Accelerated carbonation of basic oxygen furnace slag and the effects on its mechanical properties

Construction and Building Materials 98 (2015) 286–293

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Accelerated carbonation of basic oxygen furnace slag and the effects on its mechanical properties Ming-Sheng Ko a,⇑, Ying-Liang Chen b, Jhih-Hua Jiang a a b

Department of Materials and Mineral Resources Engineering, National Taipei University of Technology, 10608 Taipei, Taiwan Department of Environmental Engineering, National Cheng Kung University, 70101 Tainan, Taiwan

h i g h l i g h t s
Temperature, CO2 content, and relative humidity all affect carbonation reactions.
The appropriate carbonation conditions for BOF slag were found out in this study.
Consumption of free CaO is highly related to decreases in pH value of BOF slag.
The carbonated shell on BOF slag can be up to about 200 lm in thickness.
Carbonation is beneficial to the mechanical performance of BOF slag.

a r t i c l e

i n f o

Article history: Received 29 August 2014 Received in revised form 28 July 2015 Accepted 9 August 2015 Available online 24 August 2015 Keywords: Steel slags Accelerated carbonation Mechanical properties Free CaO Particle size

a b s t r a c t Basic oxygen furnace (BOF) slag, a byproduct of steel-making processes, is mainly composed of calcium compounds and thus has high potential for carbonation. The purpose of this study was to use an accelerated carbonation process to treat BOF slag and to examine the effects on the slag properties. Three BOF slags with different particle sizes (3.5–7 mm, 7–15 mm, and 15–25 mm) were tested, and the effects of carbonation temperature, CO2 content, and relative humidity (RH) were investigated. It was found that the pH value of the BOF slag samples decreased after carbonation, a result which was mainly attributed to the transformation from CaO to CaCO3. The appropriate carbonation conditions selected from this study were 200 °C of temperature, 40% of CO2 content, and 60% of RH. The X-ray diffraction (XRD) and Fourier transform infrared (FT-IR) spectroscopy analyses both demonstrated the formation of carbonates in the treated slag. By observing cross-sections of the carbonated slags with an optical microscope, it was revealed that the thickness of a carbonated shell on the BOF slags can achieve about 200 lm. The mechanical properties of BOF slag, including bearing strength and particle cylindrical crushing strength, were improved after carbonation, and this shows that the carbonation of BOF slag can not only capture CO2 but also have benefits to its mechanical performance. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction The manufacture of iron and steel is generally a fundamental industry for any country since it provides basic materials for infrastructures, vehicles, buildings, industrial facilities, and daily necessities. In addition to iron and steel products, the manufacturing processes generate many kinds of solid byproducts, which are discharged in the form of dust, sludge, ash, and slag. Basic oxygen furnace (BOF) slag is one of the byproducts from steel-making processes, and producing one ton of steel normally generates about 0.1–0.15 tons of BOF slag. In 2013, the annual output of BOF slag in ⇑ Corresponding author at: No. 1, Sec. 3, Chunghsiao E. Rd., 10608 Taipei, Taiwan. E-mail address: [email protected] (M.-S. Ko). http://dx.doi.org/10.1016/j.conbuildmat.2015.08.051 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.

Taiwan was about 1.5 million tons. BOF slag is mainly composed of calcium, silicon, and iron compounds, and it has high specific gravity, mechanical strength, and abrasion resistance [1–3]. Many studies have reported that BOF slag has potential for uses in cement production, road construction, asphalt concrete, and land reclamation in marine areas [4–6]. However, some research has indicated that the high free CaO content in BOF slag can cause volume expansion problems, thus reducing its applicability for construction materials [2,7]. Eqs. (1)–(6) present the chemical reactions in a CaO–H2O–CO2 system. When CaO reacts with H2O to form Ca(OH)2, the density decreases from 3.35 g/cm3 to 2.21 g/cm3, and the volume thus increases by about 100%. As Ca(OH)2 converts to CaCO3, the volume will further increase by 46%. Altun and Yılmaz [8] reported that the

M.-S. Ko et al. / Construction and Building Materials 98 (2015) 286–293

free CaO in BOF slag can react with H2O or CO2 for up to several months under normal atmospheric conditions, so weathering the slag outdoors is a feasible method for improving its volume stability.

