Indian Journal of Engineering & Materials Sciences Vol. 15, October 2008, pp. 433-440

Characterization of fly ash and it effects on the compressive strength properties of Portland cement Özlem Çelika*, Erdem Damcıa & Sabriye Pişkinb a

Department of Civil Engineering, Istanbul University, Istanbul, Turkey

b

Department of Chemical Engineering, Yıldız Technical University, Istanbul, Turkey Received 11 May 2007; accepted 1 September 2008

In this study, the structure of different type of fly ash samples collected from different thermal power plants (Soma Unit IV/ type C, Çatalagzi/type F, Çayırhan / type C, Tunçbilek/ type F) in Turkey has been investigated. The chemical and physical properties, mineralogical composition and particle size distributions of the samples and their effects on the compressive strength properties of ordinary Portland cement (OPC), have been determined by FTIR, XRD, Mastersizer and SEM. Characterization results show that either Soma Unit IV and Tunçbilek or Çatalağzı and Çayırhan fly ash samples give similar structure of peaks in FTIR/ATR separately. Particle size distribution and SEM results also support each other. There are overlaps between characterization results of fly ash samples and compressive strength results of fly ash mortars. The cement mortars are prepared and tested for compressive strength according to the European Standards (EN 196-1). Results indicate that fly ash samples in the ratio of 15% in clinker markedly increases the compressive strength value (61.1 N/mm2) at 90 days. It indicates that decreasing the particle size of fly ash in blended Portland cement causes an increase in compressive strength. This means that fineness is a more effective parameter than chemical composition in improving the strength development of fly ash mortars and it is suggested that fine fly ash can be used to obtain higher compressive strength values. Keywords: Portland cement; Fly ash; XRD; SEM; FTIR/ATR; Mastersizer

Annually, 15 million tones of fly ash is produced as industrial by-product from the combustion of lignite in the thermal power plants in Turkey and 1% of this amount is utilized in the cement and brick industry. Properties of fly ash depend primarily on the type of coal burned, the type of combustion equipment used and the type of fly ash collection mechanism employed1. Fly ash with very fine particles could be used as a significant source of pozzolanic material. It is mainly of silica component with some impurities. The impurities are mainly quartz (SiO2), clay minerals, calcite and aragonite (CaCO3), hematite (Fe2O3), pyrite (FeS2), and gypsum (CaSO4.2H2O). The occurrence of hematite and magnetite results from the oxidation of pyrite in combustion units. Original structure of quartz and hematite continue during the combustion of coal in thermal power plants. During the combustion, calcium oxide occurs from the decomposition of carbonate minerals and combined with sulfur dioxide forms anhydrite2. It can be seen that the various impurities in coal are the most important factor in the determination of chemical properties and mineralogical compositions of ashes. Because fly ash causes storage problems, its uses as _________ *For correspondence (E-mail address: [email protected])

a mineral additive in the production of cement and mortar seems to be a potential alternative for disposal and environmental protection. The properties of fly ash make it good binding agent and a suitable substitute for natural pozzolan in the amelioration of clinker. It provides superior properties and favorably contributes to the quality of concrete. Therefore, it is an accepted beneficial ingredient in the construction industry widely used in blended cements3,4. Many investigators have studied the use of fly ash in cement production as an additive material. In general, granulometry and fineness of fly ashes have an important effect on mortar strength. Fine particles behave as nucleus initiating and accelerating the hydration reactions5. Also, compressive strength values of mortars are affected by the ratio of crystalline/amorphous in particles6. Early studies show that calcium content and particle size distribution of the fly ashes are the most important parameters governing the strength development rate in normally cured Portland cement-fly ash mixtures. For an effective utilization of fly ash, it is necessary to determine their physical and chemical characteristics and to determine the effect on strength developments of Portland cement-fly ash mixtures7.

