Preparation and characterization of Portland cement clinker from iron ore tailings

Construction and Building Materials 197 (2019) 152–156

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Preparation and characterization of Portland cement clinker from iron ore tailings G. Young a,b, Mei Yang b,⇑ a b

School of Sciences, Wuhan University of Technology, Wuhan 430070, China Faculty of Materials Science and Chemistry, China University of Geosciences (Wuhan), Wuhan 430074, China

h i g h l i g h t s  The IOT were characterized by techniques like XRD and XRF.  The raw materials with different IOT addition were synthesized cement clinkers.  The addition of IOT did not qualitatively affect the mineralogical phases.  The optimum IOT addition for producing good clinker were investigated.

a r t i c l e

i n f o

Article history: Received 2 June 2018 Received in revised form 13 November 2018 Accepted 23 November 2018

Keywords: Iron ore tailings Raw material Optimum addition Cement clinker

a b s t r a c t We report a type of high-magnesium and low-silicon iron ore tailings (IOT) utilized as a raw material replacing clay to produce cement clinkers by conventional sintering process. Properties of the cement clinkers sintered at 1420 °C with different IOT addition, from 0 to 20 wt%, were investigated. The chemical and mineralogical analysis, and microscopic examination showed that IOT addition, up to 10 wt%, has little effect on formation of mineralogical phases. Furthermore, physico-mechanical tests showed the IOT addition within 10 wt% did not negatively affect the quality of the produced cement clinkers, on the contrary, could produce better quality clinkers than without IOT. These findings suggest that the highmagnesium and low-silicon IOT has a huge potential as cement raw materials instead of partial natural resources. Ó 2018 Published by Elsevier Ltd.

1. Introduction Tailings are generally considered by-products of several extractive industries, including those for iron, coal, oil sands, uranium and precious and base metals [1]. Large amounts of tailings not only occupy the land, but also contain potentially hazardous contaminants, which can bring some environmental problems [2–4]. According to official statistics, the annual discharge of iron ore tailings in China was 0.3 billion tons and the comprehensive utilization rate of IOT is still less than 10% [5,6]. In order to alleviate environmental pressures and attain sustainability, one feasible solution is to reuse the waste materials. It is well known that various solid wastes have been utilized as alternative raw materials in cement productions such as steel slag, fly ash and wastes sludge [7–10]. With the benefit of high content of silica and iron, iron ore tailings have the potential to be utilized as silicate or iron ⇑ Corresponding author at: Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan, Hubei 430070, China. E-mail address: [email protected] (M. Yang). https://doi.org/10.1016/j.conbuildmat.2018.11.236 0950-0618/Ó 2018 Published by Elsevier Ltd.

corrective material to produce green and sustainable cement products. Previous studies have shown that IOT are potential to be effectively utilized in cement productions and concrete materials. Luo et al. [11] reported that the cement product using IOT as alumina-silicate raw material has higher reactivity and lower hydration heat than the ordinary product. Shettima et al. [12] carried out a research on concrete using IOT as replacement for fine aggregate. They found the mechanical strengths of concrete with IOT are improved, while the workability is reduced. Moreover, Zhao et al. [13] utilized IOT to produce the ultra-high performance concrete, the maximum additional quantity of IOT reached to 40 wt%. However, the chemical components of IOT from different ore districts are various. A large number of research show a great potential for reuse, but this kind of IOT containing high magnesium oxide are seldom mentioned in the previous studies, which motivated us to carry on this research. Therefore, a series of experiments were performed to utilize IOT from Yeshan mine as raw materials to produce Portland cement production.

