UTILIZATION of COPPER MINE TAILINGS AS ROAD CONSTRUCTION MATERIALS THROUGH GEOPOLYMERIZATION by Lino Francisco Manjarrez Montano

Utilization of Copper Mine Tailings as Road Construction Materials through Geopolymerization

Item Type text; Electronic Dissertation

Authors Manjarrez Montano, Lino Francisco

Publisher The University of Arizona.

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Link to Item http://hdl.handle.net/10150/631338 UTILIZATION OF COPPER MINE TAILINGS AS ROAD CONSTRUCTION MATERIALS

THROUGH GEOPOLYMERIZATION

Lino Francisco Manjarrez Montano

______

A Dissertation Submitted to the Faculty of the

DEPARTMENT OF CIVIL AND ARCHITECTURAL ENGINEERING AND MECHANICS

In Partial Fulfillment of the Requirements

For the Degree of

DOCTOR OF PHILOSOPHY

In the Graduate College

THE UNIVERSITY OF ARIZONA

2018

STATEMENT BY AUTHOR

This dissertation has been submitted in partial fulfillment of the requirements for an advanced degree at the University of Arizona and is deposited in the University Library to be made available to borrowers under rules of the Library.

Brief quotations from this dissertation are allowable without special permission, provided that an accurate acknowledgement of the source is made. Requests for permission for extended quotation from or reproduction of this manuscript in whole or in part may be granted by the copyright holder.

SIGNED: Lino Francisco Manjarrez Montano

ACKNOWLEDGEMENTS I would like to express my sincere gratitude to all those who have helped me in some way to complete this dissertation. I would like to specially thank my advisor Dr. Lianyang Zhang for his valuable guidance and support. During the last four and a half years he has taught me how to analyze and solve problems in a very efficient way. I sincerely thank him for sharing his expertise and showing me how to conduct research which will benefit me throughout my life. I would also like to thank my committee members Dr. John Kemeny, Dr. George N. Frantziskonis and Dr.

Hongki Jo for their advice which is very helpful in improving the quality of this work.

My gratitude is also expressed to the Mexican agencies which funded my doctoral studies: Consejo

Nacional de Ciencia y Tecnologia (CONACYT) and Universidad Autonoma de Sinaloa (UAS).

With scholarships, they support many Mexican students in Mexico and around the world to achieve their academic goals. Undoubtedly, without their financial support my doctoral studies could have not been possible.

I would like to thank all my friends from Mexico, Tucson and around the world. Believe it or not, you all gave emotional stability and contributed directly or indirectly to the development of this work.

Finally, I would like to thank my family, because I know they will always be there.

DEDICATION I would like to dedicate this dissertation to Juan Ignacio Velazquez Dimas†. With his support from the very beginning he encouraged me to overcome any challenge in my graduate student life. Finally, I would like to dedicate this dissertation to my parents Ana Alicia Montaño Ruiz and Lino Manjarrez Quintero. I am completely sure that without their endless guidance and emotional support I could never have reached the highest level of academia.

TABLE OF CONTENTS ABSTRACT ...... 7 1. INTRODUCTION ...... 11

1.1. STATEMENT OF ISSUES ...... 11 1.2. OBJECTIVES AND SCOPE OF THE STUDY ...... 12 1.3. LITERATURE REVIEW ...... 13 1.4. RESEARCH APPROACH ...... 18 1.5. MACRO-SCALE STUDY...... 19 1.5.1. Unconfined Compression Test ...... 19 1.5.2. Setting Time Test ...... 20 1.5.3. Leaching Analysis ...... 20 1.5.4. Durability Test...... 21 1.6. MICRO-SCALE STUDY ...... 21 1.6.1. XRF Analysis ...... 21 1.6.2. XRD Analysis...... 22 1.6.3. SEM/EDS Analysis ...... 22 1.7. DISSERTATION LAYOUT ...... 23 2. PRESENT STUDY ...... 24

2.1. RESEARCH PERFORMED AND CONCLUSIONS ...... 24 2.2. VARIABLES AND DETERMINATION OF MAXIMUM COMPRESSIVE STRENGTH ...... 26 2.3. SUGGESTED FUTURE WORKS ...... 27 REFERENCES ...... 29 APPENDIX A ...... 34

UTILIZATION OF MINE TAILINGS AS ROAD CONSTRUCTION MATERIAL – A REVIEW ..... 34 APPENDIX B ...... 72

UTILIZATION OF COPPER MINE TAILINGS AS ROAD BASE CONSTRUCTION MATERIAL THROUGH GEOPOLYMERIZATION ...... 72 APPENDIX C ...... 118

THE EFFECT OF WET AND DRY CYCLES AND WATER IMMERSION ON THE DURABILITY OF GEOPOLYMERIZED COPPER MT AS A ROAD BASE CONSTRUCTION MATERIAL ...... 118 APPENDIX D ...... 166

EXPERIMENTAL STUDY OF GEOPOLYMER BINDER SYNTHESIZED WITH COPPER MINE TAILINGS AND LOW-CALCIUM COPPER SLAG ...... 166 APPENDIX E ...... 215

PRODUCTION OF GEOPOLYMER CONCRETE FROM COPPER MINE TAILINGS AND LOW- CALCIUM SLAG ...... 215

ABSTRACT

This research studies the feasibility of using copper mine tailings (MT) as an alternative road construction material through geopolymerization. First, an extensive literature review on the use of MT as road construction material was performed. The utilization of MT in road construction can be divided into two general categories based on the stabilization method: using conventional stabilizers and through geopolymerization. Despite of the efforts of many researchers, utilization of MT in road construction is still very limited. The possible reasons are the potential contamination from the waste, the lack of relevant standards and the slow acceptance on utilization of waste materials in construction by public and industry. Further research is needed on such aspects as environmental, technical, economic, government policy and public education related to utilization of MT in road construction.

Second, a systematic study on the utilization of copper mine tailings (MT) as road base construction material through geopolymerization was performed. Specifically, MT was mixed with different amount of sodium hydroxide (NaOH) solution at various concentrations from 0 to

11 M, compacted and then cured at 35 C. After 7 days’ curing, unconfined compression tests were performed on the specimens to determine their unconfined compressive strength (UCS). Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction

7 moisture content of the initial compacted MT. At a constant moisture content, the UCS of geopolymerized MT increases with higher NaOH concentration up to a certain level and then decreases. This behavior is simply related to the effect of NaOH content or Na/Al ratio on the geopolymerization. For specimens prepared at the same NaOH concentration, the highest UCS does not necessarily occur at the optimum water content or the maximum dry unit weight, emphasizing the contribution of geopolymerization to the UCS. Moreover, this study demonstrates that by selecting appropriate moisture content and NaOH concentration, the geopolymerized MT can meet the strength requirements for road base by different State DOTs and the FHWA in the

United States.

Third, the durability characteristics of the geopolymerized MT for road base were studied. To improve the mechanical properties of the geopolymerized MT, low-calcium slag (SG) was incorporated to the MT. MT/SG-based geopolymer specimens were produced using several SG contents, 0%, 5%, 10%, 30% and 50%, by total MT/SG solid weight. Sodium hydroxide (NaOH) solution at 7 M concentration was used as the alkaline activator. Mixtures were prepared at a moisture content of 14% and then compacted. After compaction, the specimens were cured in the oven at 35°C and 60°C for 7 and 14 days, respectively. The durability characteristics were determined by wet and dry (w-d) cycles and water immersion. Unconfined compression tests were performed on the specimens to determine the unconfined compressive strength (UCS). The UCS was obtained after curing, at dry and saturated conditions (0th cycle), after the 1st, 3rd, 7th and 12th w-d cycle, and after water immersion. The loss of mass and pH were also recorded after each cycle.

Additionally, leaching tests based on the TCLP method were performed to investigate the release of heavy metals. Scanning electron microscopy/energy-dispersive X-ray spectroscopy

(SEM/EDS) and X-ray diffraction (XRD) analyses were also performed to study the

8 microstructure and chemical composition of the specimens at different conditions. The results showed that the w-d cycles and water immersion significantly affect the UCS of the specimens.

This effect is more evident on the specimens cured at 35°C. However, the compressive strength of the specimens improves with curing temperature and SG content. The SEM/EDS results showed that after the w-d cycles Na is still present in the geopolymer microstructure although at a lower content. Based on the results, the geopolymerized MT/SG can be used as road base construction material.

Fourth, MT/SG-based geopolymer binder was experimentally studied. The geopolymer binder was produced with copper mine tailings (MT) and low-calcium slag (SG). The effect of water to solid

(w/s) ratio, SG content (0, 25 and 50%), sodium hydroxide (NaOH) concentration (5, 10 and 15

M), and sodium silicate (SS) to sodium hydroxide ratio (0.0, 0.5, 1.0 and 1.5) on the unconfined compressive strength (UCS) were studied systematically. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) were also performed to characterize the microstructure and phase composition of the geopolymer specimens. . The results show that the inclusion of SG improves the UCS and reduces the initial water content required for achieving a certain workability of the geopolymer paste. The geopolymer binder specimens prepared at 50% SG, 10 M NaOH, SS/NaOH = 1.0 and cured at 60 C for 7 days reached the highest UCS of 23.5 MPa. The geopolymer paste prepared at 50% SG, 15 M NaOH concentration and SS/NaOH ratios of 0.5 and 1.0 showed flash setting which led to poorer quality specimens and lower UCS. The SEM, EDS and XRD analyses clearly show the participation of iron dissolved from SG in the formation of geopolymer gels. This research helps to promote the reuse of MT and

SG through geopolymerization and contributes to the knowledge of geopolymer materials.

Finally, the production of geopolymer concrete by using copper mine tailings (MT) and low- calcium slag (SG) as both the cementitious binder and aggregates was studied. 50% MT and 50%

SG, both passing sieve No. 200, were used together as the binder material, and MT and SG, with a particle size larger than sieve No. 200 and at designed combination, were used as the aggregates.

Combined sodium hydroxide (NaOH) and sodium silicate (SS) were used as the alkaline activator.

A curing temperature of 60°C was used to prepare the geopolymer concrete specimens. The effect of different factors including water to binder (w/b) ratio, NaOH concentration, SS/NaOH ratio, cement/aggregate ratio, curing time, and curing conditions on the compressive strength of geopolymer concrete were experimentally studied. The results showed that w/b = 0.26, 10 M

NaOH concentration, SS/NaOH = 1.0 and cement/aggregate = 0.19 are the optimum values.

This research promotes the utilization of mining wastes in road construction and contributes to the reduction of CO2 emissions.

1. INTRODUCTION

1.1. Statement of Issues Roads are one of the main infrastructure systems that contribute significantly to economic development. In the United States, there are about 4 million miles of roads [1]. These roads need maintenance every 2 to 5 years and their lifetime is only between 20 and 40 years [2]. The maintenance and construction of roads need to use large amount of natural materials. On average,

20,000 tons of aggregates are used for each mile of road [3]. It is estimated that 58% of the total natural aggregates consumed in the United States are used in road construction [2]. Approximately

90% of the aggregates used in road construction are virgin or directly extracted from quarries.

There is already a shortage of high quality natural construction materials in many areas of the

United States [2]. Currently, there are two possible solutions in those areas: finding new quarries, which significantly increases the final project cost, and using stabilizers, commonly ordinary

Portland cement (OPC) and lime, to enhance the mechanical properties of low-grade natural construction materials such as soils. Even though the use of OPC and lime is effective in improving the mechanical properties of low-grade natural construction materials, they are accompanied with different negative environmental issues such as high energy consumption and greenhouse gas emissions [4]. For example, in 2016, about 8% of all carbon dioxide emissions generated worldwide are attributed to the production of OPC [5]. Therefore, it is important to find viable alternative materials to replace natural aggregates, OPC and lime for road construction.

Tremendous amount of mine tailings (MT) are generated by the mining industry every year [6]. In current practice, MT are used as backfill in underground mines, stored in open pits, dried and stacked, or pumped into tailings dams on site [7]. The disposal of MT is not only expensive but has also resulted in various ecological and environmental problems. The on-site tailings dams

11 occupy large areas of land, while at the same time represent a threat to the environment by the possible contamination of air, surface water, groundwater and soils. If MT are used as a road construction material, the exploitation of quarries can be reduced and the problems related to MT disposal can be eliminated.

1.2. Objectives and Scope of the Study The main goal of this research is to promote the utilization of mine tailings as a road construction material by using a unique stabilization technology called geopolymerization. Geopolymerization is the chemical reaction of aluminosilicates in a highly alkaline silicate or hydroxide solution, creating a very stable material called geopolymer which has a polymeric structure with interconnected Si-O-Al-O-Si bonds [8–12]. Geopolymer materials have a number of advantages over OPC, including rapid development of mechanical strength, high acid resistance, no/low alkali-silica reaction related expansion, immobilization of toxic and hazardous materials, and significantly reduced greenhouse gas emissions [13–15]. Through geopolymerization, MT can be used as road base material and, together with low-calcium slag (SG), as pavement concrete.

Specifically, the research has the following objectives:

1. Study the feasibility of using geopolymerized mine tailings as a road base construction

material in terms of compressive strength.

2. Investigate the durability properties of geopolymerized mine tailings to be used as a road

base construction material.

3. Study the feasibility of producing geopolymer binder using mine tailings and low-calcium

slag fines.

4. Study the feasibility of producing geopolymer pavement concrete using mine tailings/slag

fines-based geopolymer binder and mine tailings/slag aggregates.

1.3. Literature Review Many researchers have studied the stabilization of MT alone as road construction material. Sultan

[16] investigated the feasibility of utilizing copper MT stabilized with OPC and asphalt emulsion as road construction material. Different proportions of OPC (2-12%) and asphalt emulsion (4-

20%) were mixed with the MT. The mixtures were compacted at their optimum moisture content

(OMC) and the maximum dry density (MDD) determined, using the standard Proctor method for the OPC stabilized MT and the modified AASHTO static compaction method for the asphalt emulsion stabilized MT. The OPC specimens were cured in a humid room for 7, 28 and 90 days, respectively, and the asphalt emulsion specimens were cured at ambient temperature for 7 and 28 days, respectively. Both types of specimens were soaked in tap water for 4 hours and then tested to measure the unconfined compression strength (UCS), tensile strength, shear strength, compressibility, permeability and erodibility by rainfall. The results showed that at OPC contents greater than 10%, the increase of UCS was not significant at the same curing period. At 8% OPC, the stabilized MT can achieve a 7-day UCS of 3.45 MPa. For the asphalt emulsion stabilized MT, a 7-day UCS of 0.98 MPa can be achieved at the optimum asphalt emulsion amount of 8%. The author concluded that the copper MT stabilized with either OPC or asphalt emulsion have the acceptable engineering properties and can be adapted for use in road construction.

Indraratna et al. [17] investigated the use of compacted coal tailings from West Cliff Colliery, New

South Wales, Australia in mine access roads and pavements. The obtained optimum moisture content and maximum dry unit weight were 14% and 13.7 kN/m3, respectively, at the standard

Proctor compaction energy (600 kN-m/m3) and 12% and 15.5 kN/m3, respectively, at the modified

Proctor compaction energy (2500 kN-m/m³). California bearing ratio (CBR) tests were also performed on the compacted specimens and CBR values close to 20% were obtained for specimens

13 compacted at the standard Proctor energy and at moisture contents below 12%. CBR values higher than 80% were obtained for specimens compacted at the modified Proctor energy and at similar moisture contents. The obtained CBR for the specimens compacted at the modified Proctor energy is comparable to those of compacted granular materials used in the construction of base and subbase for airfields and roads. The results met the minimum strength requirement by the National

Association of Australian State Road Authorities.

Mahmood and Mulligan [18] studied the stabilization of 6 different types of MT with OPC (type

I) for their use as road base material for unpaved roads. Mixtures were prepared by adding a constant amount of OPC to each MT and varying the water content in order to evaluate the effect of water/cement (w/c) ratio on UCS. After a mixture was prepared, it was poured into wooden cubical molds of 25 mm in side length. The specimens were left to cure at room temperature for

24 h and then placed in a moisture chamber at 23.5 C and 98% humidity. Finally, unconfined compression test was performed on the cubic specimens. The results showed that the optimum w/c ratio was 0.3 at which a UCS of 9.5 MPa was reached. Based on the optimum w/c ratio, the

AASHTO nomograms were utilized to determine the structural layer coefficient. The results indicated that the stabilized MT can sustain more than the required stress and meet the requirements for structural layer coefficients by ten state Departments of Transportation in the

United States.

Qian et al. [19] studied the stabilization of granite MT with OPC as pavement subbase material.

The OPC was incorporated into the granite MT in a range of 3-6%. OPC-stabilized crushed stone specimens were also prepared for comparison. The specimens were made according to the Chinese specifications and were cured at ambient temperature for 7, 28 and 90 days, respectively.

Laboratory tests were conducted to evaluate the UCS, static and dynamic moduli, split tensile

14 strength, and thermal and drying shrinkage. The results showed that the UCS increases with higher

OPC content and at 5% OPC, the granite MT have comparable UCS, split tensile strength, and static and dynamic moduli to the OPC-stabilized crushed stone. The UCS values of the 5% OPC-

MT mixtures after 7, 28 and 90 days’ curing were 4.37 MPa, 6.39 MPa and 7.17 MPa, respectively, whereas for the 5% OPC-crushed stone mixtures, the UCS values were 5.53 MPa and 7.37 MPa after 28 and 90 days’ curing, respectively. The split tensile strength reached 0.49 MPa and 0.66

MPa after 90 days’ curing for the OPC-MT and OPC-crushed stone mixtures, respectively. The static and dynamic modulus after 28 days’ curing were 1590 MPa and 1526 MPa, and 2176 MPa and 1940 MPa for the OPC-MT and OPC-crushed stone mixtures, respectively. The authors concluded that the granite MT stabilized with 5% OPC can be effectively used as road subbase material.

Xu [20] evaluated the stabilization of iron MT with blast furnace slag (BFSG) cement as an alternative construction material for base and subbase courses of low-grade highways. The BFSG was added into the iron MT in a range of 5-17%. The specimens were compacted using the standard

Proctor method and placed in a curing room at 20 ± 2 C with a relative humidity of 96% for 7 days. Laboratory tests were performed to evaluate the UCS of the specimens. The results showed that the UCS of specimens with a BFSG content of 15% and 17% is 2.69 and 3.01 MPa, respectively. These values met the requirement for low-grade base materials (> 2.5 MPa) based on the Technical Specification for Construction of Highway Pavement. An economic analysis comparing the use of the MT/15% cement mixture against a gravel/4% cement mixture in a 12 m wide road showed 30% savings in the final cost.

Researchers have also studied the utilization of MT together with soil as road construction material. Osinubi et al. [21] investigated the use of iron MT and OPC to stabilize black clay (CH

15 according to the USCS) to evaluate its applicability as road subgrade material. OPC-clay mixtures were first prepared by adding 1-4% of OPC by dry weight of soil. Then iron MT was added at 0-

10% to the OPC-clay mixtures. The specimens were compacted at the British Standard light energy level (596 kN-m/m3) at three layers in a 1000 cm3 British Standard mold and cured for 7 and 28 days, respectively. Laboratory tests were performed to evaluate the index properties and shear strength parameters of the mixtures. The results showed an optimum OPC and iron MT content of

4% and 6%, respectively. The liquid limit and plasticity index of the optimum mixture were 54% and 36%, respectively. Both values were above the corresponding maximum values of 50% and

30% required by the Nigerian General Specifications for subgrade materials. The shear strength of the optimum mixture was 0.10 MPa. The authors concluded that although the addition of iron MT to the OPC-clay mixtures represented an enhancement of the properties, the results did not meet the minimum requirements of the Nigerian General Specifications for Highways.

Etim et al. [22] evaluated the feasibility of using lime and iron MT to stabilize black cotton soil for its use as subbase material in the construction of low volume roads. According to the USCS, the soil was classified as CH. Mixtures were prepared by incorporating 0-8% lime and 0-10% iron

MT, by dry soil weight, into the soil. Specimens were compacted at the British Standard light energy level. Laboratory tests were carried out to evaluate the UCS, CBR, durability and leaching.

The UCS specimens were wax cured for 7, 14 and 28 days, whereas the CBR specimens were wax cured for 6 days and then immersed in water for 1 day before testing. The durability test was performed on specimens wax cured for 7 days and then dewaxed top and bottom and immersed in water for 7 days. The results showed an optimum mixture containing 8% lime and 8% iron MT.

The optimum mixture reached a UCS of 1.07, 1.80 and 2.03 MPa after 7, 14 and 28 days’ curing, which met the criteria of the Nigerian standards for soil-lime stabilization (> 1 MPa). The CBR of

16 the optimum mixture was 50% and exceeded the requirement for subbase material of light traffic roads (> 40%). The durability analysis showed a UCS decrease of 37% after water immersion, which is acceptable according to the Nigerian standards. The leaching analysis proved that the optimum mixture leached 0.272 mg/l of iron, which was less than the acceptable value of 0.3 mg/l.

The authors concluded that lime and iron MT can be used to effectively stabilize black cotton soil for use as a subbase material of light traffic roads.

Ojuri et al. [7] investigated the use of lime, OPC and iron MT to stabilize lateritic soil (SC according to USCS) as a road construction material. The iron MT was added at 0-50% to the soil, and then the lime-OPC binder, at a 1/2 ratio, was added to the soil-MT mixtures at 0-10%.

Laboratory tests were performed to evaluate the index properties, UCS and CBR of the mixtures.

The results showed that the addition of iron MT modified some basic geotechnical properties of the soil, such as the Atterberg limits and the fine grain size distribution, which made the mixture suitable for use in road construction. Also, the addition of the lime-OPC binder increased the UCS of the mixtures. The CBR results showed that all the mixtures, except that at a soil/MT ratio of

90/10, met the minimum CBR (80%) required by the Nigerian General Specifications for highways. The optimum mixture was found at a soil/MT ratio of 70/30 with 8% of lime-OPC binder, resulting in a UCS of 0.42, 0.47 and 0.55 MPa after 7, 15 and 30 days’ curing, respectively, with a CBR of 115%. The authors concluded that lime, OPC and iron MT can be used to effectively stabilize the lateritic soil as road construction material.

The studies summarized above are focused on the stabilization of MT alone or MT together with soils using conventional stabilizers. Even though the use of conventional stabilizers has demonstrated acceptable performance, their use has the drawbacks of high-energy consumption and release of large amount of CO2. Therefore, researchers have studied the stabilization of MT,

17 alone or in combination with other materials, based on geopolymerization [23–29].

Geopolymerization is the chemical reaction of aluminosilicates in a highly alkaline silicate or hydroxide solution, creating a stable material called geopolymer. Geopolymer has a three- dimensional aluminosilicate structure consisting of linked AlO4 and SiO4 tetrahedra by sharing the oxygen atoms between them [8–12]. The chemical composition of geopolymer can be represented by the following formula [9,10]:

푀푛[−(푆𝑖푂2)푧 − 퐴푙푂2 −]푛 ∙ 푤퐻2푂 (1) where, M is an alkaline element or cation such as sodium, potassium or calcium; “ – ” represents a bond; n is the degree of polymerization or polycondensation; and z is the Si/Al ratio. Water is an essential part of geopolymerization which is released after the reaction has taken place and eventually gets evaporated creating micro size voids [30,31]. Geopolymer materials have several advantages over OPC, including abundant raw material resources, rapid development of mechanical strength, high acid resistance, no/low alkali-silica reaction related expansion, immobilization of toxic and hazardous materials, and significantly reduced energy consumption and greenhouse gas emissions [13–15]. These advantages have attracted the attention of the research community about the use of geopolymer as a potential sustainable material.

1.4. Research Approach This research comprises an innovative multi-scale and multi-disciplinary approach consisting of systematic experimental studies as presented in Fig. 1.

Macro-scale study Micro-scale study

Uniaxial XRF compression test XRD Setting time test SEM/EDS

Leaching test EDS EDS point mapping Durability test

Wet/dry Water cycles immersion

Linking macro-scale behavior and micro-scale properties to better understand how MT can be utilized in road construction through geopolymerization

Fig. 1. Research program for investigating geopolymerization of copper mine tailings

1.5. Macro-Scale Study The macro-scale study focuses on the investigation of mechanical and environmental properties of three types of geopolymer products: i) geopolymerized MT, ii) geopolymerized MT and SG fines as a binder, and iii) MT/SG fines-based geopolymer binder together with MT/SG aggregates as a geopolymer concrete. Unconfined compression and setting time tests are performed to study the mechanical properties of the geopolymer products, and wet-dry cycle, immersion and leaching tests are conducted to evaluate their durability and environmental performance.

1.5.1. Unconfined Compression Test

The unconfined compression tests are performed to evaluate the effect of various factors on the mechanical strength of geopolymer products. Geopolymerized MT specimens are prepared by

19 mixing MT with NaOH solutions at different NaOH concentrations. Geopolymer binder specimens are prepared using MT and SG fines as the source material and combined sodium hydroxide

(NaOH) and sodium silicate (SS) solution as the alkaline activator. Geopolymer concrete specimens are prepared by using MT/SG fines as the binder and MT/SG as the aggregates. The

MT-based geopolymer specimens and the MT/SG-based geopolymer binder specimens are tested on an ELE tri Flex 2 loading machine and the geopolymer concrete specimens are tested on a

Tinius Olsen loading machine.

1.5.2. Setting Time Test

The setting time test is performed on the geopolymer binder to study the effect of various factors on the initial and final setting times. The initial and final setting times of geopolymer binder have a great effect on the workability of the geopolymer product. Geopolymer binder specimens are prepared at different MT/SG ratios, NaOH concentrations, and SS/NaOH ratios. The setting time tests are performed using the Vicat apparatus and following ASTM C191 [32].

1.5.3. Leaching Analysis

Leaching analysis is performed to evaluate the environmental performance of the geopolymer products. The analysis is performed by soaking a small portion (about 20 grams) of geopolymer powder into distilled water with a liquid-to-solid mass ratio of 20:1. The liquid-solid mixture is placed in sealed plastic vessels and end-over-end rotation is performed for 18 hours. After that, the liquid is filtered through a 0.45-µm filter. Finally, a Perkin Elmer Elan DRC-II ICP-MS is used to measure the concentration of heavy metals in the filtrate based on the inductively coupled plasma mass spectrometry technique (ICP-MS).

1.5.4. Durability Test

The durability properties of the geopolymerized MT/SG specimens are evaluated by wet and dry

(w-d) cycles and water immersion. The specimens are cured in the oven at 35°C and 60°C for 7 and 14 days. The w-d cycles are performed according to the ASTM D559 [33]. The UCS is obtained after curing (0th cycle) in dry and saturated conditions and after the 1st, 3rd, 7th and 12th cycle. The water immersion test is evaluated by means of the resistance to loss in strength. The resistance to loss in strength is calculated as the ratio of the UCS of specimens cured in oven for 7 days and later immersed in water for 7 days to the UCS of specimens cured in oven for 14 days.

1.6. Micro-Scale Study Mine tailings are rich in amorphous silica and alumina components. The silica and alumina are expected to dissolve in an alkali solution and form an amorphous geopolymer gel material. This geopolymer gel can serve as a cementitious binder. It is of great importance to understand how the microstructure and chemical composition of the geopolymer gel affect the properties of the geopolymer products. In this regard, scanning electron microscope imaging (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) and X-ray fluorescence (XRF) analysis are performed.

1.6.1. XRF Analysis

XRF analysis is performed to determine the elemental composition of MT and SG. The elemental composition is important to analyze the Si, Na, Al and Fe content in the geopolymer product, to further study the effect of Si/Al, Na/Al, Si/(Fe+Al) and Na/(Fe+Al) ratios on the geopolymer compressive strength. The samples are prepared according to the following protocol: 4 gr of ground samples are mixed with 1 gr of Licowax C micropowder (spectro) and pressed into pellets at a pressure of 22 tons for 120 seconds and analyzed using a polarized energy-dispersive X-ray

21 fluorescence spectrometer. Measurements are carried under Helium atmosphere and 4 secondary targets (HOPG, Mo, Al2O34 and Co) are used to provide different excitation conditions at different voltage and current settings. Acquisition time is set to 300 seconds for each secondary target.

1.6.2. XRD Analysis

The effect that geopolymerization causes on the phase composition of aluminosilicate materials is observed by XRD analysis. MT contains both amorphous and crystalline phases. Specifically, the amorphous phase plays a key role in the geopolymerization process so that its identification is important. First, XRD analysis is performed in the raw materials and then in the geopolymer products. The analyzes are performed with a Panalytical X’pert pro MPD instrument equipped with a programmable incident beam slit using Ni-filtered, Cu, Kα and λ=1.5418 Å, as X-ray radiation.

1.6.3. SEM/EDS Analysis

To investigate the effect of NaOH concentration on MT, and the effect of both NaOH and SS on

MT/SG fine binder, SEM/EDS imaging are performed on the tested specimens. The SEM imagining elucidates the formation of amorphous gel, whereas the EDS investigates the change of elemental composition between unreacted and reacted (geopolymer gel) areas. SEM and EDS are performed on freshly fractured specimens without polishing to avoid surface contamination.

