and 10,000 load cycles increased in the slag mix, which indicates a lower resistance to permanent deformations. This also coincides with the higher deformation values obtained during the Marshall Test. The resistance to plastic deformation of the slag mix was found to be slightly lower than the limit specified in Spanish regulations for roads with the heaviest traffic load levels. However, because of resistance to fracturing and to water action showed by the mix, it was decided to use it to pave a test road section located on a service road with light vehicle traffic. Accordingly, after completing the mix design phase, our objective was to evaluate the possible use of this waste in HMA for city streets and service roads since this would reduce the consumption of natural aggregate.
3. On-site testing 3.1. Reproducibility of the mix in an asphalt plant After the laboratory phase, which evaluated the recycling of thermal power plant slag in HMA, it was decided to manufacture
the mix and try to reproduce its mechanical properties in an actual asphalt mixing plant. For this purpose, the same laboratory materials and dosages were used to elaborate both the slag mix and the reference mix. The process used to manufacture both mixes was also the same. There was thus no need to use any auxiliary machinery or to modify the asphalt plant in any way, which greatly facilitated the reuse of this waste. Accordingly, the slag was fed into the plant as though it were sand. It was then blended in with the rest of the aggregate and added to the mixture with the binder and filler. Fig. 2 shows the manufacturing process of the slag mix: (A) discontinuous asphalt mixing plant where the HMA were manufactured; (B) aggregate feed hoppers; (C) slag dosage on the conveyor belt; (D) mix exit point. The manufacturing temperature for the mixes was the same as in the laboratory (160 °C). To monitor the reproducibility of the mechanical performance of the mixes manufactured in the asphalt plant, samples were taken and subsequently compacted in the laboratory. These test specimens were subjected to the same tests as the previous
Fig. 3. Spreading and compaction of the AC 16 S slag mix.
Table 9 Density, stiffness, and water sensitivity of the mixes used to pave the road sections. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/m3)
Indirect tensile strength (kPa)
Indirect tensile strength ratio (%)
Stiffness at 15 °C (kPa)
Density (kg/m3)
Indirect tensile strength (kPa)
Indirect tensile strength ratio (%)
Stiffness at 15 °C (kPa)
2373 2364 2389 2375 2380 2361 2369 2370
1419.5 1357.6 1538.9 1438.7 1369.1 1211.4 1280.5 1287.0
89.5
6687.5
2334 2364 2351 2350 2305 2371 2355 2344
1062.0 1281.0 1176.0 1173.0 811.9 1166.0 982.6 986.9
84.1
5801.7
Table 10 Density and water sensitivity of the mixes after 6 months from the construction of the road section. Conditioning
Dry Set
Wet Set
Specimen
1 2 3 Mean 4 5 6 Mean
AC 16 S (Reference)
AC 16 S (5% Slag)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
Density (kg/ m3)
Indirect Tensile Strength (kPa)
Indirect Tensile Strength Ratio (%)
2325 2335 2319 2326 2333 2366 2295 2331
1090 1530 1920 1520 1530 1430 1280 1410
93.1
2401 2413 2383 2399 2409 2388 2378 2392
1680 1900 1720 1760 1750 1810 1670 1740
98.9
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laboratory test specimens. Tables 7 and 8 show the test results obtained for both mixes. As can be observed, the reference mix improved its resistance to water though its indirect tensile strength values remained the same as in the laboratory. On the other hand, the slag mix showed better indirect tensile strength values than in the laboratory as well as a slightly higher retained strength. Once again, this highlights the good mechanical performance of the slag mix. It also confirms the improved cohesiveness of the bituminous mastic, due to the pozzolanic properties of slag, as described in other research [33]. In the wheel-tracking test (Table 8), permanent deformations slightly increased in the slag mix as well as in the reference mix. Nevertheless, the mean deformation slope values of the mixes showed that both were apt for use in the construction of service roads, which are generally not subjected to heavy traffic loads. 3.2. Spreading and compacting As the final step in the assessment of the viability of reusing thermal power plant slag in HMA, a test section of a service road was built near the power plant that produced the slag. This section was a two-way road, five meters wide. The slag mix was placed on a subsection with a length of 500 m and the reference mix, on a contiguous subsection of identical length. This made it possible to compare the response of both mixes to the same traffic loads. The placing of the slag mix was performed with the same machinery as the reference mix: a paver, vibratory road roller, and a pneumatic road roller. The compaction temperature was monitored with a thermometer so that it coincided with the laboratory compaction temperature (155 °C). The same compaction energy was thus employed for the slag mix and the reference mix. Fig. 3 shows the phases of the construction process of the test road section: (A) spreading; (B) compaction; (C) newly constructed road; (D) temperature monitoring from a truck before spreading; (E) temperature monitoring during compaction; (F and G) extraction of pavement cores. 