Preparation and characterization of cement treated road base material utilizing electrolytic manganese residue

Accepted Manuscript Preparation and characterization of cement treated road base material utilizing electrolytic manganese residue Yuliang Zhang, Xiaoming Liu, Yingtang Xu, Binwen Tang, Yaguang Wang, Emile Mukiza PII:

S0959-6526(19)31894-3

DOI:

https://doi.org/10.1016/j.jclepro.2019.05.352

Reference:

JCLP 17117

To appear in:

Journal of Cleaner Production

Received Date: 21 December 2018 Revised Date:

25 April 2019

Accepted Date: 29 May 2019

Please cite this article as: Zhang Y, Liu X, Xu Y, Tang B, Wang Y, Mukiza E, Preparation and characterization of cement treated road base material utilizing electrolytic manganese residue, Journal of Cleaner Production (2019), doi: https://doi.org/10.1016/j.jclepro.2019.05.352. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

ACCEPTED MANUSCRIPT

Preparation and characterization of cement treated road base material utilizing electrolytic manganese residue Yuliang Zhanga, Xiaoming Liua*, Yingtang Xua, Binwen Tanga, Yaguang Wanga,

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Emile Mukizab a: School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing 100083, China

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Technology Beijing 100083, China

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b: School of Energy and Environmental Engineering, University of Science and

Abstract

The utilization of electrolytic manganese residue (EMR) in synergy with red mud (RM) and other solid wastes as road base materials (RBM) was investigated in this

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research. The chemical composition, mechanical properties, the hydration behavior, pore structure and environmental friendliness performance of the RBM with different Ca/Si ratios were investigated. The hydration characteristics were analyzed by XRD, 29

Si MAS-NMR and SEM-EDX technique; the pore structure was

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FTIR, TG,

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investigated by MIP, and environmental properties were texted by ICP. The results showed that the optimal proportion of Ca/Si is within the range of 0.91-1.17. After curing for 7 d, the unconfined compressive strength (UCS) of RBM can reach the highest strength requirement (class I) of 5-7 MPa for road base in Chinese standards. The best mechanical properties and polymerized structure were obtained for a Ca/Si ratio = 0.95 and the main hydration products are C-A-S-H gel, ettringite (AFt) and CaAl2Si2O8·4H2O, which promote the development of strength. The leaching test

ACCEPTED MANUSCRIPT results indicated that the heavy metals can be efficiently solidified, and meet the Chinese groundwater standards GB14848-2017. This study provides a direction for the large-scale and effective utilization of EMR.

characteristics; leaching.

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Keywords: Electrolytic manganese residue; road base material; hydration

*Corresponding authors at: Room 317, School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China.

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E-mail address: [email protected] (X. Liu).

1 Introduction

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Electrolytic manganese residue (EMR) is the industrial waste generated in the process of manganese production by electrolysis. Major hazardous substances in EMR are heavy metals and ammonia nitrogen [1]. China is the largest producer of

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electrolytic manganese in the world with about 1.55 Mt produced in 2017 [2]. Every ton of manganese produced generates 10-12 tons of EMR [3], implying that more than 15.5 Mt EMR will be produced per year. Currently, most of the EMR produced is

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stockpiled; and this not only occupies extensive land, but also causes heavy metals to seep into the soil and seriously pollute the surface and groundwater [4]. Therefore, it

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is imminent to develop methods for comprehensive utilization of EMR. Many studies have been done on the utilization of EMR, including

solidification/stabilization of EMR using phosphate resource and low-grade MgO/CaO [5]. Preparation of building materials such as autoclaved bricks [1], ecological cement [6] and geopolymers [7]. It has also been used as a soil fertilizer [8], chemical raw materials [9], as well as in metal recovery [10]. However, those processes consume a small amount of EMR and thus, do not practically solve the 2

ACCEPTED MANUSCRIPT hazards caused by EMR. The road base is a layer on the surface of the subgrade (soil-based), with a single or mixture of materials according to certain technical measures. As shown in Fig. 1, it

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can be divided into the upper base and the subbase. The base is the main load-bearing layer paved with high-quality materials directly under the asphalt surface layer, and it is an important part of the pavement structure. The utilization of EMR as road

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construction materials, especially in road base materials is believed to consume a

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large amount of EMR and alleviate disposal issues. Qiao [11] used EMR-fly ash-acetylene sludge in rural road construction and it was observed that this mixture has high strength. Yang [12] studied the use of EMR as filler for sulfur cement concrete; high mechanical strength and durability were obtained. Shen [13] also

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investigated the use of solidified wastes as road base materials; he reported that the

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mixture showed high resilience modulus and splitting tensile strength.

Fig. 1. Pavement structure

Previous works show that cementitious materials have high activity when the Ca/Si= 1.1-1.3 or the proportion of (MgO + CaO)/(Al2O3 + SiO2) is range 0.75-0.90 3

ACCEPTED MANUSCRIPT [14]. Meanwhile, Liu [15] investigated the hydration and heavy metals solidification by using a variety of solid wastes. It was shown that the optimal ratio of (MgO + CaO)/(Al2O3 + SiO2) is at the range of 0.76-0.88. Also, hydration characteristics were

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investigated, the main hydration products are AFt and C-S-H gel, which can promote the development of strength and better solidify the harmful elements [14-17]. The hydration products of EMR were also investigated in EMR’s utilization. Du [1] found

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that AFt and C-S-H gel are the main hydration products in steam-autoclaved bricks

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produced by using EMR.