External geopolymer surfaces are not analyzed due to high potential on excessive oxidation. The

SEM imaging and EDS analysis are performed in the FEI INSPECT-S/Thermo-Fisher Noran 6 microscope.

1.7. Dissertation Layout The dissertation follows the manual of graduate college at the University of Arizona and consists of two chapters and five appendices. The first chapter includes the statement of issues, objectives and scope of the study, literature review, research approach and dissertation layout. The second chapter summarizes the main findings from the study presented in the appendices.

Appendix A is a paper ready for submission and is about the literature review of the research on utilization of MT as road construction materials. Appendix B is an already published paper in the

Journal of Materials in Civil Engineering and studies the feasibility of using geopolymerized MT as road base construction materials through geopolymerization. Appendix C is a paper ready for submission and investigates the durability properties of geopolymerized MT as road base construction material. Appendix D is a paper already submitted to the Journal of Materials in Civil

Engineering and systematically studies the geopolymer binder produced from copper mine tailings

(MT) and low-calcium slag (SG) fines. Finally, Appendix E is a paper prepared for submission and studies the production of geopolymer concrete by using MT and SG as both the geopolymer binder and the aggregate.

2. Present Study

The methods, results and conclusions of the research have been documented in the five papers presented in the appendices. Appendix B is a paper already published in the Journal of Materials in Civil Engineering. Appendix D is a paper already submitted to the Journal of Materials in Civil

Engineering in the special issue named “Recent advances in geomaterial evaluation and applications in civil infrastructure projects”. Appendices A, C and E are the most recent findings of this research, and the results are ready to be submitted for publication.

2.1. Research Performed and Conclusions A summary of the important findings of this research is presented below. • In Appendix A an extensive literature review about the use of mine tailings as road

construction material was performed. The work is divided into two general categories

based on the stabilization method: using conventional stabilizers and through

geopolymerization. Although a lot of research have been performed in this field, the

utilization of MT in road construction is still very limited. It could be due to the potential

contamination from the waste, the lack of relevant standards and the slow acceptance on

utilization of waste materials in construction by public and industry. Further research is

needed on such aspects as environmental, technical, economic, government policy and

public education related to utilization of MT in road construction.

• In appendix B, the feasibility of using copper mine tailings (MT) as road base construction

material through geopolymerization was investigated. The MT was mixed with different

amount of sodium hydroxide (NaOH) solution at various concentrations. The results show

that the maximum dry unit weight of the compacted MT is influenced by the NaOH

24 concentration, higher NaOH concentration leading to larger maximum dry unit weight.

Moreover, this study demonstrates that by selecting appropriate moisture content and

NaOH concentration, the geopolymerized MT can meet the strength requirements for road base by different State DOTs and the FHWA in the United States. In appendix C, the durability characteristics of the geopolymerized road base MT were studied. Slag was mixed with dry MT to improve the mechanical properties of the geopolymer product. The results showed that the w-d cycles and water immersion affect significantly the UCS of the specimens. However, the compressive strength of the specimens improves with curing temperature and SG content. The SEM/EDS results showed that after the w-d cycles, Na is still present in the geopolymer microstructure although at a lower content. Based on the results, the geopolymerized MT/SG can be effectively used as road base construction material.In appendix D, geopolymer binder was produced with MT and low-calcium SG.

Various effects on the UCS such as water to solid ratio (w/s), SG content, NaOH concentration, and sodium silicate (SS) to sodium hydroxide ratio were studied systematically. The results show that the inclusion of SG improves the UCS and reduces the initial water content required for achieving certain workability of the geopolymer paste.

Some of the prepared geopolymer pastes led to flash setting and is believed it was due to the high alkalinity of the solution and the quick reaction with the source materials. Based on the SEM/EDS analysis, the geopolymer gel formed in the optimum specimen is solid- like, in contrast with the rest of the analyzed specimens. Curiously, iron from the SG’s chemical composition participated in the formation of geopolymer gel. In appendix E,

MT/SG-based geopolymer concrete was produced. The specimens were prepared with MT and SG. The geopolymer binder was obtained in the study from appendix D. The effect of

different factors including water to binder (w/b) ratio, NaOH concentration, SS/NaOH

ratio, cement/aggregate ratio, curing time and curing conditions on the compressive

strength of geopolymer concrete are studied. The results show that MT/SG geopolymer

concrete could reduce the final project cost due to the use of recycled materials. This

research promotes the utilization of wastes and contributes to the reduction of CO2

emissions.

2.2. Variables and Determination of Maximum Compressive Strength In this work, the selection of variables was determined based on the effect they have on the compressive strength of the specimens. In the evaluation of each variable, one of them was varied while the others were kept constant. The variable that was varied and resulted in the highest compressive strength of the specimens was fixed for the next variable evaluation. The combination of variables that resulted in the highest UCS were determined as the optimum. This approach was followed because of the large number of factors which need to be considered. In this work, a total of 363 specimens were tested.

The maximum compressive strength was determined based on the stress-strain plots. For example,

Fig. 1 shows six stress-strain plots corresponding to two sets of tests, each consisting of three specimens. For each set, the average of the three compressive strength values was used for the analysis.

UCSmax = 4.8 MPa Specimen A Specimen A* 5 Specimen B Specimen B* UCSmax = 4.3 MPa Specimen C Specimen C* UCSmax = 4.2 MPa 4

UCS(MPa) UCSmax = 1.2 MPa 2 UCSmax = 1.1 MPa

1 UCSmax = 0.9 MPa

0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 Axial strain (%)

Fig. 1. Typical stress-strain plots where the maximum compressive strength value was obtained

2.3. Suggested Future Works The results presented in this study show that MT can be used as a road construction material through geopolymerization. It is also shown that fine SG can be used as an additive to improve the performance of geopolymerized MT and coarse SG as aggregates to produce geopolymer concrete.

To promote the utilization of MT in real road construction, the following future works are suggested.

1. Researchers have investigated the effect of Ca on geopolymerization. When a little amount

of Ca is included, the mechanical properties of the final geopolymer product can be

improved due to the formation of CASH gels which work in synergy with geopolymer

gels. In this regard, the next step can include Ca rich materials such as CKD which is also

a waste to further enhance the properties of MT/SG-based geopolymer products for road

construction.

2. Build a large-scale road section with MT and SG and monitor its performance during a

long period of time.

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APPENDIX A

UTILIZATION OF MINE TAILINGS AS ROAD CONSTRUCTION MATERIAL – A

REVIEW

Lino Manjarrez1 and Lianyang Zhang, Ph.D., P.E., M.ASCE2

1Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, Arizona 85721, USA

2 Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, AZ 85721, USA (Corresponding author). Email: [email protected]

* Corresponding author: Tel.: 1 520 6260532; fax: 1 520 6212550.

E-mail address: [email protected]

ABSTRACT

Construction of roads needs to use large amount of natural materials extracted from quarries and, in many cases, conventional stabilizers such as ordinary Portland cement and lime. In many regions of the United States, the exploitation of quarries has been so excessive that there exists already a shortage of natural construction materials. In those regions, the stabilization of natural materials using conventional stabilizers is a common technique. However, the use of conventional stabilizers represents an important source of contamination for the environment. So, the use of alternative materials and stabilization methods in road construction is highly advisable. This paper presents a state-of-the-art review of research on utilization of mine tailings (MT) as an alternative road construction material. The research can be divided into two general categories based on the stabilization method: using conventional stabilizers and through geopolymerization. Despite of the efforts of many researchers, utilization of MT in road construction is still very limited. The possible reasons are the potential contamination from the waste, the lack of relevant standards and the slow acceptance on utilization of waste materials in construction by public and industry. The creation of new standards is strongly required for a wider application of MT in road construction. Further research is also needed on such aspects as environmental, technical, economic, government policy and public education related to utilization of MT in road construction. Utilization of MT as road construction material not only reduces the exploitation of quarries but also represents a step toward sustainability and “green” engineering.

Key words: Mine tailings; road base; stabilization; geopolymerization; compressive strength

1. Introduction Roads are one of the main infrastructure systems that contribute significantly to economic development. In the United States, there are about 4 million miles of roads connecting the country from north to south and east to west [1]. The construction of roads needs to use large amount of natural materials extracted from quarries. On average, 20,000 tons of aggregates are used for each mile of road [2]. There is already a shortage of high quality natural construction materials in many areas of the United States. Therefore, stabilizers, commonly ordinary Portland cement (OPC) and lime, have been often used to enhance the mechanical properties of low-grade natural construction materials such as soils. Even though the use of OPC and lime is effective in improving the mechanical properties of low-grade natural construction materials, they are accompanied with different negative environmental issues such as high energy consumptions and greenhouse gas emissions [3]. For example, about 7% of all carbon dioxide emissions generated worldwide are attributed to the production of OPC and lime [4,5].

On the other hand, tremendous amount of wastes such as mine tailings (MT) are generated by the mining industry every year [6]. The disposal of MT is not only expensive but has also resulted in various ecological and environmental problems such as the occupation of large areas of land, generation of windblown dust, contamination of surface and underground water, and failure of tailing dams [7–9].

A viable path to address the above problems in a sustainable and environmentally-friendly way is the use of MT as road construction material. In this regard, much research has been conducted on utilization of MT alone or in combination with natural material(s) such as soils in road construction through stabilization. Although the studies have demonstrated the potential of MT as an alternative road construction material, the utilization of MT in road construction is still very limited due to various reasons.

This paper presents a state-of-the-art review of the research on utilization of MT as road construction material. The advantages and disadvantages of the various methods for stabilizing

MT are described. The concerns related to the use of MT in road construction are also discussed.

2. Utilization of Mine Tailings (MT) Through Conventional Stabilization To use MT, alone or in combination with natural material(s), as road construction material, they need to be stabilized. Much research has been conducted on stabilization of MT alone or MT together with soils using conventional stabilizers such as ordinary Portland cement (OPC), lime, asphalt, cement kiln dust (CKD), fly ash (FA) and/or slag (SG), as detailed below.

2.1. Utilization of MT Alone Many researchers have studied the stabilization of MT alone as road construction material (see

Table 1). Sultan [10] investigated the feasibility of utilizing copper MT stabilized with OPC and asphalt emulsion as road construction material. Different proportions of OPC (2-12%) and asphalt emulsion (4-20%) were mixed with the MT. The mixtures were compacted at their optimum moisture content (OMC) and the maximum dry density (MDD) determined, using the standard

Proctor method for the OPC stabilized MT and the modified AASHTO static compaction method for the asphalt emulsion stabilized MT. The OPC specimens were cured in a humid room for 7, 28 and 90 days, respectively, and the asphalt emulsion specimens were cured at ambient temperature for 7 and 28 days, respectively. Both types of specimens were soaked in tap water for 4 hours and then tested to measure the unconfined compression strength (UCS), tensile strength, shear strength, compressibility, permeability and erodibility by rainfall. The results showed that at OPC contents greater than 10%, the increase of UCS was not significant at the same curing period. At 8% OPC, the stabilized MT can achieve a 7-day UCS of 3.45 MPa. For the asphalt emulsion stabilized MT, a 7-day UCS of 0.98 MPa can be achieved at the optimum asphalt emulsion amount of 8%. The

37 author concluded that the copper MT stabilized with either OPC or asphalt emulsion have the acceptable engineering properties and can be adapted for use in road construction.

Indraratna et al. [11] investigated the use of compacted coal tailings from West Cliff Colliery, New

South Wales, Australia in mine access roads and pavements. The obtained optimum moisture content and maximum dry unit weight were 14% and 13.7 kN/m3, respectively, at the standard

Proctor compaction energy (600 kN-m/m3) and 12% and 15.5 kN/m3, respectively, at the modified

Proctor compaction energy (2500 kN-m/m³). California bearing ratio (CBR) tests were also performed on the compacted specimens and CBR values close to 20% were obtained for specimens compacted at the standard Proctor energy and at moisture contents below 12%. CBR values higher than 80% were obtained for specimens compacted at the modified Proctor energy and at similar moisture contents. The obtained CBR for the specimens compacted at the modified Proctor energy is comparable to those of compacted granular materials used in the construction of base and subbase for airfields and roads. The results met the minimum strength requirement by the National

Association of Australian State Road Authorities.

Wasiuddin et al. [12] studied pile raw chat in hot mix asphalt for surface and base course pavement applications. The raw chat was mixed with conventional aggregates at a percentage of 40, 60 and

80% for surface course, and 40, 50 and 70% for base course, respectively. The aggregates were preheated at 163 C and then placed in a heated mixing bowl to be mixed with the asphalt binder.

Once a uniform mixture was obtained, it was placed in an oven at 149 C for two hours (short- term-aged). Subsequently, the mixture was compacted with a superpave gyratory compactor to produce specimens of 115 mm in height and 150 mm in diameter. Indirect tensile strength, moisture sensitivity, rutting and permeability tests were conducted. The results showed that the tensile strength ratio (TSR) decreases with higher raw chat content. However, the values at 60 and

80% raw chat are 98 and 92%, respectively, which are higher than the TSR of 80% required by the Oklahoma Department of Transportation (ODOT). The rutting tests showed that the average rut depth increases from 1.0 mm to 1.8 mm when the raw chat content goes from 40 to 80%. Only the specimens prepared at 70% raw chat did not meet the 5 mm rut depth requirement by ODOT.

The permeability test results showed that the permeability decreases with higher raw chat content and the tested specimens all meet the ODOT permeability requirement of less than 125×10 ̄ ⁵ cm/sec. The authors concluded that raw chat in hot mix asphalt has a great potential for both surface and base course applications.

Teredesai et al. [13] studied the stabilization of pile raw chat with class F fly ash (FA) and cement kiln dust (CKD) to evaluate its applicability as a road base material. Mixtures were made by incorporating either FA or CKD at different proportions (0-20%) to the pile raw chat. Also, mixtures containing 40% of pile raw chat and 60% of limestone aggregates were prepared. The mixtures were casted in cylindrical molds of 152 mm diameter and 305 mm height and compacted at their OMC. The specimens were cured for 14 and 28 days, respectively, in a moist room with a controlled temperature of 22 C and a relative humidity of approximately 95%. Unconfined compression tests and SEM imagining were performed. The results showed that the raw chat stabilized with 15% of FA reached a UCS of 2.1 MPa and 5.3 MPa after 14 and 28 days’ curing, respectively, while the raw chat stabilized with 15% CKD reached a UCS of 1.2 MPa and 2.0 MPa after 14 and 28 days’ curing, respectively. A similar behavior was observed for the raw chat- limestone blend, indicating the effectiveness of FA as a stabilizer. The authors concluded that the pile raw chat stabilized with FA is a promising road base material based on the ODOT guidelines.

Mahmood and Mulligan [14] studied the stabilization of 6 different types of MT with OPC (type

I) for their use as road base material for unpaved roads. Mixtures were prepared by adding a

39 constant amount of OPC to each MT and varying the water content in order to evaluate the effect of water/cement (w/c) ratio on UCS. After a mixture was prepared, it was poured into wooden cubical molds of 25 mm in side length. The specimens were left to cure at room temperature for

United States.

Swami et al. [15] investigated the use of kimberlite tailings as road construction material. The MT was stabilized with OPC for their use in subbase course layers and mixed with bitumen for their use in bituminous macadam base course layers. The specimens were compacted at the modified

Proctor energy and the OMC and MDD were 11.5% and 20.6 kN/m3, respectively. CBR and UCS were determined for compacted specimens at 0, 3 and 5% OPC, respectively. The CBR specimens were cured in wet sand for 7 days and immersed in water for 4 days, whereas the UCS specimens were cured in wet sand for 7 and 28 days, respectively. The results showed that the CBR of the pure MT (0% OPC) specimens was 70%, and it was higher than 100% for both the 3 and 5% OPC specimens. The CBR results met the minimum 20% requirement by the Indian Roads Congress

Specifications for coarse-graded subbase material. After 7 and 28 days’ curing, the 5% OPC specimens reached UCS values of 1.33 MPa and 2.05 MPa, respectively. The UCS results met the strength requirements by the Indian Roads Congress Specifications (>1.50 MPa after 28 days’ curing). For the use of kimberlite tailings in bituminous mixtures, different tests such as grain size distribution, water absorption, aggregate impact value, stripping value, flakiness and elongation

40 index were evaluated. The results showed that they failed to meet the water absorption, soundness and stripping requirement. However, the authors still recommended their use in bituminous mixes in areas with low to moderate rain conditions, especially for low-volume roads with low axle-load vehicles. Finally, they concluded that the kimberlite tailings are suitable to be used as subbase course layers in road construction.

Qian et al. [16] studied the stabilization of granite MT with OPC as pavement subbase material.

Laboratory tests were conducted to evaluate the UCS, static and dynamic moduli, split tensile strength, and thermal and drying shrinkage. The results showed that the UCS increases with higher

OPC content and at 5% OPC, the granite MT have comparable UCS, split tensile strength, and static and dynamic moduli to the OPC-stabilized crushed stone. The UCS values of the 5% OPC-

Xu [17] evaluated the stabilization of iron MT with blast furnace slag (BFSG) cement as an alternative construction material for base and subbase courses of low-grade highways. The BFSG

41 was added into the iron MT in a range of 5-17%. The specimens were compacted using the standard

Oluwasola et al. [18–20] investigated the suitability of using electric arc furnace slag (EAFSG) and copper MT to replace conventional aggregates in production of stone mastic asphalt (SMA).

Specifically, they used different amount of EAFSG and copper MT to replace granite aggregates in the different mix designs. EAFSG was used as both coarse and fine aggregates, while MT was used as only fine aggregate; the mixtures followed the SMA 14 mix designation and PG75 binder was used as the bitumen. The specimens were compacted in a Marshall mold and 75 blows were applied on each side of the specimen. Marshall stability, moisture susceptibility, dynamic creep, indirect tensile resilient modulus, rutting and leaching tests (TCLP) were performed to evaluate the performance of the mixtures. The results showed that the use of EAFSG and copper MT to replace conventional aggregates improves the performance of the SMA by reducing the drain down values, increasing the indirect tensile resilient modulus and improving the resistance to permanent deformation. The optimum mix design contains 80% EAFSG and 20% MT. The leaching analysis results showed that the concentrations of leached heavy metals are below the EPA limits. The leachability analysis further showed that even with the incorporation of bitumen to the aggregates, the concentrations of leached heavy metals were below the EPA limits. The authors concluded that

EAFSG and copper MT are promising alternate aggregates to be used in asphalt pavement material.

Gopez et al. [21] investigated the utilization of copper-gold MT as fine aggregate in roller compacted concrete. Mixtures were prepared according to ASTM C 33 [22] and using OPC of

200-500 kg per cubic meter. Two types of mixtures were prepared based on the fine aggregate: regular sand and copper-gold MT. Laboratory tests were performed to determine the compressive strength and durability. The specimens were cured for 7 and 28 days at ambient temperature. The results showed that the specimens made with regular sand have higher UCS compared to those prepared with copper-gold MT. This behavior occurred because a higher water to cement ratio was used in the copper-gold MT specimens due to the fineness of the MT particles. The optimum/economical OPC amount of 400 kg/m³ resulted in a UCS of 21.7 MPa and 25.5 MPa after 7 and 28 days’ curing, respectively, for the copper-gold MT specimens, meeting the requirement for concrete pavement by the Philippines Standards. The durability test showed that the UCS of the copper-gold MT specimens after 3, 9 and 15 wet-dry cycles was 20.9, 19.5 and

18.7 MPa, respectively. The authors concluded that the copper-gold MT are a viable alternative fine aggregate material to regular sand in roller compact concrete for pavement applications.

Augusto et al. [23] stabilized iron MT with OPC, lime and steel making SG for utilization in road construction. Mixtures were prepared by adding 1, 2, 5 and 10% of stabilizer by total solid weight to the iron MT. The specimens were compacted using an intermediate Proctor energy (1263 kN- m/m3). Laboratory tests were performed to evaluate the CBR, UCS, expansion and leaching of heavy metals. The CBR specimens were cured in a moisture chamber for 0, 3 and 7 days, while the UCS specimens were cured in both moisture chamber and open air for 7 days. The optimum stabilizer content was found at 5%, 10% and 10% for the OPC, lime and SG, respectively. The

43 results showed that OPC was the most effective stabilizer and the OPC stabilized iron MT reached a CBR of 180% and a UCS of 1.32 MPa, after 7 days curing. The expansion of the OPC-MT mixtures was less than 0.15%, which is very low compared to the maximum limits for subgrade

(2%), subbase (1%) and base (0.50%) established by the NBR 7207:82. The leaching test was performed on the iron MT and SG based on the NBR 10004:2004. The results showed that the leachate concentrations were low compared to the limits established by the norm. For example, arsenic and lead concentrations were < 0.004 mg/L and < 0.005 mg/L for the iron MT and were not detected for the SG, while the limits are 1 mg/L in both elements. The authors concluded that the iron MT stabilized with OPC represent an appropriate road construction material.

Gorakhki and Bareither [24] investigated the effect of FA and OPC on synthetic and natural MT for their use as road subbase material. The synthetic MT consisted of a mixture of kaolin clay, silica flour and angular sand from road base material, and the natural MT were obtained from copper and garnet mines. Mixtures were prepared by adding either FA or OPC to the MT in a range of 0 to 20% by the dry weight of MT. The specimens were either compacted using the standard

Proctor energy or poured in cylindrical molds of 102 mm in diameter and 203 mm in height. After compaction, the specimens (including the mold) were wrapped with plastic sheet and cured at 25

C for 7 days. Laboratory tests were then performed to evaluate the UCS. The results showed that the OPC as a stabilizer leads to better mechanical performance than the FA. The UCS of MT-FA specimens ranged from 0 to 0.59 MPa, whereas the UCS of MT-OPC specimens ranged from 1.1 to 2.2 MPa. According to ARA (Applied Research Associates) and ACAA (American Coal Ash

Association), the UCS range of MT-OPC specimens is representative of materials for road subbase construction.

Consoli et al. [25] stabilized gold MT with OPC for its use as subbase material for low volume roads. Mixtures were prepared by adding OPC at 3, 5 and 7% by dry MT weight. Compressive strength, stiffness and durability tests were performed to evaluate the performance of the specimens. The specimens were compacted using the standard Proctor energy in molds of 50 mm in diameter and 100 mm in length for the compression test, and in molds of 100 mm diameter and

127.3 mm in height for the durability and stiffness tests. The specimens were cured in a humid room at 23 ± 2 C and relative moisture of about 95% for 7 days. The results were presented as a function of the porosity/cement index. The compressive strength and stiffness tests showed that the UCS and initial shear modulus increase with the reduction of the porosity/cement index. The highest UCS and initial shear modulus of 1.80 MPa and 1170 MPa, respectively, were found in specimens with 7% OPC. The durability test results showed that after 12 wetting-drying cycles, the 3% OPC specimens show the largest loss of mass of about 5% whereas the 7% OPC specimens the smallest loss of mass of about 1%. The authors concluded that the porosity/cement index is directly related to the compressive strength, stiffness and durability of the gold MT-OPC specimens and it can be modified based on project convenience to satisfy the requirements of subbase material for low volume roads.

2.2. Utilization of MT Together with Soil Researchers have also studied utilization of MT together with soil as road construction material

(see Table 2). Ramesh et al. [26] studied the effect of lime on the compaction and strength behavior of clay-gold MT mixtures. They first investigated clay-MT mixtures at different proportions with no lime and found that the optimum clay/MT ratio was 9/1. The specimens had a MDD and OMC of 16.1 kN/m3 and 23.4%, respectively. After compaction, the specimens were enclosed in plastic bags and placed in desiccators for 7 and 30 days. The results showed a UCS of 0.23 MPa and 0.27

MPa after 7 and 30 days’ curing, respectively. Then the effect of lime on the behavior of clay-MT

45 mixtures was studied and it was found that the optimum percentage of lime in the clay-MT mixture was 3%. The MDD decreased to 15.2 kN/m3, while the OMC remained at 23.4%. The 3%-lime specimen gave a UCS of 0.66 MPa and 0.84 MPa after 7 and 30 days’ curing, respectively. The results showed that gold MT can be used in combination with lime for soil stabilization.

Osinubi et al. [27] investigated the use of iron MT and OPC to stabilize black clay (CH according to the USCS) to evaluate its applicability as road subgrade material. OPC-clay mixtures were first prepared by adding 1-4% of OPC by dry weight of soil. Then iron MT was added at 0-10% to the

OPC-clay mixtures. The specimens were compacted at the British Standard light energy level (596 kN-m/m3) at three layers in a 1000 cm3 British Standard mold and cured for 7 and 28 days, respectively. Laboratory tests were performed to evaluate the index properties and shear strength parameters of the mixtures. The results showed an optimum OPC and iron MT content of 4% and

6%, respectively. The liquid limit and plasticity index of the optimum mixture were 54% and 36%, respectively. Both values were above the corresponding maximum values of 50% and 30% required by the Nigerian General Specifications for subgrade materials. The shear strength of the optimum mixture was 0.10 MPa. The authors concluded that although the addition of iron MT to the OPC-clay mixtures represented an enhancement of the properties, the results did not meet the minimum requirements of the Nigerian General Specifications for Highways.

Etim et al. [28] evaluated the feasibility of using lime and iron MT to stabilize black cotton soil for its use as subbase material in the construction of low volume roads. According to the USCS, the soil was classified as CH. Mixtures were prepared by incorporating 0-8% lime and 0-10% iron

MT, by dry soil weight, into the soil. Specimens were compacted at the British Standard light energy level. Laboratory tests were carried out to evaluate the UCS, CBR, durability and leaching.

The UCS specimens were wax cured for 7, 14 and 28 days, whereas the CBR specimens were wax

46 cured for 6 days and then immersed in water for 1 day before testing. The durability test was performed on specimens wax cured for 7 days and then dewaxed top and bottom and immersed in water for 7 days. The results showed an optimum mixture containing 8% lime and 8% iron MT.

The optimum mixture reached a UCS of 1.07, 1.80 and 2.03 MPa after 7, 14 and 28 days’ curing, which met the criteria of the Nigerian standards for soil-lime stabilization (> 1 MPa). The CBR of the optimum mixture was 50% and exceeded the requirement for subbase material of light traffic roads (> 40%). The durability analysis showed a UCS decrease of 37% after water immersion, which is acceptable according to the Nigerian standards. The leaching analysis proved that the optimum mixture leached 0.272 mg/l of iron, which was less than the acceptable value of 0.3 mg/l.

The authors concluded that lime and iron MT can be used to effectively stabilize black cotton soil for use as a subbase material of light traffic roads.

Ojuri et al. [29] investigated the use of lime, OPC and iron MT to stabilize lateritic soil (SC according to USCS) as a road construction material. The iron MT was added at 0-50% to the soil, and then the lime-OPC binder, at a 1/2 ratio, was added to the soil-MT mixtures at 0-10%.

Laboratory tests were performed to evaluate the index properties, UCS and CBR of the mixtures.

3. Utilization of MT Through Geopolymerization The studies summarized above are focused on the stabilization of MT alone or MT together with soils using conventional stabilizers. Even though the use of conventional stabilizers has demonstrated acceptable performance, their use has the drawbacks of high-energy consumption and release of large amount of CO2. Therefore, researchers have studied the stabilization of MT, alone or in combination with other materials, based on geopolymerization [30–36].

Geopolymerization is the chemical reaction of aluminosilicates in a highly alkaline silicate or hydroxide solution, creating a stable material called geopolymer. Geopolymer has a three- dimensional aluminosilicate structure consisting of linked AlO4 and SiO4 tetrahedra by sharing the oxygen atoms between them [37–41]. The chemical composition of geopolymer can be represented by the following formula [38,39]:

푀푛[−(푆𝑖푂2)푧 − 퐴푙푂2 −]푛 ∙ 푤퐻2푂 (1) where, M is an alkaline element or cation such as sodium, potassium or calcium; “ – ” represents a bond; n is the degree of polymerization or polycondensation; and z is the Si/Al ratio. Water is an essential part of geopolymerization which is released after the reaction has taken place and eventually gets evaporated creating micro size voids [42,43]. Geopolymer materials have several advantages over OPC, including abundant raw material resources, rapid development of mechanical strength, high acid resistance, no/low alkali-silica reaction related expansion, immobilization of toxic and hazardous materials, and significantly reduced energy consumption and greenhouse gas emissions [44–46]. These advantages have attracted the attention of the research community about the use of geopolymer as a potential sustainable material. So far, only few researchers have investigated the geopolymerization of MT as a road construction material

(see Table 3).

Ahmari et al. [30] studied the utilization of copper MT as road base material through geopolymerization. Sodium hydroxide (NaOH) at 0-6% of the dry weight of MT was dissolved in water and then mixed with the MT. The mixtures were compacted using the modified Proctor energy to produce specimens and the specimens were then cured for 7, 28 and 90 days, respectively, at ambient temperature, before tested. The results showed that the specimens cured for 7 days reached a maximum UCS of 2.5 MPa at 2% of NaOH. The results also showed that no significant UCS was gained after 28 days’ curing. The study demonstrated the potential of copper

MT as road base construction material through geopolymerization.