CaO þ H2 O ! CaðOHÞ2

ð1Þ

CaO þ CO2 ! CaCO3

ð2Þ

CaðOHÞ2 þ CO2 ! CaCO3 þ H2 O

ð3Þ

CO2 þ H2 O ! H2 CO3

ð4Þ

CaO þ H2 CO3 ! CaCO3 þ H2 O

ð5Þ

CaðOHÞ2 þ H2 CO3 ! CaCO3 þ 2H2 O

ð6Þ

287

2. Materials and methods 2.1. Materials The BOF slag used in this study was generated from a steel mill in southern Taiwan, and the slags was crushed and sieved to four particle sizes, i.e. <3.5 mm, 3.5–7 mm, 7–15 mm, and 15–25 mm, which approximately accounted for 51.3 wt.%, 6.4 wt.%, 24.3 wt.%, and 18.0 wt.%, respectively. The <3.5 mm BOF slag normally is recycled as a raw material for iron-making, and therefore the other three particle sizes of slags were used as research subjects in this study. The specific gravity, pH value, moisture, and free CaO content of the BOF slags with different particle sizes were analyzed immediately. After drying to constant weight in an 105 °C oven, the chemical compositions of the BOF slags were determined by using an alkaline digestion process with lithium tetraborate (Li2B4O7) in platinum melting pots at 1100 °C, followed by dissolving the molten material in hydrochloric acid (HCl) solution and measuring the element concentrations with an inductively coupled plasma-atomic emission spectrometer (ICP-AES, Seiko Instruments Inc., SPS 7800). 2.2. Accelerated carbonation process

Although the outdoor weathering process is feasible, it normally takes a long time and requires a large amount of land space for slag piles, and these both increase the treatment costs of BOF slag. Many studies have shown that some artificial methods, such as steam aging, autoclaving, and carbonation technologies, are capable of accelerating the chemical reactions related to free CaO, and therefore improve the volume stability of steel slags and expand the possible applications [9–13]. In comparison to hydroxides, carbonates are more stable products of the conversion of free CaO, and accordingly carbonation methods should be better than steam aging or autoclaving. Santos et al. [13] used a hot-stage carbonation approach to treat BOF slag, and the results showed that this method can reduce the free CaO content and thus stabilize the slag volume. Bertos et al. [10] reported that steel slag can be reused as aggregates or armor stones after treatment with an accelerated carbonation technology. Moreover, the CO2 capture by calcium-based materials, such as steel slags, has been followed with interest in recent years because of their high CO2 reactivity, high capacity, and low material cost [14]. Several researchers have studied the CO2 capture of steel slags by using high-temperature sorption [13,15] or aqueous carbonation processes [16]. However, there is little information available on the changes in mechanical properties of the carbonated slag [17] and the influence on the subsequent slag utilization. Accordingly, the purpose of this study was to use an accelerated carbonation process to treat BOF slag and to investigate the effects of carbonation conditions, including temperature, CO2 content, and relative humidity (RH), and the changes in the characteristics of the slags.

Fig. 1 shows the schematic diagram of the accelerated carbonation devices used in this study. A rotary kiln made of stainless steel was employed as a carbonation reactor, whose operating temperature range is from ambient temperature to 450 °C. Compared to static reactors, it was posited that using this rotary kiln should improve the level of contact between the gas and solid phases. In order to control the gas composition, a gas mixing chamber was installed in front of the rotary kiln, and three kinds of gases, namely air, CO2(g), and H2O(g), were mixed together in specific proportions and subsequently introduced into the kiln. To prevent H2O(g) from condensing into droplets, the gas mixing chamber was heated as hot as the rotary kiln. During the carbonation process, the weighed BOF slag samples were put into the rotary kiln, and the retention time was set at 24 h to provide sufficient reaction time. In this study, three carbonation conditions, i.e. kiln temperature (25–250 °C), CO2 content (0–40%), and RH (0–80%), were controlled to examine their effects on the characteristics of the BOF slags with different particle sizes. 2.3. Material testing and analyses To determine the pH value of the BOF slags, deionized water was mixed with a slag sample at a liquid-to-solid ratio of 1.0 L/kg, and after stirring for 20 min, the mixture was tested with a pH electrode. The free CaO content of the BOF slags was determined by the chemical extraction method described in ASTM C 114 [18]. A slag sample was extracted with a boiled glycerin-ethanol solvent first, and the extract was titrated with a standard ammonium acetate (CH3COONH4) solution equivalent to 0.005 g CaO/mL. To analyze the mineralogical compositions of the original and carbonated BOF slags, the X-ray diffraction (XRD) method was conducted by using an X-ray diffractometer (Rigaku, D-MAX 2000) with Cu Ka radiation. The XRD patterns were recorded and compared with the standard diffraction cards from the International Centre for Diffraction Data-Powder Diffraction File (ICDD-PDF) to identify which compounds were present in the slags. A Fourier transform infrared spectroscope (FT-IR, Bruker, Vector 22) was used to examine the mineral structures of the carbonated BOF slags. The FT-IR analysis was performed in a wavenumber range of 4000–400 cm1 with a spectral resolution of 1 cm1. The slag samples were further immersed in epoxy resin to prepare the specimens for microstructure observation. After the epoxy resin solidified, the specimens containing slags were cut open, and the cross-sections were polished and then investigated with an optical microscope. To test the mechanical properties