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In this study, the mineralogy, composition and chemical characteristics of different fly ashes were investigated for an effective utilization. For this purpose, structures of fly ash samples were collected from different thermal power plants (Soma Unit IV, Çatalağzı, Çayırhan and Tunçbilek) in Turkey. These samples were analyzed by FTIR/ATR, XRD and SEM. Also, Mastersizer-X technique and their effects on the compressive strength properties of ordinary Portland cement (OPC) were investigated.

apparatus was used for the determination of functional groups distribution in fly ashes. The samples were scanned in the region of 4000-400 cm-1. XRD analyses were carried out using a Cu-Kα radiation (40 kV and 20 mA) on a Philips diffractometer equipment. In addition to these methods, Mastersizer equipment (Malvern, UK) and scanning electron microscope SEM (JEOL JSM-5410 Model) were used for measurement of particle size distribution and observation of the surface microstructure of the samples.

Materials and Methods Preparation and testing of cement samples

Raw materials

Fly ash samples were used in this work. The fly ash samples were supplied from different thermal power plants, Soma Unit IV, Çatalağzı, Çayırhan and Tunçbilek in Turkey. Clinker and cement (PC 42.5) were provided from Bursa Cement Plant in Bursa, Turkey. The chemical composition and some physical properties of these materials are presented in Table 1. As it can be seen from the Table 1, SiO2+Al2O3+Fe2O3 (S+A+F) values indicated that fly ash samples obtained from Soma Unit IV and Çayırhan thermal power plants are classified as type C; Çatalağzı and Tunçbilek fly ashes are classified as type F according to the ASTM 618. In Table 1, it can be seen that the chemical compositions of Çatalağzı and Tunçbilek fly ashes have a strong acidic character, having a high (S+A+F) content, 81.4, 85.3 respectively. Characterization of fly ash samples by XRD, FTIR/ATR, SEM, Mastersizer

Before the preparation of the mortar specimens fly ash samples were investigated by a different method, as described. Infrared spectroscopy of the powdered sample was applied using the Perkin-Elmer FTIR/ATR system (typically 16 scans; 4 cm−1). ATR

Mortar mixtures were prepared by intergrinding the Portland cement clinker with gypsum (5%) according to TS-EN 196-1. Composition of the clinker (wt/wt) was adjusted to contain 15, 25 and 35% fly ash. The mortar samples prepared were mixed with 450 g of cement blended with fly ash, 1350 g of sand and 225 mL water in mortar mixer. In the experiments, a three-cell mold (4×4×16) was used. Mortar mixtures were put in a mold to obtain three specimens which were stored in a moisture room under 20±1°C and 90% humidity for 24 h. After demolding specimens were stored under water at 20±1oC until test day 2, 7, 28, 56 and 90, which were measured in test machine. Results and Discussion Functional groups of fly ashes by FTIR/ATR

The FTIR/ATR spectrum of the fly ash samples is shown in Fig. 1. The results show a broad band between 800 and 1400 cm−1. Three characteristic bands centered at around 1100, 1000 and 660 cm−1 have been identified. The strong and broad band at 1000 cm−1 is due to (Si-O-Si) asymmetric stretching vibration. Tunçbilek fly ash sample centered at this band has the highest SiO2 content. The other two

Table 1 — Chemical composition (%) and some physical properties of the materials used Constituents

Soma Unit IV

Çatalağzı

Çayırhan

Tunçbilek

Clinker

Portland Cement

SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2O K2O TiO2 P2O5 Specific gravity (g/cm3)