Sangiorgi et al. [33] investigated the utilization of alkali activated tungsten MT and its applicability as construction material in transportation infrastructure. The applications of the alkali activated tungsten MT were the development of artificial aggregates for their use in bituminous mixtures and the production of precast segments. The results showed that the new aggregate materials presented strong (low Los Angeles) and durable (high freeze-thaw resistance) properties with polyhedric shapes which led to a good compaction array and the reduction of stress concentrations under traffic loading on the bituminous layers. Also, the precast segments such as culverts, curbs, gutters and safety barriers, showed promising results. The authors concluded that alkali activated tungsten MT can be used as an alternative sustainable material in transportation infrastructure.

Manjarrez and Zhang [36] conducted a detailed study on utilization of copper MT as road base material through geopolymerization. The MT were first mixed with a NaOH solution at a concentration of 0, 3, 5, 7 or 11 M and at a moisture content within the range of 11 to 19%. The mixture was compacted in cylindrical molds of 33.4 mm in diameter and 71.5 mm in height at the modified Proctor energy and the compacted specimens were then cured in an oven at 35 C for 7 days before tested. Laboratory tests were conducted to evaluate the UCS of the specimens and

SEM/EDS analysis to investigate the geopolymer microstructure and chemical composition. The results showed that the maximum dry unit weight increased with higher NaOH concentration, from

16.9 kN/m3 at 0 M NaOH to 17.6 kN/m3 at 11 M NaOH. The optimum moisture content decreased slightly from 16% to 15% when the NaOH concentration increased from 0 to 11 M NaOH. At a constant moisture content, the UCS increased with higher NaOH concentration up to a certain level and then decreased. The highest UCS of 5.32 MPa was found at a moisture content of 11% and 11

M NaOH. In addition, the highest UCS at a certain NaOH concentration did not occur at the optimum moisture content or the maximum dry density, highlighting the contribution of geopolymerization to the UCS. The SEM/EDS results showed that higher dissolution of aluminum from the unreacted MT led to a stronger geopolymer gel and thus a higher UCS. For example, the specimens prepared at 11M/11% and 7M/14% showed very high Si/Al ratios in the unreacted MT,

63.10 and 36.71, respectively, and reached the two highest UCS among all specimens, 5.32 and

4.41 MPa, respectively, whereas the specimens prepared at 7M/16% showed a Si/Al ratio of 2.33 in the unreacted MT and reached a UCS of only 0.75 MPa. The authors concluded that by selecting the appropriate preparation conditions (NaOH concentration and moisture content), the stabilized

MT through geopolymerization can meet the strength requirements for cement treated base materials by the transportation agencies in the United States.

4. Type of Mine Tailings and Properties Several types of mine tailings have been used as road construction materials. The characteristic that differentiates MT is the mineral they were derived from. Iron [17,23,28,29,47] and copper MT

[18–21,24,48] are the most used tailings in road construction followed by coal [11], garnet [24], tungsten [33,49], granite [16,50,51] and gold MT [21,25,26]. The mineral determines the chemical composition, for example, iron MT are richer in SiO2 and Fe2O3, whereas copper MT are richer in

SiO2 and Al2O3.

The geotechnical properties are defined by the mineral and zone they were collected. For example, iron MT have the highest specific gravity 3.08-3.74 [52,53] among the investigated MT which range lies between 2.03 (Coal MT) to 3.07 (Garnet MT). The zone where MT were collected plays an important role in the geotechnical properties because it is closely related with the grain particle size. The deeper the MT are collected the finer they are in grain size. This is because of the way they were deposited with time. In addition, MT collected near the slurry area are also finer in size compared to those located in the periphery of the dam. Mostly, the MT used as road construction materials are dominant in sand size with about 28% to 70%, D50 ranging from 0.03 to 0.81 µm and

USCS classification of ML, SM, SP or SC [11,24,25,48,53–56]. However, coarse MT have also been studied as road construction materials with 36% gravel size and D50 around 7000 µm. Since the sand size is predominant in these mine tailings, they are not plastic in nature. Nevertheless, some plasticity index between 9% and 15% have also been reported [24,52]. Specifically, around the 63% of these mine tailings have particles finer than 75 µm. Table 4 and 5 summarizes the chemical composition and geotechnical properties, respectively, of the different types of MT used as road construction material.

5. Discussion The extensive laboratory research on using MT as an alternative road construction material has demonstrated promising results, as described in the previous section. In these studies, various experiments were conducted on the stabilized MT to evaluate their properties following the different standards on road design and construction. The unconfined compressive strength (UCS) is the most common parameter specified by the various standards. Table 5 summarizes the UCS required by the standards of several countries for cement treated materials in road base and subbase layers. The standards presented in table 5 and the 7-day UCS values by the studies listed in Tables

1-3 are compared in Fig. 1. It is observed that many of the stabilized MT satisfy the standards of a

51 few countries, which indicates the feasibility of using MT as road construction material in terms of compressive strength.

Some attempts have also been made on using MT as road construction material in the field. For example, Wasiuddin et al. [51] conducted a field performance evaluation on a 960 m long chat- asphalt test road constructed in Cardin, Oklahoma, using 80% raw chat in the surface course and

50% raw chat in the base course. Spectral surface waves analysis and falling weight deflectometer and ground penetration radar tests were performed right after construction and after two and a half years of operation. The results showed that the chat-asphalt road has a performance comparable to the traditional hot mix asphalt pavement according to the Oklahoma Department of Transportation.

The other example is the work conducted by Qian et al. [16] which evaluated the performance of a 20 km subbase layer section built with cement stabilized granite MT, in Shenzhen, China. During the first two years of operation, a field survey demonstrated an excellent pavement performance according to the Chinese specifications.

Although the laboratory and field studies have both demonstrated the feasibility of using MT as road construction material, their application in practice is still very limited. The possible reasons are the potential contamination from MT, the lack of relevant standards, and the slow acceptance of stabilized waste materials in construction by public and industry, as discussed below.

Like any other wastes, the use of MT as road construction material needs to consider the environmental aspects such as the leachate of heavy metals. It is extremely important to conduct leaching analysis following USEPA, ASTM and/or other standard methods and ensure that the contaminants in MT are effectively and safely immobilized [57]. Researchers, including Augusto et al. [23], Etim et al. [28], Ojuri et al. [29], Oluwasola et al. [20], and Grubb et al. [58] have studied the leaching behavior of heavy metals in stabilized MT intended for used in road base

52 and/or subbase layers. Their results demonstrated that the heavy metals can be effectively and safely immobilized after stabilization.

The limited utilization of MT in road construction is also related to the absence of relevant standards and the slow acceptance by industry and public. It is known that standardization plays a key role in disseminating knowledge, reducing time to market for innovations and exploiting research results [59]. However, there exists a lack of standards about the use of wastes as road construction materials. To the knowledge of the authors, the standards related to the use of wastes as road construction materials are limited and focused on the use of fly ash and/or slag as cementing materials or slag as aggregates. To promote the utilization of MT in road construction, relevant standards should be developed. Moreover, the “waste” feature of MT and the potential for causing contamination and being unsafe adversely affect the acceptance of MT as road construction material by industry and public. Therefore, more work needs to be done to promote the utilization of MT in road construction, not only on the technical, environmental and economic aspects but also on the public education and government policy related to waste recycling and sustainable development.

6. Conclusions Based on the systematic review of studies on the use of mine tailings (MT) as road construction material, the following conclusions can be drawn:

• MT represent a potential material for road construction. The main advantages of utilizing MT

in road construction are the reduction of exploitation of natural construction materials and the

elimination of large areas of land occupied by MT storage and related economic and

environmental costs.

• The extensive research on utilizing MT as road construction material can be divided in three

general categories: i) MT alone stabilized with conventional stabilizers, ii) MT together with 53

soils and stabilized with conventional stabilizers, and iii) MT alone stabilized through

geopolymerization. The use of conventional stabilizers has the drawbacks of high energy and

large carbon footprint consumption, while geopolymerization represents a step towards

sustainability.

• Although much research has been carried out, the use of MT in real road construction is still

very limited. The reasons might be the lack of standards about the use of wastes as construction

material, the potential contamination from wastes, and the slow acceptance of stabilized waste

materials in construction by public and industry.

• The use of MT as road construction material based on geopolymerization has a promising

future. More research needs to be conducted to fully cover every aspect related to its

performance under both laboratory and field conditions.

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Table 1. Utilization of MT alone improved with conventional stabilizers

Waste Curing UCS No. material Grain size Stabilizer Tests conducted Ref. condition (MPa) (wt.%) OPC: moist- cured for 7, 28 1.0 - Unconfined and 90 days 0.001 compressive, OPC (2- and then mm, 41% tensile and 3.45 Copper 12%) and soaked in passing shear strength, (8% 1 MT asphalt water for 4 h; [10] No. 200 compressibility, OPC) – (100%) emulsion Asphalt sieve permeability, 7 days (4-20%) emulsion: (0.0075 and erodibility ambient mm) by rainfall temperature for 7 and 28 days 85% fine CBR, Coal MT sand compressibility 2 - - - [11] (100%) particles and collapse (SM) potential Indirect tensile 79% < Asphalt Short-term- strength, Raw chat 4.75 mm emulsion aged in an moisture 3 and 3.48% - [12] (40- – PG 64- oven at 149 ̊C susceptibility, < 0.075 80%) 22 for 2 hours rutting and mm permeability 2.1 (15% 60% < Moist room at Unconfined FA) 4.75 mm FA and 22 °C, Raw chat compressive and 1.2 4 and 10% CKD humidity 95%, [13] strength and (15% (0-80%) < 0.075 for14 and 28 (0-20%) SEM CKD), mm days 14 days 22% < Moisture Unconfined MT No. 200 OPC chamber at 5 compressive 9.5 [14] (100%) sieve 23.5 C, 98% strength (SM) humidity 1.33 (7 CBR and Cured in wet days), Kimberli 0.075- OPC unconfined 6 sand for 7 and 2.05 [15] te MT 0.020 mm (5%) compressive 28 days (28 strength days) Granite 97% < OPC 7 Ambient Unconfined 4.37, [16] MT 4.75 mm (3-6%) temperature for compressive 6.39

7, 28 and 90 strength, static and days and dynamic 7.17 moduli, split (5% tensile strength, OPC), and thermal and 7, 28 drying and 90 shrinkage days Curing room at 98% < Unconfined 20 ±2 °C for 7 4.75 mm BFSG compressive 2.7 days with 8 Iron MT and 39% cement strength and (15% [17] relative < 0.075 (5-17%) economic BFSG) humidity about mm analysis 96% Mixing temperature (170-185 °C), Copper compacting Marshall MT EAFSG temperature stability, drain (15%) (coarse (140-160 °C). down and 9 and PG 76 - [18] and fine) After indirect tensile EAFSG MT (fine) compaction, resilient (40- specimens modulus 80%) were placed in an oven at 85 °C for 5 days Before compaction, Marshall mixtures were Copper stability, placed in an MT EAFSG Bitumen: moisture oven for 4 h at (20%) (coarse PG 76 susceptibility, 10 135 °C. After - [19] and and fine) and 80- indirect tensile compaction, EAFSG MT (fine) 100 resilient specimens (40%) modulus and were placed in dynamic creep an oven at 85 °C for 5 days Bitumen: Before PG 76, Copper compaction, Moisture 80-100 MT EAFSG mixtures were susceptibility, and 80- (20%) (coarse placed in an dynamic creep, 11 100 - [20] and and fine) oven for 4 h at rutting tests and (modifie EAFSG MT (fine) 135 °C. After leaching d with (40%) compaction, analysis 5% specimens EVA) were placed in

an oven at 85 °C for 5 days 25.5 98% < 2.5 At ambient Unconfined MPa Copper- mm and temperature for compressive 12 OPC (OPC [21] gold MT 19% < 7, 28 and 63 strength and 400 0.15 mm days durability kg/m³) CBR: moisture CBR, OPC, Iron MT chamber for 7 unconfined 1.32 85% < lime and 13 days; UCS: compressive (5% [23] (90- 0.075 mm steel SG open air for 7 strength and OPC) 99%) (1-10%) days expansion Synthetic MT: 86% Synthetic < 0.075 Entire Standard MT, mm; specimen Proctor copper Fly ash 2.2 Copper (including compaction and 14 MT and and OPC (10% [24] MT: 55% mold) wrapped unconfined garnet (0-20%) OPC) sand in plastic for 7 compressive MT (80- content; days at 25 C strength 100%) Garnet MT (SP) Specimens were placed in 28% < Unconfined a humid room 1.8 Gold MT 0.425, compressive OPC (3- at 23 ±2 ̊C and MPa 15 mm 71% strength, pulse [25] (93- 7%) relative (7% < 0.075 velocity test and 97%) moisture of OPC) mm durability test about 95% for 7 days

Table 2. Utilization of MT together with soil improved with conventional stabilizers

Waste materia Grain Stabilize Curing Tests UCS No. Soil Ref. l size r condition conducted (MPa) (wt.%) Room 0.66 and Gold temperatur 0.84 (10% 70% unconfined MT Lime e in MT and 1 silt Clay compressiv [26] (10- desiccators 3% lime) size (1-6%) e strength 90%) for 0, 7 and for 7 and 30 days 30 days Tropica Iron < No. l black OPC 7 and 28 2 MT (0- 200 UU, SEM - [27] clay days curing 10%) sieve (1-4%) (CH) Wax cured Unconfine 1.07, 1.85 for 7, 14 d and 2.1 Black Iron < No. and 28 compressiv (8% MT cotton Lime 3 MT (0- 200 days (UCS) e strength, and 8% [28] soil 10%) sieve (0-8%) and 6 days CBR, lime) for (CH) and soaked durability 7, 14 and 24 h (CBR) and SEM 28 days CBR, 0.42, 0.47 Room unconfined and 0.55 Iron Lateriti OPC- temperatur compressiv (8% OPC- 4 MT (0- - c soil lime e for 7, 15 e strength [29] lime) for 50%) (SC) and 30 and (0-10%) 7, 15 and days leaching 30 days analysis

Table 3. Utilization of MT improved through geopolymerization

Waste Grain Alkali Curing UCS Ref No. material Tests conducted size activator condition (MPa) . (wt.%) Copper 36% < Ambient NaOH 2.5 (2% 1 MT 0.075 temperature UCS, SEM [30] (0-6%) NaOH) (100%) mm for 7 days Los Angeles abrasion 100% < 2 MT - - test and freeze-thaw - [33] 2 mm resistance NaOH Copper 100% < Oven at 35 5.32 (0, 3, 5, 3 MT 0.420 °C for 7 UCS, SEM (11 M [36] 7 and 11 (100%) mm days NaOH) M)

Table 4. Chemical composition of different types of mine tailings Chemical Composition (%) Material Ref. Al2O3 SiO2 Na2O SO3 K2O Fe2O3 Fe3O4 CaO TiO2 MnO MgO LOI Tungsten MT 14.90 55.60 - 5.80 3.5 14.60 ------[49] Iron MT 3.36 45.64 0.41 - 0.61 47.70 - 0.61 0.24 0.07 0.39 3.00 [28] Iron MT 3.40 41.95 - - - 31.32 18.95 1.12 0.18 - 0.42 2.65 [29] Copper MT 13.96 71.52 4.12 - 1.82 3.64 - 0.16 0.013 0.07 0.49 2.19 [56] Bauxite MT 14.00 1.20 - - 30.90 - 2.50 4.50 1.70 - - [60] Kimberlite MT 4.09 34.26 - 0.37 - 9.68 - 11.51 - - 19.55 14.00 [15] Panesqueira MT 18.27 68.54 - - 5.24 5.64 - - 1.17 - - - [33] Gold MT 15.05 60.40 - 0.30 0.40 6.60 - 6.90 0.20 - 1.70 8.40 [56] Tungsten MT 16.66 53.48 0.62 3.10 7.65 12.33 - - 1.39 - 1.27 - [61] Copper MT 14.10 55.90 3.02 2.22 3.89 3.07 - 2.27 0.49 0.06 1.78 - [48]

Table 5. Geotechnical properties of different types of mine tailings

Ref. [54] [25] [55] [24] [11] [26] [53] [48] Cop Gar Copper Gold Nora Mussle Golden Louvic Cop Mont Coal Gold Iron Copper Iron MT per net MT MT nda white giant ourt per wright MT MT MT MT MT MT Specific 3.08 2.76 2.86 3.68 3.26 2.97 3.33 3.40 2.76 2.72 3.07 2.03 2.78 3.74 2.83 Gravity Liquid limit 28 28 ------25.2 18.8 - 44.0 - - (%) Plastic limit 19 13 ------11.5 18.4 - - - - (%) Plasticity Non- Non- 9 15 ------13.7 0.4 - - - index (%) plastic plastic 0.02 0.00 0.00 D10 (mm) 0.005 0.005 - 0.060 0.005 0.004 0.005 0.147 0.053 - 0.005 0.003 0 4 4 0.04 0.05 0.01 D30 (mm) 0.012 0.028 - 0.400 0.012 0.018 0.018 0.200 0.140 - 0.020 0.025 0 0 8 0.06 0.08 0.04 D50 (mm) 0.030 0.060 0.060 0.810 0.016 0.016 0.046 0.255 0.250 - 0.011 0.070 2 0 5 0.08 0.12 0.06 D60 (mm) 0.045 0.074 - 1.190 0.023 0.055 0.070 0.300 0.310 - 0.007 0.110 4 0 0 Cu 8.82 14.8 - 18.89 5.00 13.75 13.94 4.22 2.04 30 15 5.85 - 1.50 36.67 Cc 0.59 2.12 - 2.11 1.39 1.44 0.93 0.95 0.91 5.21 1.35 1.19 - 10.67 1.89 MDW - - 17 ------16.7 18.6 15.5 15.7 - 16.9 (kN/m3) OMC (%) - - 17 ------13.3 9.7 12.0 21.5 - 16 Sand size - - 28 92 25 30 38 43 98 54.7 36.7 70 17 - 55 (%) Finer than 75 - - 72 8 75 70 62 57 2 45.3 63.3 15 83 - 45 micron (%) SW- USCS - - ML SM SM SM SM SP SC ML SM - - - SM

Table 6. Requirements for cement treated materials as road base/subbase in different countries (modified from [62])

Country OPC content (%) 7-day UCS (MPa) – (base/subbase) Australia 3 - 8 0.6 – 1.0 (base) [63] Brazil ̴ 4 > 3.5 (base) > 4 (road-mix method) > 2 (subbase) China > 5 (central-plant mixing) > 4 (base) Spain 3.5 – 6 4.5 – 6 (base) UK 2 - 5 2.5 – 4.5 (base) 2.5 – 5.5 – gyratory compactor (base) Italy 2 - 4 2.5 – 4.5 – Proctor hammer (base) South Africa 1.5 – 3.0 1.5 – 3.0 (base) France 0.75 – 1.50 5 – 10 (base) Nigeria > 5 1.03 (subbase) [28] 1.03 – 2 .75 (base) [64] 3.50 (base) [65–67] USA 3 - 10 5.20 (base) [68] 4.15 (base) [69] 2.06 – 5.51 (base) [70]

11 10 9 China China 8 Brazil Nigeria France base subbase 7 6 5 Spain Italy UCS UCS (MPa) 4 UK 3 USA South 2 Africa 1 Australia 0 1A 2B C3 D4 E5 F6 G7 H8 I9 J10 K11 L12 M13 N14 References

A. Sultan [10] H. Gorakhi and Bareither [24] B. Teredesai [13] I. Ramesh et al. [26] C. Mahmood and Mulligan [14] J. Etim et al. [28] D. Swami et al. [15] K. Ojuri et al. [29] E. Qian et al. [16] L. Ahmari et al. [30] F. Xu [17] M. Consoli et al. [25] G. Augusto et al. [23] N. Manjarrez and Zhang [36]

Fig. 1. Unconfined compressive strength (UCS) requirement of cement treated materials for road base and subbase by standards of various countries and obtained UCS values of stabilized MT by different researchers.

APPENDIX B

UTILIZATION OF COPPER MINE TAILINGS AS ROAD BASE CONSTRUCTION

MATERIAL THROUGH GEOPOLYMERIZATION

Lino Manjarrez1, and Lianyang Zhang, Ph.D., P.E., M.ASCE2

1Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, Arizona 85721, USA

2 Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, AZ 85721, USA (Corresponding author). Email: [email protected]

ABSTRACT

This paper investigates the utilization of copper mine tailings (MT) as an alternative road base construction material through geopolymerization. Specifically, MT was mixed with different amount of sodium hydroxide (NaOH) solution at various concentrations from 0 to 11 M, compacted and then cured at 35 C. After 7 days’ curing, unconfined compression tests were performed on the specimens to determine their unconfined compressive strength (UCS). Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction

(XRD) analyses were also performed to study the microstructure and chemical composition of the specimens at different conditions. The study has systematically investigated the effect of two main factors, NaOH concentration and moisture content, on the behavior of geopolymerized MT. The results show that the maximum dry unit weight of the compacted MT is influenced by the NaOH concentration, higher NaOH concentration leading to larger maximum dry unit weight. The behavior of the final geopolymerized MT depends strongly on the NaOH concentration and moisture content of the initial compacted MT. At a constant moisture content, the UCS of geopolymerized MT increases with higher NaOH concentration up to a certain level and then decreases. This behavior is simply related to the effect of NaOH content or Na/Al ratio on the geopolymerization. For specimens prepared at the same NaOH concentration, the highest UCS does not necessarily occur at the optimum water content or the maximum dry unit weight, emphasizing the contribution of geopolymerization to the UCS. Moreover, this study demonstrates that by selecting appropriate moisture content and NaOH concentration, the geopolymerized MT can meet the strength requirements for road base by different State DOTs and the FHWA in the

United States. However, before the field application of MT in road base construction, a

73 comprehensive leachate study based on the new EPA standard should be performed to ensure that the MT is environmentally safe.

Key words: Mine tailings; geopolymer; sodium hydroxide; moisture content; compressive strength; road base

1. Introduction Road construction uses tremendous amount of natural materials extracted from quarries (FHWA

2015). There is already a shortage of natural construction materials in many areas of the United

States. On the other hand, significant amount of wastes such as mine tailings (MT) are generated by the mining industry every year [2]. MT are finely ground rock that are left over after the metal- bearing minerals have been extracted in the milling process. The disposal of the enormous amount of MT is not only expensive but has also resulted in various ecological and environmental problems such as the occupation of large areas of land, generation of windblown dust, contamination of surface and underground water, and failure of tailing dams [3–5]. Obviously, a viable way to address the above two problems is to use MT as road construction material, which not only addresses the lack of natural construction materials in many areas, but also eliminates the economic and environmental costs related to disposal and management of MT.

To use MT as road construction material, alone or in combination with natural materials, they need to be stabilized [6–13]. One of the first studies that addressed this problem might be the one by

Sultan [6] who investigated copper MT stabilized with ordinary Portland cement (OPC) as an alternative road construction material. The highest unconfined compressive strength (UCS) of 3.45

MPa was found at 8% of OPC by the MT solid weight. The results showed that the OPC stabilized

MT have acceptable engineering properties and can be adapted in road construction. Indraratna et al. [7] investigated the use of compacted coal tailings from West Cliff Colliery, New South Wales,

Australia. They used both standard (600 kN-m/m3) and modified (2700 kN-m/m3) Proctor compaction energies. The optimum moisture content and maximum dry unit weight at the standard and modified Proctor compaction energies were 14% and 13.7 kN/m3, and 12% and 15.5 kN/m3, respectively. California bearing ratio (CBR) tests were also performed on the specimens and CBR values higher than 80% were obtained when the modified Proctor compaction energy was used, which met the strength requirements of the National Association of Australian State Road

Authorities. Swami et al. [10]stabilized kimberlite tailings with 5% OPC for their use in base and subbase layers. The specimens were compacted with modified Proctor energy and the obtained optimum moisture content and maximum dry unit weight were 11.5% and 20.6 kN/m3, respectively. After 7 and 28 days’ curing, the specimens reached a UCS of 1.33 MPa and 2.05

MPa, respectively, which met the strength requirements of the Indian Roads Congress

Specifications. They concluded that the kimberlite tailings are suitable to be used for base and subbase course layers in road construction. Qian et al. [11] studied granite MT stabilized with 5% cement for their use in road subbase layer. The stabilized specimens reached a UCS of 4.37 and

7.17 MPa after 7 and 29 days’ wet curing, respectively. In fact, a 20.4 km highway was constructed in Shenzhen, China by using the stabilized granite MT. A field survey conducted on the highway during the first two years of operation found that the pavement showed an excellent performance according to the Chinese specifications. Xu [12]studied iron tailings stabilized with blast furnace slag cement as an alternative construction material for base and subbase courses of low-grade highway. The specimens were compacted with the standard Proctor energy and cured for 7 days.

The optimum cement content was found at 15% by the tailings solid weight and resulted in a UCS of 2.7 MPa. An economic analysis comparing the use of the tailings/15% cement mixture against a gravel/4% cement mixture in a 12 m wide road showed 30% savings in the final cost. Augusto

75 et al. [13] stabilized iron tailings with OPC, high-calcium hydrated lime and steel making slag for their use as road construction material. The specimens were compacted at the intermediate Proctor energy (1263 kN-m/m3). The results showed that after 7 days’ curing, the tailings/OPC mixture at

5% OPC by solid weight reached the highest UCS of 1.32 MPa. The authors concluded that the iron tailings stabilized with OPC represent an appropriate road construction material.

Researchers have also studied the stabilization of MT combined with other natural materials such as soils for use in road construction [14–16]. For example, Ramesh et al. [14] studied the effect of lime on the compaction and strength behavior of clay-gold MT mixtures. They first investigated clay-MT mixtures at different proportions with no lime and found that the optimum clay/MT ratio was 9/1 which gave a UCS of 0.23 MPa after 7 days’ curing. Then they studied the effect of lime on the behavior of clay-MT mixtures and found that the optimum percentage of lime in the clay-

MT mixture was 3% which gave a UCS of 0.68 MPa. The results showed that the gold MT can be effectively used in combination with lime for soil stabilization. Ojuri et al. [16] studied the stabilization of a lateritic soil with mixtures of lime-OPC and iron MT. CBR and UCS tests were performed on the specimens to evaluate their use as a highway construction material. The results showed that when MT was incorporated into the soil, some basic geotechnical properties such as the Atterberg limits and the grain size distribution were modified, making it a suitable road base material for highway construction according to the Nigerian General Specifications for Highways.

The CBR results (>80% to 170%) showed that the mixtures at all lime-OPC ratios, except those at a soil/MT ratio of 1/0 and 9/1, met the minimum requirements of the Nigerian General

Specifications for Highways. The optimum mixture was found to be at a soil/MT ratio of 7/3 and a lime-OPC content of 8%, which gave a UCS of 0.56 MPa.

The existing studies are mainly focused on the use of OPC and/or lime as the MT or MT/soil mixture stabilizer. Even though the use of OPC and/or lime as a stabilizer is effective, they are accompanied with different negative environmental issues such as high energy consumptions and greenhouse gas emissions [17]. For example, about 7% of all carbon dioxide emissions in the world are attributed to the production of cement and lime [18,19]. Since MT contain large amount of silica and alumina, they can be potentially stabilized by geopolymerization, with no use of OPC or lime [20–26]. Geopolymerization is the chemical reaction of aluminosilicates in a highly alkaline silicate or hydroxide solution, creating a very stable material called geopolymer which has a polymeric structure with interconnected Si-O-Al-O-Si bonds [27–31]. The process of geopolymerization involves the dissolution of solid aluminosilicates in an alkaline solution, the formation of silica-alumina oligomers, and the polycondensation of the oligomeric species forming the inorganic polymeric material [28,29]. Geopolymer materials have a number of advantages over

OPC, including rapid development of mechanical strength, high acid resistance, no/low alkali- silica reaction related expansion, immobilization of toxic and hazardous materials, and significantly reduced greenhouse gas emissions [32–34]. Although much research has been conducted on using fly ash-based geopolymer to stabilize soils [35–39] and recycled asphalt pavement [40,41] for use as road construction material, to the best knowledge of the authors, the study of MT as road construction material through geopolymerization is still very limited. Ahmari et al. [26] conducted a preliminary study on utilization of copper MT as road base material through geopolymerization. The results showed that the specimens reached a maximum UCS of 2.5 MPa at 2% of NaOH after 7 days’ curing, demonstrating the potential of using copper MT as road construction material.

This paper is a continuation of the preliminary study by Ahmari et al. [26] and presents a comprehensive study on geopolymerization of copper MT so that they can be used as an alternative road base construction material. To do that, first the compaction behavior of the MT mixed with different amount of sodium hydroxide (NaOH) solution at various concentrations was studied systematically. Then unconfined compression tests were performed to measure the UCS of the compacted specimens after 7 days’ curing at 35 C, and scanning electron microscopy

(SEM)/energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analyses were carried out to study their microstructure and chemical composition. Finally, the UCS values of the stabilized MT specimens at different conditions were compared with the strength requirements for road base by different State DOTs and the FHWA in the United States to show the applicability of the geopolymerized MT as road base construction material.