Fig. 1. Schematic diagram of the accelerated carbonation devices.

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Table 1 Physical and chemical characteristics of the original BOF slags with different particle sizes. Characteristic

Particle size (mm)

Specific gravity Moisture content (wt.%) pH Free CaO (wt.%) Chemical composition (wt.%)

CaO SiO2 Al2O3 Fe2O3 K2O MgO MnO

3.5–7

7–15

15–25

3.16 0.52 12.28 6.28

3.30 0.29 12.00 4.90

3.41 0.21 11.90 3.94

42.28 10.99 2.12 31.60 1.30 6.21 5.47

41.71 12.75 1.80 31.05 1.42 6.10 5.15

41.11 13.40 2.59 29.54 1.55 6.52 5.63

of the BOF slags, the California bearing ratio (CBR) and particle cylindrical crushing strength were determined using the standard test methods of ASTM D1883 [19] and CNS 14779 [20], respectively.

3. Results and discussion 3.1. Characteristics of the original BOF slags with different particle sizes Table 1 shows the physical and chemical characteristics of the original BOF slags with different particle sizes. The specific gravity

A: alite, Ca3SiO5 B: larnite, β-Ca2SiO4 C: calcite, CaCO3 H: hematite, Fe2O3 L: lime, CaO

slightly increased from 3.16 to 3.41 with the particle size, a finding which may be due to some iron granules that had combined with large slag particles after cooling. Since the slag was generated from a high-temperature steel-making process, the moisture was very low (0.21–0.52 wt.%), which should be a result of the absorption of atmospheric water. In terms of pH, the 3.5–7 mm BOF slag had the highest pH value of 12.28, while the slags with larger particle sizes had lower pH values. This difference in pH is primarily due to the free CaO content in the slags. The 3.5–7 mm BOF slag contained the largest amount of free CaO (6.28 wt.%), and the free CaO content was negatively related to the particle size. With respect to chemical composition, the BOF slags with different particle sizes did not have significant differences in chemical composition. The primary composition of the slags was CaO (approximately 41–42 wt.%), and the secondary one was Fe2O3 (approximately 30–32 wt.%). In addition, the BOF slag contained small amounts of SiO2 (11–13 wt.%), MgO (6–7 wt.%), and MnO (5–6 wt.%), and traces of Al2O3 and K2O were also present. The chemical composition of a material is an important factor related to CO2 reactivity. Baciocchi et al. [21] reported that stainless steel slag containing high amounts of Ca and Mg, mostly in the form of oxides and silicates, has high potential for reacting with CO2. This suggests that BOF slag should be suitable for CO2 capture. Fig. 2 presents the XRD patterns of the original BOF slags with different particle sizes. Iron compounds including wuestite (FeO), hematite (Fe2O3), srebrodolskite (Ca2Fe2O5), and magnesium iron

M: magnesium iron oxide, Mg0.239Fe0.761O P: portlandite, Ca(OH)2 S: srebrodolskite, Ca2Fe2O5 W: wuestite, FeO

Intensity (a.u.)

15– 25 mm

7– 15 mm

M W

P A A B L H B A BH B CS

10

20

C B

30

3.5– 7 mm M W

P B

40

A

50

2θ θ (degrees) Fig. 2. XRD patterns of the original BOF slags.

L

60

70

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oxide (Mg0.239Fe0.761O) were found in the BOF slags, and the variations in diffraction intensity of these compounds between the slag samples were insignificant. Some calcium compounds, such as lime (CaO), portlandite (Ca(OH)2), calcite (CaCO3), alite (Ca3SiO5), and larnite (b-Ca2SiO4) were also present in all of the slag samples. Based on the diffraction intensity, it was determined that the amounts of CaO and Ca(OH)2 in the 3.5–7 mm BOF slag were higher than those in the other two samples. This finding is consistent with the above results showing that the smallest size slag had the highest amount of free CaO. Generally, the characteristics of the BOF slag samples in this research were found to be close to those discussed in other studies [1,3]. Although the original BOF slags contained similar kinds of minerals, the amounts of the minerals varied slightly depending on the particle size of the slags, especially in the case of CaO and Ca(OH)2.