In the study, the ratio of CaO/SiO2 abbreviated Ca/Si were used in studying the strength development mechanism of EMR based road base material. According to different ratio of Ca/Si, based on the synergy of multiple materials, the RBM was

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prepared from EMR in synergy with RM, two kinds of admixture and aggregates (different gradation) to evaluate the application of intermediate-calcium system in EMR road base materials. The mechanical properties, hydration characteristics and

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heavy metals solidification were investigated. Various techniques including X-ray

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diffraction (XRD), Fourier transform infrared (FTIR), Thermal Gravity Analysis (TG), Mercury intrusion porosimetry (MIP),

29

Si magic-angle spinning (MAS) nuclear

magnetic resonance (NMR) and scanning electron microscopy, energy dispersive X-ray analysis (SEM-EDX) were used for microstructure analysis. Inductively coupled plasma mass spectrometer (ICP-MS) was used for analysis of heavy metals content in the leachate. The investigated results provide a basis for the utilization of EMR, and conducive to reduce the pollution of EMR to the environment. 4

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2 Experimental procedures 2.1 Raw materials Raw materials used in this study were EMR, red mud, admixture (alkaline solid waste and neutral solid waste) and cement. EMR used in this experiment was from Guizhou

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electrolytic manganese plant. The main mineral composition is gypsum and quartz. RM from bauxite calcination process was supplied by Guizhou alumina refining plant.

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The ordinary Portland cement 42.5 grade was from Guizhou cement plant. The aggregates with a size ranging from 0.075-4.75, 4.75-9.5 and 9.5-19.5 mm were taken

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from Beijing sand plant. The chemical composition of raw materials was shown in Table 1. The XRD pattern of the raw materials can be seen from Fig. 2, the EMR, cement and alkaline solid waste can provide enough calcium to form AFt that can boast the UCS for RBM. RM and neutral solid waste mainly provide Si and Al that

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enhance pozzolanic activity and promote the formation of AFt and C-A-S-H gel. Though EMR has no pozzolanic activity, sulfate contained in it can be used to activate

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the mixture of pozzolanic waste [18]. The pozzolanic activity of EMR and RM determined via modified Chapelle test (NF P18-513) [19-20]. Before testing the

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pozzolanic activity, EMR and RM were dried and ground to 200 mesh. This experiment allows the quantification of Ca(OH)2 consumed by 1 g of raw materials mixed with 2 g CaO and distilled CO2 free water, stirred at 90

for 16 h. After

filtration, it was mixed with sucrose solution and titrated with HCl to calculate the final value. The results showed that there is no pozzolanic activity in EMR, and the pozzolanic activity [mg Ca(OH)2/1 g pozzolan] of RM is 264.29 in ordinary temperature. 5

ACCEPTED MANUSCRIPT Table 1. Chemical composition of raw materials (mass fraction/%). EMR

RM

Cement

SiO2 SO3 Al2O3 CaO Fe2O3 MnO K2O MgO Na2O TiO2 LOI

31.38 18.58 10.71 9.45 7.53 4.82 3.40 1.61 0.77 0.57 10.02

18.71 1.73 22.72 13.96 20.48 -2.08 0.71 8.61 3.34 7.02

24.25 1.77 10.52 52.8 2.62 0.21 0.90 4.27 0.38 0.59 1.49

1. gypsum

2. SiO2 3. (NH4)2SO4 4. CaCO3 5. Al(OH)3

10

6 6

85

10

10 12 1110 11

2 6 4 8727 6 8

11

10

1

2 1

10

45 2 8 79 878

1 2 2 13 2 2 1 2 3

20

10

12

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Intensity

2

7. Fe2O3 8. Ca3Al2(SiO4)(OH)8 9. Ca(OH)2 10. C3S 11. C2S 12. C3A 6. Na3((OH)SiO3)(H2O)

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10 10 11

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11

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Oxides

2

10

11 10

72

3 2

40

60

Cement

Red mud

2

EMR

80

2θ/(°)

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Fig. 2. Mineralogical phases of granulated raw materials.

2.2 Pre-treatment of EMR with alkaline solid waste

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Previous researches show that pretreatment of EMR with lime has the effect of

consolidating manganese and removing ammonia [21]. So, the admixture (a kind of alkaline solid waste) was used to pre-treat EMR in this research, its calcium oxide and calcium hydroxide content are high and can replace lime. The use of the admixture in place of lime not only consumes this solid waste but also saves cost related to lime use. 2.3 Production of road base material specimens 6

ACCEPTED MANUSCRIPT Based on the different ratio of Ca/Si, EMR was used as the main raw material, supplemented by red mud, admixture and cement. According to the different mass ratio of Ca/Si, the experiment was designed. Fig. 6 shows the UCS changes with

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different Ca/Si ratios. It is obvious that with the increase of Ca/Si ratios, the UCS curve has a peak value, corresponding Ca/Si ratio at peak is 0.95. Therefore, three group E, F and G near the peak are selected for further analysis. The detailed data are

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shown in Table 2. The proportions of these materials and the Ca/Si are shown in Table

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2. It can be seen that the Ca/Si ratios increased from 0.91 to 1.17.

