reconstruction of broken si–o–si bonds in iron ore...

International Journal of Minerals, Metallurgy and Materials Volume 26, Number 10, October 2019, 329 https://doi.org/10.1007/s12613-019-1811-z

Corresponding author: Yu-cheng Zhou E-mail: [email protected]

© University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2019

Reconstruction of broken Si–O–Si bonds in iron ore tailings (IOTs) in concrete

Juan-hong Liu1,2,3), Yu-cheng Zhou1,2,3), Ai-xiang Wu1,3), and Hong-jiang Wang1,3)

1) School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China

2) Beijing Key Laboratory of Urban Underground Space Engineering, University of Science and Technology Beijing, Beijing 100083, China

3) State Key Laboratory of High-efficient Mining and Safety of Metal Mines, Ministry of Education, University of Science and Technology Beijing, Beijing 100083,

China

(Received: 19 October 2018; revised: 9 January 2019; accepted: 11 January 2019)

Abstract: This paper reports a study on the reconstruction of broken Si–O–Si bonds in iron ore tailings (IOTs) in concrete. Limestone and IOTs were used to investigate the influence of different types of coarse aggregates on the compressive strengths of concrete samples. The dif-ferences in interfacial transition zones (ITZs) between aggregate and paste were analyzed by scanning electron microscopy (SEM) and ener-gy-dispersive spectroscopy (EDS). Meanwhile, X-ray diffraction (XRD) and infrared spectroscopy (IR) were used to study microscopic changes in limestone and IOTs powders in a simple alkaline environment that simulated cement. The results show that the compressive strengths of IOTs concrete or paste are higher than those of limestone concrete or paste under identical conditions. The Ca/Si atom ratios in the ITZs of IOTs con-crete samples are lower than those of limestone concrete; the diffraction peak of the calcium silicate phase at 2θ = 29.5°, as well as the bands of Si–O bonds shifting to lower wavenumbers, indicates reconstruction of the broken Si–O–Si bonds on the surfaces of IOTs with Ca(OH)2.

Keywords: iron ore tailings; broken Si–O–Si bonds; alkaline environment; reconstruction

1. Introduction

The development of the mining industry has led to the production of abundant iron ore tailings (IOTs). The total amount of IOTs around Beijing in China has reached more than 20 billion tons, and more than 1 billion tons is generat-ed each year. Stored IOTs occupy a large amount of land and pollute water [1–2]. Similarly, the environmental and ecological damage caused by the acquisition of building materials through rock blasting is incalculable [3]. Given this deterioration of the ecological environment and the shortage of high-quality natural resources, a new approach to eliminate the accumulation of solid waste by using tailing waste rocks as raw materials in the preparation of building materials has been proposed.

The use of tailings as a substitute for aggregates in con-crete is very effective. Thomas et al. [4] found that concrete samples still exhibited appropriate compressive strengths and durability when tailings replaced nearly 60wt% of the aggre-gate content. Furthermore, Zhao et al. [5] regarded IOTs as fine aggregates in the preparation of ultra-high-performance

concrete. Han et al. [6] and Cheng et al. [7] utilized ground IOTs as a cementitious material. Cai et al. [8], Ma et al. [9], and Huang et al. [10] researched the hydration characteris-tics of autoclaved aerated concrete containing IOTs and found that the main products consisted of C–S–H gel and tobermorite.

In fact, IOTs have been proven to be inert materials [6,11–12], and many broken bonds are present on mineral surfaces be-cause of cleavage. Moon and Fuerstenau [13] calculated the broken bond strength and concluded that the cleavage of minerals occurred along the weakest plane. Gao et al. [14], Hu et al. [15], and Longo et al. [16] floated complex minerals according to the properties of broken bonds on mineral sur-faces. When mineral surfaces interacted with reagent mole-cules, the broken cationic particles on the mineral surfaces formed very strong chemical bonds with the oxygen atoms in reagents [17]. Therefore, the adsorption behaviors of water molecules and pharmaceutical molecules on mineral surfaces are determined by the properties of broken cationic particles on mineral surfaces, and the density of broken bonds affects the adsorption behaviors of mineral surfaces [18]. Further-

1330 Int. J. Miner. Metall. Mater., Vol. 26, No. 10, Oct. 2019

more, the rebonding of broken bonds is specifically applied in the aerospace and nuclear power industries [19].

