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Utilization of municipal solid waste bottom ash and recycled aggregate in concrete
Waste Management 26 (2006) 1436–1442 www.elsevier.com/locate/wasman
Utilization of municipal solid waste bottom ash and recycled aggregate in concrete B. Juricˇ
a,b
, L. Hanzˇicˇ
a,*
, R. Ilic´
a,c
, N. Samec
d
a
d
Faculty of Civil Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia b Projekt d.d. Nova Gorica, Kidricˇeva 9a, 5000 Nova Gorica, Slovenia c J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia Accepted 28 October 2005 Available online 31 January 2006
Abstract In the combustion process of municipal solid waste (MSW), bottom ash (BA) represents the major portion of the solid residue. Since BA is composed of oxides, especially SiO2 and CaO, the feasibility of its application in concrete as a substitute for cement was tested. It was found that at the age of 28 days, the flexural and compressive strengths of the binder linearly decrease at the rate of 0.03 and 0.02 MPa per wt% of BA in the binder, respectively. According to the results it may be recommended to replace up to 15 wt% of cement by BA and to use such binder where a low strength of concrete elements is required. Furthermore, the aggregate used for low strength concrete need not be of a very good quality. Therefore, gravel aggregate was partially replaced by recycled aggregate (RA). Consistency measured by slump was significantly reduced (>50%) when BA or/and RA were introduced into the mixture. However, concrete density and compressive strength were not affected and were 2300 kg/m3 and 40 MPa, respectively. 2005 Elsevier Ltd. All rights reserved.
1. Introduction In the last few decades of the 20th century it became obvious that the large quantities of waste produced by the modern consumer society cause serious environmental damage when they are disposed of without any treatment. While a small portion of waste (less than 1% (EU, 2004)) is hazardous and requires expensive treatment, some waste, like construction and demolition (C&D) debris, can be considered as inert. Municipal waste, although classified as non-hazardous, still causes air, water and soil pollution during decay. Hence, modern landfills for municipal solid waste (MSW) have a complex design and should be able to carry out several processes such as leachate and gas management and monitoring. This makes the disposal space for a volume unit of waste rather expensive. In order
*
Corresponding author. Tel.: +386 2 22 94 331; fax: +386 2 25 24 179. E-mail address: [email protected] (L. Hanzˇicˇ).
0956-053X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2005.10.016
to minimise disposal space, it has become common practice to sort, reuse and incinerate waste materials. In Slovenia, approximately 600–1000 kg of C&D debris is produced per person per year (RS, 2001), which is comparable to other European countries (EU, 1999). Additionally, Slovenian households create about 535 kg of MSW per person each year (RS, 2002a). Landfills in Slovenia are mostly full and because of strong public resistance and the NIMBY (Not In My BackYard) syndrome, it is almost impossible to find new locations. In order to meet modern standards for waste management, Slovenian society will have to consider the construction of incineration plants. Within the framework of this issue, an experimental two-chamber plant was recently built in Vransko, Slovenia (Samec et al., 2001). Although two-chamber plants have been usually employed for the incineration of hazardous waste, lately they have gained significance for combustion of MSW as well. Two-stage combustion results in lower entrainment of particulate matter in the flue gases, which significantly
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
reduces the investment and operational costs for flue gas treatment devices. In the primary chamber of the KIVVransko incinerator (Fig. 1), the temperature ranges between 600 and 800 C, and about 70% of the required air is added. These conditions ensure warming, drying and semi-pyrolitic gasification of waste, where most of the metals cannot evaporate and remain in the bottom ash (BA). Flue gases are led to the second chamber, called the thermoreactor. Here, gases are mixed with 150–200% of the stoichiometrically required air and the temperature is raised to 1250 C. This results in the total destruction of all organic compounds. The capacity of the plant is 25 kg of waste per hour. During the combustion process approximately 10 wt% of solid residue is formed from the input mass. It is convenient for this type of plant if the calorific value of the waste is at least 16 MJ/kg. The KIV-Vransko incinerator is described in detail elsewhere (Samec et al., 2001). By incineration, the volume of waste is reduced by 70– 90%. The residual BA has a low specific weight (300 kg/m3) and therefore requires some special measures when deposited in landfills, for instance stabilization with cement. On the other hand, further applications can be considered and BA can be used as a substitute for raw materials for the manufacture of other products. Namely, its chemical composition strongly resembles the composition of cement. The use of different kinds of ash, such as coal and MSW fly ash and BA, for concrete production was recently reported (Pera et al., 1997; Kula et al., 2001; Targan et al., 2002; Re´mond et al., 2002; Filipponi et al., 2003; Cheerarot and Jaturapitakkul, 2004). Most of the ashes used in these studies contain 40–60 wt% of SiO2. For cement this value typically amounts to only 17– 25 wt%. All of the reviewed ashes contain essentially much less CaO (2–20 wt%) than cement (60–67 wt%). Since BA
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from KIV-Vransko contains 24 wt% of SiO2 and 39 wt% of CaO, it can be considered as a very appropriate substitute for cement. Cement, which is a hydraulic binder, is the fundamental ingredient of concrete. In general, it represents about 15% of concrete volume and about 45% of its total cost. Concrete is one of the prime materials in the modern construction industry; about 4.5 billion metric tons of concrete are cast each year worldwide (Su and Miao, 2003). In Slovenia, this value amounts to 1.5 · 106 tons (6 · 105 m3) (RS, 2002b), of which approximately 90% represents concrete with a characteristic compressive strength of 40 MPa or less. The aim of the present research was to investigate some vital properties of concrete made of waste materials – BA from the experimental MSW incinerator and C&D debris. By incorporating waste materials into permanent compounds, their interaction with the environment can be diminished, and simultaneously the required disposal space is reduced. 2. Experimental The study was designed in two steps. First, the binding abilities of BA were investigated and an acceptable level of BA in total binder was determined. In the second step, concretes with the formerly determined level of BA and different portions of recycled aggregate (RA) were studied for their consistency, density and compressive strength. The binding abilities of BA were tested according to the standard EN 196-1 (1994). Specimens were prepared with 450 g of binder and 225 g of water, giving a water/binder (W/B) ratio of 0.5. The aggregate (1350 g) was quartz sand with grain sizes from 0 to 2 mm, which fulfils EN 196-1 (1994) standard requirements. For the preparation of a reference mixture, pure cement, designated as CEM I 42.5 R,
Fig. 1. KIV-Vransko experimental two-chamber incineration plant, Slovenia (Samec et al., 2001).