The process of preparing road base materials is shown in Fig. 3. The pre-treated EMR together with red mud, neutral solid waste and aggregate were blended in a mechanical mixer uniformly and water was added based on the optimum moisture

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content. According to Chinese standard T0843-2009 to prepare the sample. The mixture was sealed in plastic bag for 18 h to make the moisture uniformity, and then predetermined quantity of cement was added and the specimens were moulded into a

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Φ50×50 mm cylinder. The forming pressure was 20 MPa and the degree of

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compaction was 98%. The pressure was maintained for 2 minutes and then the sample was demoulded, sealed in airproof plastic bags and cured in a cabinet at 20 ± 2℃ and 95% humidity until the appropriate age. The maximum dry density and the optimum moisture content were obtained as per T0804-1994 [22]. The UCS was measured conforming to test method T0805-1994 [22]. After curing for 7 d, the samples were immersed in water at 20℃ for 18 h before measuring the UCS. The solution for pH test was prepared according to GB/T 50123-1999 China [23]. The sample was dried, 7

ACCEPTED MANUSCRIPT ground to 2 mm. 5 g of powdered sample was mixed with 50 ml distilled water and stand for 30 minutes after stirring for 3 minutes. The supernatant was taken for pH measurement. To study the hydration characteristics of medium calcium system, the

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pastes were prepared without aggregates using water to solid proportion of 0.5. The resulting paste was moulded in 20 mm × 20 mm × 20 mm frame and cured at 20 ± 2

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and 95% humidity then the hydration mechanism was studied.

Fig 3. Preparation technology of road base material

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Table 2. Designed proportion of RBM intermediate-calcium system (mass fraction/%). EMR

RM

Admixture

Aggregate

Cement

Ca/Si

Ratio E

30

15

15

40

3

1.17

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Component

Ratio F

30

10

12

48

3

0.95

Ratio G

30

10

10

50

3

0.91

The gradation of the aggregate was 0.075-4.75, 4.75-9.5 and 9.5-19.5 mm (3:1:2), respectively. According to the standard Technical guidelines for construction of highway roadbases (JTG/T F20-2015, China), the gradation curves of standard and designed are shown in Fig. 4. It can be seen that the gradation curves are similar, and 8

ACCEPTED MANUSCRIPT the gradation of EMR road base material meet the requirement of grading curves in standards.

Gradation curve standard Gradation curve designed

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100

60

40

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Percente passing/%

80

0 0.01

0.1

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20

1

10

100

Particle diameter/mm

Fig. 4 The gradation of aggregate

2.4 Detection method

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Chemical characterization of raw materials was carried out by X-ray fluorescence (XRF). The phase analysis by XRD was done using Bruker D8

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ADVANCE X-ray diffractometer with CuKa radiation, voltage 40 kV, current 200 mA and 2θ scanning, ranging from 10° to 90°. The compressive strength was measured

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using the automatic pressure-testing machine (BC300D, Beijing Constant-stress Science &Technology CO.LTD, China) at a loading rate of 0.2 kN/s. A pH meter (pH-10/100) was used for pH measurement. TG-DTA helped to analyze the mass loss of different components in the road base matrix. The FTIR spectrum in the range of 4000-500 cm-1 was recorded using Nicolet iS10 FTIR spectrometer. The

29

Si MAS

NMR spectra were acquired with a Varian VXR600 spectrometer operating with a magnetic field at 118.578 MHz and a spinning rate of 9 kHz. SEM micrographs were 9

ACCEPTED MANUSCRIPT observed using JSM-6460 LV scanning electron microscope. The EDX-system was coupled with SEM using mixed BSE (back scatter electron) + LSE (lateral secondary electron) signal detectors. The pore structure of samples tested by Autopore IV 9510

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Mercury intrusion porosimetry. The leaching characteristics of both EMR and the optimum mixture were evaluated as Chinese standard HJ/T 300-2007 using ICP-OES Agilent 7500ce.

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3 Result and discussions

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3.1 Mechanical properties

Five different densities of materials were used to prepare RBM samples, especially the ratio of EMR and RM affect greatly its maximum dry density. Fig. 5 shows the maximum dry density and optimum moisture content of sample E, F and G.

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The maximum dry density of sample E, F and G was 1.861 g/cm3, 1.887 g/cm3 and 1.878 g/cm3 respectively. The optimum water content of sample E, F and G was

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16.2%，14.5% and 14.9%, respectively.

E F G

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-3 Maximum dry density/g*cm

1.88

1.86

1.84

1.82

1.80

0.12

0.14

0.16

0.18

0.20

Water content /% Fig 5. Dry density and water content of mixture materials 10

0.22

ACCEPTED MANUSCRIPT Fig. 6 shows the relationship between UCS for RBM of different ratio of Ca/Si. It can be observed from Fig 6 that the UCS for E, F and G was 4.2 MPa, 6.3 MPa and 5.4 MPa corresponding to 0.91, 0.95 and 1.17 respectively. This revealed that when

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the Ca/Si = 0.95, the RBM shows good mechanical properties. It is worth noting that all the UCS values recorded meet 3-5 MPa road base strength requirement in Chinese standards.

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The pH values for E, F and G were 10.11, 8.64 and 9.1 respectively, indicating

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that in a certain range of alkalinity, with a lower pH value, the sample shows good performance in UCS. The best 7 days UCS was measured on sample F with pH = 8.64 and made of 30% EMR, 10% red mud, 12% admixture, 48% aggregate and 3% of the additional cement.

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6

G

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5

E

4

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Unconfined compressive strength/MPa

7

3

2

0.6

0.8

1.0

1.2

1.4

1.6

Ca/Si

Fig 6. UCS of different ratios

3.2 Shrinkage cracking behavior and durability The freeze-thaw cycles and wet-dry cycles test were carried out to investigate the cracking behavior and durability of sample F after curing for 28 d. 11

ACCEPTED MANUSCRIPT The samples F were cured for 28 d as required by the standards prior to freezing-thawing cycles test. One cycle consists of freezing specimens at the temperature of -18

for 16 h, and then thawed in water at 20

for 8 h. The water is at

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least 20 mm higher than the surface of the specimens [23]. After completing 5 cycles, the UCS test was carried out on frozen specimens. The results were shown in Table 3. After freezing and thawing, the loss rate of mass and UCS are 1.5% and 14.4%,

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respectively. No cracking on the surface and the UCS also meets the standard of road

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base material.