With further advances in research, the artificial manu-facture of broken bonds has become a popular research topic. The broken bonds on the surfaces of silica particles can be used to form consolidated bodies, as revealed by Nakashima et al. [20], whose findings further showed that the broken Si–O–Si bonds could absorb OH− in water. Borouni et al. [21] investigated the crystallization of amorphous silica nano-particles by mechanical activation in the presence of pure aluminum. During mining crushing, a large number of Si–O–Si bonds on the surfaces of IOTs are broken. If these unsaturated coordination bonds of Si atoms can form bond-ing phenomena in a particular environment, they would have an inestimable influence on the effective utilization of IOTs and reduce consolidation costs.

In this paper, limestone and IOTs were used to investi-gate the influence of various types of coarse aggregates on the compressive strengths of concrete samples at a low wa-ter/binder (w/b) mass ratio. The differences in interfacial transition zones (ITZs) between aggregates and hardened paste were analyzed by scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Meanwhile, X-ray diffraction (XRD) and infrared spectroscopy (IR) were used to study the phase changes in limestone and IOTs powders in a simple alkaline environment that simulated cement. Reconstruction of the broken Si–O–Si bonds of IOTs in concrete was also revealed.

2. Experimental

2.1. Raw materials

The materials used consisted of P.I 42.5 Portland cement

conforming to Chinese National Standard GB 175–2007, S105 ground granulated blast-furnace slags conforming to Chinese National Standard GB/T18046–2008, and Class II fly ash conforming to Chinese National Standard GB/T 1596–2005. The specific surface area of the slag was 560 m2/kg. River sands with a fineness modulus of 2.1 were used. The main chemical compositions of the cement, slag, and fly ash are shown in Table 1.

Coarse aggregates of limestone with sizes ranging from 5 to 25 mm were used in the tests. IOTs were supplied by the Miyun Waste Rock Accumulation Plant in Beijing, China. Because of the large sizes of the IOTs, 5–10 mm li-mestone aggregate was blended with the IOTs to ensure the same gradation as limestone. The limestone mainly con-sisted of calcite, whereas the IOTs comprised quartz, albite, and white mica. The crushing value and bulk porosity of the two aggregates were similar to eliminate interference of the aggregate properties on the measurements of the compres-sive strengths of the concrete, as shown in Table 2. The main chemical compositions of the limestone and IOTs are shown in Table 3.

Table 1. Main chemical compositions of the cementitious materials wt%

Materials SiO2 Al2O3 Fe2O3 CaO MgO SO3 Na2Oeq

Cement 21.73 4.60 3.45 64.55 3.56 0.46 0.59

Slag 30.63 14.57 0.56 39.06 9.20 2.35 0.44

Fly ash 51.61 35.69 3.90 3.21 0.87 0.73 0.25

Table 2. Parameters of the limestone and IOTs

Materials Crushing value / wt% Bulk porosity / vol%

Limestone 15.6 43.8

IOTs 16.3 44.5

Table 3. Main chemical compositions of the limestone and IOTs wt%

Materials SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2Oeq TiO2 P2O5 LOI

Limestone 7.74 1.23 0.85 46.83 2.97 ― ― ― ― 39.97

IOTs 58.94 15.89 8.58 5.89 3.34 3.19 2.66 0.60 0.28 ―

Note: LOI—Loss on ignition.

