Construction and Building Materials 52 (2014) 42–51

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Construction and Building Materials

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Influence of bacterial treated cement kiln dust on the propertiesof concrete

0950-0618/$ - see front matter � 2013 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.conbuildmat.2013.11.034

⇑ Corresponding author. Tel.: +91 9988385367.E-mail addresses: kunal_pau@yahoo.co.in ( Kunal), siddique_66@yahoo.com

(R. Siddique), anitarajor@yahoo.com (A. Rajor).

Kunal a,⇑, Rafat Siddique b, Anita Rajor a

a Department of Biotechnology and Environmental Sciences, Thapar University, Patiala, Punjab, Indiab Department of Civil Engineering, Thapar University, Patiala, Punjab, India

h i g h l i g h t s

� Paper presents study on properties of concrete containing bacterial treated CKD.� Bacterial treatment reduces the alkalinity of CKD up to 67% in leachate.� Increase in compressive strength and decrease in water absorption and porosity is observed.� XRD confirms formation of non-expansive ettringite and calcium silicate in CKD-concrete.

a r t i c l e i n f o

Article history:Received 5 July 2013Received in revised form 1 November 2013Accepted 12 November 2013

Keywords:AlkalinityBacteriaCement kiln dustCompressive strengthEttringite

a b s t r a c t

During cement manufacturing, cement kiln dust (CKD) is generated which represents significant environ-ment concern related to its emission, disposal and reuse due to high alkalinity. This study presents theeffect of bacterial (Bacillus halodurans strain KG1) treated cement kiln dust on the compressive strength,water absorption and porosity (at 7, 28 and 91 days) of concrete after reducing the alkalinity. Concretespecimens were prepared with 0%, 5%, 10% and 15% untreated and treated CKD replacing cement. Testresults indicated that 7.15% and 26.6% increase in strength of concrete was achieved at 28 and 91 days,respectively, with the addition of bacterial treated 10% CKD whereas reduction in water absorption (20%)and porosity (12.35%) was observed at 91 days. X-ray diffraction (XRD) and scanning electron microscopy(SEM) results suggested that in bacterial treated 10% CKD concrete increased calcium silicate hydrate andformation of non-expansive ettringite in pores dense the concrete structure resulted in increased com-pressive strength.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Rapidly growing population generates the need for industriali-zation and urbanization which in turn increases the demand ofbuilding and construction material for infrastructure development.Concrete is one of the most durable and widely used constructionmaterial in the world with annual consumption estimated between21 and 31 billion tones in 2006 [1]. Concrete is made from coarseaggregates (gravel or crushed stone), fine aggregates (sand), water,cement and admixtures. Cement plays an important role inconcrete due to its binding properties. Increasing urbanization con-tinuously driving the cement industry to keep growing. Accordingto Oss [2], 3700 million tones of cement (3400 million tones ofclinker) was generated in 2012 worldwide, whereas in India 250million tones of clinker was generated. There is 2.78% and 11.78%increase in cement production was observed in year 2012

compared to year 2011 and 2010, respectively. This increase in ce-ment production results in towering collection of cement kiln dustfrom cement plants. Cement kiln dust (CKD) is a fine powderymaterial generated in large quantities from air pollution controldevices (e.g. cyclone, bag house, or electrostatic precipitator) dur-ing the production of cement clinker. The generation of CKD hasbeen estimated to be 15–20% of clinker production [3], whichput world wide CKD generation at an estimated 510–680 milliontones for the year 2012 and Indian production at 37.5–50 milliontones.

Cement kiln dust is a very heterogenous mix both by chemistryand particulate size. The chemical composition of CKD dependsupon the raw materials, fuels, kiln type, overall equipment layout,and type of cement being used. The concentration of free lime,sulfates and alkalies in CKD mainly dependent upon the size of par-ticles collected near to the kiln. Coarser particles of CKD containhigh content of free lime while the fine particles usually exhibithigher concentration of sulfates and alkalies and lower limecontent [4]. Cement kiln dust is generated as a measure to control

Table 1Properties of cement.