Table 3 Results of the freeze-thaw test Before freezing-thawing cycle

After freezing-thawing cycle

loss rate

211.51 g 10.4 MPa

208.27 g 8.9 MPa

1.5 % 14.4 %

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Average mass Average UCS

The 28 d cured specimens of sample F were used for wet-dry cycles test. One

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cycle consisted of soaking specimens in water at room temperature for 5 hours, then dry them at 70

for 42 h, and then dry them in air for 1 hour (48 h). After attaining

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the desired number of cycles, specimens were soaked for 2 h and dried for at least 2 h before testing the UCS [24]. The results were shown in Table 4. After 20 wetting-drying cycles, there is no cracking on the surface of road base, and the UCS was 8.1 MPa, exceeding the requirement of highway standard. Therefore, EMR road base material has excellent durability. Table 4. Results of the wet-dry test Before Wetting-drying cycle

After Wetting-drying cycle 12

loss rate

ACCEPTED MANUSCRIPT Average mass Average UCS

211.51 g 10.4 MPa

188.73 g 8.1 MPa

10.8 % 22.1 %

3.3 Hydration characteristics

5

8 4

2

5 7

77

4 73 3

85

5 6

3

1 66 5

5

F E G

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5. SiO2 1. Ca(OH)2 6. CaCO3 2. AFt 3. C-A-S-H 7. CaSO4·H2O 4. CaAl2Si2O8·4H2O 8. KAl Si AlO (OH) 2 3 10 2

4

5

Intensity

E

7

2

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F

10

20

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G

30

40

50

60

70

2θ/(°)

Fig. 7. XRD patterns of the hydrated pastes at the age of 7 d.

Fig. 7 shows the XRD spectrum of RBM hydrated pastes at the age of 7 d. It was

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observed that the hydration products with the highest content in the RBM sample are AFt, C-A-S-H gel, Ca(OH)2, KAl2Si3AlO10(OH)2 and a kind of zeolite like (CaAl2Si2O8·4H2O). The remaining phases, like quartz, calcite, gypsum as well as

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muscovite, can be obtained from raw materials. As can be seen from Fig. 6 that the

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peak intensity of C-A-S-H gel and AFt in sample F (Ca/Si = 0.95) is higher than that of sample G (Ca/Si = 0.91) and sample E (Ca/Si = 1.17). The reason is that calcium ions gradually combine with silica as well as alumina to generate AFt and C-A-S-H gel with the increase of Ca, but high sulfate content in hydration pastes, would inhibit the hydration and reduce the strength. AlO2- , SO42- and Ca2+ that were leached from pozzolanic materials react to form AFt. The four-coordinated aluminum structure changed to six-coordinated aluminum. 13

ACCEPTED MANUSCRIPT The involved reactions are as below: (1)

[Al (OH)6]3-+6Ca2++3SO42-+26H2O=Ca6Al2(SO4)3(OH)12·26H2O

(2)

1. Ca(OH)2 2. AFt

4 5

5. SiO2 6. CaCO3

3. C-A-S-H

4

66 5

20

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1

5

30

40

F-28d 5

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22

4 7 7 77 3 18 8 4 4 8

7

10

7. CaSO4·H2O 8. KAl2Si3AlO10(OH)2

7 3

7

Intensity

8 2

4. CaAl2Si2O8·4H2O

5 7

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AlO2-+2OH-+2H2O = [Al (OH)6]3-

50

60

F-7d

70

2θ/(°)

Fig. 8. XRD patterns of sample F pastes hydrated at the age of 7 and 28 d.

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As shown in Fig. 8, C-A-S-H gel, CaAl2Si2O8·4H2O and AFt increased with increasing of hydration age. Also, at 10°and 28° of the 28 d spectrum, the new peak of C-A-S-H gel and AFt appeared, which indicates with increasing hydration time, more

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and more SiO2, Al2O3 and SO3 take part in the reaction to generate C-A-S-H gel and

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AFt. The peaks of Ca(OH)2 and CaSO4·H2O decreased gradually and even disappeared, which prove that SO42- and Ca2+ participated in formation of AFt, C-A-S-H gel and CaAl2Si2O8·4H2O. 3.4 IR analysis

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Raw materials

E-7d

3612 3377

798 778 660 599 472

Transmittance(%)

Transmittance(%)

1620 1430

G-7d F-7d 1621

660

798

1428

3611

874 599

3425

3542

472

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1151 1092

1151

1091

4000

3500

3000

2500

2000

1500

1000

4000

500

3500

3000

2500

2000

1500

1006

1000

500

-1

-1

Wavenumbers/cm

Wavenumbers/cm

Fig. 9. IR spectra of EMR and sample E, F, G hydrated pastes at the age of 7 d.