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2. Raw materials and methods 2.1. Raw materials The IOT, coming from Yeshan ore district, Jiangsu, China, were supplied from Nanjing Iron and Steel Group. The mineralogical phases of IOT were performed by X-ray diffraction (XRD, Bruker AXS D8-Focus diffractometer with Cu Ka radiation, k = 1.5406 Å). The crystalline phases identified by matching the peak positions of the intense peaks with PCPDF standard cards are given in Fig. 1. The main crystalline phases are dolomite, serpentine, magnetite, K-feldspar, and quartz. The chemical analyses of the used raw materials performed by X-ray fluorescence (XRF, Axios, Netherlands) are presented in Table 1. The results show that the IOT primarily consist of SiO2, Fe2O3, MgO and CaO, specifically, contains high-MgO and lowSiO2 compared with other reported IOT used in building materials [7,10–13]. 2.2. Sample preparations and test techniques The production of cement clinker is controlled by clinker modulus. Here, lime saturation ratio (KH), silica modulus (SM), and alumina modulus (IM) were set to 0.90, 2.30, and 1.50, respectively. The IOT were dried in an oven at 105 °C for 3 h. Then 5, 10, 15, and 20 wt% IOT were mixed in raw meal and adjusted to clinker modulus values, as well as the raw meal without IOT as a reference for comparisons. Remarkably, the content of MgO in the final raw meal should not exceed 3.5 wt%. The quantities were calculated according to the chemical component of raw materials in Table 1. The results showed the MgO in the final raw meal with the maximum IOT addition is about 3.3 wt%, which remains in the safe range. Mixtures were ground in a laboratory oscillating mill for 5 min. After grinding, these raw materials were burned at 1420 °C for one hour. At the end of sintering process, the produced clinkers were taken out from resistance furnace immediately and cooled rapidly

to room temperature with an air blower in order to avoid the formation of c-C2S. The clinkers produced were analyzed by chemical analysis, Xray diffraction and metalloscope (LV150NL, Nikon, Japan). Free lime (f-CaO) content was detected by the glycerol-ethanol method. Furthermore, the microstructure of cement clinkers were studied on scanning electron microscope (SEM, Quanta200, FEI, Netherlands). The samples were polished and then sputtered with gold coatings. The sintered clinkers were crushed and pulverized with 2 wt% gypsum in oscillating mill for 10 min to produce Portland cement. The fineness of cement was tested with sieving method [14] after pulverizing, and the result showed the sieving residue of 200mesh sieve (74 mm) was about 2.6 wt%. The specified value in Chinese National Standard GB/T 21372-2008 [15] is 4.0 wt%. The cement pastes were prepared with one part by mass of cement, three parts of standard sand and one half part of water for determination of cement strength according to the Chinese National Standard GB/17671-1999. Then cement mortars were cast into steel molds and compacted on a vibration table for 60 s to eliminate bubbles. Prismatic specimens, dimensions 40  40  160 mm, were prepared for mechanical test. The specimens were de-molded after curing for 24 h in a standard curing box, then transferred to a moisture chamber with controlled temperature and relative humidity (temperature 20 ± 2 °C, relative humidity 95%) until prescribed ages. The compressive and flexural strengths of specimens were tested by micro-controlled electronic universal testing machine according to Chinese National Standard GB/T 17671-1999 [16]. The loading rates applied in the compressive and flexural strengths were 2400 N/s and 50 N/s respectively. Besides, other physical properties, including water requirement of normal consistency, setting time and soundness, were examined based on Chinese National Standard GB/T1346-2011 [17]. 1-Alite(C3S) 2-Belite(C2S)

1,2

3-Aluminate(C3A)

1,2

160

4-Ferrite(C4AF)

1,2

1

1,2

140

1-dolomite 2-serpentine 3-magnetite 4-quartz 5-K-feldspar 6-chlorite 7-muscovite 8-amphibole

1,2,4

Intensity(cps)

120 100 80

5-MgO

1,2

3 4

1

1

1 4

2

2

1,2 4

1,2

1

3,4

5

1,3

2,3 2 1,3,4

1,4

60

PC20

PC10

8 2,8

40 2,8

2

2,6

7

20

2

6

2,4,6

4,7 2,7 5 2 3 5

3,5 1,3

1,6

1,2,4

3,4

0

PC00 5

10

15

20

25

30

35

40

45

50

55

10

15

20

25

30

2

35

40

45

50

55

60

65

2

Fig. 1. X-ray diffraction pattern of iron ore tailings.

Fig. 2. X-ray diffraction patterns of cement clinkers.