36.56 21.08 4.2 25.72 1.24 3.53 0.4 1.21 0.67 0.14 1.98

49.8 23.2 8.4 2.99 1.61 0.31 0.4 4.09 1.25 0.091 2.19

47 10 9.8 15.9 2.7 4.6 2.9 2 0.7 0.4 2.21

57.39 18.27 9.65 3.34 3.63 0.5 0.4 1.9 0.81 0.17 1.97

22 5.9 3.6 65.1 1.2 0.2 0.2 1 — — —

20.1 5.8 3.3 65.8 0.8 2.7 0.4 0.5 — — 3.13

ÇELIK et al.: CHARACTERIZATION OF FLY ASH

435

Fig. 1 — FTIR/ATR spectra of the fly ash samples

bands at 1100 and 660 cm−1 are attributed to (Si-O) and (Al-O) vibrations respectively. The bands at 900 and 1160 cm−1 could be described for the SO4−2 groups8. All fly ash samples have these bands but Soma IV and Tuncbilek fly ash samples have different structure from other fly ash samples, and they have the highest and second highest compressive strength values respectively. Other fly ash samples have hump peaks at this range. When examining the studies about hydration products of cement mortars with fly ash in FTIR, the following results were obtained: The FTIR spectra of the OPC and composite cement hydrated up to 28 days are presented in a study and the major changes of the FTIR spectra in the hydrated cement pastes are: The C–O bending vibration (ν2) at about 872 cm−1 and the C–O stretching (ν3) at around 1417 cm−1 are the characteristic band of CO32−. The bands at 617-623 cm−1 are assigned ettringite, in agreement with XRD analysis. The bands appeared at around 1106-1116 cm−1 can be assigned to the stretching vibration (ν3) of gypsum SO42−. A broad band centered at ~ 3400 cm−1 is due to the symmetric and anti-symmetric stretching vibration of water bound in hydrations products. A small yet defined peak appeared at 3637-3641 cm−1 and can be attributed to the OH band from calcium hydroxide. The intensity of the corresponding peak, in the samples containing FA, is lower than that of the other tested sample9. In another study, it is revealed that hydrated minerals portlandite Ca(OH)2 and

tobermorite (CSH), at (3650 cm−1), at (3630 cm−1) respectively8. Mineralogical composition of fly ashes by XRD

The experimental results of XRD analysis on Soma Unit IV fly ash, as shown Fig. 2, indicate that the main mineralogical constituents of Soma Unit IV fly ash sample are: quartz (SiO2), coesite (SiO2), sodium aluminum silicate (Na(AlSi2O6)), tricalcium magnesium orthosilicate (Ca3Mg(SiO4)2), magnesium silicate (Mg2SiO4) and calcium aluminum silicate ((Ca3Al2(SiO4)3). The main mineralogical constituents of the Catalagzı fly ash sample are: quartz (SiO2), mullite (Al4Si2O10) and ferric oxide (Fe2O3). It is shown that Cayırhan fly ash contains mainly albite (NaAlSi3O8), quartz (SiO2) and also ferric oxide (Fe2O3). Tuncbilek, the last fly ash sample, mainly consists of quartz (SiO2), mullite (Al4Si2O10) and magnetite (Fe3O4) (Fig. 2). XRD patterns indicate that the main hydration products for the blended cement (fly ash cement) are CSH gel10-12, ettringite13 and small amounts of Ca (OH)214. Strength development strongly depends on the type and volume of the hydrates formed and thus on the interactions between solid and liquid components15. Particle size distribution of fly ashes by Mastersizer-X technique

Mastersizer-X technique was used for measuring the particle size distribution. The wide range of particle size distribution in fly ash is shown in Fig. 3

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Fig. 2—XRD pattern of fly ashes

and in Table 2. Granulometric data for original fly ashes and their size fractions in Table 2 show that d(90) and d(50) values of fly ash samples are 85.53-329.6 µm and 28.43-100.85 µm, respectively. The size of particle range from 5 µm to well over 320 µm. It can be seen that there are some significant differences in the particle size distribution of the fly ash samples, and mean diameters of the fly ash samples are 40.6, 64.06, 88.26 and 138.44 µm respectively. It is known that the fineness is easily determined by means of the residue left on standard sieves, such as 75 µm and 45

µm. It is generally agreed that cement particles larger than 45 µm are difficult to hydrate and those larger than 75 µm hardly hydrate completely16,17. As it can be seen from the mean diameters, Çatalağzı fly ash has not been hydrated adequately and the mortars with Çatalağzı fly ashes gives the lowest compressive strength values for all curing times. Compressive strength values are 61.1, 52, 55.2 and 56.6 on mortars with 15% fly ash for 90 days. Soma Unit IV fly ash has finer particles than the other samples. It is determined that fineness properties