2. Experimental Studies 2.1. Materials The materials used in this investigation were copper mine tailings (MT), reagent grade 98% sodium hydroxide (NaOH) and de-ionized water. The MT were supplied by a major mine company in Tucson, Arizona. The MT were received in the form of damp solids. After drying, the lumped

MT were pulverized and screened through No. 40 sieve so that no particles are lumped together and all MT are utilized. Table 1 shows the chemical composition of the MT based on XRF analysis and it is observed that silica and alumina are the main components. The particle size distribution was determined following the ASTM D6913-04 [42] and ASTM D422-63 [43] as shown in Fig.

1. The MT have a mean particle size around 90 µm with 40% particles passing No. 200 (75 µm) sieve. The MT particles have irregular shapes and the fine particles are attached to each other and to the surface of the coarse particles (see Fig. 2). It is noted that the MT particles are finer than the

78 gradation requirement for base material by ASTM D124-15 [44]. However, this study investigates the utilization of stabilized MT through geopolymerization as road base material, which is very similar to the use of stabilized soils with cement/lime as road base material [14–16,45]. Fig. 3 shows the XRD patterns of the MT, showing mainly the presence of crystalline materials as quartz

[SiO2], albite [NaAlSi3O8], sanidine [K(AlSi3O8)] and gypsum [CaSO4.2H2O]. A weak amorphous phase, centered at 28 is observed in the XRD pattern. The main reactive phase for geopolymerization is the amorphous phase, however, as seen in [46], the crystalline phase also participates in the geopolymerization process.

The NaOH was purchased from Hill Brothers Chemical Co. in Tucson, Arizona. The sodium hydroxide solution is prepared by dissolving the NaOH flakes in de-ionized water.

2.2. Specimen Preparation Five NaOH solution concentrations, 0, 3, 5, 7 and 11 M NaOH, and five moisture contents, 11, 14,

16, 17 and 19%, were used in the MT compaction test and specimen preparation. The moisture content is the percentage of the weight of water in the alkaline solution to the total weight of solids

(both MT and NaOH) in the mixture. The compaction was performed using the Harvard Miniature

Compaction cylindrical molds of 33.4 mm diameter and 71.5 mm height. The compaction energy used is equivalent to that of the modified Proctor by adjusting the hammer drop height. First, the

NaOH solution was slowly added to the dry MT and thoroughly mixed for 5 minutes to ensure homogeneity. The amount of added solution depends on the desired moisture content as previously described. Then, the generated MT-NaOH mixture was used for the compaction test. Three compaction tests were performed at each NaOH concentration and moisture content. After compaction, the specimens were demolded and placed in an oven at 35 C for curing. A

79 temperature around 35 oC can be easily achieved on the ground surface in the southwest region of the United States [47] and thus the selected curing temperature is practically applicable.

2.3. Methodology Unconfined compression tests were performed on the compacted specimens after 7 days’ curing to measure their unconfined compressive strength (UCS) and evaluate the effect of NaOH concentration and moisture content on the UCS. The unconfined compression tests were conducted using an ELE tri Flex 2 loading machine at a constant loading rate of 0.1 mm/min. Before conducting the compression tests, the specimens were polished at the end surfaces to make sure that they were accurately flat and parallel.

To investigate the microstructure and chemical composition of the geopolymerized MT, SEM imaging, EDS and XRD analyses were performed on some of the specimens. The SEM imaging and EDS analysis were carried out in SE conventional mode using the FEI INSPECT-S/Thermo-

Fisher Noran 6 microscope. The XRD analysis was performed with a Panalytical X’pert pro MPD instrument equipped with a programmable incident beam slit using Ni-filtered, Cu, Kα and

λ=1.5418 Å as X-ray radiation.

Table 2 summarizes the tests conducted at different conditions.

3. Results and Discussion 3.1. Effect of NaOH Concentration on Dry Unit Weight and Optimum Moisture Content Fig. 4 shows the variation of dry unit weight with moisture content of the compacted MT at different NaOH concentrations. It can be clearly seen that the compaction curves are NaOH concentration dependent and the maximum dry unit weight increases with higher NaOH concentration. The maximum dry unit weight increases from 16.9 kN/m3 at 0 M NaOH (deionized water) to 17.6 kN/m3 at 11 M NaOH. The optimum moisture content tends to decrease slightly

80 with higher NaOH concentration, changing from close to 16% at 0, 3 and 5 M NaOH to around

15% at 7 and 11 M NaOH. The increase of the maximum dry unit weight and slight decrease of the optimum moisture content at higher NaOH concentration is possibly due to the filling of the voids by the NaOH and the lubrication provided by the NaOH. The results are in agreement with the studies by other researchers [15,16,48], which showed that the inclusion of waste (fly ash, rice husk ash, or MT) and stabilizer (OPC or lime) increased the maximum dry unit weight and slightly decreased the optimum moisture content of the compacted soil-waste-stabilizer mixtures.

3.2. Unconfined Compressive Strength (UCS) 3.2.1. Effect of NaOH Concentration Fig. 5 shows the variation of the 7-day UCS with NaOH concentration at various moisture contents. Under each moisture content, the UCS increases with higher NaOH concentration (which means higher NaOH content at the same moisture content) up to a certain level and then decreases.

For moisture contents of 11% and 14%, the highest strength occurs at 11 and 7 M NaOH concentration, respectively, while for moisture contents of 16%, 17% and 19%, the highest UCS occurs at about the same 5 M NaOH concentration. This behavior is simply related to the effect of

NaOH content on the dissolution of silica and alumina species and the polycondensation process as observed by many other researchers [25,49–53]. For example, Panagiotopoulou et al. [50] studied the dissolution of silica from various industrial minerals and by-products (kaolin, metakaolin, fly ash, natural pozzolana from Milos, zeolite and furnace slag) under different NaOH solutions and observed that the amount of dissolved silica increases with the solution alkalinity up to a certain level and then decreases. Similar results were also observed by Rattanasak and

Chindaprasirt [53] who evaluated the leaching of silica and alumina from fly ash activated with

NaOH solution at different concentrations (5, 10 and 15 M). The results showed that the highest dissolution of silica and alumina was at 10 M. Yunfen et al. [51] used sodium silicate and NaOH 81 solution at different concentrations and molar ratios to activate fly ash. The results showed that regardless of the molar ratio, the strength of the geopolymer increased with the alkaline solution concentration to a certain level and then started to decrease. Somna et al. [52] studied the geopolymerization of fly ash at NaOH concentrations up to 16.5 M. The strength of the fly ash- based geopolymer increased with the NaOH concentration up to 14 M and then decreased. They attributed this behavior to the excess of the hydroxide ion causing the dissolution of aluminosilicate minerals at early stages to polarize the geopolymerization process.

The effect of the NaOH content (related to both the NaOH concentration and the moisture content here) can be further explained based on the Na/Al ratio. Fig. 6 shows the variation of UCS with

Na/Al ratio at various moisture contents, from which an optimum Na/Al ratio (a ratio at which the

UCS is the largest) at each moisture content can be identified. These optimum Na/Al ratios are useful to define the trend at the dry side and the wet side of the compaction curve, respectively.

The dry side trend corresponds to the two moisture contents (11% and 14%) located at the left side of the optimum moisture content, while the wet side trend includes the two moisture contents (17% and 19%) at the right side of the optimum moisture content (see Fig. 4). The optimum moisture content of about 16% is considered a pivot point for both trends. On the dry side, an increase in the moisture content leads to a decrease of the optimum Na/Al ratio up to the pivot point, while on the wet side, the opposite occurs and an increase in the moisture content causes a slight increase in the optimum Na/Al ratio. The UCS at the optimum Na/Al ratio decreases when the moisture content is higher. The lowest UCS is found on the wet side and is less than 1 MPa at the highest moisture content of 19%. This behavior was also observed by Sadat et al. [54] who studied the effect of molecular water on the behavior of geopolymer materials with molecular dynamic simulations. According to [54], at higher moisture contents, the available Na cations could be

82 diffused, without participating entirely in the geopolymerization process and thus adversely affect the strength of the geopolymer product. On the other hand, the highest UCS of 5.32 MPa is found at the lowest moisture content of 11% with an optimum Na/Al = 0.79 (11 M NaOH). According to different researchers, the optimum Na/Al ratio for geopolymerization is around 1.0. For example, Ren et al. [25] conducted a detailed study of the effect of Na/Al ratio on the UCS of

MT/aluminum sludge-based geopolymer. They observed an increase in the UCS with higher Na/Al ratio up to Na/Al = 1.1 and then a decrease. Table 3 shows a summary of the optimum Na/Al ratios from different researchers. The reason that the optimum Na/Al ratio from this study is slightly lower might be because the amount of Al used for the optimum Na/Al calculation was taken from the original MT composition but not all the Al could have been dissolved and participated in the geopolymerization process, meaning that the actual optimum Na/Al ratio related to the geopolymer should be a little higher than the calculated value. This will be further discussed later.

To further investigate the effect of NaOH concentration, SEM imaging and EDS analysis were performed on specimens at three different NaOH concentrations, 0 M, 7 M and 11 M, but at the same moisture content of 11%. These three specimens were selected in order to better understand the noticeable difference in UCS between them (see Fig. 5). Fig. 7 shows the SEM micrographs of the 0M/11% specimen prepared with pure distilled water. The material exhibits MT particles of varied sizes with irregular shapes and some small particles are either attached to larger particles or to each other (Fig. 7a). Figs. 7b and 7c show higher magnification images where the presence of large voids and a weak, or null, bonding between particles can be identified, which might be the main reason for the small UCS achieved by the specimen (0.58 MPa). The EDS analysis was performed at two locations (indicated in Fig. 7c) and the average of the obtained values was used for the discussion. The results show that the Si/Al and Na/Al ratios are 3.31 and 0.12, respectively.

The Si/Al ratio is very close to that of the source material (MT and distilled water) which is 3.36 while the Na/Al ratio is much smaller than that of the source material which is 0.35. The reason is because the Si and Al did not dissolve in the 0 M NaOH solution (pure water) but the Na did. The results are in agreement with the observation by Liu et al. [55] who performed leaching analysis of red mud specimens in distilled water to analyze the dissolution of Na.

Fig. 8 shows the SEM micrographs of the 7M/11% specimen prepared with 7 M NaOH solution.

In contrast with specimen 0 M/11%, two phases of materials can be observed in specimen 7

M/11%: unreacted MT particles and glassy geopolymer gels (reacted MT). The geopolymer gels act as a binder between the MT particles and contribute to the development of higher UCS; particularly this specimen reached a UCS of 2.46 MPa. Fig. 8b shows a higher magnification image where unreacted and reacted MT particles can be more clearly seen. EDS analysis was performed on both phases, points 1 and 2 for unreacted MT and points 3 and 4 for geopolymer gel. The obtained Si/Al and Na/Al ratios are 3.06 and 0.30 for the unreacted MT and 2.59 and 0.58 for the geopolymer gel. The initial Si/Al and Na/Al ratios are 3.36 and 0.63, respectively, for the source material (MT and 7 M NaOH solution). So the Si/Al and Na/Al ratios have decreased for both the unreacted MT and the geopolymer gel. The Si/Al ratio of the unreacted MT is only slightly lower than the initial one simply because more Si than Al has been dissolved in the alkali solution, as also observed by other researchers [22,56]. The Si/Al ratio of the geopolymer gel is much lower than the initial value possibly because of two reasons: i) the low dissolution of Al from the unreacted MT hinders the formation of geopolymer gel, as explained by Fernández-Jiménez et al.

[34], and thus the Si might remain unreacted, and ii) the initial Si/Al ratio of the whole material is much higher than that of the amorphous phase (the reactive silica and alumina) which is the main source of geopolymer [22,56,57]. The decrease of Na/Al was also observed by other researchers

[22,58]. It indicates that not all the available Na cations participated in the reaction and some of them appeared as precipitate on the unreacted MT.

Fig. 9 shows the SEM micrographs of specimen 11M/11% prepared with 11 M NaOH solution.

The material displays a more compact micro-structure compared with 0M/11% and 7M/11% specimens in Figs. 7 and 8, respectively. As in Fig. 8, two phases of materials can be identified.

The difference is that the geopolymer gels are present over larger areas (Figs. 9a and 9b) which are the reason for the high UCS of 5.33 MPa on this specimen. Figs. 9c and 9d show higher magnification images of unreacted MT and geopolymer gel areas, respectively. EDS analysis was performed on both phases, points 1 and 2 in Fig. 9c and 9d, respectively. Based on the EDS analysis the Si/Al and Na/Al ratios are 63.10 and 3.88 for the unreacted MT, and 3.30 and 1.62 for the geopolymer gel, respectively. The initial Si/Al and Na/Al ratios are 3.36 and 0.79, respectively, for the source material (MT and 11 M NaOH solution). The Si/Al ratio of the unreacted MT has increased significantly due to the dissolution of a large amount of Al in the alkali solution. It can be further confirmed by the high Na/Al ratio of the unreacted MT. According to [57], the dissolution of Al has a major role in the kinetics of geopolymer gel formation, which in turn increases the bond strength between particles. This explains that the UCS obtained on this specimen was the highest among all specimens.

Fig. 10 shows the XRD patterns of specimens 7M/11% and 11M/11%. For comparison, the XRD pattern of the source MT is also shown in the figure. The XRD pattern of 7M/11% specimen remains mainly crystalline after reaction. No significant change in the intensity of crystalline peaks is observed. This is consistent with the SEM/EDS results in Fig. 8. However, the presence of gypsum as a crystalline peak detected around 12o in the source MT disappears after geopolymerization. This behavior was also observed by [22] and [21] and is attributed most likely

85 gypsum is locked in the solution pore. The XRD pattern of 11M/11% specimen shows overall less intense peaks after reaction. An intensity reduction of crystalline peaks is identified between 26-

29o for quartz, sanidine and albite, respectively. This behavior can be attributed to the high NaOH concentration (or high initial Na/Al). It indicates that crystalline silica and alumina participated in geopolymerization, coinciding with the EDS results of Fig. 9. The weak amorphous phase centered at 28o detected in the source MT is also exhibited in the XRD pattern at 7M/11% and 11M/11%.

3.2.2. Effect of Moisture Content Fig. 11 shows the variation of UCS with moisture content at different NaOH concentrations. The specimens prepared at 0 M NaOH only show a slight variation in UCS between the moisture contents and, as expected, achieve the highest UCS at the optimum moisture content around.

However, for the specimens prepared at 3, 5, 7 and 11 M NaOH concentrations, the UCS increases with the moisture content to a certain level and then drops (the rising part at 11 M is not shown but can be expected based on the general trend). Specifically, at 3 and 5 M NaOH, the UCS starts to drop at 17% and 16% moisture content, respectively, while at 7 and 11 M NaOH, the UCS starts to drop at 14% and 11% moisture content, respectively. So the moisture content at which the highest UCS occurs decreases when the NaOH concentration is higher. This is simply related to the effect of NaOH content or Na/Al ratio on geopolymerization as discussed above. Because there exists an optimum NaOH content or Na/Al ratio at which the highest UCS occurs, the moisture content at which the highest UCS occurs is simply related to the NaOH concentration. In other words, the higher the NaOH concentration is, the lower the moisture content is required to get the highest UCS.

To further investigate the effect of moisture content on the UCS, SEM imaging and EDS analysis were performed on specimens at two moisture contents, 14% and 16%, but at the same NaOH

86 concentration 7 M. The reason to select these two combinations is because of the significant UCS drop between the selected moisture contents as shown in Fig. 11. Fig. 12 shows the SEM micrographs of specimen 7M/14%. As in Figs. 8 and 9, the two material phases, unreacted MT and geopolymer gel, can be identified in Fig. 12a. Fig. 12b shows a higher magnification image where four EDS analyses were performed: points 1 and 2 for unreacted MT and points 3 and 4 for geopolymer gel. The measured average Si/Al and Na/Al ratios are 36.71 and 1.61 for the unreacted

MT and 3.35 and 0.64 for the geopolymer gel, respectively, comparing to the initial Si/Al and

Na/Al ratios of 3.36 and 0.71 for the source material. So the changes of the Si/Al and Na/Al ratios from the initial values to those for the unreacted MT and geopolymer gel are very similar to those for the 11M/11% specimen. This is possibly why the 7M/14% specimen reached the second largest

UCS of 4.76 MPa which is slightly smaller than the UCS of the 11M/11% specimen (see Fig. 11).

Fig. 13 shows the SEM micrographs of the 7M/16% specimen. As in Figs. 8, 9 and 12, the two material phases can also be identified. Although the geopolymer gel can be observed in many spots, large MT particles remain unreacted (Fig. 13a). Fig. 13b shows a higher magnification image of the unreacted MT and geopolymer gel. EDS analyses were performed on points 1 and 2 for unreacted MT and points 3 and 4 for geopolymer gel. The EDS analyzes results show a significant difference between the two phases. The measured Si/Al and Na/Al ratios are 2.33 and

1.35 for the unreacted MT and 2.82 and 7.57 for the geopolymer gel, respectively, comparing to the initial Si/Al and Na/Al ratios of 3.36 and 0.76 for the source material. The Si/Al ratio of the unreacted MT is lower than the initial value indicating that more Si than Al has been dissolved in the alkali solution as in the 7M/11% specimen. It is also seen that the Na/Al ratio of the geopolymer gel is much higher than the initial value among, indicating the precipitation of unreacted Na cations as discussed above. This behavior was also observed by [22]. The low UCS for this specimen is

87 possibly due to the precipitation of unreacted Na cations (or the high Na/Al ratio of the geopolymer gel).

Fig. 10 shows the XRD pattern of specimens 7M/14% and 7M/16%. The XRD pattern of specimen

7M/14% is very similar to that of specimen 11M/11%. An intensity reduction of crystalline peaks can also be identified between 26-29o for quartz, sanidine and albite, respectively. The XRD pattern of specimen 7M/16% shows a significant intensity reduction of sanidine, but not of quartz or albite. Overall, the XRD patterns after geopolymerization maintain the same crystalline peaks, except for gypsum, indicating that the partially reacted particles are the main constituent of the geopolymer matrix.

4. Utilization of Geopolymerized Copper MT as Road Base Construction Material Because no guidelines are available on utilization of geopolymerized copper MT as road base construction material, the strength requirements for cement treated road base (CTRB) by several transportation agencies in the United States are adopted to do the feasibility analysis. Table 4 shows the CTRB 7 day UCS requirements by six State Departments of Transportation (DOTs),

Arizona DOT [59], Texas DOT [60], Nevada DOT [61], Illinois DOT [62], Oklahoma DOT [63] and Florida DOT [64], and the Federal Highway Administration [65,66].

Fig. 14 shows the variation of 7-day UCS with moisture content at different NaOH concentrations of the geopolymerized MT and the 7-day UCS requirements for CTRB by different transportation agencies. It can be clearly seen that at 0 M NaOH (which means no geopolymerization), the MT will not be able to meet the 7-day UCS requirements for CTRB by any of the seven transportation agencies. However, by selecting appropriate NaOH concentration and moisture content, the geopolymerized MT will be able to meet the 7-day UCS requirements for CTRB by all transportation agencies. Therefore, copper MT are a potential road base construction material

88 based on geopolymerization. For a complete evaluation of MT as a potential road base material through geopolymerization, other tests such the California bearing ratio (CBR) and resilient modulus tests should also be carried out.

To promote the utilization of copper MT in practice as a road base construction material based on geopolymerization, the environmental aspects such as the leachate of heavy metals should also be addressed. The leaching study by Ahmari and Zhang [24] demonstrated that MT-based geopolymer bricks which are very similar to the material used in this study are environmentally safe. However, since the curing condition in this study is different from that in [24], the degree of geopolymerization for the MT may be different. This means that the geopolymerized MT in this study may have different leaching behavior from the MT-based geopolymer bricks in [23].

Therefore, comprehensive leaching tests will be carried out in the next phase of this study.

5. Conclusions Experimental studies were performed to evaluate the feasibility of using copper mine tailings (MT) as an alternative road base construction material through geopolymerization. Based on the experimental results, the following conclusions can be drawn:

1. The maximum dry unit weight of compacted MT is strongly influenced by the molarity of the

NaOH solution. Higher molarity leads to increase of the maximum dry unit weight and slight

decrease of the optimum moisture content due to the filling of the voids by the NaOH and the

lubrication provided by the NaOH.

2. At a constant moisture content, the UCS of geopolymerized MT increases with higher NaOH

concentration up to a certain level and then decreases. This behavior is simply related to the

effect of NaOH content or Na/Al ratio on geopolymerization. The highest UCS of 5.32 MPa

was found at a moisture content of 11% and 11 M NaOH leading to an optimum Na/Al ratio

of 0.79 which is close to the optimum Na/Al ratios reported in the literature.

3. Specimens prepared at 0 M NaOH (which means no geopolymerization) only show a slight

variation in UCS at various moisture contents and, as expected, achieve the highest UCS at the

optimum moisture content. However, for the specimens prepared at 3, 5, 7 and 11 M NaOH,

the UCS increases with the moisture content to a certain level and then drops. The highest UCS

of specimens containing NaOH does not necessarily occur at the optimum moisture content

and is greater than that of specimens containing no NaOH, emphasizing the contribution of

geopolymerization to the UCS.

4. By selecting appropriate preparation conditions (NaOH concentration and moisture content),

the geopolymerized MT can meet the strength requirements for cement treated base by several

transportation agencies in the United States. However, before the field application of MT in

road base construction, a comprehensive leachate study based on the new EPA standard should

be performed to ensure that the MT is environmentally safe.

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Table 1. Chemical composition (wt%) of copper mine tailings (MT)

Chemical compound Composition MT (%)

Na2O 3.02

MgO 1.78

Al2O3 14.10

SiO2 55.90

P2O5 0.19

SO2 2.22

K2O 3.89

CaO 2.27

TiO2 0.49

MnO 0.06

Fe2O3 3.07

100

Table 2. Summary of specimens tested at different conditions Sample NaOH W Water NaOH Solution Na/Al UCS SEM XRD XRF (M) (%) (gr) (gr) used (gr) MT - - - - x x x 00-11 0 11 55 0 55.0 0.352 x x 00-14 0 14 70 0 70.0 0.352 x 00-16 0 16 80 0 80.0 0.352 x 00-17 0 17 85 0 85.0 0.352 x 00-19 0 19 95 0 95.0 0.352 x 03-11 3 11 55 6.6 61.6 0.471 x 03-14 3 14 70 8.4 78.4 0.504 x 03-16 3 16 80 9.6 89.6 0.526 x 03-17 3 17 85 10.2 95.2 0.536 x 03-19 3 19 95 11.4 106.4 0.558 x 05-11 5 11 55 11 66.0 0.551 x 05-14 5 14 70 14 84.0 0.606 x 05-16 5 16 80 16 96.0 0.642 x 05-17 5 17 85 17 102.0 0.660 x 05-19 5 19 95 19 114.0 0.695 x 06-16 6 16 80 19.2 99.2 0.699 x 06-17 6 17 85 20.4 105.4 0.721 x 06-19 6 19 95 22.8 117.8 0.764 x 07-11 7 11 55 15.4 70.4 0.631 x x x 07-14 7 14 70 19.6 89.6 0.707 x x x 07-16 7 16 80 22.4 102.4 0.758 x x x 07-17 7 17 85 23.8 108.8 0.783 x 07-19 7 19 95 26.6 121.6 0.833 x 08-11 8 11 55 17.6 72.6 0.671 x 09-14 9 14 70 25.2 95.2 0.808 x 95-11 9.5 11 55 20.9 75.9 0.730 x 11-11 11 11 55 24.2 79.2 0.790 x x x 11-14 11 14 70 30.8 100.8 0.909 x 11-16 11 16 80 35.2 115.2 0.989 x 11-17 11 17 85 37.4 122.4 1.029 x 11-19 11 19 95 41.8 136.8 1.108 x 13-11 13 11 55 28.6 83.6 0.869 x

101

Table 3. Summary of optimum Na/Al ratios from different researchers (modified from Ren et al. (2015))

Material Optimum Na/Al Reference

Metakaolin and fly ash 1.00 [67,68]

Metakaolin 1.00 [69]

Metakaolin 1.30 [58,70]

Metakaolin 1.00 [71]

Metakaolin 1.00 [72]

Metakaolin 1.15 [73]

Metakaolin 1.09 [74]

Copper MT and 1.10 [25] aluminum sludge

Fly ash 1.20 [51]

Fly ash 1.10 [53]

Fly ash 1.01 [52]

Soil – Fly ash 0.85 [37]

Copper MT > 0.79 Current study

102

Table 4. Strength requirements for cement treated road base by different transportation agencies in the United States

Agency 7-day UCS (MPa)

ADOT [59] 1.03 - 2.75

TDOT [60] 3.50

NDOT [61] 5.20

IDOT [62] 3.50

ODOT [63] 4.15

FDOT [64] 3.50

FHWA [65,66] 2.06 - 5.51

103

100 90 80 70 60 50 40 30 Percent Percent passing(%) 20 10 0 1 10 100 1,000 Particle size (µm)

Fig. 1. Particle size distribution of mine tailing (MT)

104

500 µm

20 µm

Fig. 2. SEM micrographs of MT powder at a) high and b) low magnifications

105

MT powder

P A G S P S S S S S S A P 10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 2θ

Fig. 3. XRD pattern of MT powder (A albite, G gypsum, P sanidine and S quartz)

106

17.8 11 M 17.5 7 M

17.3 5 M

17.0 3 M 0 M 16.8

16.5

Dry Dry weightunit (kN/m³) 16.3

16.0 9 11 13 15 17 19 21 Moisture content (%)

Fig. 4. Compaction curves of MT at different NaOH concentrations

107

6 w=11% 5 w=14% w=16% 4 w=17%

3 w=19%

UCS UCS (MPa) 2

0 0 2 4 6 8 10 12 14 NaOH concentration (M)

Fig. 5. UCS versus NaOH concentration at different moisture contents

108

w=11% 5 w=14% 4 w=16% w=17% 3 w=19%

UCS UCS (MPa) 2

0 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 Na/Al

Fig. 6. UCS vs Na/Al ratio at different moisture contents

109

MT a) b) MT

300 µm 100 µm

MT c) 20000 Si 17500 15000 Si/Al = 3.31 2 Na/Al = 0.12 12500 10000 K 7500 Al

5000 O Intensity (cps) Intensity Mg S Ca 2500 C Na 1 MT 0 0 1 2 3 4 5 30 µm KeV

Fig. 7. SEM micrographs of specimen made with 0 M NaOH solution at w = 11% and cured at 35 C for 7 days: a) low magnification image, b) higher magnification image of the square zone in a), and c) higher magnification images of the square zone in b). MT: Mine tailing particle

110

a) MT b) MT 1 4 2 GP GP

100 µm 30 µm 3

45000 45000 Si Si 40000 40000 35000 Si/Al = 3.06 35000 Si/Al = 2.59 30000 Na/Al = 0.30 30000 Na/Al = 0.58 25000 25000 K 20000 20000 Al Al 15000

15000 O Intensity (cps) Intensity 10000 O (cps) Intensity 10000 Na K 5000 Na Ca 5000 S Ca C S C 0 0 0 1 2 3 4 5 0 1 2 3 4 5 KeV KeV

Fig. 8. SEM micrographs of specimen made with 7 M NaOH solution at w = 11% and cured at 35 C for 7 days: a) low magnification image, b) higher magnification image of the square zone in a). The EDS spectra are for points 1 and 2 (left, unreacted) and 3 and 4 (right, reacted) in b). MT: Mine tailing particle, GP: Geopolymer

111

a) b)

GP MT

MT GP

200 µm 100 µm

c) d) MT 2 GP 1 1

30 µm 30 µm

120000 45000 Si Si 40000 100000 Si/Al = 63.10 Si/Al = 3.30 35000 Na/Al = 3.88 Na/Al = 1.62 80000 30000

60000 25000 20000 O Al 40000 15000 Na Intensity (cps) Intensity O Intensity (cps) Intensity 10000 Ca 20000 Mg C Na Al S K Ca 5000 C S K 0 0 0 1 2 3 4 5 0 1 2 3 4 5 KeV KeV

Fig. 9. SEM micrographs of specimen made with 11 M NaOH solution at w = 11% and cured at 35 C for 7 days: a) low magnification image, b) higher magnification image of the square zone in a), and c) and d) higher magnification images of the square zones shown in b) for unreacted and reacted MT particles, respectively. The EDS spectra are for c) and d), respectively. MT: Mine tailing particle, GP: Geopolymer 112

7M/16%

7M/14%

11M/11%

7M/11%

S P G A MT powder SA P P S S S S S S

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 2θ

Fig. 10. XRD patterns of MT powder and geopolymer specimens prepared at different NaOH concentrations and moisture contents [A: albite, G: gypsum, P: sanidine, S: quartz]

113

6 0 M 3 M 5 5 M 7 M 4 11 M

UCS UCS (MPa) 2

0 10 12 14 16 18 20 Moisture content (%)

Fig. 11. UCS vs moisture content at different NaOH concentrations

114

100 µm a) 30 µm b) MT

GP 1 3 4

70000 40000 Si Si 60000 Si/Al = 36.71 35000 Si/Al = 3.35 50000 Na/Al = 1.61 30000 Na/Al = 0.64 25000 40000 20000 30000 Al 15000 20000 O

Intensity (cps) Intensity O Intensity (cps) Intensity 10000 Ca 10000 Al Na K C Na S K Ca 5000 C S 0 0 0 1 2 3 4 5 0 1 2 3 4 5 KeV KeV

Fig. 12. SEM micrographs of specimen made with 7 M NaOH solution at w = 14% and cured at 35 C for 7 days: a) low magnification image, b) higher magnification image of the square zone in a). The EDS spectra are for points 1 and 2 (left, unreacted) and 3 and 4 (right, reacted) in b). MT: Mine tailing particle, GP: Geopolymer

115

a) b)

MT GP GP 4

MT 3

100 µm 30 µm 2 1

20000 30000 Si Na 17500 Si/Al = 2.33 25000 O Si/Al = 2.82 15000 Na/Al = 1.35 Si Na/Al = 7.57 12500 20000 10000 O K Al 15000 7500 Na Mg 10000 Al

Intensity (cps) Intensity 5000 Intensity (cps) Intensity K Ca Ca Ca 5000 Mg 2500 C S C S Ca 0 0 0 1 2 3 4 5 0 1 2 3 4 5 KeV KeV

Fig. 13. SEM micrographs of specimen made with 7 M NaOH solution at w = 16% and cured at 35 C for 7 days: a) low magnification image, b) higher magnification image of the square zone in a). The EDS spectra are for points 1 and 2 (left, unreacted) and 3 and 4 (right, reacted) in b). MT: Mine tailing particle, GP: Geopolymer

116

6 NDOT 5 ODOT 4 IDOT, TDOT, FDOT

3 FHWA

UCS UCS (MPa) 2 ADOT

0 10 12 14 16 18 20 Moisture content (%)

Fig. 14. 7 day UCS versus moisture content at different NaOH concentrations and the required 7 day UCS for cement treated road base by different transportation agencies

117

APPENDIX C

THE EFFECT OF WET AND DRY CYCLES AND WATER IMMERSION ON THE DURABILITY OF GEOPOLYMERIZED COPPER MT AS A ROAD BASE CONSTRUCTION MATERIAL Lino Manjarrez1, Arash Nikvar-Hassani1 and Lianyang Zhang, Ph.D., P.E., M.ASCE2

1Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, Arizona 85721, USA

2 Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, AZ 85721, USA (Corresponding author). Email: [email protected]

118

ABSTRACT

This paper studies the durability properties of geopolymerized copper mine tailings (MT) as a road base material. To improve the mechanical properties of the geopolymerized MT, low-calcium slag

(SG) was incorporated to the MT. MT/SG-based geopolymer specimens were produced using several SG contents, 0%, 5%, 10%, 30% and 50%, by total MT/SG solid weight. Sodium hydroxide (NaOH) solution at 7 M concentration was used as the alkaline activator. The mixtures were prepared at a moisture content of 14% and then compacted in cylindrical molds. After compaction, the specimens were cured in the oven at 35 °C and 60 °C for 7 and 14 days, respectively. The durability characteristics were determined by wet and dry (w-d) cycles and water immersion. Unconfined compression strength tests were performed on the specimens to determine the unconfined compressive strength (UCS). The UCS was obtained after curing, at dry and saturated conditions (0th cycle), after the 1st, 3rd, 7th and 12th w-d cycle, and after water immersion.