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
was used as binder. According to standard EN 197-1 (2000), designation CEM I 42.5 R stands for Portland cement containing at least 95 wt% of clinker minerals, having a compressive strength of at least 42.5 MPa at the age of 28 days but not exceeding 62.5 MPa. R denotes a rapid development of strength in the early stage of hydration. Subsequent specimens were made with a binder, which consisted of cement and BA. The percentage of BA in the total binder varied from 5 to 40 wt%. BA, which was obtained from the KIV-Vransko incineration plant, was in the form of flakes with sizes from 1 to 20 mm. In order to obtain a suitable granular composition it was ground with a ball mill and sieved through a 90 lm screen. The bulk density of such BA is 510 kg/m3, whereas its apparent specific gravity is 3100 kg/m3. It should be noted that the size and shape of the BA particles could depend on the incineration process. Pera et al. (1997) reported the use of MSW BA as an aggregate for concrete where the particles of BA were rather globular in shape with sizes from 4 to 20 mm. The composition of BA and CEM I 42.5 R was determined by X-ray diffraction (Table 1). Besides this, k0-instrumental neutron activation analysis (k0-INAA) (Jac´imovic´ et al., 2002) was applied to measure some major and trace elements in BA. Nine specimens with dimensions of 40 mm · 40 mm · 160 mm were cast from each mixture. They were kept in a mould for 24 h; then they were stored in a 100% moist environment at 22 ± 2 C. Specimens were tested for flexural and compressive strength at the age of 2, 7 and 28 days. In the second step, two series of concrete specimens were made. For the first series, pure cement was used as binder, whereas in the second series binder with 15 wt% of BA was used. The binder content was 430 kg/m3 and the W/B ratio was 0.55. For the reference concrete mixture, pure gravel aggregate with a maximum grain size of 8 mm was used. Suitable overall grading of the aggregate (optimal grading curve in Fig. 2) was determined according to the Fuller and EMPA equation (Muravljov, 2000). The bulk density of gravel with such a granular composition was 1906 kg/m3. The reference concrete mixture was designed to reach a compressive strength of 40 MPa. Aggregate for the subsequent specimens consisted of gravel and 9.1, 16.7 and 23.1 wt% of RA. Table 1 Composition of cement CEM I 42.5 R and bottom ash (BA) from the KIV-Vransko incineration plant determined by X-ray diffraction Oxide
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3
Content (wt%)
100
Percent passing by weight
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BA
22.3 5.83 2.17 60.81 2.82 0.34 0.72 2.75
24 14.8 2.7 39 1.7 0.9 0.2 –
optimal grading curve
80
recycled aggregate
70 60 50 40 30 20 10 0 0
0.063
0.125
0.25
0.5
1
2
4
8
Sieve size (mm) Fig. 2. Optimal grading curve of the aggregate determined according to the Fuller and EMPA equation and the grading curve of recycled aggregate.
RA was obtained from a mobile C&D debris recycling plant, which was stationed in the town of Koper at the time of the experiment. The applied recycling process consists of elimination of major metallic pieces and organic compounds and of grinding and sieving. The capacity of the recycling plant is 160 m3/h. Divisions of RA are stored in an open depository, where the material for the experiment was acquired. Since the composition of C&D debris depends on the type of the object being demolished, significant variations in RA composition and quality are possible. Composition of RA used in this study was analysed on six samples by sorting particles of washed RA. Mass proportions are presented in Fig. 3. For the purpose of this experiment, an RA division of 0–8 mm was selected. It was established by wet sieving that RA contains a large portion of small particles, as shown in Fig. 2, which would have a negative impact on the consistency of concrete. It was also established (Chen et al., 2003) that the use of unwashed RA resulted in a lower compressive strength of concrete. Therefore, RA was washed on a sieve with a 68 lm aperture. The retained aggregate was dried, divided into fractions and kept at room conditions where humidity was a 55 ± 10% and temperature 24 ± 2 C. Each aggregate mixture was afterwards composited according to the optimal grading curve where each fraction was combined from gravel and the required wt% of RA. The bulk density of pure washed RA with optimal grading composition was 1594 kg/m3. Water absorption capability of gravel and RA was tested
4%
CEM I 42.5 R
90
6%
1%
mineral debris bitumen debris brick wood and undefinable
89%
Fig. 3. Material composition of recycled aggregate.
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
3. Results and discussion
a Flexural strength (MPa)
9 8 7 6 5 4 3
after 2 days after 7 days after 28 days
2 1 0 0
5
10
15
20
25
30
35
40
Bottom ash (wt %)
b 60 Compressive strength (MPa)
according to EN 1097-6 (2000). Additionally, concrete with 15 wt% of BA in the binder and 100 wt% of RA was made. Fresh concrete was tested for its workability. The quality of casting and compacting depend on workability, and thereby it has a significant impact on the strength and durability of the concrete. For workability, which is best defined as the amount of useful internal work necessary to produce full compaction (Glanvile et al., 1947), there is no widespread standard test available. Thus, the slump of a concrete cone is often measured and the term consistency is used to describe this property. After executing the standard slump test for the purpose of this experiment, six cubical specimens with edge length of 100 mm were cast from each mixture. They were compacted by external vibrator for the uniform period of time. Specimens were taken out of the mould after 24 h and kept in a 100% moist environment at 22 ± 2 C. Compressive strength was determined after 7 and 28 days. Since concrete is often exposed to atmospheric precipitation, ground and underground water, leaching of heavy metals concentrated in BA could make such a concrete environmentally unacceptable. It was determined by Cai et al. (2004) that by introducing BA in concrete instead of gravel, quantities of potentially available contaminants are increased from 5 to 600 times. Therefore, concrete was tested for leaching of some selected metals. Concrete was crushed in a ball-mill to obtain particles smaller then 10 mm, and specimens weighing 100 g were tested according to DIN 38414-4 (1984). Leaching was carried out for 24 h in demineralised water, where the mass ratio of the specimen and water was 1:10. Leachate was analysed for chromium (EN 1233, 1996), arsenic (EN 26595, 1992), cadmium (ISO 5961, 1994), and for cobalt, nickel, copper, zinc and lead (ISO 8288, 1986).
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50 40 30 20
after 2 days after 7 days after 28 days
10 0 0
5
10
15
20
25
30
35
40
Bottom ash (wt%)
Fig. 4. Strength regarding wt% of bottom ash in the binder: (a) flexural strength (experimental error was estimated to be 8%) and (b) compressive strength (experimental error was estimated to be 5%). Table 2 Values of parameters defining the dependence of flexural and compressive strength on wt% of bottom ash (BA) in the binder Time (days)
Flexural
Compressive
ff0 (MPa)
kf (MPa/%BA)
fc0 (MPa)
kc (MPa/%BA)
2 7 28
4.0 6.6 8.1
0.05 0.05 0.03
28.5 39.0 52.1
0.35 0.27 0.23
ff0 and fc0 denote flexural and compressive strength at wt% BA = 0. Slopes of the curves (Eqs. (1) and (2)) are denoted by kf and kc.