Characteristics Values obtained IS: 8112-1989 ASTM C150

Fineness (%) 1% 10 max –Standard consistency 34% – –Initial setting time (min) 120 30 min –Final setting time (min) 240 600 max 375 maxSpecific gravity 3.03 – –

Table 2Properties of coarse and fine aggregates.

Characteristics Coarse aggregate Fine aggregate

Fineness (%) 6.74 2.72Specific gravity 2.59 2.62Water absorption (%) 0.80 1.02Size (mm) 12.5 max 4.75 maxMoisture content (%) Nil 0.16

Table 3Physical properties of CKD.

Physical properties Values Taha et al. [10] Collins and Emery [4]

Specific gravity 2.39 2.4 2.6–2.8Fineness modulus 2.25 4.824 –pH >12 – –

Kunal et al. / Construction and Building Materials 52 (2014) 42–51 43

product quality (low alkali clinker from high alkali raw materials)and to ensure uninterrupted operation of the plant. The majorfactor preventing return of more dust to the kilns is the high con-centration of alkalis in the dust that would cause the alkali contentof the clinker to exceed the allowable value. Additionally, the highconcentrations of volatiles develop deposits on the walls of the kilnwhich can result in frequent shut down of the plant. Hence cementplants generate CKD as a means of removing volatile alkalis, chlo-rides and sulfates from the kiln system.

The generation of large quantities of CKD is responsible for asignificant financial loss to the cement industry in terms of thevalue of raw materials, processing, energy usage and, above all, dis-posal and storage. According to US Environmental ProtectionAgency [5], approximately 15 million tons of CKD is producedannually in United States and major part of it is sent to landfills.Disposal of waste CKD is not only associated with the problem ofland use but also with contamination of ground water from leach-ate. It is estimated that over 200,000 tons a year of landfill spacecould be saved in UK if the surplus CKD could be recycled intothe clinker-making process or if alternative uses could be found[6]. Due to the cost associated with the production and disposalof CKD and the strong and strict environmental regulations onthe proper management of CKD, the cement industries now show-ing a keen interest in finding the proper utilization of CKD in amanner that protects human health and the environment.

Cement kiln dust has the potential for reuse in many differentways, but the cement making process is the best way to reuse thisby-product material and approximately 60–67 percent (8–8.4 mil-lion tons) of the total CKD generated in United States is used in thisway [7]. The most common beneficial uses of CKD are cementreplacement, soil stabilization, waste treatment, asphalt pavementand other uses. CKD’s cement like properties also makes it a poten-tial replacement for Portland cement in utilization in concrete,flowable slurry, etc. Several researchers [8–21] have reported onsome aspects of the utilization of CKD in cement paste, mortar/concrete.

Generally, the cement kiln dust is alkaline in nature (pH 12) andis considered to be caustic. If used in concrete, the alkaline natureneeds to be monitored to avoid expansive reaction between alkalisand certain aggregates, which leads to cracking and causes deteri-oration [22]. Bhatty [13–16,23] found that CKD blended cementhad reduced workability, setting times and strength. The loss ofstrength was attributed to the alkalies in the dust. The AmericanSociety for Testing and Materials (ASTM) specifies a limit of 0.6percent alkali in Portland cement [24,25]. If the amount of alkaliin the kiln dust is high, its reusing in the kiln causes the formationof potassium and sodium containing compound and free CaO, lead-ing ultimately to a reduction in the strength of cement [26] andcauses crack, deformities and reduce the quality of cement andconcrete [27,28]. Juenger and Jennings [22] examined that high al-kali content also affects the hydration and microstructure of ce-ment paste and therefore, influences the concrete properties.