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Comparative analysis of IR spectra of mixed raw materials and hardened paste F,

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E and G curing for 7 d are displayed in Fig. 9, the three spectra of E, F and G are similar with difference in absorption intensities. The band at 3611 cm-1 is due to the absorption spectrum of Ca-OH in Ca(OH)2, the peak is the sharpest in the spectrum of raw materials, indicated that the content of Ca(OH)2 is higher. The absorption band at

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3542 cm-1 is related to Si-OH absorption spectrum. Broad absorption spectrum at 3377 cm-1 is the absorption spectrum of Al-OH in raw materials. In RBM the peak shifted to 3425 cm-1, which represent the [Al(OH)6]3- octahedral structure of AFt [25]

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while the absorption band at 1621 cm-1 is for the absorption spectrum of H-O-H. With

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the increase of reaction time, the free water is gradually transformed into bound water; The absorption spectrum at 1428 cm-1 represents the absorption spectrum of CO32-, the peak is obviously widened in the RBM; the absorption spectrum at 798 cm-1 corresponds to CO32- absorption spectrum; the band at 1151 cm-1, 1091 cm-1,660 cm-1 is SO42- absorption spectrum, these peaks are the strongest in raw materials. The CaSO4 only present in EMR (see Fig.2), will release abundant SO42- and Ca2+ which will react with [Al (OH)6]3- to form AFt. The new absorption peaks at 874 cm-1 and 15

ACCEPTED MANUSCRIPT 1006 cm-1 are for Si-OH absorption spectrum, which represents the peak of C-A-S-H gel [25]. Also, the peak of sample F is sharpest in this position, which indicates that the sample F hydration products contained the highest number of C-A-S-H gel. The

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position of 472 cm-1 is the characteristic band of quartz, which represents the absorption spectrum of Si-O.

Transmittance(%)

F-7d

1621

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F-28d

798

660

1428 3425

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3611

874 599

472

1151

1091

1006

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumbers/cm

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Fig. 10. IR spectra of sample F hydrated pastes for 7 and 28 d.

The IR absorption spectra of RBM F hydrated pastes for 7 and 28 d are presented in Fig. 10. The two spectra are similar, except the difference in absorption rates. The

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peak at 3611 cm-1 decreased, which indicates that Ca(OH)2 is gradually consumed in

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the reaction process to generate more C-A-S-H gel. With the hydration reaction proceeding, more AFt is produced, which also leads to a sharp peak at position 3452 cm-1. The peaks decreased at 1151 cm-1，1091 cm-1 showing that more SO42- are consumed to form AFt.

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ACCEPTED MANUSCRIPT 100

95 E-7d G-7d F-7d

85 (3425,83.05)

80

(3542,76.74) (3425,76.12)

75

3611

(3542,74.23) (3425,72.66)

70

65 4000

3800

3600

3400

3200 -1

Wavenumbers/cm

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Transmittance(%)

90

3000

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Fig. 11. IR spectra (wavenumbers between 4000cm-1-3000cm-1.) of the hydrated pastes of sample E, F and G at the age of 7 d

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Fig.11. gives the IR spectra of wave number in 7 d hydration products of E, F and G samples at 4000 cm-1-3000 cm-1. As the formation of hydration products increases, the transmittance decreases in the IR patterns [14]. Therefore, the number of Si-OH and Al-OH in hydration products can be judged according to the

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transmittance. Fig. 11. shows the transmittance of Si-OH group at 3542 cm-1 for F, G specimens hydrated for 7 d are 74.23%, 76.74%, respectively. And there are no

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absorption spectrum bands of Si-OH group in sample E. It can be seen that the amount of Si-OH group in sample F is higher than that of sample G as the

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transmittance of Si-OH group in sample F is lower than that of sample G. So, sample F contains more C-A-S-H gel. The same way in 3425 cm-1, the Al-OH transmittance for sample F, E and G is 72.66%, 76.12%, 83.05% respectively. So, sample F has more Al-OH and thus, more AFt than E and G. 3.5 TG analysis

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ACCEPTED MANUSCRIPT 100

E

EMR 100

Mass change:-1.41%

Mass change:-5.40%

98

456

Mass change:-53.41% -0.5

90

DTG(%/min)

TG(mass loss %）

Mass change:-1.30%

85

Mass change:-0.6%

96

775

371

94

-0.2

92

90

DTG(%/min)

95

TG(mass loss %）

187

250

0.0

Mass change:-1.73%

0.0

680

400

-0.4

88 200

80 400

600

800

400

1000

F

100

G 100

0.1

529

98

0.0

Mass change:-3.28%

-0.3

180

736

94

92

-0.2

-0.3

90

88

-0.4

-0.4

86

-0.1

Mass change:-0.63%

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90

-0.2

Mass change:-2.39%

96

SC

Mass change:-1.66% 386

92

DTG(%/min)

-0.1

714

180

0.0

486

386

TG(mass loss %）

TG(mass loss %）

94

1000

0.1

415

96

800

Mass change:-2.38%

Mass change:-2.96% 98

600

Temperature/℃

Temperature/℃

DTG/(%/min)

200

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-1.0

88

200

400

600

800

-0.5 1000

200

400

600

800

-0.5 1000

Temperature/℃

Temperature/℃

Fig. 12. TG-DTG curves of EMR and hydration pastes of sample E, F and G at the age of 7 d.

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The TG-DTG results for EMR and the hardened RBM are shown in Fig. 12. DTG curves reflect the decomposition temperature of the phase. According to the mineral composition of EMR, the mass loss between 45 -250

is mainly due to the

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dehydration of free water in EMR. The mass loss between 250-400

is related to the is due

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dehydration of crystalline water in gypsum. The mass loss between 400-680

to the removal of crystalline water from minerals and the release of ammonia. When the temperature exceeds 680 , the crystalline form of CaSO4 changes and the mineral composition gradually decomposes [26]. Combining IR and XRD results, it can be seen from TG curves of RBM that the mass loss between 45 -200

is mainly due to the dehydration of hydration products

(C-A-S-H gel and AFt) [27]. The mass change of F, G, E in this temperature range is