3.1. Binding abilities of bottom ash The dependence of flexural and compressive strength on the content of BA is given in Fig. 4. A linear decrease of both flexural ff (Fig. 4a) and compressive fc (Fig. 4b) strength was observed. Hence, the following relationships were applied: ff ¼ ff0 k f C;
ð1Þ
fc ¼ fc0 k c C;
ð2Þ
where C is the wt% of BA in the binder, ff0 and fc0 denote the strength of the reference mix (without BA), whereas the slope of the straight line denoted by kf and kc represent the reduction rates of strength. Values of all parameters at the age of 2, 7 and 28 days are summarized in Table 2. Since the hydration of cementitious materials is a time dependent process, which can last up to several years, it was expected that the values of ff and fc would increase with elapsed time. However, it is interesting to observe that the values of kf and kc slightly decrease with time. This
might indicate that BA develops strength in the later stage of hydration compared to CEM I 42.5 R. A possible reason for the observed behaviour is that the temperature in the primary chamber is too low for formation of alite (3CaO Æ SiO2), which is an essential compound for the strength of concrete in the early stage of the hydration process. Furthermore, the quantity of CaO is insufficient for formation of all cementitious compounds. A study on fly ash carried out by Wihler (2000) has shown a similar effect. Namely, at an age of 28 days compressive strength of concrete in which cement was partially replaced by fly ash was lower than the strength of the reference mix, whereas at the age of 90 days the situation was reversed. Standard EN 197-1 (2000) sets two acceptance conditions for the compressive strength of cement CEM I 42.5 R: after two days it should be at least 20 MPa, at the age of 28 days it should reach at least 42.5 MPa but should not exceed 62.5 MPa.
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
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Since by increasing the wt% of BA in the binder, early strengths are more affected, the first condition is more essential. Hence, the maximum content of BA in total binder (calculated by Eq. (2)), which also satisfies the second condition, is about 25 wt%. Applying a safety factor of 0.7, the acceptable level of BA in binder was established to be 15 wt% and it was further applied for the study of concrete.
Table 5 Densities of fresh concrete mixtures
3.2. Workability of concrete
The abbreviations RA and BA denote recycled aggregate and bottom ash.
The consistency obtained by the slump test is presented in Table 3. From these results, two observations can be made. First, slump is significantly (>50%) reduced when binder contains 15 wt% of BA and second, reduction of slump also occurs when RA is introduced into the aggregate mixture. The latter is probably due to aggregate shape and texture, with gravel particles being round and smooth, while RA particles are angular and rough with high porosity and thus with higher water absorption as may be perceived in Table 4. In general, lower slump should result in poorer workability, so one would expect lower density for such concrete mixtures. The densities of concrete mixtures given in Table 5 display an interesting fact. When the binder contains cement as well as BA, the density of concrete is not altered although slump is reduced. To evaluate workability more precisely, yield strength (sy) and plastic viscosity (l) should be determined. Namely, the rheological properties of fresh concrete are approximated by the Bingham model (Illston and Domone, 2001): s ¼ sy þ lc;
ð3Þ
where s is shear stress and c is shear strain. Since the results of the slump test offer us only information about sy, it is possible that the presence of BA reduces cohesion (lowers l). Thus, fresh concrete would have an increased response to compaction by vibration and hence higher densities could be achieved. It should be noted that segregation
Table 3 Consistency of concrete mixtures measured by standard slump test RA (wt%)
0 9.1 16.7 23.1
Slump (mm) BA = 0 wt%
BA = 15 wt%
105 60 35 15
55 20 10 0
Table 4 Water absorption of gravel and recycled aggregate (RA) tested according to EN 1097-6 (2000)
0/4 4/8
Density (kg/m3) BA = 0 wt%
BA = 15 wt%
0 9.1 16.7 23.1
2310.7 2313.3 2328.1 2310.0
2324.8 2322.3 2319.6 2302.2
did not occur in any of the types of concrete examined in the present study. 3.3. Compressive strength of concrete The compressive strengths of the concrete mixtures are summarized in Table 6. Although it was established that when BA represents 15 wt% of the binder, compressive strength is reduced by approximately 10%, this was not generally observed on concrete specimens. The compressive strength of mixtures with aggregate containing 0 and 9 wt% of RA even increased when BA was used – surprisingly even at the age of 7 days. Maximum reduction (10%) of strength as the consequence of BA usage was observed in the mixture with 23 wt% of RA at the age of 28 days, but this mixture still reached a compressive strength of 41.6 MPa. When compressive strength is considered as a function of weight percentage of RA in the aggregate mixture, it can be established that up to a certain amount (17 wt%) of RA the compressive strength increases but later (between 17 and 23 wt%) it starts to decrease. As shown in Fig. 3, RA consists mainly (89 wt%) of mineral debris. Into this group particles of crushed concrete, mortar, plaster and stone were classified. Thus, a substantial amount of cement is present in RA. During grinding, which is a part of RA production process, cement particles may crack and the hitherto unhydrated core would be subjected to hydration when exposed to water. Hydration of cement particles in RA may have a favorable effect on concrete strength. 3.4. Environmental impact Cement gel has a characteristic porosity of 28 vol% (Neville, 2002) and the pore structure of concrete, which
The abbreviations RA and BA denote recycled aggregate and bottom ash.
Division
RA (wt%)
Table 6 Compressive strength of concrete mixtures (with different wt% of recycled aggregate RA and bottom ash BA) obtained at two different ages t RA (wt%)
t = 7 days
Water absorption (wt%) Gravel
RA
0.8 1.0
5.1 6.8
Compressive strength · (1 ± 0.05) MPa
0 9.1 16.7 23.1
t = 28 days
BA = 0 wt%
BA = 15 wt%
BA = 0 wt%
BA = 15 wt%
31.6 32.2 37.6 37.1
34.7 35.2 35.8 34.8
40.6 40.8 46.9 46.4
43.4 42.2 44.0 41.6
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
includes porosity, permeability and tortuosity, has a strong influence on the durability and applicability of concrete. Percolation of water through the material causes leaching of heavy metals, which are afterwards released into the environment. Concentrations of several elements in BA, obtained by the k0-INAA method, are summarized in Table 7. Beside Table 7 Results of k0-INAA analysis of bottom ash (BA) and concentration limits set for hazardous elements in waste classified as inert according to regulations valid at the time of the research (RS, 2000) Element
Concentration in BA (mg/kg)
Error/LD (mg/kg)
Concentration limits (mg/kg)
Ag Al As Au Ba Br Ca Cd Ce Cl Co Cr Cs Cu Dy Eu Fe Ga Hf Hg K La Mo Mg Mn Na Nd Ni Pb Pt Rb Sb Sc Se Si Sm Sn Sr Ta Tb Te Th Ti Tm U V W Yb Zn Zr
1.01 82,760 5.38 0.292 1294 11.7 226,940 11.0 41.2 3694 16.2 281 3.7 – 2.49 0.69 16,900 38.5 4.54 0.11 7926 22.4 10.0 21,481 574 7412 16.4 – – – 46.3 77.8 4.23 1.33 – 2.77 – 394 1.03 0.34 – 7.9 7261 – 2.99 36.0 6.69 0.87 967 178
0.15 3500 0.30 0.010 47 0.4 8770 2.1 1.5 151 0.6 12 0.1 54 0.16 0.08 593 5.4 0.16 0.02 807 0.8 1.0 1981 20 260 1.4 – – 2 1.8 2.8 0.17 0.11 250,000 0.15 36 18 0.04 0.02 2 0.3 508 0.2 0.23 4.7 0.41 0.04 34 17
– – 200 – – – – 12 – – 250 500 – 500 – – – – – 10 – – 500 – – – – 500 500 – – – – – – – – – – – – – – – – – – – 1500 –
The abbreviation LD denotes the limit of detection.