Cement kiln dust after reducing the alkalinity can be used in ce-ment-concrete system and may have positive effect on cement-concrete properties. Mohamed and El-Gamal [29] and Gebhardt[30] treated the alkaline CKD with carbon dioxide gas to removethe alkalinity, but these methods are highly expensive, laboriousand above all CKD is not reutilized rather landfilled. The betteralternative to the use of chemical process (involves use of carbondioxide gas) is the biological treatment of alkaline wastes usingbacterial system which grow well at high pH. Studies suggestedthat alkaliphilic bacteria could degrade pollutants under highlyalkaline conditions and had the significant advantage of not beingeasily contaminated by neutral microorganisms [31–33]. However,reports on the application of alkaliphilic bacteria in treatment ofsolid alkaline waste are very rare. Thus, in the present study we

have isolated an alkalitolerant bacterial strain (Bacillus sp. KG1)and utilized in reducing the alkalinity of the CKD. The treatedCKD then utilized in concrete as partial replacement to cement indifferent percentages (0–15%) and investigated the effect on themechanical properties of concrete.

2. Materials and methods

2.1. Material used

2.1.1. CementCement of Indian Standards (IS) mark 43 grade (IS mark 43 grade means that

the 28-day compressive strength of cement is 43 MPa) UltraTech brand was usedfor all mixes. Testing of cement was conducted as per IS: 8112-1989 [34]. The testresults conducted on cement are reported in Table 1.

2.1.2. Fine and coarse aggregateNatural sand with 4.75 mm maximum size was used as fine aggregate. The sand

was first sieved through 4.75 mm sieve to remove any particle greater than4.75 mm and removed the dust. Locally available coarse aggregates having the sizeof 12.5 mm were used in this work. Testing of fine and coarse aggregates was doneas per IS: 383-1970 [35]. Properties of the coarse and fine aggregates used areshown in Table 2.

2.1.3. Cement kiln dustCement kiln dust (CKD) is fine powdery material of grey-black in color and

relatively uniform in size. Table 3 represents the physical properties of CKDcompared with literature published. As CKD is derived from same raw materialsas cement clinker, despite the fact that it has similar chemical composition to thatof ordinary Portland cement, significant variation in physical and chemical compo-sition of CKDs obtained from different cement plants has been observed. Table 4shows the chemical composition of CKD and cement used in the study and typicalcomposition of CKD investigated by other researchers.

2.2. Isolation of bacteria

Alkaliphilic and/or alkalitolerant bacteria (that tolerate high pH) was isolatedfrom rhizospheric (near to root of plant) soil. The soil samples were suspended insterile saline solution (0.85% NaCl), diluted properly and plated on enrichmentmedium containing glucose (10 g/l), peptone (10 g/l), yeast extract (5 g/l), KH2PO4

(1 g/l), agar (15 g/l) and pH was adjusted to 10.5 with 1 N.

Table 4Chemical properties of cement and CKD.

Constituent (%) Cement Cement kiln dust (CKD)

Present study Reviewed literature

Control Bacterial treated Maslehuddin et al. [6] Taha et al. [10] Udoeyo and Hyee [55]

CaO 65.57 55.78 48.19 49.3 63.76 52.72SiO2 23.61 13.17 10.97 17.1 15.84 2.16Al2O3 2.16 2.38 2.19 4.24 3.57 1.09MgO 0.72 0.69 0.58 1.14 1.93 0.68SO3 1.32 1.13 0.82 3.56 1.65 0.05K2O 1.03 1.12 0.59 2.18 2.99 0.11Na2O 0.32 – – 3.84 0.33 –Fe2O3 2.41 2.62 2.41 2.89 2.76 0.54CuO 1.39 0.89 0.80 – – –ZnO 1.50 0.66 0.64 – – –LOI – – – 15.8 5.38 42.39

Fig. 1. Compressive strength of untreated concrete containing cement kiln dust. Fig. 2. Compressive strength of concrete containing bacterial treated cement kilndust.