18

ACCEPTED MANUSCRIPT 2.96%，2.38% and 1.41%, respectively, indicating that the content of C-A-S-H gel and AFt are highest in sample F than in E and G. Therefore, the sample F has the best polymerized structure, which conforms to the conclusions of the XRD and IR. At

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200-400 ℃, the mass loss of F, G, E in this temperature range was 3.28%, 2.39% and 1.73%, respectively. The reason is that CaSO4·H2O was dehydrated. It can be seen that the sample F contains more calcium sulfate which provides more SO42- and Ca2+

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in promoting AFt formation. The Ca(OH)2 was decomposed in 400-475℃ [28], and

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the mass loss of E, G, F in this temperature range was 0.67%, 0.63% and 1.66%, respectively. The fourth mass loss temperature range is 550-800℃, mainly owing to the decomposition of CaCO3 [29-30]. The next mass loss is the decomposition of muscovite and zeolite. By comparison, it can be seen that the Ca (OH)2 in sample F is

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highest, which reflects that the hydration products formation is higher in F than in E and G, indicating why sample F has better strength than E and G. 100

0.5

0.0

90

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TG(mass loss %）

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95

180℃

85

DTG(%/min)

Mass change:-4.70%

-0.5

80

75 200

400

600

800

-1.0 1000

Temperature/℃

Fig. 13. TG-DTG curves of hydrated pastes of sample F at the age of 28 d

From Fig. 13, it is observed that the C-A-S-H gel and AFt’s mass change was up to 4.70%, indicating that Ca (OH)2 is consumed and more AFt and C-A-S-H gel are 19

ACCEPTED MANUSCRIPT formed with increasing hydration age. 3.6 29Si NMR analysis NMR can effectively analyze the coordination structure of adjacent atoms. The 29

Si is conventionally represented by Qn, (n=0-4) the

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chemical environment of

number of oxygen atoms shared by each silicon-oxygen tetrahedron and its adjacent tetrahedrons. Q0 represents an isolated silicon-oxygen tetrahedron with chemical

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shifts ranging from -66 to -74 ppm [31-32]. According to the research of Puertas et al [33], the peaks at −77 ppm ~ −82 ppm corresponds to Q1 units; the peaks appearing

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near -85 ~ -89 ppm to Q2 units; the chemical shift from −92 ppm ~ −102 ppm is the resonance peak of Q3 units, the substitution of Si by Al cause the signals to 3~5 ppm to more positive values [33]. Therefore, the peak near -97 ppm is to Q3(1Al). Q4

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represents the structure of a three-dimensional network consisting of four silicon-oxygen tetrahedrons with chemical shifts ranging from -103 to -115 ppm, the peak near -99 ppm is to Q4(1Al). When enough Al participated, the chemical shift of 29

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Si NMR is related to the number of Al atoms next to each other. The more the

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number of Al atoms, the greater the chemical shift. In general, Qn (mAl) is used to represent the silicon-oxygen tetrahedron at the junction with the aluminum-oxygen tetrahedron [34]. In order to further study the theory, Zhang [35] proposed a relative bridge oxygen number (RBO), which is calculated as follows: RBO= (1×∑ )+2×∑ +3×∑ +4×∑ ) =

∑ ·

(3)

∑

Qn is the relative area of the corresponding formant 20

ACCEPTED MANUSCRIPT Therefore, RBO can be regarded as an effective method for effectively evaluating the degree of polymerization of [SiO4]. So, one can draw a conclusion that the bridge oxygen number can be expressed by the relative area of each resonance

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peak. In order to calculate the relative area of each formant, the position and intensity of the formant must be determined accurately. The superimposed formants are fitted by peaking and their relative area is calculated. Then, the RBO value of each material

29

Si NMR spectra of sample E, F and G cured for 7 d and the

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Fig. 14. shows

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is calculated to further evaluate the degree of polymerization and hydration of [SiO4].

positions of corresponding formants of Qn are listed in Table 5. From Table 5 that the Qn structures of these hydration cementitious materials are mainly Q0, Q1, Q2, Q3(1Al) and Q4 (Q4(1Al)). The Q0 and Q1 structure are caused by unreacted C2S and C3S in

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cementitious materials [36] and the other three Qn components are caused by C-A-S-H gel. The structure of Q3(1Al) and Q4(1Al) exists in the 29Si NMR diagram,

gel in RBM.

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showing that tetrahedral [AlO4] replace four coordination [SiO4] to generate C-A-S-H

29

Si NMR spectra contain

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It shows that sample E’s main chemical shifts in the

four resonance peaks, -74.65 ppm, -87.67 ppm, -97.92 ppm and 106.61 ppm. According to the chemical shift table, they belong to Q0, Q2, Q3(1Al) and Q4 structure respectively, and don’t have Q1. The main chemical shifts of the hydration products of sample G were -78.38 ppm, -89.69 ppm, -95.42 ppm, -99.08 ppm and they belong to the structure of Q1, Q2, Q3(1Al) and Q4(1Al) respectively. The chemical shifts of sample F were -79.39 ppm, -89.41 ppm, -97.07 ppm, -104.29 ppm, and also belong to 21

ACCEPTED MANUSCRIPT the structure of Q1, Q2, Q3(1Al) and Q4, respectively. Q3(1Al) and Q4(1Al) reflects the presence of C-A-S-H gel which is in agreement with XRD, IR and TG analysis. It can be seen from the diagram that the intensity of Q3(1Al) and Q4 peak in the ratio F is

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obviously higher than that of sample E and G, which indicate that C-A-S-H gel was more formed in ratio F than in E and G.