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Table 8 Results of leaching tests for concrete made of 100 wt% RA and 15 wt% BA in the binder in comparison with limits set by Slovenian regulations at the time of the research (RS, 2000) and recently accepted regulations (RS, 2004) Element
Leaching from concrete (mg/kg)
Limits for inert waste valid up to 2004 (mg/kg)
Limits for inert waste valid from 2004 (mg/kg)
As Ba Cd Cr Co Cu Hg Mo Ni Pb Sb Se Zn
<0.01 – <0.03 <1.50 <0.61 0.13 – – 0.83 <1.04 – – <0.17
1 20 0.5 10 5 10 0.05 – 5 5 1 0.5 30
0.5 10 0.04 0.5 – 2 0.01 0.5 0.4 0.5 0.06 0.1 4
other requirements, stated in Slovenian regulations valid at the time of the research (RS, 2000), maximum values of concentrations of hazardous elements in the waste were prescribed if the waste was to be classified as inert. Results show that only the cadmium concentration (11 ± 2 mg/kg) is close to the limit (12 mg/kg). The results of leaching tests for concrete are summarized in Table 8, and they are compared to concentration limits for leachate from inert waste valid at the time of the experiment (RS, 2000), as well as to the presently valid regulations (RS, 2004). For the majority of the elements measured, leaching is below the limits for inert waste. Before utilization of BA in concrete, a more detailed study would be required. Supposing that all MSW in Slovenia were incinerated with the same technology as used at KIV-Vransko, 1.1 · 105 tons of BA would be produced per year. For production of concrete with a characteristic compressive strength of 40 MPa or less and by supplementing 15 wt% of cement with BA, about 3.5 · 104 tons of BA could be used. This would reduce the landfill space required by about 7 · 104 m3. 4. Conclusions Conclusions of the presented study may be summarized as follows: 1. Satisfactory quality of concrete with low strength requirements (compressive strength up to 40 MPa) may be achieved using two waste materials, namely C&D debris and MSW BA, simultaneously. 2. The experiment showed a linear decrease of flexural and compressive strength at the rate of 0.03 and 0.02 MPa per wt% of BA in the binder, respectively. According to the results it may be recommended to replace up to 15 wt% of cement by BA.
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3. Reduction of slump was observed when BA and RA were introduced into the concrete mixture, but workability did not seem to be affected. The study indicated the possibility that BA reduces cohesion in concrete. More detailed investigation of rheological properties using the two-point workability test would be important. The study has indicated the feasibility of utilization of BA and RA in concrete; hence further research may be recommended. Subsequent studies should determine variations in composition of both materials and examine the influence of these variations on concrete properties. Since it is not possible to determine fundamental aspects of hydraulic reactivity of BA by undertaking strength tests alone, additional study regarding hydraulic/puzzolanic properties of BA is advised. Additionally, long-term tests should be carried out to establish the durability of such concrete and to specify fields of its application. In this aspect it is encouraging that a favorable effect of fly ash on the durability of concrete has been observed (Schießl et al., 2001). Nevertheless, more detailed study on leaching of hazardous elements would also be important. Acknowledgements We express our thanks to R. Jac´imovic´ for neutron activation analysis (k0-INAA) of bottom ash and to A. Hansel for technical assistance. References Cai, Z., Bager, D.H., Christensen, T.H., 2004. Leaching from solid waste incineration ashes used in cement-treated base layers for pavements. Waste Management 24, 603–612. Cheerarot, R., Jaturapitakkul, C., 2004. A study of disposed fly ash from landfill to replace Portland cement. Waste Management 24, 701–709. Chen, H.J., Yen, T., Chen, K.H., 2003. Use of building rubbles as recycled aggregates. Cement and Concrete Research 33 (1), 125–132. DIN 38414-4, 1984. Deutsche Einheitsverfahren zur Wasser-, Abwasserund Schlammuntersuchung; Schlamm und Sedimente (Gruppe S); Bestimmung der Eluierbarkeit mit Wasser (S 4) (German standard methods for the examination of water, waste water and sludge; sludge and sediments (group S); determination of leachability by water (S 4)) (in German). EN 196-1, 1994. Methods of testing cement – Part 1: Determination of strength. EN 197-1, 2000. Cement – Part 1: Composition, specifications and conformity for common cements. EN 1097-6, 2000. Tests for mechanical and physical properties of aggregates – Part 6: Determination of particle density and water absorption. EN 1233, 1996. Water quality: Determination of chromium – Atomic absorption spectrometric methods. EN 26595, 1992. Water quality: Determination of total arsenic – Silver diethyldithiocarbamate spectrophotometric method. EU, 1999. European Commission web pages. Construction and demolition waste management practices, and their economics impacts. Available from: (15.9.2004).
EU, 2004. European Commission web pages. Environment: Waste. Available from: (15. 9. 2004). RS, 2002b. Statistical Office of the Republic of Slovenia. Statistical yearbook of the Republic of Slovenia, Part 21: Construction. Available from: (15. 9. 2004). RS, 2004. Republic of Slovenia, Ministry of the Environment and Spatial Planning. Regulations for Waste Disposal (in Slovene), Ljubljana, Slovenia. Samec, N., Kokalj, F., Krecˇa, I., Hudernik, A., 2001. Advanced two-stage combustion system for MSW incineration. In Incineration 2001, 3rd International Symposium on Incineration and Flue Gas Treatment Technologies, Brussels, Belgium. Schießl, P., Wiens, U., Schro¨der, P., Mu¨ller, C., 2001. Neue Erkenntnisse u¨ber die Leistungsfa¨higkeit von Beton mit Steinkohlenflugasche (Teil 1), Untersuchungsergebnisse zum normalfesten Beton (New findings on efficiency of concrete containing fly ash (Part 1), Results of research performed on normal strength concrete). Beton 1, 10–17 (in German). Su, N., Miao, B., 2003. A new method for the mix design of medium strength flowing concrete with low cement content. Cement and Concrete Research 25 (2), 215–222. Targan, S., Olgun, A., Erdogan, Y., Sevinc, V., 2002. Effects of supplementary cementing materials on the properties of cement and concrete. Cement and Concrete Research 32 (10), 1551–1558. Wihler, H.D., 2000. Puzzolanita¨t von Steinkohlenflugasche (Pozzolanic activity of fly ash). Beton-Informationen 2/3, 36–43 (in German).