44 Kunal et al. / Construction and Building Materials 52 (2014) 42–51

NaOH. The agar plates were incubated at 37 �C for 48 h. Isolated colonies werepicked and re-streaked on same agar medium till pure colonies were obtained. Theselected colonies were then screened for their tolerance to pH 11 and 12 and per-formed on minimal (M9) medium containing sucrose (10 g/l), KH2PO4 (2.5 g/l),K2HPO4 (2.5 g/l), (NH4)2HPO4 (1 g/l), MgSO4�7H2O (2 g/l), FeSO4�7H2O (0.01 g/l),MnSO4�4H2O (0.007 g/l) and agar (15 g/l). pH was adjusted to 11 and 12 by KCl–NaOH buffer. The cultures were maintained on M9 medium and stored at 4 �C forfurther experimentation.

2.3. Bacterial treatment of CKD

On the basis of pH reduction of the alkaline medium, isolate KG1 showed thepromising results and utilized for the treatment of CKD. The CKD was mixed with0.8 OD (optical density; measured by spectrophotometer at kmax 600 nm) value ofthe selected bacterial strain KG1 (OD 1.0 � 108 cells) in the ratio CKD to culture(4:1). For bacterial treatment the CKD sample was poured into plastic tubs andbacterial culture KG1 in required proportion was added and mixed manually in away so that the culture was thoroughly distributed. The treatment mixture wasincubated at 35 ± 2 �C for 20 days and moisture was maintained by spraying waterfor the growth of bacterial strain. After 5 days of incubation, sucrose solution (10%)was added only once during the treatment of 20 days to provide carbon source forthe bacterial strain KG1. After the completion of the incubation period of 20 days,samples were collected randomly from different places, mixed with water (1:10)in conical flasks with shaking (@ 130 rpm for 1 h) to generate leachate and analyzedfor alkalinity and chloride along with control treatment [36].

To confirm the decrease in alkalinity, bacterial treated CKD sample was air driedand analyzed with energy dispersive X-ray spectrometry (EDX, JEOL JSM-6510 LV,USA) for change in chemical composition of CKD.

2.4. Concrete mix design

The bacterial treated CKD (moisture content 0.02%) was used for the concretemixtures along with series of control concrete mixtures containing untreatedCKD. Seven concrete mix proportions were made. First was the control mix(without CKD) and the other six mixtures contained CKD (three mixes each con-tained untreated and bacterial treated CKD). Cement was replaced with CKD byweight in proportion of 5%, 10% and 15%. The mix proportion of concrete usedwas water (0.5): cement (1.0): fine aggregate (1.45): coarse aggregate (2.98). The

water to cementitious material ratio was kept constant (0.5) to investigate the ef-fect of replacing cement with CKD. The control without CKD was designed as perIndian Standards specifications IS: 10262-1982 [37].

2.5. Preparation and casting of test specimens

Concrete cubes of size 150 mm of M20 grade were prepared for compressivestrength, water absorption and porosity. The casting of specimens was in accor-dance with Indian Standard IS: 516-1959 [38]. After casting, the specimens wereallowed to remain in iron molds for first 24 h at room temperature (27 ± 2 �C). Afterthat these were demolded and placed in the water tank at room temperature forcuring. The specimens were tested after 7, 28 and 91 days of curing period.

2.6. Study of concrete properties

Concrete can be made to have high compressive strength. Concrete propertieswere studied in triplicate. The compressive strength was determined as per IS:516-1959 [38] specifications. Water absorption and porosity of the cubes weredetermined by using ASTM C 642-97 [39] method at the age of 7, 28 and 91 days.

Concrete samples from each mix (after 91 days of curing) was taken from innercore of the matrix (crushed into fine powder by pestle-mortar) and analyzed inpowder X-ray diffraction (XRD; PANalytical X’Pro). The XRD spectrum was takenfrom 2 h = 5h to 2 h = 60h. The peaks in the new positions of the spectrum weremarked, compared and identified from the Joint Committee on Powder DiffractionStandards (JCPDS) data file and from the published literature. Scanning electronmicroscopic (SEM; JEOL JSM 6510 LV, USA) analysis was performed by mountingsmall broken concrete specimens (with and without bacterial treated CKD) on brassstubs using carbon tape. The samples were coated with gold and then analyzed at20 kV.