2.2. Test methods

2.2.1. Compressive strength tests Limestone and IOTs aggregates were used to prepare

concrete samples labeled No. 1–4 and No. 5–8, respectively. In these groups, slag was added in mass proportions of 20wt%, 30wt%, 40wt%, and 50wt% of the total amount of cementitious materials to replace fly ash, as shown in Table 4. Concrete samples with dimensions of 100 mm × 100 mm × 100 mm were prepared for the determination of compres-sive strengths after standard curing (20 ± 1°C and 95% RH)

for 90 d. The compressive strengths of the concrete samples at 7,

28, 56, and 90 d were measured in accordance with Chinese National Standard GB50107–2010. Each value represents the average of the results for three concrete samples. 2.2.2. SEM and EDS tests

To eliminate the influence of river sands, ITZs were ob-tained accurately. Small concrete pieces were specifically made without the addition of fine aggregates for SEM and EDS under standard curing conditions for 90 d. Meanwhile,

J.H. Liu et al., Reconstruction of broken Si–O–Si bonds in iron ore tailings (IOTs) in concrete 1331

the mix proportions of cementitious materials were the same as those for No. 1 and No. 5 in Table 4.

Table 4. Mix proportions of concrete samples kg·m3

No. Water Cement Slag Fly ash Limestone IOTs Sand

1 150 150 100 250 1209 — 741

2 150 150 150 200 1209 — 741

3 150 150 200 150 1209 — 741

4 150 150 250 100 1209 — 741

5 150 150 100 250 — 1209 741

6 150 150 150 200 — 1209 741

7 150 150 200 150 — 1209 741

8 150 150 250 100 — 1209 741

The paste was first stirred evenly; the limestone and IOTs

aggregates with an average particle size of 15 mm were then added to groups to ensure that the paste/aggregate volume ratio was 0.4. Eventually, the paste was placed into cubic molds with dimensions of 30 mm × 30 mm × 30 mm. After 90 d of standard curing, the samples were sliced into thin slices of 5-mm thickness. During the slicing process, an ob-vious interface between the aggregate and hardened paste was observed and polished. Clean and smooth parts con-taining the ITZs between the aggregate and the paste were selected for SEM and EDS by an FEI Quanta 200 FEG scanning electron microscope. To examine the ITZs be-tween the aggregate and paste, points at distances of 30, 60, and 90 μm from the interfaces of different aggregates were analyzed by EDS; the results are shown in Fig. 1. To in-crease the accuracy of chemical composition analysis, the inclusion of unhydrated fly ash and some inert particles in ITZs should be avoided. Each value was averaged from more than 20 points at distances of 30, 60, and 90 μm. 2.2.3. XRD and IR analyses

Eliminating the interference of cement hydration and creating a simple alkaline environment that simulated ce-

ment were necessary. In addition, the aggregates were ground into stone powders. IOTs powders were added to a saturated solution at 2.0 in Ca(OH)2/CaSO4·2H2O mass ra-tio, and the upper liquid was used for determination of the chemical compositions.

Fig. 1. Microstructure of sample No. 4 in Table 4 and selected points in ITZs.

The paste samples with CaSO4·2H2O and Ca(OH)2 were made according to the mixture mass ratios in Table 5. Stone powders with specific surface areas of 400 m2/kg and 600 m2/kg were formed by grinding the limestone and IOTs ag-gregates. Then, the paste samples with dimensions of 30 mm × 30 mm × 30 mm were standard cured for 56 d. Even-tually, the compressive strengths of the paste samples at 7, 28, and 56 d were determined to be similar to those of con-crete samples.

After 56 d of standard curing, XRD and IR analyses were carried out on the powders made of limestone and IOTs paste samples. The results of XRD and IR tests were com-pared to corresponding unhydrated raw materials (91wt% stone powders, 3wt% CaSO4·2H2O, and 6wt% Ca(OH)2). All of the powders were mixed evenly, and the phase com-positions of the powders were analyzed by a TTR III X-ray diffractometer and a NEXUS670 infrared spectrometer.