The loss of mass and pH were also recorded after cycles. Additionally, leaching test based on the

TCLP method were performed to investigate the release of heavy metals. Scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDS) and X-ray diffraction (XRD) analyses were also performed to study the microstructure and chemical composition of the specimens at different conditions. The results showed that the w-d cycles and water immersion affect significantly the UCS of the specimens. This effect is more evident on the specimens cured at 35 °C. However, the compressive strength of the specimens improves with curing temperature and SG content. The SEM/EDS results showed that after the w-d cycles Na is still present in the geopolymer microstructure although at a lower content. Based on the results, the geopolymerized

MT/SG can be effectively used as road base construction material.

119

Keywords: Mine tailings; Low-calcium slag; Geopolymer; Wet and dry cycles; Water immersion;

Durability; Compressive strength; Road base

1. Introduction The durability of construction materials is of great interest to civil engineers because it is directly related to the long-term performance of the infrastructure. Often, construction materials are exposed to natural weather, so the durability properties shall be evaluated accordingly. For example, in arid regions, such as the southwest of the United States, it is common to deal with continuously heavy rains followed by extreme heat during great part of the year, especially during the summer months. During this time, the materials experience a rapid increase in the water content, sometimes reaching saturation (worst case scenario) due to the heavy rain, and a rapid dry, due to the severe heat. The surface temperatures can go up to 70 °C [1]. The materials intended for road construction are one of the most affected by this effect. The main concern is the reduction

(or loss) of strength caused by water intrusion. To evaluate this effect, it is necessary to follow an approach that simulates real case conditions. According to several authors wet and dry cycles [2–

4] and water immersion [5–7] are the preferred tests.

Several researchers have investigated the durability properties of natural materials mixed with conventional stabilizers for their use as road construction materials [2,3,5,6]. For example,

Chittoori [2], investigated the durability of cement treated expansive clays to be used as subgrade foundation for pavement support by means of wet and dry cycles. The cycles were based on the

ASTM D559 where the specimens are immersed 5 h in water and then placed in the oven at 71 °C for 42 h, a total of 21 cycles were performed. The results showed that clays richer in kaolinite and illite held 80% of their compressive strength after wet and dry cycles, whereas those richer in montmorillonite are more susceptible to premature strength failures after chemical stabilization.

120

Osinubi et al. [5] investigated the durability properties of an expansive clay stabilized with lime and slag for their use as material for road foundation. The durability tests were carried out by obtaining the resistance to loss in strength which was determined as the ratio of the UCS of specimens wax-cured for 7 days, de-waxed top and bottom, later immersed in water for 7 more days to the UCS of specimens wax-cured for 14 days. The results showed that the specimens containing 8% lime and 7.5% slag experienced a resistance to loss in strength of 50% which is lower than the maximum 80% reported by Ola [8]. However, the authors concluded that due to the longer immersion period, 7 days instead of 4 (as reported by [8]), the mixtures can be assumed to have met the durability requirement and can be used as sub-base and base courses of lightly trafficked roads. Amadi [6] studied the durability of a clayey soil mixed with quarry fines and stabilized with several levels of CKD (0-16%). The specimens were cured in a humidity room at

20 °C for 14 days and then immersed in water for 14 more days. The loss of strength was calculated as the strength after immersion divided by the strength after humid curing. The results showed that after the addition of 8, 12 and 16% of CKD, the mixtures experienced a loss of strength of 20, 15 and 9%, respectively. The authors concluded that these mixtures are suitable to be used for subgrade applications according to the Manual of Contract documents for Highway Works of the

UK [9]. Lekha et al. [3] performed wet and dry cycles in a lateritic soil mixed with natural fibers and stabilized with OPC. Fiber content varied from 0.2 to 1.0% and OPC content was 3%. The durability test was carried out according to the ASTM D559 [10]. The specimens were brushed after each cycle and the loss of weight was measured after 12 cycles. The maximum loss of weight should not exceed 14%. The results showed that specimens with 1% fiber and 3% OPC satisfied the durability requirements of the ASTM [10]. The authors concluded that these mixtures can be used for low volume roads (traffic ≤ 1 million standard axles). Etim et al. [11] studied the durability

121 of a stabilized clay with lime and iron MT for use as subbase material in the construction of low volume roads. The durability test was carried out by wax-curing the specimens for 7 days, then dewaxed and immersed in water for 7 more days. The specimens were wax cured for 14 days. The resistance to loss in strength was computed as in [5]. Results showed that the optimum mixture with 8% lime and 8% iron MT experienced a resistance to loss in strength of 37.6%. Although the resistance to loss in strength was quite low compared to the limit of 80% reported in [8], the authors concluded that the optimum mixture may be acceptable for use as an improved subgrade for construction of flexible pavement.

Some researchers have investigated the durability properties of alkali activated materials mixed with soils [7] and recycled materials [4] for road construction applications. Rios et al. [7] investigated the mechanical and durability properties of a silty sand soil stabilized with alkali activated fly ash for its use as subbase material. After water immersion and wet/dry cycles, the alkali activated specimens satisfied the European existent specifications for soil-cement (UCS > 1

MPa resistance to immersion at early ages). Hoy et al. [4] conducted wet/dry cycles on recycled asphalt pavement and fly ash geopolymer mixtures to be used as pavement material. The cycles were performed according to the ASTM D559 [10]. The results showed that the UCS of the mixtures increased up to the 6th cycle and it is believed that the UCS increment is because of the temperature effect in the dry process and the formation of new geopolymer gels. The authors concluded that these mixtures can be used as an alternative sustainable pavement material.

Manjarrez and Zhang [12] investigated the feasibility of utilizing geopolymerized copper MT as an alternative road base material, focusing on their physical and mechanical properties. The results indicated that by properly compacting and selecting the preparation conditions (moisture content and NaOH concentration), copper MT can meet the strength requirements for road base by

122 different state DOTs [13–19] and the Federal Highway Administration (FHWA) [20] in the United

States. In this study, the durability properties of compacted copper MT are studied by wet and dry cycles based on the ASTM D559 and water immersion to calculate the resistance to loss in strength. To do that, the compacted specimens were cured for 7 days at 35 oC and 60 oC, then were weighted before and after each cycle, the loss of mass was computed. After curing, and after the

1st, 3rd, 7th and 12th cycle, as well as after water immersion, the unconfined compressive strength

(UCS) is obtained. Leaching test was also performed to investigate the immobilization of heavy metals on the geopolymerized specimens. In addition, scanning electron microscopy

(SEM)/energy dispersive X-ray spectroscopy (EDS) and X-ray Diffraction (XRD) analyses were carried out to study the microstructure and chemical composition.

2. Experimental Studies 2.1. Materials The materials used in this investigation were copper mine tailings (MT), copper slag (SG), reagent grade 98% sodium hydroxide (NaOH) and deionized water. The MT were supplied by a major mine company in Tucson, Arizona. The MT were received in the form of damp solids. After drying, the lumped MT were pulverized and screened through No. 40 sieve so that no particles are lumped together, and all MT are utilized. The SG was received in the form of aggregates and was pulverized for further analysis. Table 1 shows the chemical composition of the MT and SG based on X-ray fluorescence (XRF) analysis. The MT mainly contains silica and alumina, whereas the

SG consists of mainly silica and iron. The particle size distribution was determined following

ASTM D6913-04 [21] and ASTM D422-63 [22] as shown in Fig. 1. The MT have a mean particle size around 90 µm with 37% particles passing No. 200 (75 µm) sieve, while the pulverized SG is much finer with a mean particle size around 17.7 µm and 80% particles passing No. 200 (75 µm) sieve. The MT particles have irregular shapes and finer particles are attached to the coarser ones. 123

Fig. 2 shows the SEM micrographs of the MT and SG powders. The ultra-fine particles of SG are also attached to the coarser ones. The specific gravity of the MT and SG particles are respectively

2.8 and 3.8 indicating that the SG particles are heavier than the MT particles. Fig. 3 shows the

XRD patterns of the MT and SG powders. The MT are mainly crystalline materials such as quartz

(SiO2), gypsum (CaSO4), albite (NaAlSi3O8) and sanidine [K(AlSi3O8)]. A weak amorphous phase, centered at 28° is also clear. The SG is mainly composed of two crystalline phases; magnetite (Fe2O3) and fayalite (Fe2SiO4). A less ordered molecular structure is observed in magnetite peaks compared to fayalite related to the less sharp peaks in magnetite.

The alkaline solution consisted of NaOH and distilled water. The NaOH flakes were dissolved in distilled water to prepare the solution one day before making the geopolymer specimens.

2.2. Specimen Preparation Frist, the MT was dry mixed with SG at 0, 5, 10, 30 and 50%, by MT/SG dry solid weight, for about five minutes to ensure homogeneity. Then, the homogenized material was mixed with NaOH solution at a moisture content of 14%. A specific amount of mixed material was then placed in

Harvard Miniature Compaction cylindrical molds of 33.4 mm diameter and 71.5 mm height with minor compaction. The compacted specimens were compressed with an ELE Tri Flex 2 loading machine at a constant rate until the compacted material reached the 71.5 mm height of the Harvard miniature mold. After compaction, the specimens were demolded and placed in an oven at 35 C or 60 °C for 7 and 14 days until tested. This process was followed to obtain the compaction and curing conditions established in Manjarrez and Zhang [12] for the specimens prepared at 7 M

NaOH and a moisture content of 14% [see Manjarrez and Zhang [12] for compaction curve details].

124

Temperatures higher than 60 oC can be achieved on the ground surface in the southwest region of the United States [1], so the selected curing temperatures are practically applicable.

2.3. Experiments The experiments performed in this study consist of unconfined compression tests, wet and dry cycles, weight loss measurements, water immersion, SEM/EDS analysis and XRD analysis. The compressive strength test was performed to measure the 7-day unconfined compressive strength

(UCS), which would be named the 0 cycle from now and on. The combination selected to perform the wet/dry cycles and water immersion is 7M/14%. This combination gives a UCS around 4 MPa after 7 days curing at 35 °C; see Manjarrez and Zhang [12] for details. The UCS was also measured in specimens at saturated conditions after the 1st, 3rd, 7th and 12th cycle to evaluate the effect of cycles on the UCS and after 7 days immersed in water to measure the resistance to loss in strength.

At least three specimens were tested at each condition. The compressive strength test was conducted using an ELE tri Flex 2 loading machine at a constant loading rate of 0.1 mm/min.

Before conducting the compression tests, the specimens were polished at the end surfaces to make sure that they were accurately flat and parallel.

To investigate the microstructure and chemical composition of the MT/SG-based geopolymer,

SEM imaging, EDS and XRD analyses were performed on selected specimens. The SEM imaging and EDS analysis were carried out in SE conventional mode using the FEI INSPECT-S/Thermo-

Fisher Noran 6 microscope. The XRD analysis was performed with a Panalytical X’pert pro MPD instrument equipped with a programmable incident beam slit using Ni-filtered, Cu, Kα and

λ=1.5418 Å as X-ray radiation.

125

2.3.1. Wet and Dry Cycles The wet and dry (w-d) cycles were performed as per ASTM D559 [10]. A total of 12 w-d cycles were carried out in the compacted specimens to investigate their effect on the compressive strength. The specimens were subjected to w-d cycles after being cured in the oven for 7 days. One w-d cycle consists of the immersion of the specimens in water at room temperature for 5 h, then placed in an oven at 71 ±3 °C for 42 h. After the 1st, 3rd, 7th and 12th w-d cycle the UCS was obtained in saturated conditions and the strength loss was computed based on the UCS of the 0th cycle. ASTM D559 [10] is originally to evaluate the performance of soil-cement specimens, however, since no standards have been developed for geopolymerized MT/SG specimens, it was adapted for the purposes of this study. The wet and dry cycles are preferred to evaluate the durability characteristics of road construction materials in arid regions where heavy rains are followed by high temperatures and vice versa.

2.3.2. Resistance to Loss in Strength The resistance to loss in strength is calculated as the ratio of the UCS of specimens cured in the oven for 7 days and later immersed in water for 7 days to the UCS of specimens cured in the oven for 14 days as shown in Eq. 1. The compressive strength test before and after immersion is a competent indicator to evaluate the durability of soils [6]. The evaluation of the durability characteristics by the resistance to loss in strength on road construction materials is preferred in tropical conditions where constant rain could last for several days [11] and is given by:

푈퐶푆 푅푒푠𝑖푠푡푎푛푐푒 푡표 푙표푠푠 𝑖푛 푠푡푟푒푛𝑔ℎ푡 (%) = ( 푠표푎푘푒푑 ) 푥 100 (1) 푈퐶푆푢푛푠표푎푘푒푑

2.3.3. Leaching Test Leaching test were performed according to the toxicity characteristic leaching procedure (TCLP) using distilled water. To study the effectiveness of geopolymerization on immobilization of heavy

126 metals in MT, the leaching test results from the MT powder, SG powder and powder from 0% and

50% SG geopolymer specimens were compared. The test was performed on powders finer than 75

µm. A solid to liquid mass ratio of 1:20 was used for all the specimens throughout the experiment.

The mixture was placed in sealed vessels and then the vessels were placed in an agitator apparatus and end-over-end rotation was applied for 18 h at 30 ± 2 rpm. After agitation, a 10 ml sample was taken and filtered through a 0.45 µm filter. The concentration of heavy metals was obtained in the filtrate based on the ICP-MS (inductively coupled plasma mass spectrometry) technique.

2.3.4. Microscopic Analysis SEM imaging, EDS and XRD analyses were performed to investigate the change of the microstructure and phase composition of the geopolymer matrix due to the wet and dry cycles and water immersion. EDS mapping analysis were conducted to study the migration of Na, Al, Si and

Fe after w-d cycles.

3. Results and Discussion 3.1. Wet and Dry Cycles Fig. 4 shows the variation of the UCS with w-d cycles at various SG contents. The specimen conditions at the 0th cycle are dry and saturated for Fig. 4(a) and 4(b), respectively. Under each cycle, there is a significant UCS increase with SG content. For example, the UCS of the 0% SG specimens at the 0th cycle [dry condition, Fig. 4(a)] is 4.02 MPa, whereas at 5%, 10%, 30% and

50% SG content is 4.60, 5.08, 7.67 and 9.56 MPa, respectively. As observed by other studies

[23,24], the improving effect of SG is attributed to its physical and chemical properties and molecular structure. However, there is a UCS reduction after each w-d cycle for all the studied cases and it is believed is due to the depolymerization and dealumination of geopolymer gels [25].

127

The w-d cycles affected significantly the UCS. As observed by other researchers, the w-d cycles adversely affect the compressive strength of the specimens [2,26–29]. For example, Chittoori [2] found that after the w-d cycles on chemically treated expansive clay, a strength loss of about 20% was reported at the end of the cycles and concluded that the stabilized clay can be used as subgrade foundation for pavement support. However, the maximum UCS obtained in [2] was lower than 1

MPa even for the specimens at the 0th cycle (dry condition). In this study, the compressive strength was significant reduced after the 1st cycle and then was about the same (Fig. 4). The specimens prepared at 0% SG and tested in dry conditions experienced a strength loss of 87.8% after the first cycle, in other words the UCS at the 0th cycle (dry condition) was quite high (4.02 MPa) compared to the UCS after the 1st cycle (0.49 MPa). The lowest strength loss after the 1st cycle (69.9%) was achieved by the 50% SG specimens. The reason of this significant UCS decrease is because at the

0th cycle the specimens were tested in dry conditions, however, they were tested under saturated condition after subsequent cycles. To be consistent with the saturated conditions of subsequent cycles, an additional set of specimens at the 0th cycle was immersed in water for 2 h before conducting the compressive strength test [4,5,30], Fig. 4(b). For the 0% SG specimens, a UCS of

1.25 MPa was obtained at the 0th cycle (saturated condition) and a strength loss of 60.6% after the

1st cycle. The strength loss between the 0th and 1st cycle for the 50% SG specimens was 18%. It is also noted that the strength loss was reduced considerable when the specimen at the 0th cycle was in saturated condition. The results are summarized in table 3 and 4.

A UCS increase of alkali activated materials subjected to w-d cycles has also been observed in several studies [4,31,32]. For example, Hoy et al. [4] investigated the effect of w-d cycles in alkali- activated recycled asphalt mixed with fly ash. Sodium silicate and NaOH were used as the alkaline activators. The results showed that after curing, the specimens reached a UCS of 5.7 MPa. The

128 maximum UCS of 9.0 MPa was obtained at the 6th cycle and then the UCS decreased. In this study, the UCS increased from the 1st to the 3rd cycle and then the UCS decreased for the 7th and 12th cycle. For the 0% SG specimens, a UCS of 0.49 and 0.80 MPa was observed after the 1st and 3rd cycle, respectively. This behavior is attributed to the temperature used in the drying process of the w-d cycles (71 °C). It is believed that the formation of new geopolymer gels, or the strengthen of existent ones, occurred with the increased temperature and the available unreacted Na cations, which slightly increased the UCS in the 3rd cycle. The UCS reduction after the 3rd cycle might have occurred because unreacted Na cations could have been completely expelled out in the w-d process, which means that no new geopolymer gels were formed. On the other hand, the reacted

Na remained in the geopolymer gel as will be shown later in the SEM images.

To further investigate the Na availability after w-d cycles, SEM imaging, and EDS point and mapping analyses were performed on specimens at SG contents of 0%, 10% and 50%. Fig. 5 shows the SEM micrographs of the 0% SG specimen at the 0th cycle (dry condition). Two phases of the material can be observed: unreacted MT particles and glassy geopolymer gels (or reacted MT)

[Fig. 5(a)]. The EDS point analysis was performed on both phases, points 1 and 2 for unreacted

MT and points 3 and 4 for geopolymer gels, indicated in Fig. 5(b), and the average of the obtained values was used for the discussion. The initial Na:Al ratio of the source material is 0.72. The obtained Na:Al ratios are 0.20 and 1.07 for the unreacted MT and geopolymer gel, respectively. A decrease on the Na:Al ratio of the unreacted phase has also been observed by other researchers

[12,33,34] and is mainly because not all the available Na cations participated in the geopolymerization and appeared as precipitate on the unreacted MT. However, the Na:Al ratio is higher in the geopolymer phase indicating that Na was effectively incorporated to the gel by the

129 geopolymerization process. In fact, the Na:Al ratio of 1.07 agrees well with the values reported in the literature [12,35,36] and this could be the reason of the UCS of 4.02 MPa.

Fig. 6 shows the SEM micrographs of the 0% SG specimen after 12 w-d cycles. As in Fig. 5, two phases can be observed. A lower magnification image [Fig. 6(a)] shows that geopolymer gels are well distributed in the area even though the occurrence of the 12 w-d cycles. The UCS after 12 w- d cycles is 0.53 MPa. Although this value is low compared to that at the 0th cycle (dry condition), the specimen can still hold compressive strength. The EDS point analysis was performed on both phases, points 1 and 2 for unreacted MT and points 3 and 4 for geopolymer gels, indicated in Fig.

6(b). The obtained Na:Al ratio in the geopolymer gel is 0.79, which is lower than the 1.07 reported in the geopolymer gel at the 0th cycle (dry condition). This reduction can be associated with the w- d cycles since some Na could have been expelled out in the wetting processes [37]. However, it is noted that Na is still present in the geopolymer matrix as will be shown by EDS mapping.

Fig. 7 shows the EDS mapping results of the 10% SG specimens after the 0th, 3rd and 12th cycles.

Figs. 7(a.1, b.1 and c.1) show the micrographs after the 0th, 3rd and 12th cycle where both phases can be observed as in Figs. 5 and 6. Figs. 7(a.2, b.2 and b.3) exhibit the distribution of Na (magenta) of the micrographs. Na appears mostly in the whole analyzed area but is concentrated only in zones where geopolymer gels are located. In addition, Na content decreases with w-d cycles. After the

0th cycle, Na content is 5.98% and then decrease to 4.56% and 3.25% after the 3rd and 12th cycle, respectively. Therefore, the presence of Na after 12 w-d cycles is confirmed by the EDS mapping results. The results of the 0%, 10% and 50% SG specimens are summarized in table 5. Fig. 7 (a.3, b.3, c.3) shows the distribution of Al and Fig. 7 (a.4, b.4 and c.4) exhibit the distribution of Si.

Both Al and Si are concentrated in the geopolymer gel and unreacted MT particles. However, the location of unreacted MT particles is more evident by the Si concentration as can be seen in Fig.

130

7 (b.4 and c.4). This can be explained by the large presence of SiO2 (55.9%), compared to the low content of Al2O3 in the original MT composition (see table 1). Fig. 7 (a.5, b.5 and c.5) shows the

Fe distribution. SG particles are located where Fe is highly concentrated, it is noted that Fe also participated in the geopolymer gel formation as it is also located in the zones where Na, Al and Si are concentrated. The participation of Fe in the geopolymer reaction was also confirmed by Gomes et al. [38] and Shadnia and Zhang [24].

Fig. 8 shows the residual percentage of elements (Na, Al, Si and Fe) with w-d cycles at several SG contents. Fig. 8a shows the residual percentage of Na. The decrease of Na after w-d cycles is more evident in 0% SG specimens than those at 50% SG. For example, at 0% SG around 40% residual

Na continued in the microstructure, however, about 75% is present at 50% SG. This suggests that the more retained Na after the w-d cycles, the stronger the geopolymer matrix. Similarly, the residual Al, Si and Fe followed the same trend as Na, however, in a smaller proportion; Fig. 8b,

8c and 8d, respectively. A simply explanation can be related to the original MT and SG composition. As explained earlier, Al and Si are the major elements in the raw MT, whereas Fe and Si are the major elements for the raw SG. They can also be found as crystals phases in quartz

(SiO2) and sanidine [K(AlSi3O8)] in MT and as fayalite (Fe2SiO4) in SG. On the other hand, Na is added by the NaOH solution for the only purpose of the geopolymer reaction. That is why Na is easier to exit the matrix, while Al, Si and Fe remained at higher residual values. Results are summarized in table 5.

Fig. 9 shows the XRD pattern of the 10% SG specimens after the 0th, 3rd and 12th cycle. For comparison, the XRD patterns of the source MT and SG are also shown. The XRD pattern of the analyzed specimens mainly remains crystalline after reaction. No significant change in the intensity of crystalline peaks is observed. However, there is a slight reduction on the crystal peak

131 of albite with w-d cycles. Albite [NaAlSi3O8] contains Na and the intensity peak reduction might be related to the w-d cycles and the migration of Na from the crystalline phase. In addition, as observed by several authors [12,33,39] the presence of gypsum as a crystalline peak detected around 12° in the source MT disappears after geopolymerization. This behavior is attributed most likely to gypsum locked in the solution pores.

3.1.1. Loss of Mass The relationship between the loss of mass of the tested specimens versus the w-d cycles is presented in Fig. 10. The loss of mass increases significantly after five cycles, for the 0%, 5% and

10% SG specimens, and after four cycles, for the 30% and 50% SG specimens. Thereafter, the loss of mass increased gradually for the rest of the cycles, as observed by other researchers [4,28]. This behavior can be explained based on the geopolymerization degree of the specimens. The specimens containing larger amount of SG (30% and 50%) present stronger bonding between particles and thus the loss of mass is lower than those containing lesser amount of SG (0%, 5% and 10% SG). After 12 w-d cycles, the largest loss of mass of 6.27% is achieved by the 0% SG specimens, whereas the lowest of 3.17% is shown by the 50% SG specimens. Fig. 11 shows the images of the 0% and 50% SG specimens after 12 w-d cycles. The external surface deterioration of both 0% and 50% SG specimens is about the same. The 0% SG specimens show some weathering at the top and bottom of the cylinder, whereas the 50% SG specimens do not show external surface alterations. It can be said that, the lower the loss of mass, the larger the SG content and the higher the UCS.

3.1.2. pH Ph determines the acidity or basicness of the water. It ranges from 0 – 14, where a pH of 7 indicates water to be neutral. Lower values than 7 indicate acidity, whereas larger values basicness.

132

The pH was measured in the solution during the wetting process of w-d cycles for the 0% SG specimens. A total of 25 w-d cycles were performed, and in every cycle the water was replaced with new water. It was shown that right after specimen immersion, the pH increased from 7 to 11.

This increase is related to the Na content present in the alkaline solution. After each w-d cycle, the pH decreased because unreacted Na was expelled out from the specimen. Fig. 12 shows the pH reduced with w-d cycles. Based on the curve trend the pH closes to 7 where originally started. The results are consistent with Silva et al. [30] which investigated the effect of immersion in water of partially activated tungsten MT. This means that after 25 w-d cycles, mostly, the unreacted Na have been expelled out and the reacted Na is still incorporated in the geopolymer matrix as shown in the EDS mapping results.

3.2. Resistance to Loss in Strength The resistance to loss in strength was performed on specimens with SG contents of 0%, 10% and

50%. The curing temperatures used in this study are 35 °C and 60 °C. Several researchers

[6,8,11,32,40,41] have reported that the resistance to loss in strength should be a maximum of 80% for 7 days curing and 4 days soaking periods. However, Etim et al. [11] reported that it can be lower if the soaking period is extended. Table 6 and Fig. 13 summarize the results reported in the literature where the durability characteristics of road materials was analyzed.