Utilization of municipal solid waste bottom ash and recycled aggregate in concrete B. Juricˇ
a,b
, L. Hanzˇicˇ
a,*
, R. Ilic´
a,c
, N. Samec
d
a
d
Faculty of Civil Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia b Projekt d.d. Nova Gorica, Kidricˇeva 9a, 5000 Nova Gorica, Slovenia c J. Stefan Institute, Jamova 39, 1000 Ljubljana, Slovenia Faculty of Mechanical Engineering, University of Maribor, Smetanova 17, 2000 Maribor, Slovenia Accepted 28 October 2005 Available online 31 January 2006
Abstract In the combustion process of municipal solid waste (MSW), bottom ash (BA) represents the major portion of the solid residue. Since BA is composed of oxides, especially SiO2 and CaO, the feasibility of its application in concrete as a substitute for cement was tested. It was found that at the age of 28 days, the flexural and compressive strengths of the binder linearly decrease at the rate of 0.03 and 0.02 MPa per wt% of BA in the binder, respectively. According to the results it may be recommended to replace up to 15 wt% of cement by BA and to use such binder where a low strength of concrete elements is required. Furthermore, the aggregate used for low strength concrete need not be of a very good quality. Therefore, gravel aggregate was partially replaced by recycled aggregate (RA). Consistency measured by slump was significantly reduced (>50%) when BA or/and RA were introduced into the mixture. However, concrete density and compressive strength were not affected and were 2300 kg/m3 and 40 MPa, respectively. 2005 Elsevier Ltd. All rights reserved.
1. Introduction In the last few decades of the 20th century it became obvious that the large quantities of waste produced by the modern consumer society cause serious environmental damage when they are disposed of without any treatment. While a small portion of waste (less than 1% (EU, 2004)) is hazardous and requires expensive treatment, some waste, like construction and demolition (C&D) debris, can be considered as inert. Municipal waste, although classified as non-hazardous, still causes air, water and soil pollution during decay. Hence, modern landfills for municipal solid waste (MSW) have a complex design and should be able to carry out several processes such as leachate and gas management and monitoring. This makes the disposal space for a volume unit of waste rather expensive. In order
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Corresponding author. Tel.: +386 2 22 94 331; fax: +386 2 25 24 179. E-mail address: [email protected] (L. Hanzˇicˇ).
0956-053X/$ - see front matter 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2005.10.016
to minimise disposal space, it has become common practice to sort, reuse and incinerate waste materials. In Slovenia, approximately 600–1000 kg of C&D debris is produced per person per year (RS, 2001), which is comparable to other European countries (EU, 1999). Additionally, Slovenian households create about 535 kg of MSW per person each year (RS, 2002a). Landfills in Slovenia are mostly full and because of strong public resistance and the NIMBY (Not In My BackYard) syndrome, it is almost impossible to find new locations. In order to meet modern standards for waste management, Slovenian society will have to consider the construction of incineration plants. Within the framework of this issue, an experimental two-chamber plant was recently built in Vransko, Slovenia (Samec et al., 2001). Although two-chamber plants have been usually employed for the incineration of hazardous waste, lately they have gained significance for combustion of MSW as well. Two-stage combustion results in lower entrainment of particulate matter in the flue gases, which significantly
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
reduces the investment and operational costs for flue gas treatment devices. In the primary chamber of the KIVVransko incinerator (Fig. 1), the temperature ranges between 600 and 800 C, and about 70% of the required air is added. These conditions ensure warming, drying and semi-pyrolitic gasification of waste, where most of the metals cannot evaporate and remain in the bottom ash (BA). Flue gases are led to the second chamber, called the thermoreactor. Here, gases are mixed with 150–200% of the stoichiometrically required air and the temperature is raised to 1250 C. This results in the total destruction of all organic compounds. The capacity of the plant is 25 kg of waste per hour. During the combustion process approximately 10 wt% of solid residue is formed from the input mass. It is convenient for this type of plant if the calorific value of the waste is at least 16 MJ/kg. The KIV-Vransko incinerator is described in detail elsewhere (Samec et al., 2001). By incineration, the volume of waste is reduced by 70– 90%. The residual BA has a low specific weight (300 kg/m3) and therefore requires some special measures when deposited in landfills, for instance stabilization with cement. On the other hand, further applications can be considered and BA can be used as a substitute for raw materials for the manufacture of other products. Namely, its chemical composition strongly resembles the composition of cement. The use of different kinds of ash, such as coal and MSW fly ash and BA, for concrete production was recently reported (Pera et al., 1997; Kula et al., 2001; Targan et al., 2002; Re´mond et al., 2002; Filipponi et al., 2003; Cheerarot and Jaturapitakkul, 2004). Most of the ashes used in these studies contain 40–60 wt% of SiO2. For cement this value typically amounts to only 17– 25 wt%. All of the reviewed ashes contain essentially much less CaO (2–20 wt%) than cement (60–67 wt%). Since BA
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from KIV-Vransko contains 24 wt% of SiO2 and 39 wt% of CaO, it can be considered as a very appropriate substitute for cement. Cement, which is a hydraulic binder, is the fundamental ingredient of concrete. In general, it represents about 15% of concrete volume and about 45% of its total cost. Concrete is one of the prime materials in the modern construction industry; about 4.5 billion metric tons of concrete are cast each year worldwide (Su and Miao, 2003). In Slovenia, this value amounts to 1.5 · 106 tons (6 · 105 m3) (RS, 2002b), of which approximately 90% represents concrete with a characteristic compressive strength of 40 MPa or less. The aim of the present research was to investigate some vital properties of concrete made of waste materials – BA from the experimental MSW incinerator and C&D debris. By incorporating waste materials into permanent compounds, their interaction with the environment can be diminished, and simultaneously the required disposal space is reduced. 2. Experimental The study was designed in two steps. First, the binding abilities of BA were investigated and an acceptable level of BA in total binder was determined. In the second step, concretes with the formerly determined level of BA and different portions of recycled aggregate (RA) were studied for their consistency, density and compressive strength. The binding abilities of BA were tested according to the standard EN 196-1 (1994). Specimens were prepared with 450 g of binder and 225 g of water, giving a water/binder (W/B) ratio of 0.5. The aggregate (1350 g) was quartz sand with grain sizes from 0 to 2 mm, which fulfils EN 196-1 (1994) standard requirements. For the preparation of a reference mixture, pure cement, designated as CEM I 42.5 R,
Fig. 1. KIV-Vransko experimental two-chamber incineration plant, Slovenia (Samec et al., 2001).