Table 5. Mix proportions of paste samples

No. Water / wt% Stone powder / wt% Category CaSO4·2H2O / wt% Ca(OH)2 / wt%

1 30 91 400 m2·kg1 limestone powders 3 6

2 30 91 600 m2·kg1 limestone powders 3 6

3 30 91 400 m2·kg1 IOTs powders 3 6

4 30 91 600 m2·kg1 IOTs powders 3 6

3. Experimental results

3.1. Compressive strengths of concrete samples

The compressive strengths of two kinds of concrete sam-ples at 7, 28, 56, and 90 d at a w/b mass ratio of 0.3 are compared in Table 6. At the same proportion of cementi-

tious materials, the compressive strengths of the IOTs con-crete samples are 6%–33% greater than those of limestone concrete samples. Comprehensive consideration of stone porosities and crushing values indicates that IOTs are bene-ficial for improving the compressive strength of concrete samples.


Table 6. Compressive strengths of concrete samples MPa

No. 7 d 28 d 56 d 90 d

1 42.3 51.3 57.5 60.6

2 43.1 51.9 57.8 60.9

3 43.6 52.9 58.2 61.2

4 44.3 53.6 58.5 61.3

5 45.1 56.5 63.1 67.2

6 47.6 59.2 64.6 67.9

7 54.1 62.9 68.2 71.5

8 59.3 70.6 74.1 76.9

3.2. SEM and EDS analyses of concrete samples

As shown in Fig. 1 and Table 7, the Ca/Si atom ratios at the three points of the limestone and IOTs concrete samples are approximately 1.2 and 0.8, respectively. Obviously, the Ca/Si atom ratios in the ITZs of the IOTs concrete samples are lower than those in the limestone concrete samples. The

number of Ca atoms is greater than the number of Si atoms in any same length span around the limestone, whereas the IOTs concrete samples exhibit the opposite result. The gels around the limestone and IOTs are C–S–H phase with dif-ferent structures containing several elements (e.g., Mg, Al, and Fe) on the surfaces.

3.3. Changes of elements in the upper liquid

As shown in Fig. 2, no change is observed in the content of SiO2, which indicates that Si–O–Si bonds were not de-stroyed. Both the experiments and the literature [11–12] in-dicate that the IOTs are inert aggregates, thus ruling out the possibility of a reaction of the IOTs with cementitious ma-terials. Meanwhile, the concentrations of Ca2+ and OH− ex-hibit downward trends, indicating that some Ca(OH)2 ad-hered to the IOTs powders and that a physical or chemical interaction occurred between IOTs and Ca(OH)2.

Table 7. Ca/Si atom ratios and chemical compositions in ITZs

Types Distance / μm Ca/Si atom ratios Ca / at% Si / at% Al / at% Mg / at%

Limestone

30 1.32 12.11 9.17 3.41 4.84

60 1.18 13.45 11.36 7.26 2.13

90 1.25 10.06 8.07 3.86 4.32

IOTs

30 0.76 9.44 12.41 4.05 1.42

60 0.79 8.73 11.10 2.19 1.68

90 0.86 10.59 12.36 7.31 2.30

Fig. 2. Changes of elements and pH in the upper liquid.

3.4. Compressive strengths of paste samples

As shown in Fig. 3, the compressive strengths of the IOTs paste samples are obviously higher than those of the limestone paste samples at a w/b mass ratio of 0.3, which coincides with the results for the compressive strengths of the concrete samples. The characteristics of IOTs were re-flected in a simple alkaline environment.

When the sample surfaces are viewed at 5000× magni-

fication, as shown in Fig. 4, small particles are observed to be loosely stacked on the surface of the limestone paste sample. In this case, the main strength might be caused by gypsum hardening, carbonization of Ca(OH)2, or agglome-ration of powders [22]. The surface of the IOTs paste sam-ple is as dense as the C–S–H gel, and a bonding force might exist between IOTs powders.

Fig. 3. Compressive strengths of limestone paste samples and IOTs paste samples.