3. Results and discussion

3.1. Physical and chemical properties of CKD

Specific gravity of CKD was found to be observed as 2.39whereas typical specific gravity varies between 2.4 and 2.8

Fig. 3. Percent water absorption of (a) untreated and (b) bacterial treated concrete containing cement kiln dust.

Fig. 4. Percent porosity of (a) untreated and (b) bacterial treated concrete containing cement kiln dust.

Fig. 5. X-ray diffraction shown by concrete without CKD.

Kunal et al. / Construction and Building Materials 52 (2014) 42–51 45

[4,10]. Fineness modulus and pH was found to be observed as 2.25and 12, respectively.

CKDs on the average are typically characterized by higher alkaliand sulfur content which is one of the main reason from removingthe dust from kilns. As CKD is derived from the same raw materialsas ordinary Portland cement, despite that it has significant varia-tion in chemical composition obtained from different cement

plants. The chemical composition of typical CKD, CKD and cementused in this study is shown in Table 4. Compounds of lime, silica,alumina and iron constitute the major composition of CKD fol-lowed by alkali (K2O) and sulfur (SO3). Analysis of CKD showedthe absence of Na and Cl with small amounts of Cu, Mg and Zn.Lime was found to be observed as maximum value of 55.78%whereas silica was found to be observed as 13.17%. Alkalinity of

Fig. 6. X-ray diffraction shown by concrete containing 5% CKD; (a) untreated CKD, and (b) bacterial treated CKD.

46 Kunal et al. / Construction and Building Materials 52 (2014) 42–51

the CKD was mainly due to the presence of K (K2O; 1.12%) and S(SO3; 1.13%) components.

3.2. Bacterial treatment of CKD

Bacterial treated CKD sample after 20 days were analyzed foralkalinity and chloride as per method of APHA [36]. The alkalinityand chloride of control CKD leachate was 1467 mg/l and 460 mg/l,respectively, whereas after treated with bacterial strain KG1, thealkalinity and chloride content of CKD reduced significantly(p < 0.05; t-test at 95% confidence limits) to 480 mg/l (32.72%)and 73.33 mg/l (15.94%), respectively. All the treatments were per-formed in three replications and the results are the average ofthree readings.

The EDX analysis of bacterial treated CKD also revealed the sig-nificant (p < 0.0001; two way ANOVA) reduction of K2O and SO3

content by 47.32% and 24.43%, respectively, compared to controlCKD (Table 4). The fact behind the reduction of alkalinity is dueto the production of organic acid (acetic and formic acid) by bacte-rial enzymes in fermentation and respiration process of bacterialmetabolism which eventually reduces the alkalinity [33].

3.3. Effect of bacterial treated CKD on concrete properties

During fresh concrete mixing (before casting), the samples werecollected from different places and the leachate generated wasanalyzed for alkalinity of the fresh concrete mix leachate. The

alkalinity of the control concrete (0% CKD) was 1666.67 mg/l whichkeep on decreasing as the cement was replaced with untreatedCKD in different percentages of 5 (1553.33 mg/l), 10 (1526.67mg/l) and 15 (1440 mg/l). This is supported by the fact that thealkalinity of cement is 1893.33 mg/l which is higher than CKDalone and 0% CKD control concrete. When cement is replaced withdifferent percentages of untreated CKD, the alkalinity of concrete(5%, 10% and 15%) goes on decreasing instead of increasing. Afteraddition of bacterial treated CKD in concrete, 64%, 62.8% and 60%alkalinity was observed in 5%, 10% and 15% treated CKD concretecompared to control concrete mix. After casting at the age of7 days, the control mix (0% CKD) showed compressive strength of23.23 N/mm2, whereas 5%, 10% and 15% CKD (untreated) concreteshowed compressive strength of 23.78, 24.31 and 23.03 N/mm2,respectively (Fig. 1). The compressive strength of the CKD control(untreated) concrete increased with increase in curing period. Atthe age of 28 days, the compressive strength of the control CKDconcrete (0% CKD) was 34.82 N/mm2 whereas of 5%, 10% and 15%control CKD concrete was 35.78, 36.29 and 34.53 N/mm2, respec-tively. Similarly at 91 days of curing period, there was 40.44,41.89, 44.12 and 40.03 N/mm2 of compressive strength in 0%, 5%,10% and 15% CKD control concrete (Fig. 1).