Figs. 14(a) and 14(b) show the UCS vs SG content of specimens cured in the oven at 35 °C and

60 °C, respectively. Overall, the specimens cured at 35 °C achieved lower UCS than those cured at 60 °C which means that curing temperature has a significant effect on the UCS of the geopolymer specimens. The effect of curing temperature in geopolymerization has been well studied by many researchers [23,24,42–44]. The RLS increases with SG content for both curing temperatures. For the specimens cured at 35 °C, the RLS is 9.0%, 20.1% and 48.3% for 0%, 10%

133 and 50% SG content, respectively. The increase is more evident in specimens cured at 60 °C where the RLS is 22.4%, 28.7% and 61.3%, respectively. The results are summarized in table 7. Although these RLS are lower than the minimum 80% reported in [8], the geopolymerized MT/SG specimens may be acceptable for use as road base construction materials since the immersion period was 7 days and not 4 days as reported in [8]. In [11], a similar immersion period as the reported in this study was performed in mixtures containing 8% lime, 8% iron ore MT and soil. A

RLS of 37.6% was obtained and the authors concluded that the mixture can be used as an improved subgrade for construction of flexible pavement (see table 6 and Fig. 13). Based on the assumptions from [8] and [11], the MT/SG specimens can be used as road base construction materials.

4. Leaching Test Table 8 shows the concentration of different metals leached from the MT and SG powder and the geopolymer powder from 0% and 50% SG specimens after the TCLP leaching analysis. The limits and threshold concentrations by different standards are also shown in the table. The chemical composition of the MT and SG shown in table 1 is consistent with the concentration of the leached metals of the MT and SG powders. The MT powder contains substantial amount of Ca, Mg, K and

Na, whereas the SG powder contains mainly Mg, Ca, Zn and Na. For MT and SG powders the leached concentrations of Mg, Ca, Mn, Ni and Zn were reduced by the geopolymerization reaction suggesting their encapsulation in the geopolymer gel. However, the concentration of released As from the geopolymer specimens at 0% and 50% SG is higher than that from the MT and SG powders indicating that geopolymerization has and adverse effect on the immobilization of As.

These behavior is attributed by natural pH of the geopolymer gel (alkaline) since As and Mo exhibits higher solubility in alkaline conditions as observed by [45,46].

6. Conclusions

134

The durability characteristics of geopolymerized copper MT and low-calcium slag as road base construction material were studied. Based on the experimental results, the following conclusions can be drawn:

1. The wet and dry cycles affected significantly the UCS. This is due to the

depolymerization and dealumination of geopolymer gels after water immersion.

The UCS reduction is more evident in specimens without slag, indicating that the

geopolymer reaction is higher when slag is included. The incorporation of SG also

reduces the loss of mass of geopolymer specimens.

2. The migration of Na occurred after wet and dry cycles. EDS mapping results show

that after 12 wet and dry cycles, Na is still present in the structure of geopolymer

specimens. The migration of Na decreased with slag content. Al, Si and Fe also

migrated from the microstructure, however at a lower rate.

3. After 25 wet and dry cycles, the pH of the water used for specimen immersion was

around 7.0. It indicates that the unreacted Na was completely expelled out from the

microstructure after 25 cycles.

4. The resistance to loss in strength of geopolymer specimens after water immersion

is reduced with temperature. The resistance to loss in strength was lower for

specimens cured at 35°C, compared to those cured at 60°C. At a constant

temperature, the resistance to loss in strength increases with slag content.

5. Although the strength loss after wet and dry cycles and the resistance to loss in

strength after water immersion are low. The geopolymerized MT and low-calcium

slag can be recommended as an alternative road base material.

135

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Table 1. Chemical composition of MT and SG based on XRF analysis

Chemical compound MT (%) SG (%)

Na2O 3.02 N/A

MgO 1.78 N/A

Al2O3 14.10 2.43

SiO2 55.90 32.90

P2O5 0.19 N/A

SO2 2.22 N/A

SO3 N/A 1.56

K2O 3.89 N/A

CaO 2.27 2.13

TiO2 0.49 N/A

MnO 0.06 N/A

Fe2O3 3.07 37.50

Fe3O4 N/A 5.48

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Table 2 summarizes the tests conducted at different conditions.

Solu Na Wat Na Sam MT SG W tion Na: Si:A UC SE XR OH er OH ple (%) (%) (%) used Al l S M D (M) (gr) (gr) (gr) MT 100 0 ------✓ ✓ SG 0 100 ------✓ ✓ 100 0 7 14 70 19.6 89.6 0.72 3.36 ✓ 95 5 7 14 70 19.6 89.6 0.73 3.44 ✓ 7M/ 90 10 7 14 70 19.6 89.6 0.75 3.52 ✓ 14% 70 30 7 14 70 19.6 89.6 0.82 3.92 ✓ 50 50 7 14 70 19.6 89.6 0.93 4.56 ✓

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Table 3. Wet and dry cycles results (specimen at the 0th cycle in dry condition) SG content Cycle UCS (MPa) Strength loss (%) 0 4.02 - 1 0.49 87.8 0% 3 0.80 80.2 7 0.85 78.9 12 0.53 86.8 0 4.61 - 1 0.96 79.2 5% 3 1.01 78.2 7 0.77 83.4 12 0.64 86.1 0 5.08 - 1 0.87 83.0 10% 3 1.17 76.9 7 0.87 82.9 12 0.82 83.8 0 7.67 - 1 2.00 74.0 30% 3 2.14 72.1 7 1.85 75.9 12 1.70 77.8 0 9.56 - 1 2.88 69.9 50% 3 3.11 67.4 7 2.68 72.0 12 2.45 74.4

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Table 4. Wet and dry cycles results (specimen at the 0th cycle in saturated condition) SG content Cycle UCS (MPa) Strength loss (%) 0% 0 1.24 - 1 0.49 60.6 3 0.80 36.0 7 0.85 31.8 12 0.53 57.4 5% 0 1.56 - 1 0.96 38.8 3 1.01 35.7 7 0.77 51.0 12 0.64 59.0 10% 0 1.60 - 1 0.87 45.7 3 1.17 26.5 7 0.87 45.6 12 0.82 48.5 30% 0 2.87 - 1 2.00 30.6 3 2.14 25.6 7 1.85 35.6 12 1.70 40.8 50% 0 3.51 - 1 2.88 18.0 3 3.11 11.4 7 2.68 23.6 12 2.45 30.2

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Table 5. Results from EDS mapping Na, Al Si, Fe, SG Cyc. weight Residual weight Residual weight Residual weight Residual content (%) (%) (%) (%) (%) (%) (%) (%) 0 7.57 1.00 9.27 1.00 27.42 1.00 3.45 1.00 0% 3 3.58 0.47 7.56 0.82 27.25 0.99 3.21 0.93 12 3.29 0.43 7.39 0.80 26.87 0.98 2.94 0.85 0 5.98 1.00 9.71 1.00 25.48 1.00 3.30 1.00 10% 3 4.56 0.76 8.65 0.89 24.42 0.96 3.08 0.93 12 3.25 0.54 8.41 0.87 24.10 0.95 2.98 0.90 0 6.51 1.00 7.44 1.00 23.04 1.00 2.80 1.00 50% 3 5.39 0.83 7.13 0.96 22.69 0.98 2.65 0.95 12 4.87 0.75 7.00 0.94 22.39 0.97 2.51 0.90

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Table 6. Resistance to loss in strength summary Reference Material Curing condition RLS (%) Dry: 7 days in plastic Clay – rice husk ash Basha et al. [47] bags 57% - 95% – OPC Wet: 7 days in water Dry: 14 days wax cured Osinubi et al. [5] Clay – lime – slag Wet: 7 days was 50% - 70% cured plus 7 days in water Dry: 14 days in plastic bags Laterite – sand – Joel and Agbede [48] Wet: 7 days in plastic 45% - 95% OPC bags plus 7 days in water Dry: 14 days ambient Ijimdiyaa and Clay – groundnut temperature 9% - 35% Ashimiyu [49] shell ash Wet: 7 days ambient plus 7 days in water Dry: 28 days humid room Clay – quarry fines – Amadi [6] Wet: 14 days humid 9% - 20% CKD room plus 14 days in water Dry: 14 days wax cured Clay – lime – iron Etim et al. [11] Wet: 7 days wax 10% - 37.6% MT cured plus 7 days water Dry: 28 days ambient temperature Rios et al. [7] Soil – fly ash Wet: 7 days ambient 37% - 43% temperature plus 14 days in water Dry: 14 days in oven at 35°C This study MT – SG Wet: 7 days in oven 9% - 61.3% at 35°C plus 7 days in water

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Table 7. Summary of results of water immersion UCS average (Mpa) Condition 35°C 60°C 0% SG 10% SG 50% SG 0% SG 10% SG 50% SG 14 days oven 5.12 6.30 11.38 7.29 8.12 12.98 7 days oven + 7 days 0.46 1.27 5.50 1.63 2.33 7.96 water RLS (%) 9.03 20.15 48.34 22.36 28.69 61.33

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Table 8. Concentration (ppm) of leached metals from MT and SG powders and 0% and 50% SG geopolymer specimens

Na Mg Al K Ca Cr Mn Fe Co Ni Cu Zn As Se Mo Cd Pb MT powder 43.47 2.55 0.07 12.63 302.37 0.00 0.06 0.03 0.00 0.01 0.02 0.05 0.10 0.02 0.22 0.00 0.00 SG powder 3.01 0.56 0.04 0.71 4.48 0.00 1.18 0.08 0.01 0.03 0.14 0.29 0.08 0.00 0.06 0.00 0.00 0% SG 993.24 0.18 0.27 8.01 1.58 0.00 0.00 0.16 0.00 0.00 0.03 0.01 0.02 0.03 0.74 0.00 0.00 50% SG 801.77 0.03 1.03 32.59 3.40 0.01 0.00 0.07 0.00 0.00 0.29 0.07 6.58 0.26 39.78 0.13 0.01 EPA limit NA NA NA NA NA 5.0 NA NA NA 5.0 NA NA 5.0 1.0 NA 1.0 5.0 DIN NA NA NA NA NA NA NA NA NA NA 2.0-5.0 2.0-5.0 NA NA NA NA NA Greek NA NA 2.5-10.0 NA NA NA 1.0-2.0 NA NA 0.2-0.5 0.25-0.5 2.5-5.0 NA NA NA NA NA

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Table 9. Requirements for cement treated materials for road base/subbase in different countries

(modified from Autelitano and Giuliani 2016)

Country OPC content (%) 7-day UCS (MPa) – (base/subbase) Australia 3 - 8 0.6 – 1.0 (base) [51] Brazil ̴ 4 > 3.5 (base) > 4 (road-mix method) China > 4 (base) > 5 (central-plant mixing) UK 2 - 5 2.5 – 4.5 (base) 2.5 – 5.5 – gyratory compactor (base) Italy 2 - 4 2.5 – 4.5 – Proctor hammer (base) South Africa 1.5 – 3.0 1.5 – 3.0 (base) Nigeria > 5 1.03 (subbase) [11] 1.03 – 2 .75 (base) [15] 3.50 (base) [13,17,18] USA 3 - 10 5.20 (base) [14] 4.15 (base) [19] 2.06 – 5.51 (base) [20]

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100 90 80 SG 70 MT 60 50 40 30

Percent Percent passing(%) 20 10 0 0 1 10 100 1,000 10,000 Particle size (µm)

Fig. 1. Particle size distribution of MT and SG

151 a) b)

Fig. 2. SEM micrographs of MT and SG powders: a) MT and b) SG

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MT Powder P A G S A P P S S S S S S

F F F M SG Powder M F F F F F M F M

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 2θ

Fig. 3. XRD pattern of MT and SG powders (S quartz, G gypsum, A albite, S sanidine, M magnetite and F fayalite)

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10 0% SG 5% SG 8 10% SG 30% SG 50% SG 6

4 UCS(MPa)

0 0 1 3 7 12 a) w-d cycles

10 0% SG 5% SG 8 10% SG 30% SG 50% SG 6

4 UCS(MPa)

0 0 1 3 7 12 b) w-d cycles

Fig. 4. UCS vs w-d cycles at different SG contents. Specimens at 0th cycle are a) dry and b) saturated

154

a) 1 b) 4

2 100 µm 40 µm

60000 40000 Si Si 50000 Si:Al = 2.68 Si:Al = 31.05 30000 40000 Na:Al = 0.20 Na:Al = 1.07

30000 20000 O Al 20000

10000 Na Intensity (cps) Intensity

Intensity (cps) Intensity Ca 10000 O Al K C Na S K Ca C S 0 0 0 1 2 3 4 5 0 1 2 3 4 5 KeV KeV

Fig. 5. SEM micrographs of specimens made with 7 M NaOH solution at w = 14% and cured at

35°C for 7 days, or at 0th cycle: a) low magnification image; b) higher magnification image of the square zone in (a). the EDS spectra are for points 1 and 2 (unreacted) and 3 and 4 (reacted) in (b).

GP = geopolymer; and MT = mine tailing particle.

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a) b) 2 3 4

100 µm 40 µm

80000 60000 Si Si 50000 Si:Al = 8.67 Si:Al = 2.54 60000 Na:Al = 0.79 Na:Al = 0.18 40000

40000 30000 O Al O 20000 Na

20000 (cps) Intensity Intensity (cps) Intensity Al 10000 Ca C Na S K Ca Fe C S K Fe 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 KeV KeV

Fig. 6. SEM micrographs of specimens made with 7 M NaOH solution at w = 14% and cured at

35°C for 7 days and subjected to 12 w-d cycles: a) low magnification image; b) higher magnification image of the square zone in (a). the EDS spectra are for points 1 and 2 (unreacted) and 3 and 4 (reacted) in (b). GP = geopolymer; and MT = mine tailing particle.

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SG SG GP MT GP GP MT SG MT

a.1) 40 µm b.1) 40 µm c.1) 40 µm

Na=5.98% Na=4.56% Na=3.25%

a.2) b.2) c.2)

Al=9.71% Al=8.65% Al=8.41%

a.3) b.3) c.3)

Si=25.48% Si=24.42% Si=24.10%

a.4) b.4) c.4)

Fe=3.30% Si=3.08% Si=2.98%

a.5) b.5) c.5)

Fig. 7. EDS mapping results of the 10% SG specimens after the (a) 0th, (d) 3rd and (g) 12th cycle. The distribution of Na (magenta) is shown in (b), (e) and (h) and the overlapped distribution of Na, Al (light blue) and Si (dark blue) in (c), (f) and (i). GP = geopolymer; and MT = mine tailing particle.

157

1.0

0.8

0.6

0.4 0% SG

Residual Na Na Residual (%) 0.2 10% SG 50% SG 0.0 0 2 4 6 8 10 12 w-d cycles a)

1.0

0.8

0.6

0.4 0% SG

Residual Al (%) Al Residual 10% SG 0.2 50% SG

0.0 0 2 4 6 8 10 12 b) w-d cycles

158

1.0

0.8

0.6

0.4 0% SG

10% SG Residual Si (%) Si Residual 0.2 50% SG

0.0 0 2 4 6 8 10 12 c) w-d cycles

Fig. 8. Residual elements vs wet and dry cycles: a) Na, b) Al and c) Si

159

12th cycle

3rd cycle

S 0th cycle

PA G S MT Powder A P P S S S S S S

F M F F F SG Powder F M F F F M F M

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 2θ

Fig. 9. XRD patterns of SG powder, MT powder and geopolymer specimens prepared at 10% SG after the 0th, 3rd and 12th w-d cycle. A = albite; G = gypsum; P = sanidine; S = quartz; F = fayalite;

M = magnetite

160

8 0% SG 7 5% SG 6 10% SG 30% SG 5 50% SG 4

3 Loss (%) mass of Loss 2

0 0 2 4 6 8 10 12 Wet and dry cycles

Fig. 10. Loss of mass vs w-d cycles at different SG contents

161

a) b)

Fig. 11. Photos of a) 0% SG and b) 50% SG specimens after 12 w-d cycles

162

14 13 12 11

pH 10 9 8 7 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 w-d ycles

Fig. 12. pH measurements in the solution where the 0% SG specimen was immersed

163

100

RLS (%) RLS 40

0 1 A2 B3 C4 5D 6E 7F 8G 9H 10

[A] Basha et al. (2005) [E] Amadi (2014) [B] Osinubi et al. (2010) [F] Etim et al. (2017)

[C] Joel and Agbede (2011) [G] Rios et al. (2017) [D] Ijimdiyaa et al. (2012) [H] This study

Fig. 13. Resistance to loss of strength from different researchers

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14 100 to Resistancestrenght in (%)loss 14 days oven at 35°C 12 7 days oven 7 days immersed 80 10

8 60 48.3% 6 40

UCS(MPa) 20.1% 4 9.0% 20 2

0 0 a) 0% SG 10% SG 50% SG

14 100 Resistance to to Resistancestrenght in (%)loss

12 14 days oven at 60°C 7 days oven and 7 days immersed 80 10 61.3% 8 60

6 28.7% 40 UCS(MPa) 4 22.4% 20 2

0 0 b) 0% SG 10% SG 50% SG

Fig. 14. UCS vs SG content of specimens cured at curing temperatures of: a) 35°C and b) 60°C.

The variation of resistance to loss in strength vs SG content is also plotted

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APPENDIX D

EXPERIMENTAL STUDY OF GEOPOLYMER BINDER SYNTHESIZED WITH

COPPER MINE TAILINGS AND LOW-CALCIUM COPPER SLAG

Lino Manjarrez1, Arash Nikvar-Hassani2, Rasoul Shadnia3, Lianyang Zhang, Ph.D., P.E.,

M.ASCE4

1 Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, AZ 85721, USA

2 Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, AZ 85721, USA

3 Department of Civil Engineering, Hakim Sabzevari University, Sabzevar, Iran

4 Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, AZ 85721, USA (Corresponding author). Email: [email protected]

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ABSTRACT

This paper presents an experimental study on geopolymer binder produced with copper mine tailings (MT) and low-calcium slag (SG). The study systematically investigated the effects of water to solid ratio (w/s), SG content (0, 25 and 50%), sodium hydroxide (NaOH) concentration

(5, 10 and 15 M), and sodium silicate (SS) to sodium hydroxide ratio (0.0, 0.5, 1.0 and 1.5) on the unconfined compressive strength (UCS) of synthesized geopolymer binder specimens. Scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction

(XRD) analyses were also performed to characterize the microstructure and phase composition of the geopolymer specimens. The results show that the inclusion of SG improves the UCS and reduces the initial water content required for achieving a certain workability of the geopolymer paste. The geopolymer binder specimens prepared at 50% SG, 10 M NaOH, SS/NaOH = 1.0 and cured at 60 C for 7 days reached the highest UCS of 23.5 MPa. The geopolymer paste prepared at 50% SG, 15 M NaOH concentration and SS/NaOH ratios of 0.5 and 1.0 showed flash setting which led to poorer quality specimens and lower UCS. The SEM, EDS and XRD analyses clearly show the participation of iron dissolved from SG in the formation of geopolymer gels. This research helps to promote the reuse of MT and SG through geopolymerization and contributes to the knowledge of geopolymer materials.

Key words: Mine tailings; low-calcium slag; geopolymer; compressive strength; flash setting

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1. Introduction Recently, the use of sustainable materials is gaining increasing interest and recognition especially in the construction industry. Undoubtedly, ordinary Portland cement (OPC), which is a major component of concrete, is one of the most used construction materials worldwide. However, the manufacturing of OPC is accompanied with environmental and ecological problems such as the release of carbon dioxide. It is estimated that the OPC industry is responsible for about 7% of all the carbon dioxide generated worldwide [1,2]. A solution to this problem is to use waste materials such as mine tailings (MT) and furnace slags (SG) to produce cementitious binder as a replacement of OPC.

Every year, the mining industry generates significant amount of MT [3]. MT are finely ground rock that are left over after the valuable metal bearing minerals have been extracted. MT are currently used as backfill in underground mines, stored in open pits, dried and stacked, or pumped into tailings ponds on site (Hudson-Edwards et al. 2011). Usually, the on-site tailings ponds occupy large areas of land, while at the same time represent a threat to the environment by possible contamination of air, surface water, groundwater and soils nearby. Furnace slag is produced during the primary copper smelting process and is stored on site in stock piles or under the sea in several countries. According to the United States Geological Survey (USGS), in 2016, around 15 to 20 million tons of iron and steel slag were generated in the United States and 300 to 360 million tons worldwide [4].

Extensive research has been conducted on the utilization of MT [5–9] and SG [10–15] in combination with OPC as construction material. Although the studied materials have demonstrated acceptable performance, MT and SG are used in small quantities compared to OPC and the environmental and ecological problems related to OPC manufacturing remain. A viable path to

168 address the problem in an environmentally-friendly and sustainable way is to use novel technologies such as geopolymerization so that no OPC is used.

Geopolymerization is the chemical reaction of aluminosilicates in a highly alkaline silicate or hydroxide solution, creating a stable material called geopolymer which has a polymeric structure with interconnected Si-O-Al-O-Si bonds [16–20]. The process of geopolymerization involves the dissolution of solid aluminosilicates in an alkaline solution, the formation of silica-alumina oligomers, and the polycondensation of the oligomeric species forming the inorganic polymeric material [17,18]. Geopolymer materials have a number of advantages over OPC, including rapid development of mechanical strength, high acid resistance, no/low alkali-silica reaction related expansion, immobilization of toxic and hazardous materials, and significantly reduced greenhouse gas emissions [21–23]. Since MT and SG both include high content of alumina and silica, they are attractive raw materials for producing geopolymer binder to replace OPC.

Several researchers have studied the geopolymerization of MT [24–27] and SG [28–30]. For example, Manjarrez and Zhang [27] conducted a systematic study on the utilization of geopolymerized copper MT as road construction material through geopolymerization. The results showed that by selecting the appropriate preparation conditions (moisture content and NaOH concentration), the geopolymerized MT can meet the strength requirements for cement treated road base by several transportation agencies. The investigation of geopolymerized SG has mainly focused on high calclium SG [31–36]. For instance, Puliguilla and Mondal [32] studied the role of high calcium slag in the hardening of fly ash-slag based geopolymer. They found that the rate of hardening increasesd with the addition of SG. They attributed this behavior to the dissolution of

Ca present in the SG that led to the formation of CASH gels which worked in synergy with the aluminosilicate gels resulting in a denser gel-like network. So far, the investigation of

169 geopolymerized low-calcium SG is still limited [37–41]. Shadnia and Zhang (2017) conducted an experimental study on geopolymer synthesized with class F fly ash and low-calcium SG. The study showed that when the amount of SG was increased, both the UCS and the unit weight of the geopolymer binder increased. A UCS of 51 MPa was found on specimens prepared with 50% SG and cured at 75 C for 7 days. Ahmari et al. [37] conducted an experimental study on geopolymer synthesized with copper MT and low-cacium SG using sodium hydroxide (NaOH) solution at 15

M concentration. The results showed that specimens prepared with 50% SG and cured at 90 C for

7 days reached a maximum UCS of 48 MPa.

This paper systematically studies the geopolymerization of blended copper MT and low-calcium

SG, focusing on the effects of water to solid (w/s) ratio, curing temperature, NaOH concentration and sodium silicate to sodium hydroxide (SS/NaOH) ratio on the physical and mechanical properties of geopolymer specimens synthesized at different conditions. To study the microstructure and chemical composition of the geopolymer specimens, scanning electron microcopy (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) analyses were carried out. This research aims to promote the utilization of copper MT and low calcium SG through geopolymerization as a substitute of OPC in civil construction.

2. Experimental Studies 2.1. Materials The materials used in this investigation were copper mine tailings (MT), low-calcium copper slag

(SG), reagent grade 98% sodium hydroxide (NaOH) and sodium silicate (SS) solution. The MT were supplied by a major mine company in Tucson, Arizona. The MT were received in the form of damp solids. After drying, the MT were pulverized and screened through No. 40 sieve in order to utilize 100% of the material. The SG was provided by a smelter located in Hayden, Arizona.

170

The SG was received in the forms of aggregates and it was crushed and milled to powder prior to use. Table 1 shows the chemical composition of the MT and SG based on XRF analysis. The MT mainly consist of silica and alumina, whereas the SG consists mostly of silica and iron. Fig. 1 shows the particle size distribution of both the MT and SG determined using mechanical sieving and hydrometer tests following ASTM D6913 [42] and ASTM D422 [43]. The MT have a mean particle size around 90 µm with 40% particles smaller than No. 200 (75 µm), whereas the milled

SG is much finer with a mean particle size around 18 µm and 79% particles passing No. 200 sieve.

Fig. 2 shows the SEM micrographs of the MT and SG powders. In general, the MT particles are larger than the SG particles. It can also be observed that finer MT particles are attached to the surface of coarser ones. The same is observed for the SG particles, but they are finer in size. The specific gravity of MT and SG is 2.83 and 3.80, respectively, which means that the SG is heavier than the MT. Fig. 3 shows the XRD patterns of the MT and SG powders. The MT consist of mainly crystalline materials such as quartz (SiO2), gypsum (CaSO4), albite (NaAlSi3O8) and sanidine

[K(AlSi3O8)]. A weak amorphous phase, centered at 28° is also clear. The SG is mainly composed of two crystalline phases, magnetite (Fe2O3) and fayalite (Fe2SiO4).

The alkaline solution was prepared with NaOH, SS and distilled water. First, the NaOH flakes were dissolved in distilled water to prepare solutions at different NaOH concentrations of 5, 10 and 15 M. After the NaOH solution cooled down to room temperature, SS was incorporated to the

NaOH solution at different SS/NaOH ratios of 0.0, 0.5, 1.0 and 1.5.

2.2. Preparation of MT/SG-Based Geopolymer Specimens

To prepare geopolymer specimens, MT and SG were first dry mixed at a specified SG content

(MT/SG = 50/50, 75/25 or 100/0, by total solid weight) for 5 minutes using a mechanical mixer to ensure homogeneity of the mixture. Then, the alkaline solution was slowly added to the dry

171 mixture and mixing continued for 5 more minutes. The specimens were produced at a consistent workability and the 50% SG specimen prepared at a 15.8% initial water content was used as the reference workability according to a water to solids (w/s) analysis performed, as detailed later.

Since the addition of SG significantly improved the workability of the paste, the initial water content was increased when less SG was used. The initial water content values were 15.8%, 17.3% and 18.5% for 50%, 25% and 0% of SG, respectively. The mixing was performed at room temperature of about 23±2 oC.

After mixing, the resulting paste was poured into plexiglas molds of 25 mm diameter and 50 mm height. The paste was poured while the mold was shaken on a vibrator table for about 5 minutes to release trapped air bubbles. Then, the specimens were placed in an oven, including the mold, at a specified temperature for curing. After 24 hours, the specimens were demolded, wrapped in plastic sheets and placed back in the oven for six more days. To study the effect of water to solid

(w/s) ratio, the 50% SG specimens were prepared at different w/s ratios for 10 M NaOH, SS/NaOH

= 1.0 and curing temperature of 60 °C; the water present in the NaOH and SS solutions was counted in the water weight, and MT, SG, NaOH and the solid present in the SS solution were counted in the solid weight. To study the effect of curing temperature, the 50% SG specimens were prepared at the optimum w/s ratio, 10 M NaOH and SS/NaOH = 1.0, and cured at 45, 53, 60, 68 and 75 oC, respectively. To study the effect of NaOH concentration and SS/NaOH ratio, the 50% SG specimens were prepared at NaOH concentrations of 5, 10 and 15 M and SS/NaOH ratios of 0.0,

0.5, 1.0 and 1.5, and cured at 60 °C.

Unconfined compression tests were performed on the geopolymer specimens prepared at different conditions with an ELE Tri Flex 2 loading machine at a constant loading rate of 1 mm/min to measure the unconfined compressive strength (UCS). For each condition, three specimens were

172 tested and the average of the measured values was used in the analysis. Before conducting the compression tests, the specimens were polished at the end surfaces to make sure that they were accurately flat and parallel.

To investigate the effect of SG content, NaOH concentration and SS/NaOH ratio on the microstructure and chemical composition of the MT/SG-based geopolymer specimens, SEM imaging, EDS analysis and XRD analysis were carried out. The SEM imaging and EDS analysis were performed in SE conventional mode using the FEI INSPECT-S/Thermo-Fisher Noran 6 microscope. The XRD analysis was carried out in a Panalytical X’pert pro MPD instrument equipped with a programmable incident beam slit using Ni-filtered, Cu, Kα and λ = 1.5418 Å as

X-ray radiation.

Table 2 summarizes the tests conducted at different conditions.