13.0

(a)

3.5–7 mm 7–15 mm 15–25 mm

12.5

pH

12.0

11.5

11.0

10.5

10.0 50

100

150

200

250

Temperature (°C) 13.0 3.5– 7 mm 7– 15 mm 15 – 25 mm

(b) 12.5

pH

12.0

11.5

11.0

10.5

10.0 0

10

20

30

40

CO2 (%) 13.0 3.5– 7 mm 7– 15 mm 15 – 25 mm

(c) 12.5

pH

12.0

11.5

11.0

10.5

10.0 0

10

20

30

40

50

60

70

289

80

RH (%) Fig. 3. Effects of accelerated carbonation conditions on the pH value of BOF slags; (a) temperature [CO2 = 40%, RH = 60%], (b) CO2 content [temperature = 200 °C, RH = 60%], and (c) RH [temperature = 200 °C, CO2 = 40%].

3.2. Effects of accelerated carbonation conditions on the characteristics of the BOF slags Some studies have reported that carbonation processes can reduce the pH value of materials due to the consumption of alkaline components [10,22]. Fig. 3 shows the effects of the accelerated carbonation conditions, including temperature, CO2 content, and RH, on the pH value of the BOF slags. In Fig. 3(a), the pH value of the slags gradually decreased in the temperature range of 25–200 °C, but then increased above 200 °C. A possible reason for these results is that an increase in temperature enhances carbonation reactions, but at the same time it decreases the solubility of CO2 in water, thus reducing the rate of carbonation at temperatures above 200 °C. With regard to the influence of the CO2 content, Fig. 3(b), it was found that the pH value of the BOF slags dropped with increases in the CO2 content. This should result from the higher amount of CO2 in the gas phase enhancing the rate of carbonation. The slags with different particle sizes had similar pH values after carbonation at a CO2 content of 40%. In Fig. 3(c), the pH value of the slags significantly decreased when the RH increased from 0% to 60% but then slightly increased at an RH of 80%. Bertos et al. [10] noted that water is capable of promoting CO2 reactions by dissolving calcium ions from a solid, but an excessive amount of water will block the pores where CO2 passes through the solid surface. From the above results, it was determined that an RH of 60% should be suitable for the carbonation of BOF slags. Table 2 presents the free CaO content in the BOF slags treated at different carbonation conditions. Although the slags carbonated at 100–250 °C all had less free CaO than the original BOF slags, the free CaO content of the slags declined to the minimum at a temperature of 200 °C. When increasing the CO2 content, the free CaO in the carbonated slags significantly decreased. In terms of the effects of RH, the free CaO content of the carbonated slags decreased with increases in RH from 10% to 60%, but it returned to a higher level at 80% RH. In sum, the variations in the free CaO content of the carbonated slags are similar to those in related to the pH value. This shows that decreases in the pH value of BOF slags are highly related to the consumption of free CaO. It is noteworthy that there were some differences between the various slag samples in relationship to particle size. At identical carbonation conditions, the 3.5–7 mm BOF slag generally had the largest consumption of free CaO, and the consumption amount decreased with increases in the particles size of the slags. Baciocchi et al. [21] indicated that particle size is an important parameter affecting the CO2 uptake of slags, owing to the specific surface area of the particles. In this study, the finer BOF slag originally contained a higher amount of free CaO, and it also had larger specific surface area, which could provide more positions for

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From the above results, it is suggested that a temperature of 200 °C, a CO2 content of 40%, and an RH of 60% were selected as the appropriate conditions for slag carbonation in this study. In comparison to other research related to high-temperature CO2 sorption [13,23,24], in which the carbonation temperatures were between 600 and 700 °C, the carbonation temperature in this study was found to be much lower, meaning that the system should require less energy input. Furthermore, this carbonation system may reuse waste heat directly from steel mills or from other industries (e.g., municipal solid waste incinerators, power plants, and metal refineries) without additional energy for heating. Chang et al. [25] used a slurry reactor to enhance the carbonation efficiency of steel-making slags, a method in which the slags were ground into fine powder (<44 lm) and mixed with deionized water at a liquid-to-solid ratio of 10 L/kg. Although such a chemical approach can significantly improve the carbonation efficiency, it required a large amount of energy for grinding operation and had a problem with liquid-solid separation. Therefore, in comparison to wet chemical approaches, a rotary kiln reactor should be more suitable to use in a practical carbonation system.