Maslehuddin et al. [8] reported decrease in compressivestrength (>5%) of concrete mixes (10% and 15% replacement) atall ages (3, 7, 14, 28, 56 and 91 days). The authors concluded thatup to 5% CKD could be used without compromising the compres-sive strength of concrete. El-Aleem et al. [40] concluded that up

Fig. 7. X-ray diffraction shown by concrete containing 10% CKD; (a) untreated CKD, and (b) bacterial treated CKD.

Kunal et al. / Construction and Building Materials 52 (2014) 42–51 47

to 6% CKD replacement there is no significant reduction incompressive strength of hardened mortar whereas above thispercentage the compressive strength decreased sharply. Thisreduction in strength may be due to decrease in cement content,increase in free lime in cement dust, increased porosity or the for-mation of chloro and sulpho-aluminate phases which leads to soft-ening and expansion of the hydration products. In this study, up to10% CKD replacement (control CKD concrete) showed increasedstrength whereas in 15% CKD control concrete compressivestrength was decreased. The increase in compressive strength upto 10% CKD replacement levels in control concrete mix may bedue to an appropriate alkalinity that increases the dissolution ofsilicate and formation of calcium silicate hydrate which is respon-sible for increased compressive strength [41]. The decreased com-pressive strength in 15% CKD control concrete may be due toincreased alkalinity, porosity and lower cement content at 91 daysof curing. These results were in concomitant with the findings ofBhatty [13–16,23] and Rehsi and Garg [26]. Studies reported thathigh alkalinity in dust causes cracks, reduces the quality of cementand concrete, affects the hydration and microstructure of cementpaste and ultimately reduced the strength of concrete [22,27,28].

Fig. 2 shows the compressive strength of bacterial treated CKDcontaining concrete cubes (5%, 10% and 15%) at the age of 7, 28 and91 days. Similar trend of increased compressive strength was alsofound in bacterial treated CKD concrete at all curing ages. The earlystrength (compressive strength at 7 days of curing) of bacterial

treated CKD concrete showed decrease in strength values com-pared to control CKD concrete cubes. This may be due to decreasein the alkalinity of concrete mix containing bacterial treated CKD.The CKD was treated with bacterial strain KG1 to reduce the alka-linity which was then used as a replacement to cement in differentpercentages (5%, 10% and 15%). The heat of hydration is due to thealkalinity present in the cement-concrete mix which results in thepozzolanic reaction and developed the early strength. The reducedalkalinity slows the pozzolanic reaction of CKD-cement mixtureand lowers the early strength of concrete compared to control con-crete mixes.

According to Pu [42], the strength of the cement-concrete mixcontaining active mineral additives such as CKD can be consideredas composed of two parts: the first part of strength is contributedby the hydrates that are formed by the hydration of clinker in ce-ment, and the second part is contributed by the additional hy-drates obtained from the secondary reaction between activesilica and alumina oxides in mineral additives with free calciumhydroxides obtained from the hydration of clinker. This secondpart is responsible for the strength developed in bacterial treatedCKD concrete in later ages (28 and 91 days). In 10% bacterial trea-ted CKD concrete 7.15% and 2.81% increase was observed in com-pressive strength compared to 0% and 10% CKD control concrete,respectively at 28 days of curing, whereas at the age of 91 days,this increase was 26.6% and 16.09%, respectively. Above 10% CKDaddition (bacterial treated) the strength decreased and this may

Fig. 8. X-ray diffraction shown by concrete containing 15% CKD; (a) untreated CKD, and (b) bacterial treated CKD.

Fig. 9. SEM image of control concrete without CKD.

48 Kunal et al. / Construction and Building Materials 52 (2014) 42–51

be attributed to decrease in cement content and hydrationreaction.

Several studies have been reported the improvement of com-pressive strength of the cement mortar by inclusion of microorgan-isms [43–45]. Researchers proposed a new phenomenon known asbiocalcification or microbially induced calcite precipitation to im-prove the overall strength and performance of cement mortar byrepairing the cracks and pores of the structure. In this study,

bacterium was used to reduce the alkalinity of CKD and then uti-lized in partial replacement to cement in concrete. The latestrength developed after 28 days of curing and is probably due toreduced hydration reaction, increased CS/CSH gel formation (evi-denced from XRD results discussed later in the text) and depositionof bacterial cells or spores within the pores of CKD-cement-sandmatrix which plugs the pores with in the concrete. Konsta-Gdoutosand Shah [19] and Salem and Ragai [46] reported that presence ofalkali concentration plays an important role in initial hydration ofcement pastes. Cement paste containing alkaline CKD showed in-crease hydration resulted in early strength.

Water absorption and porosity are directly related to compres-sive strength of concrete. Increased water absorption and pore sizedecreased the compressive strength of concrete. Results of waterabsorption and porosity shows decrease of 20% and 12.35%, respec-tively, in 10% bacterial treated CKD concrete compared to un-treated CKD control concrete at 91 days of curing (Figs. 3 and 4).Water absorption and porosity decreases with increase in CKD con-centration but above 10% the water absorption and porosity in-creases in both control and bacterial treated CKD concrete (at theages of 7, 28 and 91 days). This is due to decrease in cement con-tent and increase in CKD concentration which decreases the bind-ing of CKD-cement-aggregate in concrete and develops pores.

X-ray diffraction (XRD) analysis of concrete samples with orwithout bacterial treated cement kiln dust shows peaks of quartz(Q), calcium silicate hydrate (CSH), calcite (C), larnite (L) andettringite (E) phases on comparing the values of 2h/d/I/I of the

Fig. 10. SEM image shows 5% cement kiln dust containing concrete; (a) untreated, and (b) bacterial treated.

Fig. 11. SEM image shows 10% cement kiln dust containing concrete; (a) untreated, and (b) bacterial treated.

Fig. 12. SEM image shows 15% cement kiln dust containing concrete; (a) untreated, and (b) bacterial treated.

Kunal et al. / Construction and Building Materials 52 (2014) 42–51 49

peaks by JCPDS data file (Figs. 5–8). Peaks of different phases intreatments shows the intensity corresponding to the strength ofconcrete. Alite (C3S) is the major mineral component (>50%) foundin cement and upon hydration forms calcium silicate hydrate orcalcium silicate which hardens the cement slurry and is responsi-ble for initial (1–3 days) and final strengths [47].

The second major component found in cement is C2S or Belite orLarnite (Ca2SiO4). Larnite reacts with water to form calcium silicatehydrate or calcium silicate and portlandite, and responsible for thedevelopment of late strength. Neville [48], Molnar et al. [49] andJumate and Manea [47] studied that hydration and hydrolysis reac-tion of C3S and C2S mineral components produce calcium silicatehydrate (also known as Tobermorite) gels and later the solid phasedevelops crystals during curing period leading to strengthening ofthe cement-concrete mixes.

In 10% bacterial treated CKD concrete (Fig. 7b) the increasedformation of CSH resulted in increased strength compared to 10%CKD control concrete (Fig. 7a). Ettringite formation in 10% bacterialtreated CKD concrete, due to less alkali content, was nonexpansiveand filled the pore structure in concrete resulted in dense structureand increased the compressive strength. Min and Mingshu [50]stated that the high concentration of hydroxyl ions (i.e. high pHvalues) due to higher alkali content of the solution results in theexpansive type of ettringite. According to Heinz and Ludwig [51],several factors affect the formation of ettringite such as sulfatecontent, pH of the solution and availability of the calciumhydroxide. Higher alkali content increased the solubility of sulfateions in solution which being absorbed by CSH resulted in forma-tion of expansive type of ettringite. The XRD results (Figs. 6 and7) shows increased intensity of CS (21, 26, 29 degree 2h) and

50 Kunal et al. / Construction and Building Materials 52 (2014) 42–51

nonexpansive ettringite (7, 17 and 34 degree 2h) in bacterial trea-ted CKD concrete (5% and 10%) responsible for the strength devel-opment in concrete where as in 15% bacterial treated CKDconcrete, reduction in cement content reduced the required alka-linity (needed for hydration reaction) which in turn decreasedthe CSH and thus reduced the strength compared to controlconcrete.