Fig. 4. Microstructure of (a) 600 m2·kg-1 limestone paste sample and (b) 600 m2·kg-1 IOTs paste sample.

3.5. XRD pattern of paste samples

Fig. 5 shows the XRD patterns of two kinds of paste samples after 56 d of standard curing; the patterns of the corresponding unhydrated raw materials are included for comparison.

The XRD patterns of limestone paste samples are similar to those of unhydrated raw materials, with the peak of Ca-CO3 appearing at 2θ = 29.4° and the peak of CaMg(CO3)2 at approximately 2θ = 31°. However, the intensities of the main diffraction peaks still differ slightly. Meanwhile, compared with the peaks of the IOTs raw materials, the peaks of SiO2 in the patterns of the IOTs paste samples de-crease obviously near 30°; by contrast, the intensities of the peaks of albite and white mica increase. However, the con-tent of stone powders in paste sample powders used for XRD tests is 91wt%; therefore, explaining the reaction in paste samples is difficult. As a result, only qualitative evi-dence can indicate the changes in reactants and products.

Fig. 5. XRD patterns of limestone paste samples and IOTs paste samples.

The diffraction peak at 2θ = 29.5° is assigned to the cal-cium silicate phase. The characteristic peak of this phase does not appear in the patterns of the IOTs raw materials. However, this peak is especially obvious in the patterns of

the IOTs paste samples. The broken Si–O–Si bonds have undergone chemisorption through Ca(OH)2, which has led to the new calcium silicate product.

3.6. IR spectra of paste samples

Information about the chemical bonds and functional groups contained in molecules can be obtained by IR spec-troscopy [23]. Fig. 6 shows the IR spectra of limestone paste samples and IOTs paste samples, where the spectra of the corresponding unhydrated raw materials are shown for comparison. The stretching absorptions of the O–H bonds are observed from 3643 cm−1. The bands at 727, 879, and 1436 cm−1 are attributed to C–O bonds. For example, the bands at 727 cm−1 are assigned to the out-of-plane bending of C–O bonds, and those at 879 cm−1 are due to the bending vibrations of C–O bonds. The bands at 1436 cm−1 are caused by the stretching vibrations of C–O bonds. The bands at 3635 and 3429 cm−1 are attributed to the stretching absorptions of O–H bonds, and the bands at 1442 cm−1 are caused by the stretching vibrations of C–O bonds. Moreover, the bands near 786 cm−1 correspond to the stretching vibra-tions of Si–O bonds and the splitting absorption bands (cleavage promotes the appearance of unsaturated bonds on the surface of the particles, resulting in band splitting [24]). The bands at approximately 1020 cm−1 correspond to Si–O bonds.

The bands at approximately 3643 cm−1 are associated with Ca(OH)2, and the bands at 3429 cm−1 are attributed to crystalline water. Ca(OH)2 is partially converted into the crystalline water of compounds in IOTs paste samples, whereas no such phenomenon occurs in the limestone paste samples. As shown in Fig. 6, both bands associated with Si–O bonds shift toward lower wavenumbers, which indi-cates that some of the Si–O bonds undergo decreases in the bond force constants compared with those of the raw mate-rials. The lengths of these Si–O bonds then increase [25], and the electron clouds between Si–O bonds shift to the


middle of the chemical bonds. This discovery indicates that the Si atom of the broken Si–O bond is connected to a high-ly electronegative atom or that the O atom of the broken bond links with an atom whose electronegativity is lower than that of a Si atom. Obviously, the former is an O atom and the latter is a Ca atom in the environment of IOTs paste samples. Different electronegative atoms around chemical bonds also lead to differences in wavenumbers of the same chemical bond between the two spectra. The relation be-tween the wavenumber and the bond force constant is given by Eq. (1).

1 1=

2π

kMmc

M m

(1)

where 1

is the wavenumber, k is the bond force constant,

c is the speed of light, M and m are the masses of the two atoms.