Table 2 Free CaO contents in the carbonated BOF slags with different particle sizes. Free CaO (wt.%)

Particle size (mm) 3.5–7

7–15

15–25

100 150 200 230 250

3.76 3.22 1.45 2.88 4.59

2.48 1.70 0.63 2.01 2.15

2.66 2.55 0.37 2.07 3.40

CO2 (%) [temperature = 200 °C, RH = 60%]

10 20 30 40

2.99 2.61 2.25 1.45

4.10 2.19 2.28 0.63

3.91 1.17 0.81 0.37

RH (%) [temperature = 200 °C, CO2 = 40%]

10 30 60 80

3.71 3.93 1.45 3.20

3.44 2.56 0.63 4.35

3.96 2.04 0.37 2.96

Temperature (°C) [CO2 = 40%, RH = 60%]

reacting with CO2. Consequently, after the accelerated carbonation process, the maximum consumption of free CaO of the 3.5–7 mm BOF slag was 4.8 wt.%, whereas those of the 7–15 mm and 15– 15 mm slags were 4.3 wt.% and 3.6 wt.%, respectively. In addition, the maximum consumption percentages of free CaO of the carbonated slags reached 77% (3.5–7 mm), 87% (7–15 mm), and 91% (15–25 mm).

B: larnite, β-Ca2SiO4 C: calcite, CaCO3 H: hematite, Fe2O3 L: lime, CaO

3.3. Microstructures of the carbonated BOF slags Fig. 4 shows the XRD patterns of the BOF slags carbonated at the selected conditions. Some calcium and iron compounds were

M: magnesium iron oxide, Mg0.239Fe0.761O P: portlandite, Ca(OH)2 S: srebrodolskite, Ca2Fe2O5 W: wuestite, FeO

Intensity (a.u.)

15–25 mm

7–15 mm

C P

10

CS

20

B

30

B H BH L C

3.5–7 mm

M W

40

L

P C

50

M W

60

2θ θ (degrees) Fig. 4. XRD patterns of the carbonated BOF slags (temperature = 200 °C, CO2 = 40%, and RH = 60%).

70

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15–25 mm

1036 712 1416

Transmittance (%)

7–15 mm

873

3419

1036

712

873

3.5–7 mm 1416

712 1416 874 4000

3500

3000

1500

1000

500

-1

Wavenumber (cm ) Fig. 5. FT-IR spectra of the carbonated BOF slags (temperature = 200 °C, CO2 = 40%, and RH = 60%).

291

observed in the carbonated slags. The mineral categories were similar to those in the original BOF slags, but they had significant differences in terms of diffraction intensity. In comparison to Fig. 2, it was obvious that calcite became the predominant mineral in the carbonated slags, while lime and portlandite almost disappeared. This finding supports the premise that carbonation reactions related to CaO and Ca(OH)2 occurred and resulted in the formation of CaCO3. The FT-IR spectra of the carbonated BOF slags are shown in Fig. 5, and their surface structures are revealed. After the accelerated carbonation process, the slags with different particle sizes basically had similar FT-IR spectra. The appearance of the strong bands near 712, 873, and 1414 cm1 was mainly attributed to the CO2 3 in calcite, and the O–H bond in portlandite was thought to be responsible for the vibration band near 3419 cm1. In addition, it can be seen that the wide vibration band corresponding to Si-O bonds is located near 1036 cm1 [26–28]. The above results obtained from the FT-IR spectra can further demonstrate the occurrence of carbonation reactions, which causes the predominant surface structure of the slags to change into carbonate bonding. Fig. 6 presents photomicrographs of the cross-sections of the original and carbonated BOF slags. The light-pink or dark-gray part on the right side of the photomicrograph is the solidified epoxy resin. In Fig. 6(a), (c), and (e), it was observed that the original BOF slags showed consistent features from the surface to the interior of the particles. This indicates that the composition of the BOF slags did not vary depending on the radius of the particles. However, after carbonation under the selected conditions, a layer of carbonation products was formed on the surface of the BOF slags, and its features were very different from those of the interior non-carbonated part. Some dark-red materials embedded in the carbonated layer should be the iron oxides. Fig. 6(b) shows that the interface between the carbonated and non-carbonated parts

Fig. 6. Photomicrographs of the cross-sections of the original and carbonated BOF slags. (a) 3.5–7 mm, original, (b) 3.5–7 mm, carbonated, (c) 7–15 mm, original, (d) 7– 15 mm, carbonated, (e) 15–25 mm, original, and (f) 15–25 mm, carbonated.