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
was used as binder. According to standard EN 197-1 (2000), designation CEM I 42.5 R stands for Portland cement containing at least 95 wt% of clinker minerals, having a compressive strength of at least 42.5 MPa at the age of 28 days but not exceeding 62.5 MPa. R denotes a rapid development of strength in the early stage of hydration. Subsequent specimens were made with a binder, which consisted of cement and BA. The percentage of BA in the total binder varied from 5 to 40 wt%. BA, which was obtained from the KIV-Vransko incineration plant, was in the form of flakes with sizes from 1 to 20 mm. In order to obtain a suitable granular composition it was ground with a ball mill and sieved through a 90 lm screen. The bulk density of such BA is 510 kg/m3, whereas its apparent specific gravity is 3100 kg/m3. It should be noted that the size and shape of the BA particles could depend on the incineration process. Pera et al. (1997) reported the use of MSW BA as an aggregate for concrete where the particles of BA were rather globular in shape with sizes from 4 to 20 mm. The composition of BA and CEM I 42.5 R was determined by X-ray diffraction (Table 1). Besides this, k0-instrumental neutron activation analysis (k0-INAA) (Jac´imovic´ et al., 2002) was applied to measure some major and trace elements in BA. Nine specimens with dimensions of 40 mm · 40 mm · 160 mm were cast from each mixture. They were kept in a mould for 24 h; then they were stored in a 100% moist environment at 22 ± 2 C. Specimens were tested for flexural and compressive strength at the age of 2, 7 and 28 days. In the second step, two series of concrete specimens were made. For the first series, pure cement was used as binder, whereas in the second series binder with 15 wt% of BA was used. The binder content was 430 kg/m3 and the W/B ratio was 0.55. For the reference concrete mixture, pure gravel aggregate with a maximum grain size of 8 mm was used. Suitable overall grading of the aggregate (optimal grading curve in Fig. 2) was determined according to the Fuller and EMPA equation (Muravljov, 2000). The bulk density of gravel with such a granular composition was 1906 kg/m3. The reference concrete mixture was designed to reach a compressive strength of 40 MPa. Aggregate for the subsequent specimens consisted of gravel and 9.1, 16.7 and 23.1 wt% of RA. Table 1 Composition of cement CEM I 42.5 R and bottom ash (BA) from the KIV-Vransko incineration plant determined by X-ray diffraction Oxide
SiO2 Al2O3 Fe2O3 CaO MgO Na2O K2O SO3
Content (wt%)
100
Percent passing by weight
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BA
22.3 5.83 2.17 60.81 2.82 0.34 0.72 2.75
24 14.8 2.7 39 1.7 0.9 0.2 –
optimal grading curve
80
recycled aggregate
70 60 50 40 30 20 10 0 0
0.063
0.125
0.25
0.5
1
2
4
8
Sieve size (mm) Fig. 2. Optimal grading curve of the aggregate determined according to the Fuller and EMPA equation and the grading curve of recycled aggregate.
RA was obtained from a mobile C&D debris recycling plant, which was stationed in the town of Koper at the time of the experiment. The applied recycling process consists of elimination of major metallic pieces and organic compounds and of grinding and sieving. The capacity of the recycling plant is 160 m3/h. Divisions of RA are stored in an open depository, where the material for the experiment was acquired. Since the composition of C&D debris depends on the type of the object being demolished, significant variations in RA composition and quality are possible. Composition of RA used in this study was analysed on six samples by sorting particles of washed RA. Mass proportions are presented in Fig. 3. For the purpose of this experiment, an RA division of 0–8 mm was selected. It was established by wet sieving that RA contains a large portion of small particles, as shown in Fig. 2, which would have a negative impact on the consistency of concrete. It was also established (Chen et al., 2003) that the use of unwashed RA resulted in a lower compressive strength of concrete. Therefore, RA was washed on a sieve with a 68 lm aperture. The retained aggregate was dried, divided into fractions and kept at room conditions where humidity was a 55 ± 10% and temperature 24 ± 2 C. Each aggregate mixture was afterwards composited according to the optimal grading curve where each fraction was combined from gravel and the required wt% of RA. The bulk density of pure washed RA with optimal grading composition was 1594 kg/m3. Water absorption capability of gravel and RA was tested
4%
CEM I 42.5 R
90
6%
1%
mineral debris bitumen debris brick wood and undefinable
89%
Fig. 3. Material composition of recycled aggregate.
B. Juricˇ et al. / Waste Management 26 (2006) 1436–1442
3. Results and discussion
a Flexural strength (MPa)
9 8 7 6 5 4 3
after 2 days after 7 days after 28 days
2 1 0 0
5
10
15
20
25
30
35
40
Bottom ash (wt %)
b 60 Compressive strength (MPa)
according to EN 1097-6 (2000). Additionally, concrete with 15 wt% of BA in the binder and 100 wt% of RA was made. Fresh concrete was tested for its workability. The quality of casting and compacting depend on workability, and thereby it has a significant impact on the strength and durability of the concrete. For workability, which is best defined as the amount of useful internal work necessary to produce full compaction (Glanvile et al., 1947), there is no widespread standard test available. Thus, the slump of a concrete cone is often measured and the term consistency is used to describe this property. After executing the standard slump test for the purpose of this experiment, six cubical specimens with edge length of 100 mm were cast from each mixture. They were compacted by external vibrator for the uniform period of time. Specimens were taken out of the mould after 24 h and kept in a 100% moist environment at 22 ± 2 C. Compressive strength was determined after 7 and 28 days. Since concrete is often exposed to atmospheric precipitation, ground and underground water, leaching of heavy metals concentrated in BA could make such a concrete environmentally unacceptable. It was determined by Cai et al. (2004) that by introducing BA in concrete instead of gravel, quantities of potentially available contaminants are increased from 5 to 600 times. Therefore, concrete was tested for leaching of some selected metals. Concrete was crushed in a ball-mill to obtain particles smaller then 10 mm, and specimens weighing 100 g were tested according to DIN 38414-4 (1984). Leaching was carried out for 24 h in demineralised water, where the mass ratio of the specimen and water was 1:10. Leachate was analysed for chromium (EN 1233, 1996), arsenic (EN 26595, 1992), cadmium (ISO 5961, 1994), and for cobalt, nickel, copper, zinc and lead (ISO 8288, 1986).