Further analysis by Mestrenova, showed that the RBO values were 50.22%,

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55.26%, 55.62% for E, G and F, first increased and then decreased with the increase

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of Ca/Si. This indicated that sample F (Ca/Si ratio is 0.95) has the highest

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polymerization degree of tetrahedral [SiO4], and so its strength [15].

22

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Fig. 14. 29Si NMR spectra of the hydrated pastes of sample E, F and G at 7 d of hydration

Table 5 The RBO values of sample E, F and G Peak position Sample (ppm) -74.65 -87.67

Q0

8.33

Q2

100.00

Q3(1Al)

13.19

-106.61

Q4

2.34

-79.93

Q1

10.03

-89.41

Q2

100.00

-97.07

Q3(1Al)

15.34

-104.29

Q4

12.87

-78.38

Q1

5.21

-88.69

Q2

100.00

-95.42

Q3

14.13

-99.08

Q4(1Al)

9.05

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RBO value

50.22%

-97.92

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F

Relative

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E

Assign

55.62%

G

55.26%

3.7 SEM analysis The SEM micrographs and EDX analysis of sample E, F and G hydrated for 7 d 23

ACCEPTED MANUSCRIPT are presented in Fig. 15. The specimens were dried for 24 h in vacuum furnace at 60 after stopping the hydration process with ethanol. They were then coated with epoxy resin and sprayed with metal on the surface before test [37,38]. Combining the main

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elements in EDX, it can be seen that the main hydration products are fibrous C-A-S-H gel, needle shaped or rod shaped AFt and calcium zeolite in the hardened paste of the three materials for 7 d. In sample F and G, the AFt and CaAl2Si2O8·4H2O are mostly

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slender acicular, while AFt and CaAl2Si2O8·4H2O in sample E are short and thick rods

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for the most part. These AFt microcrystals are used to fill the pores and make the structure more compact. The surface of the unreacted particles was covered with fibrous C-A-S-H gel, which connected all parts of the material together. The C-A-S-H gel crystallites of sample F and G are more intensive, with better connections and the

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bonding between particles are more compact, resulting in high strength.

24

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ACCEPTED MANUSCRIPT

Fig. 15. SEM and EDX analysis of E, F and G pastes hydrated at the age of 7 days.

Fig.16 shows the SEM images of E, F and G pastes hydrated for 28 d, as can be 25

ACCEPTED MANUSCRIPT seen that more rod shaped AFt and CaAl2Si2O8·4H2O formation in sample F and G, tightly bound to fibrous C-A-S-H gel, filling up the pores gradually, and thus playing an active effect on the improvement of RBM strength. In sample E, foil-like AFm and

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a large number of flake-like Ca(OH)2 appear, which has adverse effects on strength development. The content of calcium hydroxide in the 7 d hardened paste of sample E found in TG is obviously higher than that of sample F, and G. As the ratio of Ca/Si in

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sample E is 1.17 higher than sample F and G (0.95 and 0.91respectively), it has high

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density of Ca2+ and SO42- than that in sample F and G. Thus, AFt is easier to convert

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to AFm, and more calcium hydroxide production, inhibit the strength development.

Fig. 16. Microstructure of E, F and G pastes hydrated at the age of 28 days.

3.8 MIP analysis The pore structure is one of the important components of the hardened slurry. The parameters of the pore structure mainly include total pore volume, porosity, pore 26

ACCEPTED MANUSCRIPT size and distribution, etc. These parameters directly affect the deformation properties, strength and durability of the hardened paste [39]. Table 6 Pore structure of the hardened pastes of samples E, F and G hydrated for 7 d

Diameter (nm)

0.3844 0.3477 0.3633

94.8 41.6 56.3

Pore size distribution (%)

Porosity (%) 35.42 33.73 34.57

<10nm

10~50nm

50nm~1um

>1um

4.17 6.45 7.72

5.682 11.00 20.34

54.90 70.45 59.20

35.24 12.10 12.73

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E F G

Average pore

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Sample

Total pore volume (mg/L)

Table 6 shows the total pore volume, porosity, and pore size distribution of

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sample E, F and G with hydration for 7 d. It can be concluded from table 6 that the total pore volume and porosity of hardened paste of E, F and G samples are almost the same after hydration for 7 d which means that the three samples have similar

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hydration products, and then similar mechanical properties. The total pore volume decreases in sample F when the Ca/Si = 0.95. Also, the lowest average pore diameter and porosity can be observed in sample F, which indicates its best pore structure [40].

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According to the pore size distribution, the gel pores of less than 10 nm and the

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medium capillary pores of 10-50 nm are more in the hardened paste of F and G samples, indicating that the pore size of sample F and G are more compact, as the number of small pores is larger than that of big pores. This improves the macroscopic and creep properties, as well as strength and durability. Compared with F and G specimens, the content of micropore in the hardened paste of sample E is higher, at 50 nm-1 um, which indicates that the pore size is big and has an adverse effect on the strength. This is consistent with UCS results where sample E showed the least strength. 27

ACCEPTED MANUSCRIPT The pore size distribution curve is plotted as a function of log of differential intrusion and pore size diameter. The peak corresponding to the differential curve is called the most probable pore size [41]. That is, the aperture with the greatest

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probability. Fig. 17 gives the most probable pore size of E, F and G pastes hydrated for 7 d. It can be observed that the curves show only one peak, the corresponding value of E, F and G pastes are 1054 nm, 183.18 nm and 433.68 nm, respectively. It is

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generally accepted that the smaller the most probable pore size, the finer the pore

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structure [42]. Sample F has the lowest most probable pore size which indicates its excellent pore structure; due to its ratio of Ca/Si = 0.95 which is very close to 1 while sample E has the poorest pore structure due to the fact that its Ca/Si ratio= 1.17 higher than 1.

