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Usage of coal combustion bottom ash in concrete mixture
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Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 1922–1928
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Usage of coal combustion bottom ash in concrete mixture Haldun Kurama *, Mine Kaya Eskisehir Osmangazi University, Mining Engineering Department, Batı Meselik, 26480 Eskisehir, Turkey Received 20 February 2007; received in revised form 14 July 2007; accepted 16 July 2007 Available online 27 August 2007
Abstract The present study aims to determine and evaluate the applicability of an industrial bottom ash (CBA), supplied from Tunc¸bilek Power Station-Turkey, in concrete industry. In the laboratory experiments, the bottom ash was used up to 25% as a partial substitute for the Portland cement. In order to be able to reduce the unburned carbon content, CBA was treated by three different processes (particle size classification, heavy medium separation and electrostatic separation). Based on the obtained results, it was concluded that the addition of CBA up to 10% as a replacement material for Portland cement could improve the mechanical properties of concrete, and thus, could be used in the concrete industry. The effect of operating parameters on treatment processes has also been discussed in the paper. 2007 Elsevier Ltd. All rights reserved. Keywords: Coal bottom ash; Portland cement; Unburned carbon; Treatment; Concrete
1. Introduction The electricity industry, particularly coal-fired power plants, has been greatly affected by the increasing public attention being paid to the environment. Coal ash generated from power plants have become an important economic and environmental objective, and thus calls for recycling alternatives to traditional landfill option due to their high generation amount and low recycling rate. Total energy consumption in US, China, India and EU has been rising since the mid-1990s and this trend is expected to continue. European environment agency (EEA) has recently reported that fossils are presently dominating the fuel sources with an 80% share. This proportion is expected to increase slightly over the next 30 years in the EU. Despite some growth in absolute terms, renewable energy is not expected to raise its share to a significant extent, while contribution of nuclear power is projected to decline [1]. Although large amount of fly ash has already been utilized in the construction industry as a partial cement *
Corresponding author. Tel.: +90 222 2393756x3435; fax: +90 222 2393613. E-mail address: [email protected] (H. Kurama). 0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.07.008
replacement and/or mineral additive in cement production, the usage of coal bottom ash (CBA) is limited due to its relatively higher unburned carbon content and different structural properties compared to fly ash [2]. CBA, a coarse sand to fine gravel size material collected at the bottom of the boiler, is generally used as a low cost replacement material either as a base material in road construction or as a blasting grit. According to the American Coal Ash Association (ACAA), recycling rate of fly ash in concrete and concrete products has been reported to be 47%, while for CBA this rate has been reported as only 5.28% of the total recycling amount- the total CBA production being about 19.8 M tonnes for 2002. 7.6 M tonnes of this production were recycled mainly in structural fills/embankments (26.61%), road base/subbase/pavement (19.15%) and mining applications (10.43%). The same utilization profile can also be given for the EU. Nearly 89% of the produced CBA was recycled, but 54% of this amount was evaluated for reclamation and restoration [3]. However, the fused and glassy texture of coal ash makes it an ideal substitute for natural raw materials. Using siliceous ash in concrete and cement applications requires that the loss-on-ignition (LOI) content of the ash generally be less than 6% by weight in order to obtain the required pozzolanic proper-
H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928
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ties. This requirement results from the fact that properties of concrete incorporating high-carbon ash are inferior to those of concrete incorporating low-carbon ash. The amount of water and quantity of air entraining agents used in the mix increases significantly as the carbon content of the ash increases above 6% LOI level [4]. Consequently, it is critical that removal/recovery technologies should be developed so that high carbon CBA can be used more extensively for industrial applications. A number of methods have been developed and commercialized for removing the unburned carbon particles from coal ash, including combustion of the carbon at low temperatures [5–10], mechanical/particle size classification [11], gravity separation [12–14], and electrostatic separation [15–19], froth flotation [20,21] and, combinations thereof. Although, each of these methods may be used to remove carbon particles from siliceous coal ash. However, the efficiency of the employed method and pozzolanic properties of the resulting low-carbon siliceous fraction are highly dependent on the physical characteristics of the original ash. In Turkey, nearly 28% of the total primary energy is supplied from the burning of bituminous and lignite coal. The ashes produced annually from 11 power plants vary from 6.5 to 13 M tonnes. However, only 1% of these waste materials are re-used, mainly in concrete industry. The purpose of the present study is to test common methods (such as sink and float test, particle size classification, and electrostatic separation) as an effective and economical method for removing of unburned carbon from Tunc¸bilek Power Station bottom ash in order to enhance its application as a constituent in concrete production. The paper also examines the effects of pre-treated CBA additions on the final concrete properties as a replacement for Portland cement in cement mixture.
loss-on-ignition analysis (LOI) and the determined LOI is accepted as the mass of unburned carbon in the original sample, which is a common approach in cement and concrete applications. Particle size distribution of the representative sample and unburned carbon content of each size fraction are given in Table 2. According to wet screen analysis, the D80 and D50 ash size values were calculated as 2.7 and 1 mm, respectively. As seen from Table 2, the unburned carbon in CBA has mainly accumulated at coarse fractions. However, a little amount of carbon-rich fraction is also present at 0.053 mm. The specific gravity of the sample measured by the pycnometer method was 2.39 g/cm3. The crystalline mineral phases in the CBA were identified by using X-Ray Diffraction (XRD), model S5000 diffractometer, with a nickel filtered Cu Ka. Regarding the XRD analysis, it was found that CBA had a relatively simple mineralogy consisting of alumina, glass and varying amount of crystalline phases of quartz, ferrite spinel and calcite (Fig. 1). The scanning electron micrograph of ash shows spherical, rounded and irregularly shaped grains (Fig. 2).
2. Materials and methods
Particle size (mm)
(%)
R (%)
C (%)
+2.36 2.360 + 2.000 2.000 + 0.600 0.600 + 0.212 0.212 + 0.149 0.149 + 0.075 0.075 + 0.053 0.053
31.49 1.96 26.98 31.14 3.59 2.62 0.48 1.74
31.49 33.45 60.43 91.57 95.16 97.78 98.26 100.00
8.97 21.68 12.96 5.22 3.01 3.79 6.99 13.13
2.1. Characterization of material Lignite bottom ash used in this study was obtained from Tunc¸bilek Power Station, located in central Turkey. The station consumes about 2,350,000 tons of low-grade lignite coal (calorific value varying from 2250 to 3900 kcal/kg) and generates 854,670 ton ash per year. The chemical composition of CBA is given in Table 1. As can be followed from Table 1, the sum of SiO2 + Al2O3 and Fe2O3 reach 81.06% in the composition, indicating that it could be classified as a type F ash as prescribed by ASTM C 618. In this study, the total carbon content of generated ash was determined by employing the standard
2.2. Methods 2.2.1. Pre-treatment 2.2.1.1. Particle size separation. Mechanical means of removing carbon from siliceous ash is based on the relative particle size of the carbon particles and the siliceous particles in the ash. In this study, the representative 500 g of Table 2 Particle size distributions of CBA and unburned carbon (C) values of each sieve fractions
Table 1 Main oxide composition of received CBA Oxides (wt%) SiO2
Fe2O3
CaO
Al2O3
LOI
54.5
11.16
4.69
15.4
8.90
Fig. 1. X-ray diffractogram of CBA.
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Fig. 2. Morphology of the bottom ash.
CBA was subjected to laboratory impact crusher with and without using 2 mm separating sieve. The crushed product was then classified to +2.36, 2.36–2.00, 2.00–0.6, 0.6– 0.212, 0.212–0.149, 0.149–0.075, 0.075–0.053 and 0.053 mm size fractions by wet screening. The sink part of the screened fractions was separated by decantation in order to remove floated particles, mainly carbon particles, before filtration (1st stage). Each collected fraction was dried in oven at 100 C and weighed. In order to find out the crushing effect on the unburned carbon content of each size fraction, the obtained product from 1st stage crushing was then re-crushed (2nd stage). 2.2.1.2. Sink and float tests. The sink and float tests were performed on crushed samples at various densities to assess the suitability of heavy medium separation. These experiments were conducted in 250 mL glass flax with a volume of 100 mL, using an appropriate mixture of bromoform and alcohol to adjust the density of liquid between 1.0– 2.4 g/cm3. A 50 g of representative ash sample was introduced in the liquid of highest density. The floating product was removed, washed and then placed into the next lower density and so on. 2.2.1.3. Electrostatic separation tests. Electrostatic separation encompasses a number of different technologies which are based on the electrical properties of the particles to be separated. The electrostatic separation tests were carried out by using a conductor/non-conductor type of separator (Boxmag Rapid Ltd-HT150).
Table 3 Mixture proportion of bottom ash cement paste Sample
Std
BC5
BC10
BC15
BC25
Cement (g) Sand (g) CBA (g) Water (g)
450 1350 – 225
428 1350 22 225
405 1350 45 225
383 1350 67 225
338 1350 112 225
effect of ash addition in cement bodies. The amounts mentioned above were chosen so as to highlight the effects of CBA addition. These compositions were designated as Std, BC5, BC10, BC15 and BC25, respectively. The mixture proportions of the ash-cement used in this study are listed in Table 3. The pre-treated materials mixed in the required proportions were ground in a ceramic ball mill to a fineness of the 25 mass% residues on a 38 lm to improve its pozzolanic properties. The physical tests of the cement mixes were performed according to Turkish standards TS EN 197–1. The cement–water mixtures were stirred at a low speed for 30 s, and then with the addition of sand, the mixture was stirred again for 5 min. Three 40 · 40 · 160 mm prismatic specimens for compression tests were prepared from each mixture. The moulded specimens were cured at 20 C with 95% humidity for 24 h, and then after placed in a tap water and cured for 7, 28 and 56 days. 3. Results and discussion 3.1. Pre-treatment
2.2.2. Moulding of CBA paste specimens Representative cement compositions were prepared by progressive incorporation of pre-treated samples in place of Portland cement (5, 10, 15, and 25 wt%) to observe the
3.1.1. Particle size classification It is well known that in impact crushers, comminution is performed by impact effect rather than compression [22].
H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928
Impact causes immediate fracture with no residual stresses. This stress-free condition is particularly valuable in the quarrying industry. When the crushing force is applied instantaneously, the impact crushers give trouble-free crushing on ores which tend to be brittle. The impact crushers are widely used in coal preparation, coal being much more friable than the associated stones and rubbish such as wood, steel, etc. The results of crushing and screening tests are summarized in Table 4. It is clear that unburned carbon in CBA is mainly concentrated at coarse fractions such as 2.36 + 2.00 mm and 2.00 + 0.6 mm, valid for both employed crushing types (with and without separating sieve usage). Although there is a rough relationship between particle size and unburned carbon as LOI, in the case of the without separating sieve usage, the calculated cumulative weight percent for fractions of (+2.36, 0.6 + 0.212, 0.212 + 0.149 and 0.149 + 0.075) which have less than 6% LOI is 57.67%. This is more beneficial than fractions which have less than 6% LOI (51.78%) for separating sieve usage. This can be attributed to the further crushing effect of separation sieve on carbon particles. In order to find out the positive effects of other treatment methods, such as gravity and electrostatic separation, on the further decrease of unburned carbon content, the treatment tests were performed for the 2.00 + 0.6 and 0.6 + 0.212 mm size fractions of crushed samples. Considering the relatively higher particle size of 2.36 + 2.00 mm and unsuitability of the electrostatic separation method, different gravity concentration method such as jigging was also tested for this size fraction. However, a sufficient carbon content reduction could not be achieved by employing this method. 3.1.2. Sink and float test The analyzed unburned carbon content and weight distributions of the size fractions of 2.00 + 0.6 mm and 0.6 + 0.212 mm at different density ranges are given in Table 5a and b, respectively. It can be seen from columns 3 and 6 of the Table 5a that, if separation density of 1.94 is chosen, then 26.80% of ash, being lighter than 1.94, could be separated as a float product. According column 6, only 41.99% of the unburned carbon would be separated in this density range. Consequently, 58.01% of the unburned car-
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Table 5 Heavy liquid test results Specific gravity (a) 1.21 1.21–1.63 1.63–1.74 1.74–1.89 1.89–1.94 1.94–2.05 2.05–2.89 +2.89 Specific gravity (b) 1.63 1.63–1.84 1.84–1.94 1.94–2.05 2.05–2.89 +2.89
wt%
R
C (%)
Distribution C (%)
R
3.40 10.40 7.40 2.60 3.00 40.0 30.20 3.00
3.20 13.80 21.20 23.80 26.80 66.80 97.00 100.00
58.76 12.98 12.77 10.16 8.95 8.61 9.67 10.10
17.38 11.75 8.22 2.30 2.34 29.97 25.41 2.63
17.38 29.13 37.35 39.65 41.99 71.96 97.37 100.00
wt%
R
C (%)
Distribution C (%)
Cumulative C (%)
5.52 2.26 7.97 38.38 43.87 2.00
5.52 7.78 15.75 54.13 98.00 100
17.84 4.42 3.93 4.31 4.65 0.42
19.31 1.96 6.15 32.43 39.99 0.16
19.31 21.27 27.41 59.85 99.84 100.00
bon would be obtained as a sink product which accounts for the 73.2% of the total feed weight. Similarly, in Table 5b, 84.43% of the feed weight of 0.6 + 0.212 size fraction still contains a 59.85% unburned carbon for the separating density of 1.94 g/cm3. These results indicate that, in order to recover a sink product with low unburned carbon, it is not possible to obtain a certain separating density for the sink and float test. This inefficiency could have been resulted from similar densities of the unburned carbon and siliceous material, due to the porous structure of the CBA. 3.1.3. Electrostatic separation tests The electrostatic separation test results of 2.00 + 0.6 mm and 0.6 + 0.212 mm size fractions of CBA are given in Table 6a and b, respectively. The tests were carried out under constant feeding rate of 5.25 min/100 g and electrical power of 30 kW. The rotor rate was steadily increased and distribution of unburned carbon was analyzed according to the rotor rate. The results show that electrostatic separation is an ineffective method for both tested samples. This
Table 4 Crashing-classification test results of CBA with and without using separating sieve 1st stagea
1st stage Wt%
C (%)
wt%
C (%)
wt%
C (%)
wt%
C (%)
+2.36 2.36 + 2.00 2.00 + 0.6 0.6 + 0.212 0.212 + 0.149 0.149 + 0.075 0.075 + 0.053 0.053
20.88 3.01 32.20 34.58 4.19 3.43 0.43 1.28
6.23 14.09 13.38 5.71 3.28 4.34 7.40 14.75
19.42 3.75 38.07 28.98 5 4.27 0.59 1.92
5.94 12.37 11.57 5.10 3.53 4.11 8.03 15.42
– 2.0 40.31 38.68 8.92 5.94 1.19 1.96
– 10.36. 10.80 7.24 3.75 5.27 7.88 14.99
– 0.73 38.56 43.70 8.08 5.02 1.70 2.21
– 12.40 11.65 5.99 3.96 6.53 8.51 16.94
a
With separating sieve usage.
2nd stage
2nd stagea
Particle size (mm)
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Table 6 Electrostatic separation tests results of CBA Test number
Rotor rate (rpm)
Concentrate
Tailings
wt%
C (%)
wt%
C (%)
(a) 1 2 3 4 5
50 60 88 100 117
9.9 16.8 20.0 15.8 16.6
9.36 10.40 9.81 11.81 10.66
83.2 90.1 80 64.8 83.4
11.52 10.52 10.69 10.28 10.15
(b) 1 2 3 4
30 40 70 80
9.68 29.74 18.24 15.68
4.61 3.23 5.73 4.66
90.32 70.26 81.76 84.32
4.60 4.89 4.57 4.79
Table 7 Chemical composition of cement and pre-treated bottom ash Oxide composition
CBA (%)
Cement (%)
SiO2 Al2O3 Fe2O3 CaO MgO K2O SO3 Na2O LOI
55.95 16.65 9.69 4.39 5.14 1.44 0.70 0.084 4.65
20.96 5.58 3.69 63.97 1.9 – 2.84 – 1.15
can be explained by the insufficient conductivity of charged coal particles for separation from non- conductive ash particles under conductor/unconductor type separation. As a conclusion, the crushing-screening method was found to be a useful route for lowering the carbon content of the ash before performing concrete tests. By using this method, 57.67% of feed CBA was beneficiated with an unburned carbon content of 4.65%. 3.2. Concrete tests The oxide compositions of the Portland cement and pretreated CBA used in the experiments are given in Table 7. The cement used was CEM I 32.5R commercial Portland cement. Specific gravity of cement was 3.15 g/cm3. Initial and final settling times were 3 and 4 h, respectively. Its Blaine surface area was 3345 cm2/g.
The compressive and flexural strengths of the cement mixtures at various ages are given in Table 8. Table 8 indicates that both compressive and flexural strength of specimens increase with increasing amount of ash replacement up to 10%. When 10% of CBA is replaced by cement, the 56 day compressive strength increases approximately 5%, compared to the standard mixture. The ash addition higher than this amount leads to a decrease in the strength values of all specimens. This decrease is more significant for the lower curing time such as 7 and 28 days. However, for 56 days curing time, all values are higher than that of the standard mixture, except the BC25 due to the relatively low activity of ash at the beginning of the curing periods. These results agree with the previous findings in literature for fly/bottom ash usage in concrete. It is well known that, the calcium–silica–hydrate (C–S– H) is a major phase present both in the hydrated Portland cement and tri calcium silicate (C3S). The factors that influence the mechanical behaviour of C–S–H phases are: size and shape of the particles, distribution of particles, particle concentration, particle orientation, topology of the mixture, composition of the dispersed/continuous phases and the pore structure. The addition of coal ash as a supplementary cementing material causes an increase on both pozzolanic and physical properties that enhance the performance of concrete. When Portland cement hydrates it produces a quantity of alkali calcium hydroxide. Reactive silica in ash reacts with this lime to form stable calcium silicate and aluminate hydrates. These hydrates fill the voids within the concrete, removing some of the lime and thus reducing the permeability. This process improves the strength and durability of the concrete [23]. The pozzolanic reaction occurs relatively slow at normal temperatures, enhancing strength in the longer term compared to normal Portland cement concrete. A research study performed by Papadakis [24], to determine the effect of fly ash on the Portland cement system showed that, the compressive strength of fly ash specimens were similar to standard samples at 3–14 days, but higher from that of the 28 days or more. This delay was explained by the pore solution chemistry and low surface area due to the relatively larger particle size of the used fly ash. Also, Cheriaf et al. [25] reported that, after 28 days of hydration, ash particles reacted with calcium hydroxide. Recently, Hanehara et al. [26] reported that the reaction ratio of fly ash decreased with an increase in fly ash substitution ratio.