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50 40 30 20
after 2 days after 7 days after 28 days
10 0 0
5
10
15
20
25
30
35
40
Bottom ash (wt%)
Fig. 4. Strength regarding wt% of bottom ash in the binder: (a) flexural strength (experimental error was estimated to be 8%) and (b) compressive strength (experimental error was estimated to be 5%). Table 2 Values of parameters defining the dependence of flexural and compressive strength on wt% of bottom ash (BA) in the binder Time (days)
Flexural
Compressive
ff0 (MPa)
kf (MPa/%BA)
fc0 (MPa)
kc (MPa/%BA)
2 7 28
4.0 6.6 8.1
0.05 0.05 0.03
28.5 39.0 52.1
0.35 0.27 0.23
ff0 and fc0 denote flexural and compressive strength at wt% BA = 0. Slopes of the curves (Eqs. (1) and (2)) are denoted by kf and kc.
3.1. Binding abilities of bottom ash The dependence of flexural and compressive strength on the content of BA is given in Fig. 4. A linear decrease of both flexural ff (Fig. 4a) and compressive fc (Fig. 4b) strength was observed. Hence, the following relationships were applied: ff ¼ ff0 k f C;
ð1Þ
fc ¼ fc0 k c C;
ð2Þ
where C is the wt% of BA in the binder, ff0 and fc0 denote the strength of the reference mix (without BA), whereas the slope of the straight line denoted by kf and kc represent the reduction rates of strength. Values of all parameters at the age of 2, 7 and 28 days are summarized in Table 2. Since the hydration of cementitious materials is a time dependent process, which can last up to several years, it was expected that the values of ff and fc would increase with elapsed time. However, it is interesting to observe that the values of kf and kc slightly decrease with time. This
might indicate that BA develops strength in the later stage of hydration compared to CEM I 42.5 R. A possible reason for the observed behaviour is that the temperature in the primary chamber is too low for formation of alite (3CaO Æ SiO2), which is an essential compound for the strength of concrete in the early stage of the hydration process. Furthermore, the quantity of CaO is insufficient for formation of all cementitious compounds. A study on fly ash carried out by Wihler (2000) has shown a similar effect. Namely, at an age of 28 days compressive strength of concrete in which cement was partially replaced by fly ash was lower than the strength of the reference mix, whereas at the age of 90 days the situation was reversed. Standard EN 197-1 (2000) sets two acceptance conditions for the compressive strength of cement CEM I 42.5 R: after two days it should be at least 20 MPa, at the age of 28 days it should reach at least 42.5 MPa but should not exceed 62.5 MPa.
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Since by increasing the wt% of BA in the binder, early strengths are more affected, the first condition is more essential. Hence, the maximum content of BA in total binder (calculated by Eq. (2)), which also satisfies the second condition, is about 25 wt%. Applying a safety factor of 0.7, the acceptable level of BA in binder was established to be 15 wt% and it was further applied for the study of concrete.
Table 5 Densities of fresh concrete mixtures
3.2. Workability of concrete
The abbreviations RA and BA denote recycled aggregate and bottom ash.
The consistency obtained by the slump test is presented in Table 3. From these results, two observations can be made. First, slump is significantly (>50%) reduced when binder contains 15 wt% of BA and second, reduction of slump also occurs when RA is introduced into the aggregate mixture. The latter is probably due to aggregate shape and texture, with gravel particles being round and smooth, while RA particles are angular and rough with high porosity and thus with higher water absorption as may be perceived in Table 4. In general, lower slump should result in poorer workability, so one would expect lower density for such concrete mixtures. The densities of concrete mixtures given in Table 5 display an interesting fact. When the binder contains cement as well as BA, the density of concrete is not altered although slump is reduced. To evaluate workability more precisely, yield strength (sy) and plastic viscosity (l) should be determined. Namely, the rheological properties of fresh concrete are approximated by the Bingham model (Illston and Domone, 2001): s ¼ sy þ lc;
ð3Þ
where s is shear stress and c is shear strain. Since the results of the slump test offer us only information about sy, it is possible that the presence of BA reduces cohesion (lowers l). Thus, fresh concrete would have an increased response to compaction by vibration and hence higher densities could be achieved. It should be noted that segregation
Table 3 Consistency of concrete mixtures measured by standard slump test RA (wt%)
0 9.1 16.7 23.1
Slump (mm) BA = 0 wt%
BA = 15 wt%
105 60 35 15
55 20 10 0
Table 4 Water absorption of gravel and recycled aggregate (RA) tested according to EN 1097-6 (2000)
0/4 4/8
Density (kg/m3) BA = 0 wt%
BA = 15 wt%
0 9.1 16.7 23.1
2310.7 2313.3 2328.1 2310.0
2324.8 2322.3 2319.6 2302.2
did not occur in any of the types of concrete examined in the present study. 3.3. Compressive strength of concrete The compressive strengths of the concrete mixtures are summarized in Table 6. Although it was established that when BA represents 15 wt% of the binder, compressive strength is reduced by approximately 10%, this was not generally observed on concrete specimens. The compressive strength of mixtures with aggregate containing 0 and 9 wt% of RA even increased when BA was used – surprisingly even at the age of 7 days. Maximum reduction (10%) of strength as the consequence of BA usage was observed in the mixture with 23 wt% of RA at the age of 28 days, but this mixture still reached a compressive strength of 41.6 MPa. When compressive strength is considered as a function of weight percentage of RA in the aggregate mixture, it can be established that up to a certain amount (17 wt%) of RA the compressive strength increases but later (between 17 and 23 wt%) it starts to decrease. As shown in Fig. 3, RA consists mainly (89 wt%) of mineral debris. Into this group particles of crushed concrete, mortar, plaster and stone were classified. Thus, a substantial amount of cement is present in RA. During grinding, which is a part of RA production process, cement particles may crack and the hitherto unhydrated core would be subjected to hydration when exposed to water. Hydration of cement particles in RA may have a favorable effect on concrete strength. 3.4. Environmental impact Cement gel has a characteristic porosity of 28 vol% (Neville, 2002) and the pore structure of concrete, which
The abbreviations RA and BA denote recycled aggregate and bottom ash.