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437

Fig. 3 — Particle size distributions of fly ashes Table 2 — Granulometric data for fly ashes used in experiments and their size fractions Fly ash

Soma Unit IV Çatalağzı Çayırhan Tunçbilek

(a

Percentages % of particles with diameter Mean d10 (µm) d50 (µm) d90 (µm) diameter (µm) 5.95 9.59 5.01 6.06

28.43 100.85 55.81 38.93

85.53 329.6 221.71 151.42

40.6 138.44 88.26 64.06

increase in fly ash samples, Soma Unit IV, Tunçbilek, Çayırhan and Çatalağzı respectively. The results indicate that there is a strong relationship between particle size distributions and compressive strength values. When comparing mean diameters or particle size distributions of fly ash samples with the compressive strength values of mortars which obtained the same fly ash, it can be seen that the lower particle size gives the higher compressive strength value. This result is supported by other researchers5,18. The particle size distribution seems compatible with the information received from SEM micrographs and mean diameter values of the samples.

(b

(c

Microstructure of fly ashes by SEM

A typical SEM photomicrograph of the fly ash samples is shown in Fig. 4. It provides threedimensional information on an atomic or near atomic scale19. All samples are almost spherical in shape and Soma Unit IV fly ash is finer than the others. SEM micrographs show that Catalagzı fly ash has the very coarse particles in all samples. In some recent studies, which used SEM micrographs concluded that increase in the fineness of blended cement induces development of uniform pore spaces and products of hydration16,20. In another study, it is reported that there are considerable differences in morphology of the hydration products in the high- and low-calcium fly

(d

Fig. 4 — SEM micrographs (a) Soma Unit IV fly ash, (b) Çatalağzı, (c) Çayırhan and (d) Tunçbilek (X 500 and X 2000)

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Table 3 — Compressive strength values (N/mm2) of the mortars produced by fly ashes separately added to Portland cement clinker

Ratio of fly ash in clinker (%)

15

2 Days 25

35

15

7 Days 25

35

Curing time 28 Days 15 25 35

15

56 Days 25 35

15

90 Days 25 35

Compressive strength values (N/mm2)

Fly Ash Samples Soma Unit IV

19.7

18.1

14.6

36

35.5

29.5

50

49

42.1

56.5

56

47.3

61.1

58.6

51

Çatalağzı

17.4

14.4

10.9

32

26.4

21.1

43.6

37

30.5

48.5

42.1

35.3

52

46

39

Çayırhan

20.8

15.7

10.6

37

29.2

23

48

40.5

34.1

52.5

45.6

40.5

55.2

49.2

44.5

Tunçbilek

20.7

15.7

11.7

37.5

30

23.5

47.7

42.5

36.9

53.9

48.5

43.4

56.6

52.5

48

ash pastes. In the low-calcium fly ash paste the microcrystalline hydration products are present while in the high calcium ones the ettringite needle-like products are observed21. Chindaprasirt et al.22, using SEM reported these results at 28 days: fly ash particles were observed in three forms: particles with a dissolved surface, smooth surface, and a covering layer made of hydration product and pozzolanic reaction. They added to these results: The blended cement paste containing original fly ash generally exhibited a lower pore size than that of Portland cement type I paste. Compressive strength of mortars obtained different fly ashes

The compressive strength values of mortars are given in Table 3. It is observed that the mortars obtained by adding Soma Unit IV fly ash to clinker in the ratio of 15, 25, 35% have higher compressive strengths than mortars produced with other blended cements, except 2 and 7 days compressive strength values. An increase in the ratio of fly ash clinker causes a decrease in compressive strength of the mortars. Best values in strength are obtained by the addition of 15% fly ash. The 90-days strength of the mortar produced using Soma Unit IV (15%) was 61.1 N/mm2. When the changing of compressive strength values is examined the ratio of increase in decreasing of compressive strength values is unsystematic for 2 and 7 days. The ratios are extremely different for 28, 56 and 90 days. When Soma Unit IV fly ash is used, up to 25% compressive strength values did not decrease significantly. These decreasing are 2, 0.9 and 5.2 % for 28, 56 and 90 days, but between 25 and 35% decrease increases dramatically and the results are 14.1, 15.5 and 13% for the curing days respectively. Decreasing ratios for fly ash mortars, which were prepared Çatalağzı, Çayırhan and