3. Results and Discussion

3.1. Effect of w/s Ratio

Fig. 4 shows the variation of UCS with w/s ratio of specimens produced at 50% SG, 10 M NaOH,

SS/NaOH = 1.0 and cured at 60 °C for 7 days. The w/s ratio varied from 0.131 to 0.189. It can be clearly seen that the UCS slightly increases and then decreases with the w/s ratio. At w/s = 0.131, lumps were formed due to the low amount of water available in the mixture and large voids were formed in the specimens. When the w/s ratio was increased up to 0.158, a maximum UCS of 23.5

MPa was obtained. This w/s ratio was defined as the optimum w/s ratio and the workability of the corresponding paste was used as the reference for preparation of the specimens presented in the next subsections. Further increase of the w/s ratio led to a decrease on the UCS. This behavior could be related to the particle-particle interaction. The higher amount of water available in the paste is directly linked to the microstructure. For example, the interparticle distance increases and

173 the particle interference decreases, leading to a drop of UCS. The results are in agreement with the studies by other researchers [44–46], which observed that the increase of alkaline solution causes an increase of the paste flowability and a decrease of the UCS of geopolymer specimens.

3.2. Effect of Curing Temperature

Temperature has a significant effect on the mechanical properties of geopolymer as shown by the previous studies summarized in Table 3. There is an optimum temperature that gives the best performance of the geopolymer product and is determined mainly by the reactivity and fineness of the source materials. For example, Yunfen et al. [47] utilized class F fly ash together with NaOH and SS to synthesize geopolymer and found an optimum curing temperature of 80 oC, whereas

[48] used class F fly ash and ground granulated blast furnace slag (GGBFS) as source material and

NaOH and SS as alkaline reagent and found an optimum curing temperature of 60 oC.

Fig. 5 shows the variation of UCS with curing temperature for the 50% SG geopolymer specimens prepared at 10 M NaOH, SS/NaOH = 1.0 and cured for 7 days. The results indicate that the UCS increases with curing temperature and does not show an optimum curing temperature in the range of temperature from 45 to 75 oC considered in this study. The selection of the curing temperature range was based on the temperature that outdoor construction materials, such as concrete pavement, could experience in the southwest region of United States. Shadnia et al. [49] measured the temperature of ground on summer days in Arizona and found that a temperature between 45 and 68 oC (see the shaded range in Fig. 5) can be achieved for about 7 hours during a regular day.

Considering feasible application of the MT/SG geopolymer binder in practice, a curing temperature of 60 oC is used for all the specimens discussed in the flowing subsections.

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3.3. Effect of SG Content

Fig. 6 shows the variation of UCS with SG content for the MT/SG-based geopolymer specimens prepared at 10 M NaOH, SS/NaOH ratio of 0.0, 0.5, 1.0 and 1.5, and cured at 60 °C for 7 days. It can be clearly seen that the inclusion of SG increases the UCS of the MT/SG-based geopolymer specimens. For example, at SS/NaOH = 1.0, when the SG content is increased from 0% to 25% and 50%, the UCS is increased from 2.5 MPa to 12.0 MP and 23.5 MPa, respectively. It was also observed that when the SG content is increased from 0% to 25% and 50%, the required w/s ratio for achieving the desired workability is decreased from 0.185 to 0.173 and 0.158, respectively.

Therefore, the inclusion of SG not only increases the UCS, but also decreases the required initial water content, as also observed previous studies [37,38]. A lower initial water content is directly linked to the reduction of NaOH and SS from the alkaline solution, which represents a cost benefit for the final geopolymer product.

The enhancement of UCS by the SG can be attributed to its physical and chemical properties.

Compared to the MT powder, the SG powder is finer and its particles have larger surface area, which improves the chemical reaction with the alkaline solution as observed by [34] in the study of alkali-activated fly ash/slag pastes. Also, the SG has a higher reactivity than the MT due to the extremely high temperature it experienced in the smelter and the developed amorphous structure ideal for geopolymerization. The chemical composition of the source material plays a key role in the geopolymer formation and it can be analyzed by the Si/Al ratio. According to several studies

[20,38,40,50–53], the initial Si/Al ratio that determines the ideal mechanical properties of geopolymers should be within 1 to 3. However, as presented by [38,54] in the study of iron distribution in slag-based geopolymers, structural Fe+3 was found within the alumina-silicate structure suggesting its participation in the geopolymerization process. As demonstrated by

175

Komnitsas et al. (2009) and Shadnia and Zhang (2017), if Fe is present in the geopolymer reaction, the Si/Al ratio should be replaced by Si/(Al+Fe). In this study, the highest compressive strength is achieved by the specimens synthesized at 10 M NaOH and SS/NaOH = 1.0 with initial Si/Al =

4.94 and Si/(Al+Fe) = 1.53, with the later value lying within the range of reported optimum values.

Fig. 7 shows the variation of unit weight with SG content for the MT/SG-based geopolymer specimens presented in Fig. 6. As expected, the incorporation of SG increases the unit weight of geopolymer specimens. This is simply because of two reasons: 1) the SG is finer than the MT and thus create a more compacted microstructure, and 2) the SG has a greater specific gravity than the

MT.

To further investigate the effect of SG content, SEM imaging and EDS point analysis were performed on specimens prepared at 0%, 25% and 50% SG, 10 M NaOH and SS/NaOH = 1.0.

Figs. 8a, 8b and 8c show the SEM micrographs of the 100/0, 75/25 and 50/50 specimens, respectively. Two phases are observed: unreacted MT/SG particles and geopolymer gels. The

75/25 and 50/50 specimens show a more compacted microstructure than the 100/0 specimen. This is because the SG particles are finer than the MT particles and can fill in more and smaller voids.

The EDS point analysis was performed on the geopolymer gel, points 1 and 2, in the micrographs.

The obtained Si/Al, Si/(Al+Fe) and Na/Al ratios are 5.77, 4.94 and 3.36 for the 100/0 specimen,

7.98, 4.06 and 2.55 for the 75/25 specimen, and 6.53, 1.02, 3.06 for the 50/50 specimen, respectively. The initial Si/Al, Si/(Al+Fe) and Na/Al ratios are 3.70, 3.25 and 1.06 for the 100/0 specimen, 4.20, 2.20 and 1.16 for the 75/25 specimen, and 4.94, 1.53 and 1.10 for the 50/50 specimen, respectively. In the 100/0 (no SG) specimen, the difference between Si/Al and

Si/(Al+Fe) is minor because the MT contains only 3.07% Fe2O3 (see Table 1). However, in the

75/25 and 50/50 specimens, the difference between Si/Al and Si/(Al+Fe) is much larger because

176 the SG contains 37.5% Fe2O3 (see Table 1). Therefore, it is important to consider the effect of Fe in the geopolymer reaction when SG is included.

Fig. 9 compares the XRD patterns of the MT and SG powders and the 100/0, 75/25 and 50/50 geopolymer specimens prepared at 10 M NaOH, SS/NaOH = 1.0 and cured at 60 °C for 7 days.

After geopolymerization, the crystalline peaks remain the same without emergence of new peaks, indicating that the partially reacted particles are the main constituents of the geopolymer matrix.

A notable change can be observed in the amorphous phase of MT and SG powders and the geopolymer specimens. In the MT powder, the amorphous phase shows a weak broad hump from approximately 22° to 32°, whereas in the SG powder it is represented by a broad hump from approximately 10° to 62°. In the 100/0 specimen, the XRD pattern roughly shows the same weak broad hump of the MT powder, although with sharper crystalline peaks. Gypsum as a crystalline peak detected in the MT powder disappeared after geopolymerization. This is most likely because gypsum was encapsulated in the solution pores [27,37]. In the 75/25 and 50/50 geopolymer specimens, the XRD pattern shows a combination of the original patterns corresponding to MT and SG. In addition, a decrease in the intensity of crystalline peaks can be observed due to the geopolymer reaction. The broad hump in the 75/25 and 50/50 geopolymer specimen extends from approximately 10° to 50° and has a larger area below the amorphous hump than that of the MT and SG powders alone due to the combination of the original amorphous phases and the possible transformation of the crystalline phase to a new geopolymer phase.

3.4. Effect of SS and NaOH Content

Fig. 10 shows the variation of UCS with SS/NaOH ratio for the MT/SG-based geopolymer specimens prepared at 50% SG, NaOH concentrations of 5, 10 and 15 M, and cured at 60 °C for 7 days. The UCS of specimens synthesized at 5 and 10 M NaOH increases with the SS/NaOH ratio

177 up to 1.0 and then decreases. The 15 M NaOH specimens, however, experiences a slight UCS decrease from SS/NaOH = 0.0 up to SS/NaOH = 1.0 and then a UCS increase; this behavior is due to flash setting and will be discussed later. The highest UCS for both 5 and 10 M NaOH specimens occurs at SS/NaOH = 1.0 and for 15 M NaOH specimens at SS/NaOH = 1.5. These optimum

SS/NaOH ratios agree well with the observations by several authors [55–58], as shown in Table

4, although different source materials were used.

The effect of NaOH concentration and SS/NaOH ratio can be further explained based on the Na/Al ratio. The Na/Al ratios at SS/NaOH = 0.0, 0.5, 1.0 and 1.5 were 0.85, 0.84, 0.81 and 0.80, 1.41,

1.31, 1.10 and 1.03, and 1.87, 2.02, 1.90 and 1.19, for 5, 10 and 15 M NaOH concentrations, respectively. At SS/NaOH = 1.0, the highest UCS of 23.5 MPa occurred at 10 M NaOH, corresponding to Na/Al = 1.10. This is in good agreement with the observations by several authors

[27,51,59–62] who showed that the optimum Na/Al ratio is around 1.0.

The 15 M NaOH specimens experienced a different UCS versus SS/NaOH trend than the 5 and 10

M NaOH specimens (see Fig. 10). The UCS slightly decreases with SS/NaOH from 0.0 to 0.5 and

1.0 and then increases at 1.5. The unexpected trend can be attributed to the increase of w/s ratio and the formation of lumps due to flash setting during specimen preparation. To reach the defined workability as described earlier, a w/s ratio of 0.210 and 0.227 was required for the paste prepared at 15 M NaOH and SS/NaOH of 0.5 and 1.0, respectively. As discussed previously, too high a w/s ratio could lead to a porous structure and decrease of the UCS. It was also found that, at SS/NaOH

= 0.5 and 1.0, lumps were formed due to flash setting during the specimen preparation process.

The formation of lumps led to large voids which could be visually observed around the specimen surface after removing the mold. It was noted that flash setting occurred only at certain conditions defined by the NaOH concentration and the SS/NaOH ratio. For example, at SS/NaOH = 1.0, the

178 paste experienced flash setting at 15 M but not at 5 and 10 M. At 15 M concentration, the paste experienced flash setting at SS/NaOH = 0.5 and 1.0 but not at SS/NaOH = 0.0 and 1.5. The results are in good agreement with the observations by Pacheco-Torgal et al. [63] who mentioned that the

NaOH concentration and SS/NaOH ratio are the most important parameters responsible for flash setting. Table 5 summarizes several studies where flash setting occurred and the possible causes for flash setting.

Since NaOH concentration and SS/NaOH ratio both are important on the flash setting effect, further analysis is conducted on the alkaline solution. Fig. 11 shows the variation of solid NaOH to total water (Na/tw) ratio in the alkaline solution with SS/NaOH ratio at different NaOH concentrations, where the total water includes the distilled water for dissolving NaOH and the water in the SS solution. At all NaOH concentrations, the Na/tw ratio decreases with the SS/NaOH ratio. Notably, the two alkaline solutions that caused flash setting of the geopolymer paste have both the Na/tw and SS/NaOH ratios above a certain level. This is in general agreement with the observations by several other studies summarized in Table 5. The amount of NaOH and that of SS both play a key role in flash setting and should be properly considered in the production of geopolymers.

To further investigate the effect of SS/NaOH ratio, SEM imaging and EDS point and mapping analyses were performed on geopolymer specimens prepared at 50% SG, 10 M NaOH concentration, and SS/NaOH = 0.0 and 1.0. These two specimens were selected to understand the effect of SS/NaOH ratio on the microstructure and chemical composition of geopolymer at the same NaOH concentration. Figs. 12a and 12b show the SEM micrographs and EDS point analysis results of the geopolymer specimen at SS/NaOH = 0.0 and 1.0, respectively. In Fig. 12a, the material exhibits two phases: MT/SG particles of varied sizes with irregular shapes (unreacted)

179 and glassy geopolymer gels (reacted MT/SG particles). A higher magnification image is also shown and the two phases can be more clearly seen. Fig. 12b shows the SEM micrographs of the specimen at SS/NaOH = 1.0, which clearly exhibits a denser microstructure than the specimen at

SS/NaOH = 0.0. The two phases are also observed, but the geopolymer gel is solid and covers partially, or totally, the MT/SG particles. Sindhunata et al. [64] also observed solid geopolymer gels in their study of FA-based geopolymer and attributed them to the effect of SS in the geopolymer gel formation process. The EDS point analysis was performed on both phases, points

1 and 2 for unreacted MT/SG and points 3 and 4 for geopolymer gel. At SS/NaOH = 0.0, the obtained Si/(Al+Fe) and Na/Al ratios are 1.53 and 0.17 for the unreacted MT/SG, and 0.76 and

2.02 for the geopolymer gel. While at SS/NaOH = 1.0, the obtained Si/(Al+Fe) and Na/Al ratios are 0.68 and 2.45 for the unreacted MT/SG, and 1.02 and 3.06 for the geopolymer gel. As listed in

Table 2, at SS/NaOH = 0.0 and 1.0, the initial Si/(Al+Fe) and Na/Al ratios are 1.41 and 1.41, and

1.53 and 1.10, respectively. The Si/(Al+Fe) ratio of the geopolymer gel is lower than the initial one at both SS/NaOH ratios because the dissolved Al and Fe from the unreacted part was incorporated into the gel, while the Na/Al ratio of the geopolymer gel is higher than the initial one because the SS in the alkaline solution provides extra Na to the geopolymer matrix.

Fig. 13 shows the EDS mapping analysis performed on the 50% SG, 10 M NaOH and SS/NaOH

= 0.0 specimen. Fig. 13a shows the SEM micrograph of the analyzed area, Figs. 13b-13e show the distribution of Na, Al, Si and Fe, respectively, and Fig. 13f shows the overlapped distribution of

Na, Al, Si and Fe. As indicated in Table 1, Al and Si are the main elements present in MT, whereas

Si and Fe are the main elements present in SG. So MT particles can be identified in Figs. 13c and d by identifying the spots where both Al and Si are highly concentrated, such as the lower and upper middle zones of the analyzed area. Similarly, SG particles can be identified in Figs. 13d and

180 e by identifying the spots where both Si and Fe are highly concentrated. The distribution of Na is an important indicator of the geopolymer gel location [33]. Na is supplied by the alkaline solution and, together with the Al, Si and Fe dissolved from MT and SG particles, forms the geopolymer gels. The overlapped distribution of Na, Al, Si and Fe shown in Fig. 13f gives an overall picture of the location of each element in the geopolymer matrix. It is notable that Fe participated in the geopolymer gel formation, which agrees with the results of [54] in the study of iron distribution in geopolymers.

Fig. 14 shows the EDS mapping analysis of the 10 M NaOH, SS/NaOH = 1.0 and 50/50 specimen.

It shows that significant amount of Al, Si and Fe have migrated from their original position to form the geopolymer gel, as demonstrated by the Na location. The overlapped elements image in Fig.

14f further confirms that the geopolymer gels cover essentially the entire analyzed area shown in

Fig. 14a.

Fig. 15 shows the SEM micrograph and EDS point analysis results of the geopolymer specimen prepared at 50% SG, 15 M NaOH and SS/NaOH =1.0. The two phases, unreacted MT/SG and geopolymer gel, can also be clearly observed in the specimen; but the geopolymer gel looks flaky in contrast to the solid-like geopolymer gel at 10 M (see Fig. 12b). It is believed that the flaky shape of the geopolymer gel at 15 M is due to the rapid reaction (flash setting) that inhibited the formation of strong solid-like gel. EDS point analysis was performed on both phases, points 1 and

2 for unreacted MT/SG and points 3 and 4 for geopolymer gel. The obtained Si/(Al+Fe) and Na/Al ratios are 0.46 and 4.10 for the unreacted MT/SG and 0.81 and 4.39 for the geopolymer gel, respectively. The initial Si/(Al+Fe) and Na/Al ratios are 1.61 and 1.90, respectively. The

Si/(Al+Fe) of the unreacted MT/SG is lower than the initial one because Si, Al and Fe were dissolved in the alkaline solution and migrated to the geopolymer gel. The higher Na/Al ratio in

181 both phases than that at 10 M is simply related to the larger amount of NaOH in the alkaline solution.

Fig. 16 shows the EDS mapping analysis on the 50% SG, 15 M NaOH and SS/NaOH = 1.0 specimen. Comparing with what shown in Fig. 14, it can be clearly seen that less amount of geopolymer gels were formed at 15 M than at 10 M due to the flash setting at 15 M. This can be further seen from the XRD patterns of the specimens at 10 M and 15 M NaOH in Fig. 9. Both

XRD patterns show a broad hump approximately between 10° to 50° which represents the amorphous geopolymer phase; but the broad hump at 10 M has a higher magnitude than that at 15

4. Conclusions

Based on the experimental study of the MT/SG-based geopolymer binder specimens synthesized under different conditions, the following conclusions can be drawn.

1. The UCS increases with w/s ratio up to certain level and then decreases. An optimum w/s

ratio of 0.158 was found for the studied MT/SG.

2. The incorporation of SG improves the UCS of the MT/SG-based geopolymer and reduces

the water content required for a certain workability of the geopolymer paste. The reduction

of required water content decreases the consumption of NaOH and SS and thus the cost of

the final MT/SG-based geopolymer product.

3. At 5 and 10 M NaOH concentrations, the UCS of the MT/SG-based geopolymer increases

with the SS/NaOH ratio up to 1.0 and then decreases, showing an optimum SS/NaOH ratio

of 1.0 which is in good agreement with the observations by other researchers.

182

4. The MT/SG-based geopolymers prepared at 15 M NaOH do not show an optimum

SS/NaOH ratio because of the flash setting at SS/NaOH = 0.5 and 1.0. The flash setting

occurs when the amount of NaOH and that of SS in the alkaline solution both exceed a

certain limit.

5. The Fe from SG also participates in the geopolymerization process. It is important to

consider the effect of Fe in the geopolymer reaction when SG is included

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193

Table 1. Chemical composition of MT and SG based on XRF analysis

Chemical compound MT (%) SG (%)

Na2O 3.02 N/A

MgO 1.78 N/A

Al2O3 14.10 2.43

SiO2 55.90 32.90

P2O5 0.19 N/A

SO2 2.22 N/A

SO3 N/A 1.56

K2O 3.89 N/A

CaO 2.27 2.13

TiO2 0.49 N/A

MnO 0.06 N/A

Fe2O3 3.07 37.50

Fe3O4 N/A 5.48

194

Table 2. Summary of specimen properties and different types of tests Study NaOH SS/ SG Curing Na/Al Si/Al Si/ UCS SEM XRD (M) NaOH (%) temperature (Al+Fe) (oC) Overall - - 0 - - - - ✓ ✓ - - 100 - - - - ✓ ✓ w/s ratio 10 1.0 50 60 0.95 4.87 1.51 ✓ 10 1.0 50 60 1.05 4.92 1.52 ✓ 10 1.0 50 60 1.10 4.94 1.53 ✓ 10 1.0 50 60 1.14 4.96 1.54 ✓ 10 1.0 50 60 1.19 4.98 1.55 ✓ 10 1.0 50 60 1.27 5.03 1.56 ✓ Curing 10 1.0 50 45 1.10 4.94 1.53 ✓ temperature 10 1.0 50 53 1.10 4.94 1.53 ✓ 10 1.0 50 60 1.10 4.94 1.53 ✓ 10 1.0 50 68 1.10 4.94 1.53 ✓ 10 1.0 50 75 1.10 4.94 1.53 ✓ NaOH 5 0.0 50 60 0.85 4.56 1.41 ✓ concentration 10 0.0 50 60 1.41 4.56 1.41 ✓ ✓ 15 0.0 50 60 1.87 4.56 1.41 ✓ SS/NaOH 5 0.5 50 60 0.84 4.79 1.49 ✓ ratio 5 1.0 50 60 0.81 4.90 1.52 ✓ ✓ 5 1.5 50 60 0.80 4.98 1.55 ✓ 10 0.5 50 60 1.31 4.84 1.50 ✓ 10 1.0 50 60 1.10 4.94 1.53 ✓ ✓ ✓ 10 1.5 50 60 1.03 5.02 1.56 ✓ 15 0.5 50 60 2.02 4.94 1.53 ✓ 15 1.0 50 60 1.90 5.19 1.61 ✓ ✓ ✓ 15 1.5 50 60 1.19 5.02 1.56 ✓ SG content 10 0.0 25 60 1.64 3.81 1.99 ✓ 10 0.5 25 60 1.26 4.06 2.13 ✓ 10 1.0 25 60 1.16 4.20 2.20 ✓ ✓ ✓ 10 1.5 25 60 1.10 4.29 2.25 ✓ 10 0.0 0 60 1.45 3.36 2.95 ✓ 10 0.5 0 60 1.15 3.59 3.15 ✓ 10 1.0 0 60 1.06 3.70 3.25 ✓ ✓ ✓ 10 1.5 0 60 1.01 3.78 3.32 ✓

195

Table 3. Summary of optimum curing temperatures reported in the literature Optimum UCS (MPa) Source Applied Optimum alkali curing (curing Reference material alkali concentration temperature time, days) (̊C) SS and Class F fly potassium KOH 7.5 M 75 - [64] ash silicate (KOH) Class F fly NaOH and - 85 48.2 (28) [65] ash SS SS/NaOH=1.2, Class F fly NaOH and NaOH = 10.6 80 41.5 (3) [47] ash SS M NaOH and SS/NaOH=1.5, Zeolite 40 31 (7) [66] SS NaOH = 7 M Class C fly NaOH and SS/NaOH=1.5, 75 63.4 (28) [67] ash SS NaOH = 8.1 M Mine tailings NaOH 15 M 90 21 (7) [68] NaOH and Glass cullet 5 M 60 41 (7) [69] KOH MT and aluminum NaOH 5.1 M 90 44.8 (7) [62] sludge Class C fly NaOH and SS/NaOH=1.4, 100 59.4 (28) [70] ash SS NaOH = 8 M NaOH and SS/NaOH=1.2, Metakaolin 80 73.0 (28) [71] SS NaOH = 8 M Class F fly NaOH and SS/NaOH=2.5, ash and 60 37 (7) [48] SS NaOH = 12 M GGBFS Fly ash and NaOH >15 M 75 58 (7) [38] SG SS/NaOH = Waste clay NaOH and 1.6, NaOH = 90 36.2 (5) [72] brick powder SS 8.1 M Copper MT NaOH and SS/NaOH=1.0, and low > 68 24.8 (7) This study SS NaOH = 10 M calcium SG

196

Table 4. Summary of optimum NaOH concentrations and SS/NaOH ratios reported in the literature

Optimum NaOH Source material UCS (MPa) concentration, Reference SS/NaOH ratio High calcium FA 19.9 – 41.2 20 M, 1.0 [55] binder Ground granulated blast furnace slag 11.0 – 42.0 14 M, 2.5 [73] and OPC were added to class F FA Class F FA based 6.0 – 42.0 12 M, 1.0 [56] geopolymer Low calcium FA and bottom ash 0.2 – 12.6 14 M, 2.0 [74] geopolymer Class F fly ash 13.0 – 58.0 10 M, 2.5 [75] based geopolymer Class F fly ash 46.0 – 52.7 12 M, 2.5 [76] geopolymer Waste concrete 21.0 - 32.0 10 M, 1.1 [57] fines and class F FA Tungsten mine 25.0 – 60.0 24 M, 2.5 [63] waste High calcium fly 6.75 – 45.8 10 M, 1.0 [58] ash Copper MT and 2.6 – 23.5 5, 10 M, 1.0 Current study low-calcium SG

197

Table 5. Summary of studies that have reported flash setting

Source material Alkali used Flash setting Reason Reference Viscosity of SS SS/NaOH=1.5 at 8, and the increase in 10, 12, 14 and 16 M solid content of Tungsten MT SS and NaOH NaOH, and [77] NaOH at higher SS/NaOH=2.0 at 14 NaOH and 16 M NaOH concentrations Several factors such as SS, OPC SS/NaOH=2.5 at 8 contamination, Low calcium M NaOH and w/s low water SS and NaOH [78] fly ash ratios of 0.18 and geopolymer 0.20 mixtures and NaOH concentration Lower water Low calcium 14 M NaOH and SS and NaOH content specimens [79] fly ash SS/NaOH=2.5 (< 30.1 kg/m³) High calcium SS/NaOH=1.5 and SS and NaOH Viscosity of SS [80] fly ash 10 M Calcium content, 8 M NaOH and pH of the source Fly ash SS and NaOH [81] SS/NaOH=2.0 material and viscosity of SS Viscosity of SS Copper MT and 15 M NaOH and SS and NaOH and high NaOH Current study low calcium SG SS/NaOH = 0.5, 1.0 concentration

198

100

90 SG 80 MT 70 60 50 40 30

Percent Percent passing(%) 20 10 0 0 1 10 100 1,000 10,000 Particle size (µm)

Fig. 1. Particle size distribution of MT and SG

199 a) b)

30 µm 30 µm

Fig. 2. SEM micrographs of MT and SG powders: a) MT and b) SG

200

MT Powder P A G S A P P S S S S S S

F F F M SG Powder M F F F F F M F M

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 2θ

Fig. 3. XRD pattern of MT and SG powders (S quartz, G gypsum, A albite, S sanidine, M magnetite and F fayalite)

201

15 UCS(MPa) 10

0 0.12 0.13 0.14 0.15 0.16 0.17 0.18 0.19 0.20 w/s ratio

Fig. 4. UCS versus water to solid (w/s) ratio for geopolymer specimens prepared at 50% SG, 10

M NaOH, SS/NaOH = 1.0, and cured at 60 oC for 7 days

202

UCS(MPa) 10

0 40 45 50 55 60 65 70 75 80 Temperature ( C)

Fig. 5. UCS versus curing temperature for geopolymer specimens prepared at 50% SG, 10 M

NaOH, SS/NaOH = 1.0, and cured for 7 days

203

SS/NaOH=0.0 20 SS/NaOH=0.5

SS/NaOH=1.0 15 SS/NaOH=1.5

10 UCS(MPa)

0 0 25 50 Slag content (%)

Fig. 6. UCS versus SG content for geopolymers specimens prepared at 10 M NaOH, SS/NaOH ratios of 0.0, 0.5, 1.0 and 1.5, and cured at 60 °C for 7 days

204

24 SS/NaOH=0.0 SS/NaOH=0.5 22 SS/NaOH=1.0 SS/NaOH=1.5 20

Unit weight (kN/m³) weight Unit 18

16 0 25 50 Slag content (%)

Fig. 7. Unit weight versus SG content for geopolymer specimens prepared at 10 M NaOH,

SS/NaOH ratios of 0.0, 0.5, 1.0 and 1.5, and cured at 60 °C for 7 days

205

120000 a) Si 100000 Si/Al = 5.77 Si/(Al+Fe) = 4.94 GP 80000 1 2 Na/Al = 3.36 60000 O 40000 Na

Intensity (cps) Intensity Al GP 20000 Ca C S K Fe MT 0 MT 0 1 2 3 4 5 6 7 30 µm KeV

b) 100000 Si Si/Al = 7.98 80000 Si/(Al+Fe) = 4.06 60000 Na/Al = 2.55 MT 2 40000 O

Intensity (cps) Intensity 20000 Na Al K Fe GP C S Ca 1 0 SG 0 1 2 3 4 5 6 7 30 µm KeV

c) 60000 Si 50000 Si/Al = 6.53 2 Si/(Al+Fe) = 1.02 40000 Na/Al = 3.06 Fe 30000 O

GP 20000 Na Intensity (cps) Intensity C Al S 10000 K Ca 1 MT 0 0 1 2 3 4 5 6 7 30 µm SG KeV

Fig. 8. SEM micrographs of specimens prepared at 10 M NaOH, SS/NaOH = 1.0, and cured at 60 °C for 7 days: a) 0% SG, b) 25% SG, and c) 50% SG. The EDS spectra are for points 1 and 2 (reacted) in a), b) and c), respectively. MT: mine tailing particle, SG: slag particle, GP geopolymer

206

50MT/50SG - 15 M NaOH

50MT/50SG - 10 M NaOH

75MT/25SG - 10 M NaOH

S 100MT/0SG - 10 M NaOH

P G A MT Powder S A P P S S S S S S F F M F SG Powder F F F M F F M F M

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 2θ

Fig. 9. XRD patterns of MT and SG powders and geopolymer specimens prepared at 10 M NaOH and 0, 25, 50% SG, 15 M NaOH and 50% SG, and cured at 60 °C for 7 days (S quartz, G gypsum, A albite, S sanidine, M magnetite and F fayalite)

207

5 M 30 10 M 25 15 M

15 UCS(MPa)

0 0.0 0.5 1.0 1.5 SS/NaOH ratio

Fig. 10. UCS versus SS/NaOH ratio for geopolymer specimens prepared at NaOH concentrations of 5, 10 and 15 M, and cured at 60oC for 7 days; circles in red are for specimens that experienced flash setting