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was indistinct, so the thickness of the carbonated layer on the surface of the BOF slags measured approximately 150–200 lm. It is known that the carbonation of a solid is a diffusioncontrolled reaction [10,29]. CO2 diffuses into a solid through surface pores, which the carbonation products gradually form around. The pores are progressively blocked up by the carbonation products, and then the gas permeability of the solid decreases. In the end, the surface is completely covered with a carbonated layer, so the CO2 diffusion is restricted, and the carbonation reactions thus almost stop. Walton et al. [29] offered a conceptual model of the carbonation process showing that a shell of carbonated materials

90

(a)

Original Carbonated 85

CBR (%)

80

75

3.4. Effects of carbonation on the mechanical properties of the BOF slags In order to reveal the changes in mechanical properties of the BOF slags after carbonation, the CBR values and the particle cylindrical crushing strength of the original and carbonated BOF slags were determined in this study, and the results are given in Fig. 7. The CBR test is used to evaluate the strength of a compacted material, and a greater CBR value means higher bearing strength. It was found that the accelerated carbonation increased the CBR value of the BOF slags, especially for the sample with smallest particle size (3.5–7 mm). In addition, the particle cylindrical crushing strength of the carbonated BOF slags was much higher than that of the original BOF slags, increasing by 38.9–69.1%. Bertos et al. [10] reported that the accelerated carbonation technology would change the physical properties of a material, and it was suggested that the calcium carbonate can be precipitated in the open pores and thus increasing the mechanical strength of the material. In this study, a layer of carbonates was observed on the surface of the carbonated BOF slags (see Fig. 6), and it probably contributes to the increases in CBR values and particle cylindrical crushing strength. 4. Conclusions

70

65

60 3.5-7

7-15

15-25

Particle size (mm) 40

Particle cylindrical crushing strength (MPa)

surrounds an inner core of intact materials. It was observed that in this study, similar to the conceptual model, the carbonated shell of the BOF slags could achieve about 200 lm in thickness.

(b)

Original Carbonated

30

20

10

Some conclusions can be drawn from the results of this study as follows. The original BOF slags with different particle sizes basically had similar chemical compositions, but there were differences among them in terms of free CaO. Coarser BOF slags had less free CaO, thus resulting in lower pH values. Iron oxides and calcium compounds were the major minerals present in the original BOF slags, and the amounts of the CaO and Ca(OH)2 varied depending on the particle size of the slags. In the accelerated carbonation process for the BOF slags, the temperature and RH must be controlled at proper levels. Increases in temperature can promote carbonation reactions but simultaneously reduce the solubility of CO2 in water, which decreases the rate of carbonation at high temperatures. An increase in RH is beneficial for carbonation; however excessive RH will result in the water droplets blocking the surface pores of the slags, thus interfering with both the CO2 diffusion and the carbonation reactions. Furthermore, it was found that a higher amount of CO2 in the gas phase can increase the rate of carbonation. Accordingly, it was concluded that the appropriate carbonation conditions selected from this study were 200 °C of temperature, 40% of CO2 content, and 60% of RH. Under these conditions, the free CaO content of the BOF slags could be reduced to the minimum levels. The microstructure analyses for the carbonated BOF slags demonstrated the formation of CaCO3 and indicated that the thickness of the carbonated shell surrounding the BOF slags can be up to about 200 lm. The bearing strength and particle cylindrical crushing strength of the BOF slags increased after the accelerated carbonation process, and these findings suggest that carbonation of BOF slag can improve its mechanical performance. Further research may be pursued to examine the physical properties of carbonated BOF slags, such as volume expansion and abrasion hardness, when using the slags as construction materials in practice.

0 3.5-7

7-15

15-25

Particle size (mm) Fig. 7. Mechanical properties of the original and carbonated BOF slags. (a) CBR and (b) particle cylindrical crushing strength.

Acknowledgement The authors gratefully acknowledge the National Science Council, Taiwan, for its financial support of this study (contract number: NSC 99-2211-E-027-030).

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