Table 1 Chemical analysis of raw materials for cement clinkers. Raw materials

Limestone Sand Clay IOT

Oxides (wt%) SiO2

Fe2O3

CaO

Al2O3

MgO

SO3

Na2O

K2O

Lost

0.48 96.38 57.54 29.14

0.04 0.43 6.62 17.01

55.14 0.30 1.87 13.20

0.12 0.59 23.74 5.01

0.82 0.21 1.05 16.27

0.11 – 0.28 1.74

0.03 0.13 1.02 0.41

0.08 0.95 0.31 1.10

43.13 0.82 7.45 16.04

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Table 2 Chemical analysis of the produced cement clinkers. Portland Cement Clinker

PC00 PC10 PC20

Oxides (wt%) CaO

SiO2

Al2O3

Fe2O3

Na2O

K2O

MgO

SO3

f-CaO

LOI (950 °C)

66.18 65.74 63.27

21.74 20.87 20.05

6.12 6.37 6.79

3.32 3.55 3.92

0.18 0.11 0.07

0.09 0.15 0.23

0.88 1.83 3.95

0.11 0.24 0.40

0.72 0.75 1.12

0.27 0.30 0.38

Table 3 Mineralogical composition of the produced cement clinkers. Portland Cement Clinker

PC00 PC10 PC20

Crystal Phases (wt%) C3S

C2S

C3A

C4AF

55.13 57.45 48.32

20.73 16.48 21.03

10.59 10.86 11.35

10.09 10.79 11.92

Fig. 3. Microstructure of the cement clinker PC10: A) Alite crystals existing in various sizes; B) Partly alite crystals edge decompose seriously; C) Bluish rounded belite crystals; D) Interstitial melt crystallizing to mixtures of ferrite and calcium aluminate crystals.

3. Results and discussion 3.1. Chemical and mineralogical characteristics of produced clinkers The XRD analyses of produced clinkers with 0, 10, 20 wt% IOT (PC00, PC10, PC20) are presented in Fig. 2. It can be seen that main mineralogical phases, C3S, C2S, C3A and C4AF were well formed in these three clinkers. The absence of c-C2S was also confirmed by X-ray diffraction. Dicalcium silicate was detected in the form of b-C2S, which has been probably stabilized by the presence of impurity ions (Fe3+, Al3+) and rapid cooling [8]. f-CaO content is very low, so as not to be detected in the XRD patterns.

The chemical analysis and potential mineral composition (according to Bogue) of cement clinkers are given in Tables 2 and 3, respectively. As the tables present, the addition of IOT below 10 wt% did not seem to affect its mineralogical composition. When IOT quantities reached 20 wt%, it can be seen that the mineralogical composition changes obviously. The PC20 clinker contained more C4AF and less C3S. Besides, The contents of MgO, f-CaO and SO3 in PC20 are relatively high. This could be attributed to the heavy replacement of clay with the IOT. Consequently, the addition of IOT seems to have little effect on mineralogical phases as if the IOT content in raw materials does not exceed 10 wt%. However, the chemical constituents and

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Fig. 4. SEM images of the cement clinker PC10 and its energy spectrums.

potential mineral composition of the cement clinker PC20 still meet the requirements of Chinese Standard GB/T 21372-2008 [15]. 3.2. Microstructure analysis for the produced cement clinkers The microstructure of cement clinkers was examined by optical microscope in the polished sections. As shown in Fig. 3, the matrix contains prismatic C3S crystals [Fig. 3A], round C2S crystals [Fig. 3C], and irregular interstitial phase [Fig. 3D]. f-CaO is dispersed among other phases, in very low percentage. The subhedral and euhedral C3S crystallize in a variety of sizes. Whereas, partial alite edge decomposes seriously [Fig. 3B], this could be attributed to the drop in temperature was not fast enough. In the optical microscope, C2S was observed as bluish rounded crystals, with sizes between 20 and 40 lm. In most cases, C2S crystals gathered into a lump surrounded by C3S crystals. The interstitial melt took a relatively high proportion, it can be noted that the interstitial melt were crystallized to mixtures of ferrite and calcium aluminate crystals. Fig. 4 presents the microstructure of cement clinkers clinker PC10 and its energy spectrums. From Fig. 4(A), we can see that prismatic C3S crystals have different sizes and C2S appears as globular