Division
RA (wt%)
Table 6 Compressive strength of concrete mixtures (with different wt% of recycled aggregate RA and bottom ash BA) obtained at two different ages t RA (wt%)
t = 7 days
Water absorption (wt%) Gravel
RA
0.8 1.0
5.1 6.8
Compressive strength · (1 ± 0.05) MPa
0 9.1 16.7 23.1
t = 28 days
BA = 0 wt%
BA = 15 wt%
BA = 0 wt%
BA = 15 wt%
31.6 32.2 37.6 37.1
34.7 35.2 35.8 34.8
40.6 40.8 46.9 46.4
43.4 42.2 44.0 41.6
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includes porosity, permeability and tortuosity, has a strong influence on the durability and applicability of concrete. Percolation of water through the material causes leaching of heavy metals, which are afterwards released into the environment. Concentrations of several elements in BA, obtained by the k0-INAA method, are summarized in Table 7. Beside Table 7 Results of k0-INAA analysis of bottom ash (BA) and concentration limits set for hazardous elements in waste classified as inert according to regulations valid at the time of the research (RS, 2000) Element
Concentration in BA (mg/kg)
Error/LD (mg/kg)
Concentration limits (mg/kg)
Ag Al As Au Ba Br Ca Cd Ce Cl Co Cr Cs Cu Dy Eu Fe Ga Hf Hg K La Mo Mg Mn Na Nd Ni Pb Pt Rb Sb Sc Se Si Sm Sn Sr Ta Tb Te Th Ti Tm U V W Yb Zn Zr
1.01 82,760 5.38 0.292 1294 11.7 226,940 11.0 41.2 3694 16.2 281 3.7 – 2.49 0.69 16,900 38.5 4.54 0.11 7926 22.4 10.0 21,481 574 7412 16.4 – – – 46.3 77.8 4.23 1.33 – 2.77 – 394 1.03 0.34 – 7.9 7261 – 2.99 36.0 6.69 0.87 967 178
0.15 3500 0.30 0.010 47 0.4 8770 2.1 1.5 151 0.6 12 0.1 54 0.16 0.08 593 5.4 0.16 0.02 807 0.8 1.0 1981 20 260 1.4 – – 2 1.8 2.8 0.17 0.11 250,000 0.15 36 18 0.04 0.02 2 0.3 508 0.2 0.23 4.7 0.41 0.04 34 17
– – 200 – – – – 12 – – 250 500 – 500 – – – – – 10 – – 500 – – – – 500 500 – – – – – – – – – – – – – – – – – – – 1500 –
The abbreviation LD denotes the limit of detection.
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Table 8 Results of leaching tests for concrete made of 100 wt% RA and 15 wt% BA in the binder in comparison with limits set by Slovenian regulations at the time of the research (RS, 2000) and recently accepted regulations (RS, 2004) Element
Leaching from concrete (mg/kg)
Limits for inert waste valid up to 2004 (mg/kg)
Limits for inert waste valid from 2004 (mg/kg)
As Ba Cd Cr Co Cu Hg Mo Ni Pb Sb Se Zn
<0.01 – <0.03 <1.50 <0.61 0.13 – – 0.83 <1.04 – – <0.17
1 20 0.5 10 5 10 0.05 – 5 5 1 0.5 30
0.5 10 0.04 0.5 – 2 0.01 0.5 0.4 0.5 0.06 0.1 4
other requirements, stated in Slovenian regulations valid at the time of the research (RS, 2000), maximum values of concentrations of hazardous elements in the waste were prescribed if the waste was to be classified as inert. Results show that only the cadmium concentration (11 ± 2 mg/kg) is close to the limit (12 mg/kg). The results of leaching tests for concrete are summarized in Table 8, and they are compared to concentration limits for leachate from inert waste valid at the time of the experiment (RS, 2000), as well as to the presently valid regulations (RS, 2004). For the majority of the elements measured, leaching is below the limits for inert waste. Before utilization of BA in concrete, a more detailed study would be required. Supposing that all MSW in Slovenia were incinerated with the same technology as used at KIV-Vransko, 1.1 · 105 tons of BA would be produced per year. For production of concrete with a characteristic compressive strength of 40 MPa or less and by supplementing 15 wt% of cement with BA, about 3.5 · 104 tons of BA could be used. This would reduce the landfill space required by about 7 · 104 m3. 4. Conclusions Conclusions of the presented study may be summarized as follows: 1. Satisfactory quality of concrete with low strength requirements (compressive strength up to 40 MPa) may be achieved using two waste materials, namely C&D debris and MSW BA, simultaneously. 2. The experiment showed a linear decrease of flexural and compressive strength at the rate of 0.03 and 0.02 MPa per wt% of BA in the binder, respectively. According to the results it may be recommended to replace up to 15 wt% of cement by BA.
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3. Reduction of slump was observed when BA and RA were introduced into the concrete mixture, but workability did not seem to be affected. The study indicated the possibility that BA reduces cohesion in concrete. More detailed investigation of rheological properties using the two-point workability test would be important. The study has indicated the feasibility of utilization of BA and RA in concrete; hence further research may be recommended. Subsequent studies should determine variations in composition of both materials and examine the influence of these variations on concrete properties. Since it is not possible to determine fundamental aspects of hydraulic reactivity of BA by undertaking strength tests alone, additional study regarding hydraulic/puzzolanic properties of BA is advised. Additionally, long-term tests should be carried out to establish the durability of such concrete and to specify fields of its application. In this aspect it is encouraging that a favorable effect of fly ash on the durability of concrete has been observed (Schießl et al., 2001). Nevertheless, more detailed study on leaching of hazardous elements would also be important. Acknowledgements We express our thanks to R. Jac´imovic´ for neutron activation analysis (k0-INAA) of bottom ash and to A. Hansel for technical assistance. References Cai, Z., Bager, D.H., Christensen, T.H., 2004. Leaching from solid waste incineration ashes used in cement-treated base layers for pavements. Waste Management 24, 603–612. Cheerarot, R., Jaturapitakkul, C., 2004. A study of disposed fly ash from landfill to replace Portland cement. Waste Management 24, 701–709. Chen, H.J., Yen, T., Chen, K.H., 2003. Use of building rubbles as recycled aggregates. Cement and Concrete Research 33 (1), 125–132. DIN 38414-4, 1984. Deutsche Einheitsverfahren zur Wasser-, Abwasserund Schlammuntersuchung; Schlamm und Sedimente (Gruppe S); Bestimmung der Eluierbarkeit mit Wasser (S 4) (German standard methods for the examination of water, waste water and sludge; sludge and sediments (group S); determination of leachability by water (S 4)) (in German). EN 196-1, 1994. Methods of testing cement – Part 1: Determination of strength. EN 197-1, 2000. Cement – Part 1: Composition, specifications and conformity for common cements. EN 1097-6, 2000. Tests for mechanical and physical properties of aggregates – Part 6: Determination of particle density and water absorption. EN 1233, 1996. Water quality: Determination of chromium – Atomic absorption spectrometric methods. EN 26595, 1992. Water quality: Determination of total arsenic – Silver diethyldithiocarbamate spectrophotometric method. EU, 1999. European Commission web pages. Construction and demolition waste management practices, and their economics impacts. Available from:
EU, 2004. European Commission web pages. Environment: Waste. Available from: