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Construction and Building Materials 22 (2008) 1922–1928

and Building

MATERIALS

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 2007Available online 27 August 2007

Abstract

The present study aims to determine and evaluate the applicability of an industrial bottom ash (CBA), supplied from Tuncbilek PowerStation-Turkey, in concrete industry. In the laboratory experiments, the bottom ash was used up to 25% as a partial substitute for thePortland cement. In order to be able to reduce the unburned carbon content, CBA was treated by three different processes (particle sizeclassification, heavy medium separation and electrostatic separation). Based on the obtained results, it was concluded that the addition ofCBA up to 10% as a replacement material for Portland cement could improve the mechanical properties of concrete, and thus, could beused 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 powerplants, has been greatly affected by the increasing publicattention being paid to the environment. Coal ash gener-ated from power plants have become an important eco-nomic and environmental objective, and thus calls forrecycling alternatives to traditional landfill option due totheir high generation amount and low recycling rate. Totalenergy consumption in US, China, India and EU has beenrising since the mid-1990s and this trend is expected to con-tinue. European environment agency (EEA) has recentlyreported that fossils are presently dominating the fuelsources with an 80% share. This proportion is expectedto increase slightly over the next 30 years in the EU.Despite some growth in absolute terms, renewable energyis 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 uti-lized in the construction industry as a partial cement

0950-0618/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.conbuildmat.2007.07.008

* Corresponding author. Tel.: +90 222 2393756x3435; fax: +90 2222393613.

E-mail address: hkurama@ogu.edu.tr (H. Kurama).

replacement and/or mineral additive in cement production,the usage of coal bottom ash (CBA) is limited due to its rel-atively higher unburned carbon content and different struc-tural properties compared to fly ash [2]. CBA, a coarsesand to fine gravel size material collected at the bottomof the boiler, is generally used as a low cost replacementmaterial either as a base material in road construction oras a blasting grit. According to the American Coal AshAssociation (ACAA), recycling rate of fly ash in concreteand concrete products has been reported to be 47%, whilefor CBA this rate has been reported as only 5.28% of thetotal recycling amount- the total CBA production beingabout 19.8 M tonnes for 2002. 7.6 M tonnes of this produc-tion were recycled mainly in structural fills/embankments(26.61%), road base/subbase/pavement (19.15%) and min-ing applications (10.43%). The same utilization profilecan also be given for the EU. Nearly 89% of the producedCBA was recycled, but 54% of this amount was evaluatedfor reclamation and restoration [3]. However, the fused andglassy texture of coal ash makes it an ideal substitute fornatural raw materials. Using siliceous ash in concrete andcement applications requires that the loss-on-ignition(LOI) content of the ash generally be less than 6% byweight in order to obtain the required pozzolanic proper-

Table 2Particle size distributions of CBA and unburned carbon (C) values of eachsieve fractions

Particle size (mm) (%) R (%) C (%)

+2.36 31.49 31.49 8.972.360 + 2.000 1.96 33.45 21.682.000 + 0.600 26.98 60.43 12.960.600 + 0.212 31.14 91.57 5.220.212 + 0.149 3.59 95.16 3.010.149 + 0.075 2.62 97.78 3.790.075 + 0.053 0.48 98.26 6.99�0.053 1.74 100.00 13.13

H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928 1923

ties. This requirement results from the fact that propertiesof concrete incorporating high-carbon ash are inferior tothose of concrete incorporating low-carbon ash. Theamount of water and quantity of air entraining agents usedin the mix increases significantly as the carbon content ofthe ash increases above 6% LOI level [4]. Consequently,it is critical that removal/recovery technologies should bedeveloped so that high carbon CBA can be used moreextensively for industrial applications. A number of meth-ods have been developed and commercialized for removingthe unburned carbon particles from coal ash, includingcombustion of the carbon at low temperatures [5–10],mechanical/particle size classification [11], gravity separa-tion [12–14], and electrostatic separation [15–19], froth flo-tation [20,21] and, combinations thereof. Although, each ofthese methods may be used to remove carbon particlesfrom siliceous coal ash. However, the efficiency of theemployed method and pozzolanic properties of the result-ing low-carbon siliceous fraction are highly dependent onthe physical characteristics of the original ash.

In Turkey, nearly 28% of the total primary energy issupplied from the burning of bituminous and lignite coal.The ashes produced annually from 11 power plants varyfrom 6.5 to 13 M tonnes. However, only 1% of these wastematerials are re-used, mainly in concrete industry.

The purpose of the present study is to test commonmethods (such as sink and float test, particle size classifica-tion, and electrostatic separation) as an effective and eco-nomical method for removing of unburned carbon fromTuncbilek Power Station bottom ash in order to enhanceits application as a constituent in concrete production.The paper also examines the effects of pre-treated CBAadditions on the final concrete properties as a replacementfor Portland cement in cement mixture.

2. Materials and methods

2.1. Characterization of material

Lignite bottom ash used in this study was obtained fromTuncbilek Power Station, located in central Turkey. Thestation consumes about 2,350,000 tons of low-grade lignitecoal (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, indicatingthat it could be classified as a type F ash as prescribed byASTM C 618. In this study, the total carbon content ofgenerated ash was determined by employing the standard

Table 1Main oxide composition of received CBA

Oxides (wt%)

SiO2 Fe2O3 CaO Al2O3 LOI

54.5 11.16 4.69 15.4 8.90

loss-on-ignition analysis (LOI) and the determined LOI isaccepted as the mass of unburned carbon in the originalsample, which is a common approach in cement and con-crete applications.

Particle size distribution of the representative sampleand unburned carbon content of each size fraction aregiven in Table 2.

According to wet screen analysis, the D80 and D50 ashsize values were calculated as 2.7 and 1 mm, respectively.As seen from Table 2, the unburned carbon in CBA hasmainly accumulated at coarse fractions. However, a littleamount of carbon-rich fraction is also present at�0.053 mm. The specific gravity of the sample measuredby the pycnometer method was 2.39 g/cm3.

The crystalline mineral phases in the CBA were identi-fied by using X-Ray Diffraction (XRD), model S5000 dif-fractometer, with a nickel filtered Cu Ka. Regarding theXRD analysis, it was found that CBA had a relatively sim-ple mineralogy consisting of alumina, glass and varyingamount of crystalline phases of quartz, ferrite spinel andcalcite (Fig. 1).

The scanning electron micrograph of ash shows spheri-cal, rounded and irregularly shaped grains (Fig. 2).

2.2. Methods

2.2.1. Pre-treatment

2.2.1.1. Particle size separation. Mechanical means ofremoving carbon from siliceous ash is based on the relativeparticle size of the carbon particles and the siliceous parti-cles in the ash. In this study, the representative 500 g of

Fig. 1. X-ray diffractogram of CBA.

Fig. 2. Morphology of the bottom ash.

Table 3Mixture proportion of bottom ash cement paste

Sample Std BC5 BC10 BC15 BC25

Cement (g) 450 428 405 383 338Sand (g) 1350 1350 1350 1350 1350CBA (g) – 22 45 67 112Water (g) 225 225 225 225 225

1924 H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928

CBA was subjected to laboratory impact crusher with andwithout using 2 mm separating sieve. The crushed productwas 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 partof the screened fractions was separated by decantation inorder to remove floated particles, mainly carbon particles,before filtration (1st stage). Each collected fraction wasdried in oven at 100 �C and weighed. In order to find outthe crushing effect on the unburned carbon content of eachsize fraction, the obtained product from 1st stage crushingwas then re-crushed (2nd stage).

2.2.1.2. Sink and float tests. The sink and float tests wereperformed on crushed samples at various densities to assessthe suitability of heavy medium separation. These experi-ments were conducted in 250 mL glass flax with a volumeof 100 mL, using an appropriate mixture of bromoformand alcohol to adjust the density of liquid between 1.0–2.4 g/cm3. A 50 g of representative ash sample was intro-duced in the liquid of highest density. The floating productwas removed, washed and then placed into the next lowerdensity and so on.

2.2.1.3. Electrostatic separation tests. Electrostatic separa-tion encompasses a number of different technologies whichare based on the electrical properties of the particles to beseparated. The electrostatic separation tests were carriedout by using a conductor/non-conductor type of separator(Boxmag Rapid Ltd-HT150).

2.2.2. Moulding of CBA paste specimens

Representative cement compositions were prepared byprogressive incorporation of pre-treated samples in placeof Portland cement (5,10,15, and 25 wt%) to observe the

effect of ash addition in cement bodies. The amounts men-tioned above were chosen so as to highlight the effects ofCBA addition. These compositions were designated asStd, BC5, BC10, BC15 and BC25, respectively. The mix-ture proportions of the ash-cement used in this study arelisted in Table 3. The pre-treated materials mixed in therequired proportions were ground in a ceramic ball millto a fineness of the 25 mass% residues on a 38 lm toimprove its pozzolanic properties. The physical tests ofthe cement mixes were performed according to Turkishstandards TS EN 197–1. The cement–water mixtures werestirred at a low speed for 30 s, and then with the addition ofsand, the mixture was stirred again for 5 min. Three40 · 40 · 160 mm prismatic specimens for compressiontests were prepared from each mixture. The moulded spec-imens were cured at 20 �C with 95% humidity for 24 h, andthen after placed in a tap water and cured for 7, 28 and 56days.

3. Results and discussion

3.1. Pre-treatment

3.1.1. Particle size classificationIt is well known that in impact crushers, comminution is

performed by impact effect rather than compression [22].

Table 5Heavy liquid test results

Specificgravity

wt% R C(%)

Distribution C(%)

R

(a)�1.21 3.40 3.20 58.76 17.38 17.381.21–1.63 10.40 13.80 12.98 11.75 29.131.63–1.74 7.40 21.20 12.77 8.22 37.351.74–1.89 2.60 23.80 10.16 2.30 39.651.89–1.94 3.00 26.80 8.95 2.34 41.991.94–2.05 40.0 66.80 8.61 29.97 71.962.05–2.89 30.20 97.00 9.67 25.41 97.37+2.89 3.00 100.00 10.10 2.63 100.00

Specificgravity

wt% R C(%)

Distribution C(%)

Cumulative C(%)

(b)�1.63 5.52 5.52 17.84 19.31 19.311.63–1.84 2.26 7.78 4.42 1.96 21.271.84–1.94 7.97 15.75 3.93 6.15 27.411.94–2.05 38.38 54.13 4.31 32.43 59.852.05–2.89 43.87 98.00 4.65 39.99 99.84+2.89 2.00 100 0.42 0.16 100.00

H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928 1925

Impact causes immediate fracture with no residual stresses.This stress-free condition is particularly valuable in thequarrying industry. When the crushing force is appliedinstantaneously, the impact crushers give trouble-freecrushing on ores which tend to be brittle. The impactcrushers are widely used in coal preparation, coal beingmuch more friable than the associated stones and rubbishsuch as wood, steel, etc. The results of crushing and screen-ing tests are summarized in Table 4. It is clear thatunburned carbon in CBA is mainly concentrated at coarsefractions such as 2.36 + 2.00 mm and 2.00 + 0.6 mm, validfor both employed crushing types (with and without sepa-rating sieve usage). Although there is a rough relationshipbetween particle size and unburned carbon as LOI, in thecase of the without separating sieve usage, the calculatedcumulative 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 ben-eficial than fractions which have less than 6% LOI (51.78%)for separating sieve usage. This can be attributed to the fur-ther crushing effect of separation sieve on carbon particles.

In order to find out the positive effects of other treat-ment methods, such as gravity and electrostatic separation,on the further decrease of unburned carbon content, thetreatment tests were performed for the 2.00 + 0.6 and0.6 + 0.212 mm size fractions of crushed samples. Consid-ering the relatively higher particle size of 2.36 + 2.00 mmand unsuitability of the electrostatic separation method,different gravity concentration method such as jiggingwas also tested for this size fraction. However, a sufficientcarbon content reduction could not be achieved by employ-ing this method.

3.1.2. Sink and float test

The analyzed unburned carbon content and weight dis-tributions of the size fractions of 2.00 + 0.6 mm and0.6 + 0.212 mm at different density ranges are given inTable 5a and b, respectively. It can be seen from columns3 and 6 of the Table 5a that, if separation density of 1.94is chosen, then 26.80% of ash, being lighter than 1.94, couldbe separated as a float product. According column 6, only41.99% of the unburned carbon would be separated in thisdensity range. Consequently, 58.01% of the unburned car-

Table 4Crashing-classification test results of CBA with and without using separating

Particle size (mm) 1st stage 2nd stage

Wt% C (%) wt%

+2.36 20.88 6.23 19.422.36 + 2.00 3.01 14.09 3.752.00 + 0.6 32.20 13.38 38.070.6 + 0.212 34.58 5.71 28.980.212 + 0.149 4.19 3.28 50.149 + 0.075 3.43 4.34 4.270.075 + 0.053 0.43 7.40 0.59�0.053 1.28 14.75 1.92

a With separating sieve usage.

bon would be obtained as a sink product which accountsfor the 73.2% of the total feed weight. Similarly, in Table5b, 84.43% of the feed weight of 0.6 + 0.212 size fractionstill contains a 59.85% unburned carbon for the separatingdensity of 1.94 g/cm3. These results indicate that, in orderto recover a sink product with low unburned carbon, it isnot possible to obtain a certain separating density for thesink and float test. This inefficiency could have beenresulted from similar densities of the unburned carbonand siliceous material, due to the porous structure of theCBA.

3.1.3. Electrostatic separation tests

The electrostatic separation test results of 2.00 + 0.6 mmand 0.6 + 0.212 mm size fractions of CBA are given inTable 6a and b, respectively. The tests were carried outunder constant feeding rate of 5.25 min/100 g and electricalpower of 30 kW. The rotor rate was steadily increased anddistribution of unburned carbon was analyzed according tothe rotor rate. The results show that electrostatic separa-tion is an ineffective method for both tested samples. This

sieve

1st stagea 2nd stagea

C (%) wt% C (%) wt% C (%)

5.94 – – – –12.37 2.0 10.36. 0.73 12.4011.57 40.31 10.80 38.56 11.65

5.10 38.68 7.24 43.70 5.993.53 8.92 3.75 8.08 3.964.11 5.94 5.27 5.02 6.538.03 1.19 7.88 1.70 8.51

15.42 1.96 14.99 2.21 16.94

Table 6Electrostatic separation tests results of CBA

Test number Rotor rate (rpm) Concentrate Tailings

wt% C (%) wt% C (%)

(a)1 50 9.9 9.36 83.2 11.522 60 16.8 10.40 90.1 10.523 88 20.0 9.81 80 10.694 100 15.8 11.81 64.8 10.285 117 16.6 10.66 83.4 10.15

(b)1 30 9.68 4.61 90.32 4.602 40 29.74 3.23 70.26 4.893 70 18.24 5.73 81.76 4.574 80 15.68 4.66 84.32 4.79

Table 7Chemical composition of cement and pre-treated bottom ash

Oxide composition CBA (%) Cement (%)

SiO2 55.95 20.96Al2O3 16.65 5.58Fe2O3 9.69 3.69CaO 4.39 63.97MgO 5.14 1.9K2O 1.44 –SO3 0.70 2.84Na2O 0.084 –LOI 4.65 1.15

1926 H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928

can be explained by the insufficient conductivity of chargedcoal particles for separation from non- conductive ash par-ticles under conductor/unconductor type separation.

As a conclusion, the crushing-screening method wasfound to be a useful route for lowering the carbon contentof the ash before performing concrete tests. By using thismethod, 57.67% of feed CBA was beneficiated with anunburned carbon content of 4.65%.

3.2. Concrete tests

The oxide compositions of the Portland cement and pre-treated CBA used in the experiments are given in Table 7.The cement used was CEM I 32.5R commercial Portlandcement. Specific gravity of cement was 3.15 g/cm3. Initialand final settling times were 3 and 4 h, respectively. ItsBlaine surface area was 3345 cm2/g.

Table 8Strength tests results of specimens

Mixture Compressive strength (N/mm2)

7 days 28 days 56

TS EN 197–1 Minimum 16 Minimum 32.5 –Std 27.8 40.9 42.BC5 28.09 40.38 44.BC10 28.22 40.24 45.BC15 26.47 33.57 43.BC25 19.79 29.13 41.

The compressive and flexural strengths of the cementmixtures at various ages are given in Table 8.

Table 8 indicates that both compressive and flexuralstrength of specimens increase with increasing amount ofash replacement up to 10%. When 10% of CBA is replacedby cement, the 56 day compressive strength increasesapproximately 5%, compared to the standard mixture.The ash addition higher than this amount leads to a decreasein the strength values of all specimens. This decrease is moresignificant for the lower curing time such as 7 and 28 days.However, for 56 days curing time, all values are higher thanthat of the standard mixture, except the BC25 due to the rel-atively low activity of ash at the beginning of the curing peri-ods. These results agree with the previous findings inliterature 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 Portlandcement and tri calcium silicate (C3S). The factors that influ-ence the mechanical behaviour of C–S–H phases are: sizeand shape of the particles, distribution of particles, particleconcentration, particle orientation, topology of the mix-ture, composition of the dispersed/continuous phases andthe pore structure. The addition of coal ash as a supple-mentary cementing material causes an increase on bothpozzolanic and physical properties that enhance the perfor-mance of concrete. When Portland cement hydrates it pro-duces a quantity of alkali calcium hydroxide. Reactivesilica in ash reacts with this lime to form stable calcium sil-icate and aluminate hydrates. These hydrates fill the voidswithin the concrete, removing some of the lime and thusreducing the permeability. This process improves thestrength and durability of the concrete [23]. The pozzolanicreaction occurs relatively slow at normal temperatures,enhancing strength in the longer term compared to normalPortland cement concrete. A research study performed byPapadakis [24], to determine the effect of fly ash on thePortland cement system showed that, the compressivestrength of fly ash specimens were similar to standard sam-ples at 3–14 days, but higher from that of the 28 days ormore. This delay was explained by the pore solution chem-istry and low surface area due to the relatively larger par-ticle size of the used fly ash. Also, Cheriaf et al. [25]reported that, after 28 days of hydration, ash particlesreacted with calcium hydroxide. Recently, Haneharaet al. [26] reported that the reaction ratio of fly ashdecreased with an increase in fly ash substitution ratio.

Flexural strength (N/mm2)

days 7 days 28 days 56 days

Minimum 4.0 Minimum 5.5 –65 6.08 6.60 6.8608 6.27 6.75 7.351 6.07 6.62 7.6045 5.77 6.57 7.2033 4.72 6.22 6.69

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% BCand (d) 25% BC addition respectively.

H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928 1927

However, apart from the particle size, it is reported in theliterature that morphology, chemical content and the car-bonaceous solid content of the replacement material shouldalso be considered. All these factors lead to discolorationwhich is aesthetically unacceptable in certain applications,poor air entrainment behaviour and mixture segregation[4].

In practice, the utilization or substitution rate of ash ischanges between 20–30% (for high Si- ash) depending onthe type of ash and mixture composition. In the presentstudy, the maximum substitution rate of CBA was deter-mined as 10%. This lower value, when compared to thecommon practice of fly ash usage, can be attributed to dif-ferent phase distributions and higher unburned carboncontents of CBA.

The microstructural characteristics of the cement mix-tures at different curing times are illustrated in Fig. 3. Itcan be followed from Fig. 3a that, although the pozzolanicreaction starts at 28 days, there is a slight reaction betweenCBA and calcium hydroxide at early ages. When curingtime is increased to 56 days, a well-shaped C–S–H forma-tion is observed. On the other hand, for the samples havinga replacement amount of CBA higher than 10%, a lowactivity of ash could be observed for both 28 and 56 dayscuring time (Fig. 3b and d). As previously indicated in lit-erature [23], the early products in the hydration of C3S con-sist of foils and flakes, whereas in Portland cement agelatinous coating is often observed. The products of C3Swhich is a few days old consist of C–S–H fibres and partlycrumpled sheets, whereas in Portland cement partly crum-pled sheets and reticular network are observed. In our

study, C–S–H fibres or elongated particles, observed at rel-atively later stages of curing times such as 28 day and also56 days for BC10, could be attributed to the strength devel-opment as shown in Table 8.

4. Conclusions

Based on the experimental results of this study, the fol-lowing main conclusions can be drawn;

1. Compared to other pre-treatment methods such as theheavy medium separation and electrostatic separationmethods, the crushing-screening method was found tobe a more useful route for lowering the carbon contentof the considered CBA. By using this method, 57.67%of feed CBA was beneficiated with an unburned carboncontent of 4.65%.

2. In concrete tests, although the compressive and flexuralstrengths of specimens cured at 56 day increase withincreasing amount of ash replacement up to 15%, themaximum substitution rate of CBA was determined as10%. When 10% of CBA is replaced by cement, the com-pressive strength of CBA-concrete increases from42.65 N/mm2 to 45.1 N/mm2. This relatively lower sub-stitution ratio compared to the common practice of flyash usage, can be attributed to the different phase distri-butions and higher unburned carbon contents of CBA.

3. The observed C–S–H fibres or elongated particles on theSEM micrograph of BC10 clearly indicate the pozzola-nic effect of CBA substitution on improving the strengthof concrete.

1928 H. Kurama, M. Kaya / Construction and Building Materials 22 (2008) 1922–1928

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