or ellipsoidal shape. In Fig. 4(B), it is shown that C4AF and C3A melt together and interweave each other being interstitial phase. These characteristics are in accordance with those observed by optical microscope. 3.3. Physical and mechanical characteristics of the produced OPC Table 4 shows the physical and mechanical properties of the produced cement clinkers. The ‘‘water demand” is usually considered to be the quantity for water required to prepare a standard consistency cement paste [8]. The obtained values indicated that the values for water demand, setting time and expansion have little difference between the PC00 and PC10 clinkers. Contrast to the PC00 clinker, the PC20 needs more water and shorter time to become a standard consistency cement paste, and the expansion goes worse. Though these properties change distinctly, but still qualified according to Chinese National Standard GB/T 1346-2011 [17]. The compressive strengths of mortars at age of 7 and 28 days curing are listed in Table 4. It can be seen that both the PC00 and PC10 clinker present similar compressive strengths, but the PC20 clinker declines sharply. This observed decrease in the compressive

Table 4 Physical and mechanical properties of the produced cement clinkers. Portland Cement Clinker

PC00 PC10 PC20

Water Demand/wt%

21.2 21.4 26.3

Setting Time/min

Expansion/mm

Initial Time

Final Time

136 132 105

190 185 142

0.8 0.8 1.4

Compression Strength/MPa 3 Days

28 Days

21.2 24.1 16.6

50.6 52.3 38.4

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3d 28d

Flexural strength (MPa)

8

50 7

45 40

6

35 5

30 25

4

20 3

Conflict of interest

55

None. Compressive strength (MPa)

9

Acknowledgements Financial support from Yeshan Mining Co., Ltd affiliated with Nanjing Iron & Steel Group, is gratefully acknowledged. The authors are grateful to Wu Ye, a researcher from school of sciences, Wuhan university of technology, for helping to improve the manuscript. The authors also would like to express their appreciation to the anonymous reviewers for some constructive suggestions.

15

2

10 0

5

10

15

20

IOT Content (%) Fig. 5. Flexural and compression strengths of cement clinkers with different ITO addition.

strength could be attributed to the significant reduction of C3S which plays leading role in the compressive strength. 3.4. Optimum ITO addition The strengths of produced cement clinkers with different quantities of IOT are shown in Fig. 5. The strengths are raised slightly as the percentage of IOT goes up, but when the content exceeds 10 wt %, the strengths descend sharply. The observed improvement in strengths could be partly ascribable to the affect of MgO and SO3. Li et al. [18] reported that additions of MgO can contribute to the formation of alite by lowering the clinker’s formation temperature. They also found that SO3 and MgO impact the interstitial phase content, specifically, C4AF increases with SO3 addition. Staneˇk’s [19] research drew similar conclusions that the alite is formed after reaching the burning temperature on condition that MgO content is high and SO3 content is low. And positive influence of MgO is observed only in presence of alkalis, specifically K2O. Based on Theodor Staneˇk’s conclusion, the positive effect of MgO could be affirmed due to the existence of K2O in the IOT. When a small quantity of IOT, no more than 10 wt%, are added into raw materials, MgO could promote the alite formation. With IOT addition increase, the quantity of SO3 would be elevated, which would result in interstitial phase increasing according to Li’s conclusion. Mineralogical composition listed in Table 4 and XRD diffraction patterns presented in Fig. 2 can prove the correctness of the conclusion sufficiently. 4. Conclusions The research highlighted the feasibility of recycling IOT with producing cement clinker. Results show that the raw meal within 10 wt% IOT addition sintered at 1420 °C for one hour can produce better quality cement clinker than without IOT. XRD analysis, optical microscope and SEM examination showed that IOT addition did not qualitatively affect the formation of mineralogical phases. The mechanical properties of produced cement products are comparable with those of 42.5R grade and the physical properties are also qualified according to Chinese National Standard GB 175-2007 [20]. Thus, we have, for the first time, applied high-MgO and low-SiO2 IOT as raw materials to produce cement clinkers. We expect these results to be very useful to mining company and environmental protection agency disposing IOT in the near further.

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