3.7. Model of reconstruction of the broken Si–O–Si bonds

Fig. 7 shows the model of the reconstruction of the brea-

kage of Si–O–Si bonds in IOTs concrete or paste. The Ca–O bonds or O–H bonds of Ca(OH)2 are connected to unsatu-rated Si atoms, accompanied by the formation of partial crystalline water. The polymerization of Si–O–Ca bonds, Si–O–Si bonds, and O–H bonds, as well as crystalline water, results in the formation of the calcium silicate phase. More details of this reconstruction should be researched further.

Fig. 6. IR spectra of limestone paste samples and IOTs paste samples.

Fig. 7. Model of reconstruction of the broken Si–O–Si bonds

4. Discussion

There is little information regarding research on IOTs in the present literature. Shettima et al. [26] replaced fine ag-gregates in concrete with IOTs and found that the mechani-cal properties and modulus of elasticity were consistently higher than those of conventional concrete, irrespective of the substitution rate. This result is similar to the findings in the present study. The small difference in compressive strengths might be due to the different sizes of IOTs. Saito et al. [27] showed that afwillite and tobermorite could be synthesized mechanochemically by room-temperature grinding of a powder mixture of Ca(OH)2 and silica gel. The distinction in the present work is that the reaction of the si-lica gel is dramatic, whereas the change in IOTs occurs only on the surface. However, the two new products are similar. Zapata and García-Ruiz [28] discriminated among nitrate,

chlorate, and perchlorate salts using IR; they speculated that new covalent cation–anion interactions caused the shift of wavenumber. However, they did not explore the exact causes.

A series of mechanochemical effects are produced by solid matter under mechanical force, resulting in defects in the solid matter, such as distortion of lattices and a decrease in grain crystallization. Then, an amorphous substance is formed in the outermost 5–10 nm, which is called the Beilby layer. The solid surface can attract external molecules be-cause it is unsaturated. Eventually, chemical reactions be-tween substances are triggered [29–31]. The broken bonds caused by mining crushing have a mechanical activation ef-fect on the surfaces of the IOTs [32], which could form very strong chemical bonds with Ca(OH)2. However, IOTs are considered an inert ore. The complete Si–O tetrahedra in-side are very stable, and corrosion by an alkaline environ-


ment does not result in the breakage of chemical bonds. Therefore, the reaction of SiO2 and Ca(OH)2 in IOTs sam-ples is not a process of depolymerization and repolymeriza-tion as is true for geopolymers [33–35]. As shown in Fig. 7, the broken bonds on the surfaces of IOTs are directly con-nected to Ca(OH)2. Finally, a calcium silicate phase is formed.

In this paper, reconstruction of the broken Si–O–Si bonds in IOTs in concrete was demonstrated by myriad micro-structural information. However, further experiments should be designed and carried out to investigate the internal cause of reconstruction of broken Si–O–Si bonds.

5. Conclusions

(1) In the comparison of groups with the same propor-tions of cementitious materials, the compressive strengths of IOTs concrete samples are higher than those of limestone concrete samples at a w/b mass ratio of 0.3. This phenome-non is more obvious in tests of paste samples.

(2) The Ca/Si atom ratios in the ITZs of IOTs concrete samples are lower than those of limestone concrete samples, which illustrates that gels around limestone and IOTs are C–S–H phases with different structures.

(3) Small particles are stacked loosely on the surfaces of limestone paste samples, whereas the surfaces of IOTs paste samples are as dense as C–S–H gel. However, a bonding force between IOTs powders might exist.

(4) The qualitative evidence from XRD analyses and the bands of Si–O bonds shifting to lower wavenumbers indi-cate that reconstruction of the broken Si–O–Si bonds on the surfaces of IOTs with Ca(OH)2 results in the formation of a calcium silicate phase. A model for reconstruction of the broken Si–O–Si bonds in IOTs concrete or paste is pre-dicted.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Nos. 51678049 and 51834001).

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