208

0.7 5 M 0.6 10 M

0.5 15 M

0.4

0.3

NaOH/Total water NaOH/Total 0.2

0.1

0.0 0.0 0.5 1.0 1.5 SS/NaOH ratio

Fig. 11. Solid NaOH/total water ratio versus SS/NaOH ratio for alkaline solution at different NaOH concentrations

209

GP 4 SG GP

3 SG MT MT 2

100 µm 10 µm 1

60000 60000 Si 50000 Si/(Al+Fe) = 1.53 50000 Si Si/(Al+Fe) = 0.76 40000 Na/Al = 0.17 40000 Na/Al = 2.02 O 30000 30000 Fe O Al K 20000 20000 Al Mg Fe Na

10000 Ca 10000 S Ca Intensity (cps) Intensity C Na S Ti (cps) Intensity C K Ti 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 KeV KeV

b) 2

GP MT SG 3 MT 1

GP SG 100 µm 30 µm

50000 60000 Si Si Si/(Al+Fe) = 0.68 40000 50000 Si/(Al+Fe) = 1.02 Na/Al = 2.45 40000 Na/Al = 3.06 30000 Fe 30000 O Fe 20000 O 20000 NaAl S Na Al S Ca

10000 Ca 10000 Intensity (cps) Intensity C K (cps) Intensity C K 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 KeV KeV

Fig. 12. SEM micrographs of geopolymer specimen prepared at 50% SG, 10 M NaOH, a) SS/NaOH = 0.0 and b) SS/NaOH = 1.0 and cured at 60 °C for 7 days. The left EDS spectra are for points 1 and 2 (unreacted) and the right EDS spectra are for points 3 and 4 (reacted). MT: mine tailing particle, SG: slag particle, GP geopolymer

210

MT Na SG GP

GP MT 20 µm a) b)

Al Si

c) d)

Fe Na, Al, Si and Fe

e) f)

Fig. 13. EDS mapping of geopolymer specimens prepared at 50% SG, 10 M NaOH, SS/NaOH = 0.0, and cured at 60 °C for 7 days: a) SEM micrograph, b) sodium, Na, c) aluminum, Al, d) silicon, Si, e) iron, Fe, and f) overlapped elements. MT: mine tailing particle, SG: slag particle, GP geopolymer

211

GP MT a) SG 20 µm b)

Al Si

c) d)

Fe Na, Al, Si and Fe

e) f)

Fig. 14. EDS mapping of geopolymer specimens prepared at 50% SG, 10 M NaOH, SS/NaOH = 1.0, and cured at 60 °C for 7 days: a) SEM micrograph, b) sodium, Na, c) aluminum, Al, d) silicon, Si, e) iron, Fe, and f) overlapped elements. MT: mine tailing particle, SG: slag particle, GP geopolymer 212

100 µm 30 µm 2 SG

4 1 MT GP GP

MT 3

14000 75000 Si Fe Si 12000 Si/(Al+Fe) = 0.81 Si/(Al+Fe) = 0.46 60000 10000 O Na/Al = 4.39 8000 Na/Al = 4.10 45000 O 6000 30000 Fe 4000 Na Ca Na Ca

Al S K 15000 Al K Intensity (cps) Intensity 2000 C (cps) Intensity C S 0 0 0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7 KeV KeV

Fig. 15. SEM micrographs of geopolymer specimen prepared with 50% SG, 15 M NaOH, SS/NaOH = 1.0, and cured at 60 °C for 7 days. The left EDS spectra are for points 1 and 2 (unreacted) and the right EDS spectra are for points 3 and 4 (right, reacted) in a). MT: mine tailing particle, SG: slag particle, GP geopolymer

213

MT GP

20 µm a)

Al Si

Fe Na, Al, Si and Fe

Fig. 16. EDS mapping of geopolymer specimens prepared at 50% SG, 15 M NaOH, SS/NaOH = 1.0, and cured at 60 °C for 7 days: a) SEM micrograph, b) sodium, Na, c) aluminum, Al, d) silicon, Si, e) iron, Fe, and f) overlapped elements. MT: mine tailing particle, SG: slag particle, GP geopolymer

214

APPENDIX E

PRODUCTION OF GEOPOLYMER CONCRETE FROM COPPER MINE TAILINGS

AND LOW-CALCIUM SLAG

Lino Manjarrez1, Arash Nikvar-Hassani1 and Lianyang Zhang, Ph.D., P.E., M.ASCE2

1Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, Arizona 85721, USA

2 Department of Civil and Architectural Engineering and Mechanics, University of Arizona,

Tucson, AZ 85721, USA (Corresponding author). Email: [email protected]

215

ABSTRACT

This paper investigates the production of geopolymer concrete from copper mine tailings (MT) and low-calcium slag (SG). 50% MT and 50% SG, both passing sieve No. 200, were used together as the binder material, and MT and SG, with particle size larger than sieve No. 200 and at a designed combination, were used as the aggregates. Combined sodium hydroxide (NaOH) and sodium silicate (SS) were used as the alkaline activator. A curing temperature of 60°C was used to prepare the geopolymer concrete specimens. The effect of different factors including water to binder (w/b) ratio, NaOH concentration, SS/NaOH ratio, cement/aggregate ratio, curing time, curing conditions and water exposure on the compressive strength of geopolymer concrete were experimentally studied. The results showed that w/b = 0.26, 10 M NaOH concentration, SS/NaOH

= 1.0 and cement/aggregate = 0.19 are the optimum values. This research promotes the utilization of wastes and contributes to the reduction of CO2 emissions.

216

1. Introduction Concrete is by far the most widely construction material used in the world. Due to its importance in civil construction, its use has been standardized by many associations including the American

Concrete Institute (ACI) and the American Society for Testing and Materials (ASTM). Concrete is composed of coarse and fine aggregates, cement and water. Gravel and sand are the most used coarse and fine aggregates, respectively, and ordinary Portland cement (OPC) is used as binder.

Because of its high demand, a shortage of natural materials accompanied with environmental problems have called the attention of the scientific community.

According to Sonawane & Pimplikar [1] more than 26 billion tons of aggregates have been used only in 2012. It is expected that the demand of aggregates will increase twice due to its rate of construction in the next two or three decades [2,3], which might increase the shortage of natural aggregates, and in turn will increase the hauling distances and the final cost of the project. In addition, the manufacturing of OPC produces about 7% of all carbon dioxide emissions generated worldwide [4,5]. Even though OPC has shown well performance as cementing material, its production is accompanied with different negative environmental issues such as high energy consumptions and greenhouse gas emissions [6]. A solution for these problems is the utilization of wastes, as partial or complete replacement in concrete mixtures, that could mostly cover the needs of conventional concrete. The preferred wastes are those from the family of large-volume wastes, such as, copper mine tailings and slag.

On the other hand, tremendous amount of wastes such as mine tailings (MT) and slag are generated by the mining industry [7] and metal smelter operations every year [8]. For example, the disposal of MT is not only expensive but has also resulted in various ecological and environmental problems such as the occupation of large areas of land, generation of windblown dust,

217 contamination of surface and underground water, and failure of tailing dams [9–11]. Similarly, the disposal of SG is accompanied with various problems such as the occupation of land on the smelter site, in fact, SG has been disposed under the sea water in several countries [12]. According to the

United States Geological Survey (USGS), in 2016 around 15 to 20 million tons of iron and steel slags were generated in the United States and 300 to 360 million tons worldwide [8]. Therefore, the re-use of these wastes as potential source materials to produce sustainable concrete has attracted the attention.

Extensive research has been conducted to investigate the effect of MT and SG on the properties of conventional concrete [13–17]. For example, Onuaguluchi & Eren [13,14] studied the use of copper MT as substitution of OPC in conventional concrete. The mechanical and durability properties were investigated. The MT was incorporated from 0% to 15% by OPC total solid weight. The water to cement ratios were 0.50 and 0.57. The results showed that with the addition of MT the UCS decreased. The UCS specimens at 0% MT and w/c=0.50 was 23.0, 36.0 and 44.1

MPa after 7, 28 and 90 days’ curing, whereas in specimens at 15% MT the UCS was 19.7, 29.5 and 38.2 MPa, respectively. The highest UCS of 31 and 36 MPa were found after 28 and 90 days curing and at w/c=0.57. The durability properties showed that concrete with up to 15% MT can be used in aggressive chemical environments. The authors concluded that the use of copper MT in concrete mixtures as partial substitution of OPC is promising. Gupta [60] investigated the utilization of copper MT as substitution of sand in the production of durable concrete. The MT was added in the range of 0-80% by sand solid weight. Crushed stone was used as coarse aggregates and OPC as binder. The water to cement ratio was 0.48 for all the mixtures. The results showed that the UCS increased with MT content up to 30%, then a slightly UCS decrease was observed. The specimens without MT reached UCS of 26.0, 30.9, 32 and 35 MPa, while the

218 specimens with 30% MT reached 26.0, 31.0, 32.7 and 35.5 MPa, after 7, 28, 56 and 90 days, respectively. Boakye et al. [17] investigated the use of pulverized copper slag as partial replacement of OPC in concrete mixtures. The SG was incorporated to the mixture at 0, 2.5, 5, 10 and 15% by total solid weight of OPC. Regular aggregates were used in the preparation of the mixtures. Also, 1.5% of hydrated lime was used for the pozzolanic reaction activation of the SG.

A w/c ratio of 0.5 was used in the mixtures. The results showed that the UCS decreased with SG content. Control specimens (0% SG), were prepared and after 90 days curing a UCS of about 50

MPa was achieved, while specimens with 15% SG reached a UCS of about 42 MPa. The authors stated that the UCS reduction might be because of the high glassy content of the SG (about 99.3%).

As shown in these studies, the substitution of conventional aggregates (or OPC) by copper MT and copper SG is on small quantities (less than 30%), however, there are still the drawbacks of the use of tremendous amount of aggregates and OPC manufacturing. Alternative techniques to increase the amount of aggregate substitution and the complete replacement of OPC have been studied recently.

Research has also been conducted to investigate the utilization of different types of waste, as aggregates or binders materials, based on geopolymerization technology [18–23]. The research of geopolymerization on concrete have been focused mainly on the partial replacement of aggregates and high-calcium materials such as fly ash or ground granulated blast furnace slag (GGBFS) as binder. Bakharev et al. [20] investigated the properties of alkali-activated slag concrete using

GGBFS as binder. Conventional aggregates were used and NaOH and SS were used as the alkaline activators. The alkaline solution was prepared at 4% NaOH and SS/NaOH=0.75, at a water to binder ratio of 0.5. The specimens were cured in a water bath at 70°C for 6 h and then the heating element was turned off and the water temperature decreased to room temperature. The UCS was

219 obtained after 1, 7 and 31 days of curing. The results showed that the UCS was 16, 36 and 46 MPa after 1, 7 and 31 days, respectively. The results are in agreement with concrete specimens prepared with OPC grade 40 as binder, which reported UCS of 6.6, 35 and 45 MPa, after the same curing days, highlighting the rapid strength development of alkali activated slag concrete. Kong &

Sanjayan [19] studied the effect of elevated temperatures on geopolymer concrete made with SG coarse aggregates and fly ash as binder. The alkaline activators were potassium hydroxide (KOH) and SS. The concentration of KOH solution was 7 M and SS/KOH=2.0. All the mixtures were prepared at a solid to liquid ratio of 3. One set of specimens was exposed to elevated temperatures up to 1200°C and the other was unexposed. Results showed that the UCS of unexposed specimens is higher than those exposed to elevated temperature, 70 to 26.7 MPa, respectively. Hadi et al. [18] investigated the use of geopolymer concrete with GGBFS. They analyzed the effect of binder content, alkaline activator to binder content ratio, SS/NaOH ratio and NaOH concentration (14 M) on the UCS and setting time of geopolymer concrete. They observed that the alkaline activator to binder content ratio is the most significant parameter that influences geopolymer concrete. An alkaline activator to binder content ratio of 0.35 led to the highest UCS of 56.1 MPa after 7 days cured in ambient temperature.

To the knowledge of the authors, research on geopolymer concrete produced with copper MT and low-calcium copper SG has not been carried out yet. Recycling and utilization of both MT and SG are important because they are two major type of wasted produced in large volumes. Therefore, this paper studies the production of geopolymer concrete from copper MT and low-calcium SG.

The MT/SG are used as fine aggregates and binder, whereas the coarser SG is used as coarse aggregates. The research systematically investigates the effects of synthesis parameters such as water to binder (w/b) ratio, NaOH concentration, SS/NaOH ratio, cement/aggregate ratio and

220 curing time on geopolymer concrete specimens. The research promotes the reuse of wastes through geopolymerization.

2. Experimental Study 2.1. Materials The materials used in this investigation were copper mine tailings (MT), low-calcium copper slag

(SG), reagent grade 98% sodium hydroxide (NaOH), sodium silicate solution (SS) and distilled water. The MT were supplied by a major company in Tucson, Arizona. The MT were received in the form of damp solids. After drying, the lumped MT were pulverized and screened through No.

40 sieve so that no particles are lumped together, and all MT are utilized. The SG was supplied by a local copper smelter in Arizona. The SG was received in the form of aggregates with particle size ranging from 1 ½ in to about 75 µm. The SG particles were sieved and separated by grain size to meet the grain size distribution (GSD) requirements for coarse and fine aggregates in concrete according to the AASHTO M-43 [25]. The particle size distribution was determined following

ASTM D6913-04 [26] and ASTM D422-63 [27]. Since MT has about 30% of fine aggregate size

(particle size larger than sieve No. 200 and lower than sieve No. 4), it was mixed with SG particles passing sieve No. 4 and retained in sieve No. 200 to use 100% of MT. In Fig. 1 is shown the GSD of MT, SG powder, SG coarse aggregates, SG fine aggregates and SG+MT fine aggregates, also the upper and lower boundaries for coarse and fine aggregates are plotted according to [25]. As observed, the SG coarse aggregates curve meets the AASHTO M-43 requirements for coarse aggregates. However, the SG+MT fine aggregates curve does not meet the requirements for fine aggregates in meshes No. 50 and No. 100, but it was accepted as correct for simplicity. Fig. 2 shows the coarse and fine SG aggregates. Table 1 shows the chemical composition of the MT and

SG based on X-ray fluorescence (XRF) analysis, where silica and alumina are the main components of MT, whereas for the SG are silica and iron. Fig. 3 shows the SEM micrographs of 221 the MT and SG powders. In general, the MT particles are larger than SG particles. It is observed that finer MT particles are attached to the surface of coarser ones. The same is observed in SG particles, but they are finer in size. The specific gravity of MT and SG is 2.83 and 3.80, respectively, which means that the SG is heavier than the MT. Fig. 4 shows the XRD patterns of the MT and SG powders. The MT are mainly crystalline materials such as quartz (SiO2), gypsum

(CaSO4), albite (NaAlSi3O8) and sanidine [K(AlSi3O8)]. A weak amorphous phase, centered at 28° is also clear. The SG is mainly composed of two crystalline phases; magnetite (Fe2O3) and fayalite

(Fe2SiO4). A less ordered molecular structure is observed in magnetite peaks compared to fayalite related to the less sharp peaks in magnetite.

The alkaline solution consisted of NaOH, SS and distilled water. First, the NaOH flakes were dissolved in distilled water to prepare solutions at different NaOH concentrations of 7.5, 10, 13 and 15 M. After the NaOH solution cooled down to room temperature, SS was incorporated to the

NaOH solution at different SS/NaOH ratios of 0.5, 1.0, 1.5 and 2.0.

2.2. Preparation of Geopolymer Concrete Specimens To prepare geopolymer concrete specimens, first coarse and fine SG aggregates were weighted and mixed, respectively, to meet the requirements described in AASHTO M-43 [25] for concrete mixtures. Then, the required amount of MT was weighted (including powder), as well as, the SG powder. Thereafter, the MT and SG powder were mixed in a mechanical mixer for 5 min to ensure homogeneity. Then, the mixture was added to the SG fine aggregates and hand mixing was performed for 5 more min., or until a homogeneity was observed. After this, the SG coarse aggregates were incorporated to the mixture and hand mixing continued for 5 more min. Finally, the alkaline solution already prepared was added and hand mixing continued for 5 more minutes.

Right after mixing, the fresh geopolymer concrete was cast into the cylindrical molds of 6 in height

222 and 3 in diameter in two layers. For compaction of the specimens, each layer was given 25 manual strokes using a rodding bar of 10 mm diameter. The specimens were then placed in the oven at 60

°C for 24 h. Thereafter, the specimens were demolded, sealed in plastic bags and placed back in the oven until the day of testing. The combination of variables considered in this study are summarized in Table 2. Table 3 also summarizes the detail ingredients of geopolymer concrete specimens.

3. Results 3.1. Water to Binder Ratio The water to binder (w/b) ratio is one of the most important factors on the compressive strength of geopolymer concrete. The water present in the NaOH and SS solutions was counted in the water weight, and NaOH, the solid present in SS solution, MT and SG powders were counted in the binder weight. Fig. 5 shows the variation of UCS with w/b ratio of specimens produced at 10 M

NaOH, SS/NaOH = 1.0 and cured at 60 °C for 7 days. The binder is composed of 50% MT and

50% SG, both passing sieve No. 200. The SG is used as coarse aggregates and, SG and MT are used as fine aggregates; MT occupies the finer part of the fine aggregates (particle size between

297 µm and 75 µm). It is seen that the UCS slightly increases and then decreases with w/b ratio.

When the w/b ratio was increased up to 0.26, a maximum UCS of 9.27 MPa is obtained. This w/b ratio was defined as the optimum and the workability of the corresponding concrete mixture was used as the reference for the preparation of the specimens presented in the next subsections. The

UCS after w/b = 0.26 decreases. According to several authors [22,28,29], excessive amount of water during the geopolymer reaction produces a porous microstructure which in turn decreases the compressive strength.

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3.2. NaOH Concentration Fig. 6 shows the variation of UCS with NaOH concentration of specimens produced at w/b = 0.26,

SS/NaOH = 1.0 and cured at 60 °C for 7 days. It is seen that the UCS increases with NaOH concentration up to 10 M, and then decreases. This behavior can be related to the excess of Na present in the geopolymer reaction. At 7.5 M, 10 M and 13 M NaOH the w/b ratio was 0.26, however, at 15 M NaOH the amount of solution used was increased, up to get a w/b = 0.37, because the mixture was too dry to make the specimens. Right after mixing, the mixture experienced flash setting, and the specimen compaction was difficult to perform. After demolding, the specimens were poorly compacted and large voids were observed in the specimen surface as shown in Fig. 7; for comparison, specimens at 10 M NaOH are also shown.

The flash setting effect in MT/SG geopolymer binder was studied by Manjarrez et al. (2019 – under review). The geopolymer paste prepared with 50% SG and 50% MT at 10 M NaOH and

SS/NaOH = 1.0 also experienced flash setting. The authors stated that the possible causes of flash setting are the high NaOH concentration and interaction of NaOH and SS.

3.3. SS/NaOH Ratio Fig. 8 shows the variation of UCS with SS/NaOH ratio of specimens synthesized at w/b = 0.26,

10 M NaOH and cured at 60 °C for 7 days. There is a significant UCS increase from SS/NaOH ratio of 0.5 to 1.0. After SS/NaOH = 1.0, the UCS continued increasing, however at a slower rate.

These results agree well with several authors [21,24,30] that showed that the incorporation of soluble SS improves the geopolymerization reaction and the UCS of the specimens.

A SS/NaOH = 1.0 was selected to produce further geopolymer concrete specimens. It is clearly seen that the UCS at SS/NaOH = 1.0 is lower than at 2.0, however, the difference is less than 1

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MPa. By selecting a SS/NaOH = 1.0, SS solution is saved which will reduce the cost of the final geopolymer product.

3.4. Cement/Aggregate Ratio Fig. 9 shows the variation of UCS with the cement/aggregate (cem/agg) ratio of specimens produced at w/b = 0.26, 10 M NaOH, SS/NaOH = 1.0 and cured at 60 °C for 7 days. It is seen that the UCS decreases with cem/agg ratio. The maximum UCS of 10.33 MPa was obtained at a cem/agg = 0.19.

3.5. Curing Time Fig. 10 shows the variation of UCS with curing time at different curing conditions of specimens produced at w/b = 0.26, 10 M NaOH, SS/NaOH = 1.0. One set of specimens was cured in the oven at 60 °C and a different set was cured outdoor at ambient conditions. For the later set, the surface temperature was recorded at random days for a period of 12 hours (starting around 9:00 am) with a thermocouple data logger. It is clearly seen in Fig. 10 that the specimens cured in the oven reached higher UCS compared to those cured outdoor. After 7, 14 and 28 days, the specimen cured in the oven reached 9.3, 12.0 and 12.2 MPa, respectively, whereas the specimens cured outdoor reached 3.3, 7.9 and 9.1 MPa, respectively.

The temperature in the oven was always constant (60 °C) and it varied outdoor. Fig. 11 shows the recorded temperature at random days during the 28 days curing. A peak of 70 °C was reached on

07/03/2018, although for a short period of time. Temperatures about 60 °C were recorded for an average of 5 hours on a regular day.

4. Conclusions Based on the experimental study of the geopolymer concrete produced from MT and SG, the following conclusions can be obtained.

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1. The UCS increased with w/b ratio up to a certain level and then decreases. An optimum

w/b = 0.26 was found for the studied geopolymer concrete.

2. The UCS increased with NaOH concentration up to 10 M NaOH and then decreases. At 15

M NaOH, the geopolymer mixture experienced flash setting and it is believed is due to the

interaction between NaOH and SS, and the high NaOH concentration.

3. A SS/NaOH = 1.0 was considered as optimum although the UCS continued to increase

with SS/NaOH ratios. However, the UCS increment was below 1 MPa.

4. Specimens cured in oven developed higher UCS compared to those cured outdoor. This is

because the temperature in the oven is constant, while outdoor fluctuates along the day.

Temperatures above 45 °C is reached on the surface of outdoor constructions for about 7

hours on a regular day in Arizona.

5. Even though the geopolymer concrete specimens did not reach compressive strength higher

than 20 MPa, after curing for 28 days as required for conventional pavement concrete. This

study represents a step forward the state of the art of the production of geopolymer concrete

from copper MT and low-calcium SG.

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Table 1. Chemical composition of MT and SG based on XRF analysis

Chemical compound MT (%) SG (%)

Na2O 3.02 N/A

MgO 1.78 N/A

Al2O3 14.10 2.43

SiO2 55.90 32.90

P2O5 0.19 N/A

SO2 2.22 N/A

SO3 N/A 1.56

K2O 3.89 N/A

CaO 2.27 2.13

TiO2 0.49 N/A

MnO 0.06 N/A

Fe2O3 3.07 37.50

Fe3O4 N/A 5.48

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Table 2. Summary of specimen characteristics

NaOH Specimen w/binder SS/NaO Cement/Aggre Study objective Curing time concentration ID ratio H ratio gate ratio (M) 0.22-1.0-7- 0.22 1.0 7 0.22 10 0.22-Mix1 0.26-1.0-7- w/b ratio 0.26 1.0 7 0.22 10 0.22-Mix1 0.30-1.0-7- 0.30 1.0 7 0.22 10 0.22-Mix1 0.26-0.5-7- 0.26 0.5 7 0.22 10 0.22-Mix1 0.26-1.5-7- SS/NaOH ratio 0.26 1.5 7 0.22 10 0.22-Mix1 0.26-2.0-7- 0.26 2.0 7 0.22 10 0.22-Mix1 0.26-1.0-7- 0.26 1.0 7 0.19 10 0.19-Mix2 Cement/Aggregate 0.26-1.0-7- 0.26 1.0 7 0.25 10 ratio 0.25-Mix3 0.26-1.0-7- 0.26 1.0 7 0.29 10 0.29-Mix4 0.26-1.0- 14-0.22- 0.26 1.0 14 0.22 10 Curing time (oven Mix1 and ambient 0.26-1.0- temperature) 28-0.22- 0.26 1.0 28 0.22 10 Mix1 0.26-1.0-7- 0.26 1.0 7 0.22 7.5 0.22-Mix1 0.26-1.0-7- NaOH concentration 0.26 1.0 7 0.22 13 0.22-Mix1 0.26-1.0-7- 0.26 1.0 7 0.22 15 0.22-Mix1 Water exposure

time (day) 0.26-1.0-7- 0.26 1.0 0 0.22 10 0.22-Mix1 0.26-1.0-7- Water exposure 0.26 1.0 7 0.22 10 0.22-Mix1 0.26-1.0-7- 0.26 1.0 14 0.22 10 0.22-Mix1 0.26-1.0-7- 0.26 1.0 28 0.22 10 0.22-Mix1

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Table 3. Ingredients of geopolymer concrete specimens

Fine Coarse Sample SS/NaOH Water NaOH MT SG w/b SS (gr) Agg. Agg. ID ratio (gr) (gr) (gr) (gr) (gr) (gr) 0.22-1.0- 7-0.22- 0.22 126.8 50.7 177.5 470 470 1286 3000 Mix1 0.26-1.0- 7-0.22- 1.0 0.26 150.7 60.3 211.0 470 470 1286 3000 Mix1 0.30-1.0- 7-0.22- 0.30 176.1 70.4 246.6 470 470 1286 3000 Mix1 0.26-0.5- 7-0.22- 0.5 197.7 79.0 138.3 470 470 1286 3000 Mix1 0.26-1.5- 7-0.22- 1.5 0.26 118.6 47.4 249.0 470 470 1286 3000 Mix1 0.26-2.0- 7-0.22- 2.0 98.8 39.5 276.7 470 470 1286 3000 Mix1 0.26-1.0- 7-0.19- 1.0 0.26 138.2 55.2 193.4 430 430 1371 3200 Mix2 0.26-1.0- 7-0.25- 1.0 0.26 160.7 64.2 224.9 500 500 1200 2800 Mix3 0.26-1.0- 7-0.29- 1.0 0.26 170.0 68.0 238.0 530 530 1114.3 2600.0 Mix4 0.22-1.0- 7-0.22- 1.0 0.26 153.8 46.2 200.0 470 470 1286 3000 Mix1 (7.5 M) 0.22-1.0- 7-0.22- 1.0 0.26 148.0 77.0 225.0 470 470 1286 3000 Mix1 (13 M) 0.22-1.0- 7-0.22- 1.0 0.26 146.9 88.1 235.0 470 470 1286 3000 Mix1 (15 M)

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100 90 SG coarse Agg. 80 SG fine Agg.

70 SG+MT Fine Agg.

60 SG powder

50 MT 40 30 Percent Percent passing(%) 20 10 0 0.1 1 10 100 1000 10000 100000 Particle size (µm)

Fig. 1. Particle size distribution of MT and SG

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a) b) Fig. 2. Slag aggregates: a) coarse and b) fine

235 a) b)

Fig. 3. SEM micrographs of MT and SG powders: a) MT and b) SG

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MT Powder P A G S A P P S S S S S S

F F F M SG Powder M F F F F F M F M

10 14 18 22 26 30 34 38 42 46 50 54 58 62 66 70 2θ

Fig. 4. XRD pattern of MT and SG powders (S quartz, G gypsum, A albite, S sanidine, M magnetite and F fayalite)

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4 UCS(MPa)

0 0.20 0.22 0.24 0.26 0.28 0.30 0.32 w/b ratio

Fig. 5. UCS versus water to binder (w/b) ratio for geopolymer concrete specimens prepared at 10

M NaOH, SS/NaOH=1.0, and cured at 60 °C for 7 days

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4 UCS(MPa)

0 6 8 10 12 14 16 NaOH concentration (M)

Fig. 6. UCS vs NaOH concentration for geopolymer concrete specimens prepared at w/b=0.26,

SS/NaOH=1.0, and cured at 60 °C for 7 days

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a) b)

Fig. 7. Geopolymer concrete specimens prepared at SS/NaOH=1.0 and cured at 60 °C for 7 days: a) 10 M NaOH and b) 15 M NaOH

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6 UCS(MPa) 4

0 0.5 1.0 1.5 2.0 SS/NaOH ratio

Fig. 8. UCS vs SS/NaOH ratios for geopolymer concrete specimens prepared at w/b=0.26, 10 M

NaOH, and cured at 60 °C for 7 days

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6 UCS(MPa) 4

0 0.18 0.20 0.22 0.24 0.26 0.28 0.30 cement/aggregate ratio

Fig. 9. UCS vs cement/aggregate ratios for geopolymer concrete specimens prepared at w/b=0.26,

10 M NaOH, SS/NaOH = 1.0 and cured at 60 °C for 7 days

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12 Ambient

10 Oven

6 UCS(MPa)

0 0 5 10 15 20 25 30 Time (days)

Fig. 10. Effect of curing time and curing conditions on the UCS of geopolymer concrete

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40 Temperature 30 7/3/2018 7/2/2018 7/5/2018 7/6/2018 20 7/13/2018 7/16/2018 7/27/2018 10 9:00 12:36 16:12 19:48 23:24 Time

Fig. 11. Measured outdoor temperature on the surface of geopolymer concrete cylinders

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