Tunçbilek are more dramatic than Soma Unit IV between the range of 15-25% and 25-35%. In the range of 15-25, 25-35% for 28 days; 28, 56 and 90 days except samples with Soma Unit IV, the decreasing results are: 14.6, 17.6; 13.2, 16.5, 11.5 and 15.2% for Çatalağzı; 15.6, 15.8: 13.1,11.2, 10.9 and 9.6 % for Çayırhan; 10.9, 13.2; 10, 10.5, 7.24 and 8.57 % for Tunçbilek respectively. As it can be seen from Table 3, Soma Unit IV fly ash is more efficient in increasing the compressive strength than other fly ashes due to its fineness and chemical properties. According to the European Standard (EN 196-1), suitable compressive strength values are in the range of ≥42.5 N/mm2 and ≤ 62.5 N/ mm2. It is shown that the compressive strength values of mortars prepared by fly ash samples are suitable as they are well in the range of the EN 196-1 standards. Mortars with Soma Unit IV fly ash (type C) have the best mechanical strength in terms of chemical compositions, functional groups, mineralogical compositions, particle size distribution, microstructure and pozzolanic reaction to compressive strength. Type C fly ash mortar gives better compressive strength values than type F fly ash23-25. When it is examined in FTIR results, they can be described relatively with other parameters. All fly ash samples give mainly the same peaks, but Soma Unit IV and Tunçbilek fly ash samples have a different structure from other fly ash samples and mortars with these ashes give the highest and second highest compressive strength values respectively. Compressive strength has been obtained from CSH gels which have been defined by XRD in the investigations26,27. It can be seen from Fig. 4 that Soma Unit IV and Tuncbilek fly ash have both fine and spherical particles according to the others. Pozzolanic reaction accelerates with fineness of fly ash. This is supported the investigators28,29. Both

ÇELIK et al.: CHARACTERIZATION OF FLY ASH

pozzolanic reaction and compressive strength properties also increase with the increase of fineness properties30,31. Conclusions The following conclusions can be drawn from this study: (i) The mortar obtained from the cement with Soma Unit IV (15%, 61.1 N/mm2) fly ash gives better mechanical strengths than cement prepared using other fly ashes. (ii) It was determined that the most important parameter is particle size distribution on compressive strength properties, not chemical composition. Although Tuncbilek fly ash has a lower calcium content than the Cayırhan fly ash; it affected the strength of the cement mortars due to its fine particles. Mortars with Cayırhan fly ash have higher compressive strength than the mortars with Catalagzı fly ash. (iii) FTIR showed that Soma Unit IV and Tunçbilek fly ashes have different results, which have the highest, and the second highest compressive strength values, from the other fly ash samples. (iv) The microstructure of fly ash samples support compressive strength value due to their shape and size differences. (v) It was observed that ash amounts in clinker increased from 15% to 35% causing a decreasing effect on the compressive strength of the mortars. As a result, it is suggested that Soma Unit IV fly ash samples can be used as cement additives up to 25% (48-49 N/mm2). It was determined that the utilization of fly ash in production of cement is possible by mixing clinker up to 15%. In the follow-up studies, mineral additives ratios are going to use higher ratios than this study. The purpose of this is to utilize a greater volume fly ash and to contribute to economical and environmental aspects and for these purposes, different types of mineral additives and chemical admixtures will be used. (vi) When type C and type F fly ash fineness properties are improved, higher compressive strength values can be obtained by preparing different mixtures. Acknowledgements The author would like to thank Bursa Cement Factory Quality Control Manager Guler Yurter and laboratory chief Sabiha Kan for assistance in the experimental observations.

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