3.3. Results of the test road section and quality control In order to monitor the performance of the test road section, pavement cores were extracted at the construction site after each mix was compacted. The water sensitivity (UNE-EN 12697-12) [30] and stiffness (UNE-EN 12697-26 at 15 °C) [34] of the samples were measured to control the quality of the mixes used to pave the road surface. Table 9 shows the apparent density and the stiffness of the mixes (manufactured in the asphalt plant, compacted at the site, and then extracted as samples). It also shows the results of the water sensitivity test. Although the retained strength values for the two mixes were slightly lower than the values obtained for laboratory mixes, these mixes were still apt for the pavement of service roads. These lower retained strength values could be due to the lower density of the mixes compacted at the road construction site, which also contributed to the decrease in indirect tensile strength. Furthermore, the stiffness values of the cores show that the addition of slag to the AC 16 S mix reduced the stiffness modulus. This might be related to the fact that in comparison to natural aggregate, thermal slag has a lower resistance to mechanical forces. Even so, the stiffness values of both mixes are acceptable since they have sufficient bearing capacity for the vehicle traffic level typically associated with service roads. Six months after the construction of the test road section, and with the mixes under traffic loads, new cores were extracted in order to analyze its evolution. The results obtained in terms of density and water sensitivity have been shown in Table 10. As it can be
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observed, after several months of service life, the density of the cores has not change significantly, but the indirect tensile strength has been increased in both cases (reference mix, and slag mix), due to a possible improvement of the cohesion of the mix caused by action of the traffic loads. Furthermore, the retained strength has been also increased, proving that the performance of slag mixtures during its service life and after the winter is good (the effect of the traffic and environmental agents does not affect the mechanical properties of the mix).
4. Conclusions This article describes a research study on the viability of reusing thermal power plant slag in bituminous mixes for road surfaces. The use of these wastes in the same region can help to create symbiosis between regional industries, reduce the consumption of natural aggregate, and lower the costs of manufacturing asphalt mixes. The phases of this research project included the characterization and design of the mix in the laboratory, its on-site placing on a test road section, and the subsequent monitoring and evaluation of its performance. Of the two AC 16 S mixes in the study, one was manufactured with 5% slag whereas the reference mix was manufactured only with natural aggregate. The main conclusions that can be derived from this research are the following: Boiler slag from thermal power plants can be used as a substitute for the fine aggregate fraction in hot bituminous mixes since its properties comply with the official requirements for this type of material. The mechanical performance of the slag mix was found to be comparable to that of the reference mix. In fact, the slag mix had a better resistance to water though it had a slight increase in permanent deformations. Despite its lower resistance to plastic deformations, its mechanical performance was still apt for the pavement surfaces of roads that are not subjected to heavy traffic loads. This research has demonstrated the viability of manufacturing and placing thermal power plant slag mixes with the same machinery used for conventional bituminous mixes. This is an important advantage and heightens the valorization of this waste material since its use does not entail extra production costs. This research has shown that the reuse of thermal power plant slag in bituminous mixes is a promising alternative to natural quarry aggregate. This waste can be considered cost-effective when it is recycled in the same geographic region and applied to city streets and service roads subjected to light vehicle traffic loads (especially in rehabilitation and maintenance work). Not only does the recycling of slag in HBMs reduce the environmental impact caused by the consumption of natural resources, but it also mitigates the problems associated with the generation, deposit, and accumulation of waste material at landfills.
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