0.52

0.39

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0.65

G F E

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Log Differential Intrusion (ml/g)

0.78

0.26

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0.13

0.00

5

10

4

3

10

10

2

10

1

10

0

10

Pore size Diameter (nm)

Fig. 17 Mercury intake under different pore size diameter

3.9 Leaching test and environment analysis The EMR and RBM leaching tests were determined by Solid Waste-Extraction procedure for leaching toxicity Horizontal vibration method (China HJ557—2010). The leaching tests were performed by mixing 100g of RBM below 9.5 mm in size or 28

ACCEPTED MANUSCRIPT EMR with 1 L distilled water in an extracting bottle, and then fixed vertically on the horizontal oscillating device. The oscillation frequency was 110 ± 10 min-1 and the amplitude was 40 mm. After 8 hours of shaking at room temperature, the mixture was

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allowed to settle down for 16 h [5, 43, 44, 45]. The filtrate of the solution was detected.

The leaching test values of EMR and RBM are presented in Table 7. As shown in

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Table 7, it can be seen that the content of Mn2+ and NH3-N2 in EMR are high, but the

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heavy metal contents are relatively low. For the RBM, the concentration of all parameters met the standard requirements in GB/T 14848-2017 for groundwater [46]. The content of Mn2+ and NH3-N2 in RBM were only 0.0013 mg/L and1.06 mg/L respectively. In RBM the rate of Mn2+ solidification is 97.99% and the rate of NH3-N2

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removal is 96.13%, indicating that RBM is environmentally friendly. The RBM intermediate-calcium system can effectively solidify heavy metals by adsorption on C-A-S-H gel and AFt.

Content

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Table 7. Leaching text results

pH

Mn2+

Na+

Total Cr

Se

Class

Class

Class III

Class

NH3-N2

Cd2+

Pb2+

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（mg/L） GB/T

Class

Class

Class

Class

14848-20

6.5-8.5

0.05

1.5

100

0.05

0.01

0.0001

0.005

EMR

6.86

1267

27.44

0

0.147

0.769

0.056

0.363

RBM

8.44

0.0013

1.06

56.38

0.014

N. D

0.0001

0.0016

17

The field testing concerning this study was conducted in Songtao, Guizhou, 29

ACCEPTED MANUSCRIPT China. Using ratio F made of 30% EMR, 10% red mud, 12% admixture, 48% aggregates and 3% cement. A road 300 m long and 6 m wide was built. The pictures of the road construction are shown in Fig. 18. After drilling cores at three different

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locations, heavy metals and UCS were tested respectively. The leaching test results showed in Table 8, depict that the content of heavy metals and NH3-N2 also met the standard requirements in GB/T 14848-2017 for groundwater, and the UCS was 8.8

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MPa. It is worth noting that this strength is far higher than the road base strength

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requirement of 3-5MPa in Chinese standards which proves that the designed process to prepare RBM from solid waste materials is feasible and effective. Therefore, the application of RBM not only consumes the stockpiled industrial solid wastes that could pollute the environment, but also saves both the cost and the extensive land that

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could be used for solid wastes disposal.

Table 8. Field leaching test results

Cr(mg/L)

Mn(mg/L)

Cd(mg/L)

Hg(mg/L)

Pb (mg/L)

NH3-N2 (mg/L)

1

0.00678

<0.00048

<0.0001

<0.0001

0.00203

0.10337

0.00719

<0.00048

<0.0001

<0.0001

0.00205

0.07891

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2

EP

Sample:

3

0.00778

<0.00048

<0.0001

<0.0001

0.0018

0.07027

Average

0.00725

<0.00048

<0.0001

<0.0001

0.00196

0.084183

GB/T 14848-2017

0.01( )

0.05( )

0.0001( )

0.0005( )

0.005( )

0.2(III)

30

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Fig.18. Photograph of practical construction of RBM

4. Conclusions

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In this paper, the mechanical properties, environmental friendliness performance and hydration behavior of RBM were investigated based on different Ca/Si ratios.

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The following conclusions can be drawn: （1） EMR can be fully utilized in RBM. The best UCS recorded after

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hydrating for 7d was 6.3 MPa; corresponding to Ca/Si = 0.95 ratio. The UCS exceeded the strength requirement (3-5 MPa) for highway road base in Chinese standards.

（2） The main hydration products of RBM prepared by EMR were C-A-S-H gel, AFt and CaAl2Si2O8·4H2O, and it was seen that they were highest and has the best polymerized structure in sample F. The products increased with hydration age. 31

ACCEPTED MANUSCRIPT （3） The leaching behavior of NH3-N2, Mn2+ and other heavy metals in RBM and field sample were below the maximum requirements by the Chinese Standard for groundwater quality, indicating that RBM has acceptable

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environmental performance. （4） The field testing showed that RBM ratio F is a valuable alternative for natural materials in road base construction purposes. The EMR based road

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base materials not only provides a practical solution to achieve the

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large-scale comprehensive utilization of EMR, but also own economic benefits. Acknowledgments

The authors gratefully acknowledge the financial support from the National Natural Science Foundation of China (No.51574024) and Fundamental Research Funds for

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the Central Universities (FRF-TP-18-005B1). References

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37

ACCEPTED MANUSCRIPT Highlights: 1. The application of road base material utilizing EMR was proposed. 2. The road base material shows excellent mechanical properties when Ca/Si=0.95.

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3. The road base material has well polymerized [SiO4] structure. 4. Leaching test shows the road base material is environmentally friendly.

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5. This research is well applied to practical production.