Table 8 Strength tests results of specimens Mixture
TS EN 197–1 Std BC5 BC10 BC15 BC25
Compressive strength (N/mm2)
Flexural strength (N/mm2)
7 days
28 days
56 days
7 days
28 days
56 days
Minimum 16 27.8 28.09 28.22 26.47 19.79
Minimum 32.5 40.9 40.38 40.24 33.57 29.13
– 42.65 44.08 45.1 43.45 41.33
Minimum 4.0 6.08 6.27 6.07 5.77 4.72
Minimum 5.5 6.60 6.75 6.62 6.57 6.22
– 6.86 7.35 7.60 7.20 6.69
H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928
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Fig. 3. SEM micrograph of the hydrated bottom ash-calcium hydroxide mixture at 28 and 56 days curing times for: (a) 10% BC, (b) 25% BC, (c) 10% BC and (d) 25% BC addition respectively.
However, apart from the particle size, it is reported in the literature that morphology, chemical content and the carbonaceous solid content of the replacement material should also be considered. All these factors lead to discoloration which is aesthetically unacceptable in certain applications, poor air entrainment behaviour and mixture segregation [4]. In practice, the utilization or substitution rate of ash is changes between 20–30% (for high Si- ash) depending on the type of ash and mixture composition. In the present study, the maximum substitution rate of CBA was determined as 10%. This lower value, when compared to the common practice of fly ash usage, can be attributed to different phase distributions and higher unburned carbon contents of CBA. The microstructural characteristics of the cement mixtures at different curing times are illustrated in Fig. 3. It can be followed from Fig. 3a that, although the pozzolanic reaction starts at 28 days, there is a slight reaction between CBA and calcium hydroxide at early ages. When curing time is increased to 56 days, a well-shaped C–S–H formation is observed. On the other hand, for the samples having a replacement amount of CBA higher than 10%, a low activity of ash could be observed for both 28 and 56 days curing time (Fig. 3b and d). As previously indicated in literature [23], the early products in the hydration of C3S consist of foils and flakes, whereas in Portland cement a gelatinous coating is often observed. The products of C3S which is a few days old consist of C–S–H fibres and partly crumpled sheets, whereas in Portland cement partly crumpled sheets and reticular network are observed. In our
study, C–S–H fibres or elongated particles, observed at relatively later stages of curing times such as 28 day and also 56 days for BC10, could be attributed to the strength development as shown in Table 8. 4. Conclusions Based on the experimental results of this study, the following main conclusions can be drawn; 1. Compared to other pre-treatment methods such as the heavy medium separation and electrostatic separation methods, the crushing-screening method was found to be a more useful route for lowering the carbon content of the considered CBA. By using this method, 57.67% of feed CBA was beneficiated with an unburned carbon content of 4.65%. 2. In concrete tests, although the compressive and flexural strengths of specimens cured at 56 day increase with increasing amount of ash replacement up to 15%, the maximum substitution rate of CBA was determined as 10%. When 10% of CBA is replaced by cement, the compressive strength of CBA-concrete increases from 42.65 N/mm2 to 45.1 N/mm2. This relatively lower substitution ratio compared to the common practice of fly ash usage, can be attributed to the different phase distributions and higher unburned carbon contents of CBA. 3. The observed C–S–H fibres or elongated particles on the SEM micrograph of BC10 clearly indicate the pozzolanic effect of CBA substitution on improving the strength of concrete.
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References [1] EEA signals 2004. European Environment Agency. Copenhagen. www.eea.eu.int. [2] Manz OE. Worldwide production of coal ash and utilization in concrete and other products. Fuel 1997;76(8):691–6. [3] Barnes I, Sear L. Ash Utilisation from coal-based power plants. (Report No. COAL R274 DTI/PubURN-04/1915. www.dti.gov.uk/ energy/coal/cfft/cct/pub/pdfs. [4] Freeman E, Yu-Ming G, Hurt R, Suuberg E. Interactions of carboncontaining fly ash with commercial air-entraining admixtures for concrete. Fuel 1997;76(8):761–5. [5] Cochran JW. Method and product of fly ash beneficiation by carbon burnout in a dry bubbling fluid bed. US patent no. US 5,160,539. [6] Cochran JW, Kirkconnell SF. Method of fly ash beneficiation and apparatus. US patent no. US 5,399,194. [7] John RE. Thermal processing of fly ash. US patent no. US 5,390,611. [8] Martinez MP. Apparatus and process for removing unburned carbon in fly ash. US patent no. US 5,555,821. [9] Bachik A. Apparatus and process for carbon removal from fly ash. US patent no. US 5,749,308. [10] Trerice DN. Method and apparatus for reduction of fly ash carbon by microwave. US patent no. US 4,663,507. [11] Gray ML, Champagne KJ, Finseth DH. Continuous air agglomeration method for high carbon fly ash beneficiation. US patent no. US 6,126,014. [12] Kirchen G, Lehrke J. Method for the utilization of ash from coalfired plants. US patent no. US 5,797,496. [13] Nelson RD, Heavilon JL, Styron RW, Fletcher BG. Method and apparatus for reducing carbon content in particulate mixtures. US patent no. US 5,299,692.
¨ zdemir O, Ersoy B, C [14] O ¸ elik MS. Separation of pozzolonic material from lignite fly ash of Tunbßilek power station. In: Proceedings of the international ash utilization symposium; 2001. www.flyash.info [15] Whitlock DR. Electrostatic separation of unburned carbon. In: Tyson SS, editor. Proceedings of the Symposium Tech Mgr, vol. 2. Ash Use R& D and Clean Coal By-Products, Orlando, FL, p. 70-1–70-12. [16] Heavilon JL, Pike CW, Savage DR, Styron RW. Method and apparatus for carbon content in fly ash. US patent no. US 5,513,755. 1996. [17] Stencel JM, Li TX, Gurupira T, Jones C, Neathery JK, Ban H. Technology development for carbon-ash beneficiation by pneumatic transport, triboelectric processing. In: Proceedings of the international ash utilization symposium; 1999. www.flyash.info [18] Bittner JD, Dunn TM, Hrach JR, Frank J. Method and apparatus for separation of unburned carbon from fly ash. US patent no. US 6,074,458. [19] Hurst JV, Styron RW. Fly ash beneficiation process. US patent no 4.121.945. [20] Aunsholt KEH. Process for separation of coal particles from fly ash by flotation. US patent no. 4.426,282. [21] Hwang JY. Wet process for fly ash beneficiation. US patent no. 5,047,145. [22] Wills BA. Mineral processing technology. fourth ed. Oxford: Pergomon Press; 1988. p. 231–53. [23] Ramachandran VS, Beaudoin JJ. Handbook of analytical techniques in concrete, concrete science. New York: Chem Tec Publishing; 2001. [24] Papadakis VG. Effect of fly ash on Portland cement systems part I. Low-calcium fly ash. Cem Concr Res 1999;29:1727–36. [25] Cheriaf M, Cavalcante JR, Pera J. Pozzolanic properties of pulverized coal combustion bottom ash. Cem Concr Res 1999;29:1387–91. [26] Hanehara S, Tomosawa F, Kobayakawa M, Hwang KR. Effects of water/powder ratio, mixing ratio of fly ash and curing temperature on pozzolanic reaction of fly ash in cement paste. Cem Concr Res 2001;31:19–31.
Construction and Building
MATERIALS
Construction and Building Materials 22 (2008) 1922–1928
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Usage of coal combustion bottom ash in concrete mixture Haldun Kurama *, Mine Kaya Eskisehir Osmangazi University, Mining Engineering Department, Batı Meselik, 26480 Eskisehir, Turkey Received 20 February 2007; received in revised form 14 July 2007; accepted 16 July 2007 Available online 27 August 2007
Abstract The present study aims to determine and evaluate the applicability of an industrial bottom ash (CBA), supplied from Tunc¸bilek Power Station-Turkey, in concrete industry. In the laboratory experiments, the bottom ash was used up to 25% as a partial substitute for the Portland cement. In order to be able to reduce the unburned carbon content, CBA was treated by three different processes (particle size classification, heavy medium separation and electrostatic separation). Based on the obtained results, it was concluded that the addition of CBA up to 10% as a replacement material for Portland cement could improve the mechanical properties of concrete, and thus, could be used in the concrete industry. The effect of operating parameters on treatment processes has also been discussed in the paper. 2007 Elsevier Ltd. All rights reserved. Keywords: Coal bottom ash; Portland cement; Unburned carbon; Treatment; Concrete
1. Introduction The electricity industry, particularly coal-fired power plants, has been greatly affected by the increasing public attention being paid to the environment. Coal ash generated from power plants have become an important economic and environmental objective, and thus calls for recycling alternatives to traditional landfill option due to their high generation amount and low recycling rate. Total energy consumption in US, China, India and EU has been rising since the mid-1990s and this trend is expected to continue. European environment agency (EEA) has recently reported that fossils are presently dominating the fuel sources with an 80% share. This proportion is expected to increase slightly over the next 30 years in the EU. Despite some growth in absolute terms, renewable energy is not expected to raise its share to a significant extent, while contribution of nuclear power is projected to decline [1]. Although large amount of fly ash has already been utilized in the construction industry as a partial cement *
Corresponding author. Tel.: +90 222 2393756x3435; fax: +90 222 2393613. E-mail address: [email protected] (H. Kurama). 0950-0618/$ - see front matter 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2007.07.008
replacement and/or mineral additive in cement production, the usage of coal bottom ash (CBA) is limited due to its relatively higher unburned carbon content and different structural properties compared to fly ash [2]. CBA, a coarse sand to fine gravel size material collected at the bottom of the boiler, is generally used as a low cost replacement material either as a base material in road construction or as a blasting grit. According to the American Coal Ash Association (ACAA), recycling rate of fly ash in concrete and concrete products has been reported to be 47%, while for CBA this rate has been reported as only 5.28% of the total recycling amount- the total CBA production being about 19.8 M tonnes for 2002. 7.6 M tonnes of this production were recycled mainly in structural fills/embankments (26.61%), road base/subbase/pavement (19.15%) and mining applications (10.43%). The same utilization profile can also be given for the EU. Nearly 89% of the produced CBA was recycled, but 54% of this amount was evaluated for reclamation and restoration [3]. However, the fused and glassy texture of coal ash makes it an ideal substitute for natural raw materials. Using siliceous ash in concrete and cement applications requires that the loss-on-ignition (LOI) content of the ash generally be less than 6% by weight in order to obtain the required pozzolanic proper-
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ties. This requirement results from the fact that properties of concrete incorporating high-carbon ash are inferior to those of concrete incorporating low-carbon ash. The amount of water and quantity of air entraining agents used in the mix increases significantly as the carbon content of the ash increases above 6% LOI level [4]. Consequently, it is critical that removal/recovery technologies should be developed so that high carbon CBA can be used more extensively for industrial applications. A number of methods have been developed and commercialized for removing the unburned carbon particles from coal ash, including combustion of the carbon at low temperatures [5–10], mechanical/particle size classification [11], gravity separation [12–14], and electrostatic separation [15–19], froth flotation [20,21] and, combinations thereof. Although, each of these methods may be used to remove carbon particles from siliceous coal ash. However, the efficiency of the employed method and pozzolanic properties of the resulting low-carbon siliceous fraction are highly dependent on the physical characteristics of the original ash. In Turkey, nearly 28% of the total primary energy is supplied from the burning of bituminous and lignite coal. The ashes produced annually from 11 power plants vary from 6.5 to 13 M tonnes. However, only 1% of these waste materials are re-used, mainly in concrete industry. The purpose of the present study is to test common methods (such as sink and float test, particle size classification, and electrostatic separation) as an effective and economical method for removing of unburned carbon from Tunc¸bilek Power Station bottom ash in order to enhance its application as a constituent in concrete production. The paper also examines the effects of pre-treated CBA additions on the final concrete properties as a replacement for Portland cement in cement mixture.
loss-on-ignition analysis (LOI) and the determined LOI is accepted as the mass of unburned carbon in the original sample, which is a common approach in cement and concrete applications. Particle size distribution of the representative sample and unburned carbon content of each size fraction are given in Table 2. According to wet screen analysis, the D80 and D50 ash size values were calculated as 2.7 and 1 mm, respectively. As seen from Table 2, the unburned carbon in CBA has mainly accumulated at coarse fractions. However, a little amount of carbon-rich fraction is also present at 0.053 mm. The specific gravity of the sample measured by the pycnometer method was 2.39 g/cm3. The crystalline mineral phases in the CBA were identified by using X-Ray Diffraction (XRD), model S5000 diffractometer, with a nickel filtered Cu Ka. Regarding the XRD analysis, it was found that CBA had a relatively simple mineralogy consisting of alumina, glass and varying amount of crystalline phases of quartz, ferrite spinel and calcite (Fig. 1). The scanning electron micrograph of ash shows spherical, rounded and irregularly shaped grains (Fig. 2).
2. Materials and methods
Particle size (mm)
(%)
R (%)
C (%)
+2.36 2.360 + 2.000 2.000 + 0.600 0.600 + 0.212 0.212 + 0.149 0.149 + 0.075 0.075 + 0.053 0.053
31.49 1.96 26.98 31.14 3.59 2.62 0.48 1.74
31.49 33.45 60.43 91.57 95.16 97.78 98.26 100.00
8.97 21.68 12.96 5.22 3.01 3.79 6.99 13.13
2.1. Characterization of material Lignite bottom ash used in this study was obtained from Tunc¸bilek Power Station, located in central Turkey. The station consumes about 2,350,000 tons of low-grade lignite coal (calorific value varying from 2250 to 3900 kcal/kg) and generates 854,670 ton ash per year. The chemical composition of CBA is given in Table 1. As can be followed from Table 1, the sum of SiO2 + Al2O3 and Fe2O3 reach 81.06% in the composition, indicating that it could be classified as a type F ash as prescribed by ASTM C 618. In this study, the total carbon content of generated ash was determined by employing the standard
2.2. Methods 2.2.1. Pre-treatment 2.2.1.1. Particle size separation. Mechanical means of removing carbon from siliceous ash is based on the relative particle size of the carbon particles and the siliceous particles in the ash. In this study, the representative 500 g of Table 2 Particle size distributions of CBA and unburned carbon (C) values of each sieve fractions
Table 1 Main oxide composition of received CBA Oxides (wt%) SiO2
Fe2O3
CaO
Al2O3
LOI
54.5
11.16
4.69
15.4
8.90
Fig. 1. X-ray diffractogram of CBA.
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Fig. 2. Morphology of the bottom ash.
CBA was subjected to laboratory impact crusher with and without using 2 mm separating sieve. The crushed product was then classified to +2.36, 2.36–2.00, 2.00–0.6, 0.6– 0.212, 0.212–0.149, 0.149–0.075, 0.075–0.053 and 0.053 mm size fractions by wet screening. The sink part of the screened fractions was separated by decantation in order to remove floated particles, mainly carbon particles, before filtration (1st stage). Each collected fraction was dried in oven at 100 C and weighed. In order to find out the crushing effect on the unburned carbon content of each size fraction, the obtained product from 1st stage crushing was then re-crushed (2nd stage). 2.2.1.2. Sink and float tests. The sink and float tests were performed on crushed samples at various densities to assess the suitability of heavy medium separation. These experiments were conducted in 250 mL glass flax with a volume of 100 mL, using an appropriate mixture of bromoform and alcohol to adjust the density of liquid between 1.0– 2.4 g/cm3. A 50 g of representative ash sample was introduced in the liquid of highest density. The floating product was removed, washed and then placed into the next lower density and so on. 2.2.1.3. Electrostatic separation tests. Electrostatic separation encompasses a number of different technologies which are based on the electrical properties of the particles to be separated. The electrostatic separation tests were carried out by using a conductor/non-conductor type of separator (Boxmag Rapid Ltd-HT150).
Table 3 Mixture proportion of bottom ash cement paste Sample
Std
BC5
BC10
BC15
BC25
Cement (g) Sand (g) CBA (g) Water (g)
450 1350 – 225
428 1350 22 225
405 1350 45 225
383 1350 67 225
338 1350 112 225
effect of ash addition in cement bodies. The amounts mentioned above were chosen so as to highlight the effects of CBA addition. These compositions were designated as Std, BC5, BC10, BC15 and BC25, respectively. The mixture proportions of the ash-cement used in this study are listed in Table 3. The pre-treated materials mixed in the required proportions were ground in a ceramic ball mill to a fineness of the 25 mass% residues on a 38 lm to improve its pozzolanic properties. The physical tests of the cement mixes were performed according to Turkish standards TS EN 197–1. The cement–water mixtures were stirred at a low speed for 30 s, and then with the addition of sand, the mixture was stirred again for 5 min. Three 40 · 40 · 160 mm prismatic specimens for compression tests were prepared from each mixture. The moulded specimens were cured at 20 C with 95% humidity for 24 h, and then after placed in a tap water and cured for 7, 28 and 56 days. 3. Results and discussion 3.1. Pre-treatment
2.2.2. Moulding of CBA paste specimens Representative cement compositions were prepared by progressive incorporation of pre-treated samples in place of Portland cement (5, 10, 15, and 25 wt%) to observe the
3.1.1. Particle size classification It is well known that in impact crushers, comminution is performed by impact effect rather than compression [22].
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Impact causes immediate fracture with no residual stresses. This stress-free condition is particularly valuable in the quarrying industry. When the crushing force is applied instantaneously, the impact crushers give trouble-free crushing on ores which tend to be brittle. The impact crushers are widely used in coal preparation, coal being much more friable than the associated stones and rubbish such as wood, steel, etc. The results of crushing and screening tests are summarized in Table 4. It is clear that unburned carbon in CBA is mainly concentrated at coarse fractions such as 2.36 + 2.00 mm and 2.00 + 0.6 mm, valid for both employed crushing types (with and without separating sieve usage). Although there is a rough relationship between particle size and unburned carbon as LOI, in the case of the without separating sieve usage, the calculated cumulative weight percent for fractions of (+2.36, 0.6 + 0.212, 0.212 + 0.149 and 0.149 + 0.075) which have less than 6% LOI is 57.67%. This is more beneficial than fractions which have less than 6% LOI (51.78%) for separating sieve usage. This can be attributed to the further crushing effect of separation sieve on carbon particles. In order to find out the positive effects of other treatment methods, such as gravity and electrostatic separation, on the further decrease of unburned carbon content, the treatment tests were performed for the 2.00 + 0.6 and 0.6 + 0.212 mm size fractions of crushed samples. Considering the relatively higher particle size of 2.36 + 2.00 mm and unsuitability of the electrostatic separation method, different gravity concentration method such as jigging was also tested for this size fraction. However, a sufficient carbon content reduction could not be achieved by employing this method. 3.1.2. Sink and float test The analyzed unburned carbon content and weight distributions of the size fractions of 2.00 + 0.6 mm and 0.6 + 0.212 mm at different density ranges are given in Table 5a and b, respectively. It can be seen from columns 3 and 6 of the Table 5a that, if separation density of 1.94 is chosen, then 26.80% of ash, being lighter than 1.94, could be separated as a float product. According column 6, only 41.99% of the unburned carbon would be separated in this density range. Consequently, 58.01% of the unburned car-
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Table 5 Heavy liquid test results Specific gravity (a) 1.21 1.21–1.63 1.63–1.74 1.74–1.89 1.89–1.94 1.94–2.05 2.05–2.89 +2.89 Specific gravity (b) 1.63 1.63–1.84 1.84–1.94 1.94–2.05 2.05–2.89 +2.89
wt%
R
C (%)
Distribution C (%)
R
3.40 10.40 7.40 2.60 3.00 40.0 30.20 3.00
3.20 13.80 21.20 23.80 26.80 66.80 97.00 100.00
58.76 12.98 12.77 10.16 8.95 8.61 9.67 10.10
17.38 11.75 8.22 2.30 2.34 29.97 25.41 2.63
17.38 29.13 37.35 39.65 41.99 71.96 97.37 100.00
wt%
R
C (%)
Distribution C (%)
Cumulative C (%)
5.52 2.26 7.97 38.38 43.87 2.00
5.52 7.78 15.75 54.13 98.00 100
17.84 4.42 3.93 4.31 4.65 0.42
19.31 1.96 6.15 32.43 39.99 0.16
19.31 21.27 27.41 59.85 99.84 100.00
bon would be obtained as a sink product which accounts for the 73.2% of the total feed weight. Similarly, in Table 5b, 84.43% of the feed weight of 0.6 + 0.212 size fraction still contains a 59.85% unburned carbon for the separating density of 1.94 g/cm3. These results indicate that, in order to recover a sink product with low unburned carbon, it is not possible to obtain a certain separating density for the sink and float test. This inefficiency could have been resulted from similar densities of the unburned carbon and siliceous material, due to the porous structure of the CBA. 3.1.3. Electrostatic separation tests The electrostatic separation test results of 2.00 + 0.6 mm and 0.6 + 0.212 mm size fractions of CBA are given in Table 6a and b, respectively. The tests were carried out under constant feeding rate of 5.25 min/100 g and electrical power of 30 kW. The rotor rate was steadily increased and distribution of unburned carbon was analyzed according to the rotor rate. The results show that electrostatic separation is an ineffective method for both tested samples. This
Table 4 Crashing-classification test results of CBA with and without using separating sieve 1st stagea
1st stage Wt%
C (%)
wt%
C (%)
wt%
C (%)
wt%
C (%)
+2.36 2.36 + 2.00 2.00 + 0.6 0.6 + 0.212 0.212 + 0.149 0.149 + 0.075 0.075 + 0.053 0.053
20.88 3.01 32.20 34.58 4.19 3.43 0.43 1.28
6.23 14.09 13.38 5.71 3.28 4.34 7.40 14.75
19.42 3.75 38.07 28.98 5 4.27 0.59 1.92
5.94 12.37 11.57 5.10 3.53 4.11 8.03 15.42
– 2.0 40.31 38.68 8.92 5.94 1.19 1.96
– 10.36. 10.80 7.24 3.75 5.27 7.88 14.99
– 0.73 38.56 43.70 8.08 5.02 1.70 2.21
– 12.40 11.65 5.99 3.96 6.53 8.51 16.94
a
With separating sieve usage.
2nd stage
2nd stagea
Particle size (mm)
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Table 6 Electrostatic separation tests results of CBA Test number
Rotor rate (rpm)
Concentrate
Tailings
wt%
C (%)
wt%
C (%)
(a) 1 2 3 4 5
50 60 88 100 117
9.9 16.8 20.0 15.8 16.6
9.36 10.40 9.81 11.81 10.66
83.2 90.1 80 64.8 83.4
11.52 10.52 10.69 10.28 10.15
(b) 1 2 3 4
30 40 70 80
9.68 29.74 18.24 15.68
4.61 3.23 5.73 4.66
90.32 70.26 81.76 84.32
4.60 4.89 4.57 4.79
Table 7 Chemical composition of cement and pre-treated bottom ash Oxide composition
CBA (%)
Cement (%)
SiO2 Al2O3 Fe2O3 CaO MgO K2O SO3 Na2O LOI
55.95 16.65 9.69 4.39 5.14 1.44 0.70 0.084 4.65
20.96 5.58 3.69 63.97 1.9 – 2.84 – 1.15
can be explained by the insufficient conductivity of charged coal particles for separation from non- conductive ash particles under conductor/unconductor type separation. As a conclusion, the crushing-screening method was found to be a useful route for lowering the carbon content of the ash before performing concrete tests. By using this method, 57.67% of feed CBA was beneficiated with an unburned carbon content of 4.65%. 3.2. Concrete tests The oxide compositions of the Portland cement and pretreated CBA used in the experiments are given in Table 7. The cement used was CEM I 32.5R commercial Portland cement. Specific gravity of cement was 3.15 g/cm3. Initial and final settling times were 3 and 4 h, respectively. Its Blaine surface area was 3345 cm2/g.
The compressive and flexural strengths of the cement mixtures at various ages are given in Table 8. Table 8 indicates that both compressive and flexural strength of specimens increase with increasing amount of ash replacement up to 10%. When 10% of CBA is replaced by cement, the 56 day compressive strength increases approximately 5%, compared to the standard mixture. The ash addition higher than this amount leads to a decrease in the strength values of all specimens. This decrease is more significant for the lower curing time such as 7 and 28 days. However, for 56 days curing time, all values are higher than that of the standard mixture, except the BC25 due to the relatively low activity of ash at the beginning of the curing periods. These results agree with the previous findings in literature for fly/bottom ash usage in concrete. It is well known that, the calcium–silica–hydrate (C–S– H) is a major phase present both in the hydrated Portland cement and tri calcium silicate (C3S). The factors that influence the mechanical behaviour of C–S–H phases are: size and shape of the particles, distribution of particles, particle concentration, particle orientation, topology of the mixture, composition of the dispersed/continuous phases and the pore structure. The addition of coal ash as a supplementary cementing material causes an increase on both pozzolanic and physical properties that enhance the performance of concrete. When Portland cement hydrates it produces a quantity of alkali calcium hydroxide. Reactive silica in ash reacts with this lime to form stable calcium silicate and aluminate hydrates. These hydrates fill the voids within the concrete, removing some of the lime and thus reducing the permeability. This process improves the strength and durability of the concrete [23]. The pozzolanic reaction occurs relatively slow at normal temperatures, enhancing strength in the longer term compared to normal Portland cement concrete. A research study performed by Papadakis [24], to determine the effect of fly ash on the Portland cement system showed that, the compressive strength of fly ash specimens were similar to standard samples at 3–14 days, but higher from that of the 28 days or more. This delay was explained by the pore solution chemistry and low surface area due to the relatively larger particle size of the used fly ash. Also, Cheriaf et al. [25] reported that, after 28 days of hydration, ash particles reacted with calcium hydroxide. Recently, Hanehara et al. [26] reported that the reaction ratio of fly ash decreased with an increase in fly ash substitution ratio.
Table 8 Strength tests results of specimens Mixture
TS EN 197–1 Std BC5 BC10 BC15 BC25
Compressive strength (N/mm2)
Flexural strength (N/mm2)
7 days
28 days
56 days
7 days
28 days
56 days
Minimum 16 27.8 28.09 28.22 26.47 19.79
Minimum 32.5 40.9 40.38 40.24 33.57 29.13
– 42.65 44.08 45.1 43.45 41.33
Minimum 4.0 6.08 6.27 6.07 5.77 4.72
Minimum 5.5 6.60 6.75 6.62 6.57 6.22
– 6.86 7.35 7.60 7.20 6.69
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Fig. 3. SEM micrograph of the hydrated bottom ash-calcium hydroxide mixture at 28 and 56 days curing times for: (a) 10% BC, (b) 25% BC, (c) 10% BC and (d) 25% BC addition respectively.
However, apart from the particle size, it is reported in the literature that morphology, chemical content and the carbonaceous solid content of the replacement material should also be considered. All these factors lead to discoloration which is aesthetically unacceptable in certain applications, poor air entrainment behaviour and mixture segregation [4]. In practice, the utilization or substitution rate of ash is changes between 20–30% (for high Si- ash) depending on the type of ash and mixture composition. In the present study, the maximum substitution rate of CBA was determined as 10%. This lower value, when compared to the common practice of fly ash usage, can be attributed to different phase distributions and higher unburned carbon contents of CBA. The microstructural characteristics of the cement mixtures at different curing times are illustrated in Fig. 3. It can be followed from Fig. 3a that, although the pozzolanic reaction starts at 28 days, there is a slight reaction between CBA and calcium hydroxide at early ages. When curing time is increased to 56 days, a well-shaped C–S–H formation is observed. On the other hand, for the samples having a replacement amount of CBA higher than 10%, a low activity of ash could be observed for both 28 and 56 days curing time (Fig. 3b and d). As previously indicated in literature [23], the early products in the hydration of C3S consist of foils and flakes, whereas in Portland cement a gelatinous coating is often observed. The products of C3S which is a few days old consist of C–S–H fibres and partly crumpled sheets, whereas in Portland cement partly crumpled sheets and reticular network are observed. In our
study, C–S–H fibres or elongated particles, observed at relatively later stages of curing times such as 28 day and also 56 days for BC10, could be attributed to the strength development as shown in Table 8. 4. Conclusions Based on the experimental results of this study, the following main conclusions can be drawn; 1. Compared to other pre-treatment methods such as the heavy medium separation and electrostatic separation methods, the crushing-screening method was found to be a more useful route for lowering the carbon content of the considered CBA. By using this method, 57.67% of feed CBA was beneficiated with an unburned carbon content of 4.65%. 2. In concrete tests, although the compressive and flexural strengths of specimens cured at 56 day increase with increasing amount of ash replacement up to 15%, the maximum substitution rate of CBA was determined as 10%. When 10% of CBA is replaced by cement, the compressive strength of CBA-concrete increases from 42.65 N/mm2 to 45.1 N/mm2. This relatively lower substitution ratio compared to the common practice of fly ash usage, can be attributed to the different phase distributions and higher unburned carbon contents of CBA. 3. The observed C–S–H fibres or elongated particles on the SEM micrograph of BC10 clearly indicate the pozzolanic effect of CBA substitution on improving the strength of concrete.
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