Figs. 9–12 show the SEM analysis of control (0% CKD) and,untreated and bacterial treated CKD (5%, 10% and 15%) contain-ing concrete. In 0% CKD concrete the SEM image shows the for-mation of calcium silicate hydrate and the hydration reactionformed dense structure resulted in increased compressivestrength. Similar type dense structures was also observed in 5%and 10% CKD concrete but are less porous than 0% control at28 days of curing period. In 15% CKD control concrete(Fig. 12a) voids were shown resulted in highly porous structureand exhibited decrease in strength compared to other controltreatments. Samoui et al. [52] observed more reticular and por-ous structure in high alkali cement paste compared to low alkalipaste. In 10% bacterial treated CKD concrete less porous andhighly dense structure was formed at 28 days of curing(Fig. 11b) due to reduced hydration and alkalinity, compactnessof the materials in concrete and this could possibly explain theincrease in strength compared to untreated CKD concrete. In15% bacterial treated CKD concrete (Fig. 12b) highly porous nat-ure and voids due to less cement content and CSH formationthat do not bind actively the materials, thus resulted in reducedstrength in concrete. At the age of 28 days, in 15% bacterial trea-ted CKD concrete needle shaped ettringites were seen on theinterface of the cement paste-aggregate (Fig. 12b) which gener-ates localized pressure and causes expansion of the crystallizedstructure and reduces the strength of concrete whereas in 10%bacterial treated CKD concrete, at 28 days of curing ettingite for-mation was observed in voids (Fig. 11b) which increases thedensity, reduces porosity and strengthens the concrete. These re-sults were in accordance with the findings of Divet and Pavoine[53] and Famy et al. [54] indicating the expansive nature ofettringites on the outer surface of calcium silicate hydrate sur-face at the interfaces of cement paste-aggregate and non-expan-sive nature of ettringite in the voids of the concrete i.e.microporous zones in the cement paste, pores or bubbles. Theexpansive ettringite exerts pressure to the aggregates forminggaps at the interfaces resulting in increased porosity and reducedstrength.

These results suggest that bacterial treatment of CKD reducedits alkalinity and improved the strength of concrete (up to 10%CKD) in later ages due to increased calcium silicate hydrate gel for-mation and formation of non expansive ettringite.

4. Conclusions

1. Treatment of cement kiln dust with Bacillus haloduransstrain KG1 has positive effects on the properties of CKDconcrete.

2. Increase in 7.15% and 26.6% compressive strength of con-crete having 10% bacterial treated CKD after 28 and 91 days,respectively, and decrease in water absorption (20%) andporosity (12.35%) at 91 days was achieved whereas above10%, decrease in strength was observed due to reducedhydration reaction and lower cement content.

3. The late strength development in bacterial treated CKDconcrete is probably due to reduced hydration reaction,increased calcium silicate hydrate gel formation and depo-sition of bacterial cells or spores within the pores ofconcrete.

4. XRD and SEM analysis reveals the increased formation ofCSH gel and nonexpansive ettringite formation which sup-ports the increased compressive strength in 10% bacterialtreated CKD concrete.

5. Further investigation is necessary to identify the effect ofdecreased alkalinity on durability properties of bacterialtreated CKD concrete.

Acknowledgements

The authors wish to express their gratitude to SERB, Depart-ment of Science and Technology, Govt. of India for the support inthis research work. Authors also acknowledge the support ofDepartment of Biotechnology & Environmental Sciences andDepartment of Civil Engineering, Thapar University, Patiala (India)for infrastructure and making this research possible.

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