Chemistry and Corrosion Effects of Steels Embedded in High-Density

Home , Calcium sulfide

CHEMISTRY AND CORROSION MECHANISMS OF STEELS EMBEDDED IN HIGH-DENSITY SLAG CONCRETE FOR STORAGE OF USED NUCLEAR FUEL

Parthiban Nadarajah

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Department of Chemical Engineering and Applied Chemistry University of Toronto

ABSTRACT

Chemistry and Corrosion Mechanisms of Steels Embedded in High-Density Slag Concrete for Storage of Used Nuclear Fuel

Parthiban Nadarajah

Master of Applied Science

Department of Chemical Engineering and Applied Chemistry University of Toronto

2011

The chemistry and corrosion mechanisms associated with reduced sulfur compounds such as calcium sulfide, present in ground granulated blast-furnace slag (GGBFS), have been studied in high-density concrete, mortar and simulated pore-water environments. The high-density concrete and mortar samples were produced to replicate the high-density GGBFS concrete, in the dry storage containers (DSCs), used for radiation shielding from used nuclear fuel. Electrochemical measurements on embedded steel electrodes in high-density GGBFS concrete and mortar samples, showed that sulfide is capable of consuming oxygen to create a stable, reducing environment, though not in all cases, and the high-frequency electrolyte resistance increases with hydration time.

Ion chromatography on simulated pore-water environments determined that thiosulfate is quite kinetically stable as a sulfide oxidation product and magnetite is capable of oxidizing sulfide.

Microscopy has also been used to provide visual evidence of GGBFS hydration and elemental quantification of the hydrating microstructure in different environments.

ACKNOWLEDGEMENTS

My appreciation goes to my supervisor Professor Roger C. Newman who has allowed me to contribute to this field of study and for his excellent guidance throughout the research project. I would like to thank Dr. Anatolie G. Carcea, Nick Senior and the Corrosion and Advanced

Materials research group for their assistance throughout the course of my research.

I would like to thank John Balinski, Bruce Cornelius and Barry Shenton from AMEC Earth and

Environmental and Jim Sato from the Ontario Power Generation, for their generous collaboration and efforts, as the contributing industry partners in the research project. I would like to acknowledge financial support from the Department of Chemical Engineering and Applied

Chemistry at the University of Toronto, Natural Sciences and Engineering Research Council

(NSERC) Canada and the University Network of Excellence in Nuclear Engineering (UNENE).

Finally, I would like to thank my family and friends for their continuous inspiration, motivation and support during the course of my Master’s degree studies.

iii

Table of Contents

Abstract ...... ii Acknowledgements ...... iii Table of Contents ...... iv List of Tables ...... vii List of Figures ...... viii List of Appendices ...... xi CHAPTER 1 - INTRODUCTION ...... 1 1.1 Objectives and Motivations ...... 1 CHAPTER 2 - BACKGROUND AND LITERATURE REVIEW ...... 3 2.1 Nuclear Waste Management ...... 3 2.1.1 Dry Storage Container Processes and Challenges ...... 3 2.1.2 Radiation and Shielding ...... 5 2.2 High-Density Concrete and Mortar ...... 6 2.2.1 Ordinary Portland Cement (OPC) ...... 6 2.2.2 Ground Granulated Blast-Furnace Slag (GGBFS) ...... 8 2.2.3 Coarse and Fine Aggregates ...... 10 2.2.3.1 Aggregate Properties and Test Methods ...... 11 2.2.4 Concrete and Mortar Microstructure ...... 13 2.2.4.1 Hydration Products ...... 14 2.2.4.2 Aggregates and Hydrated Cement Paste ...... 17 2.2.4.3 Pore Solution ...... 18 2.2.4.4 Kinetics of Oxygen Diffusion ...... 19 2.2.5 Admixtures ...... 20 2.3 Steel Corrosion in Concrete ...... 21 2.3.1 Corrosion Mechanisms ...... 22 2.3.1.1 Carbonation ...... 24 2.3.1.2 Chloride Attack ...... 25 2.3.2 Measurement Techniques ...... 27 2.3.3 Reference Electrodes ...... 30

CHAPTER 3 - EXPERIMENTAL DETAILS ...... 31 3.1 High-Density Concrete and Mortar Sample Preparation ...... 31 3.1.1 Cementitious Materials Specifications ...... 31 3.1.2 Coarse and Fine Aggregate Specifications and Testing ...... 32 3.1.3 Admixtures ...... 34 3.1.4 Mix Designs ...... 34 3.1.5 Mixing and Casting Procedures ...... 35 3.1.6 Concrete to Mortar Procedure and Theory ...... 37 3.1.7 High-Density Concrete and Mortar Sample List Summary ...... 38 3.2 Electrochemistry Experiments ...... 39 3.2.1 Electrochemical Cell ...... 39 3.2.2 Reference Electrode Preparation ...... 40 3.2.3 Corrosion Potential Measurements ...... 40 3.2.4 Electrochemical Impedance Spectroscopy Measurements ...... 40 3.2.5 Cyclic Voltammetry Measurements ...... 41 3.2.6 Coarse Aggregate Resistance ...... 41 3.3 Ion Chromatography Experiments ...... 42 3.3.1 Ion Chromatography Column ...... 42 3.3.2 GGBFS in Water ...... 43 3.3.3 GGBFS in Basic Solutions...... 44 3.3.4 Aggregate and GGBFS in Basic Solutions ...... 44 3.3.5 Ion Chromatography Sample List Summary ...... 45 3.4 Microscopy Experiments ...... 46 3.4.1 Environmental Scanning Electron Microscope (ESEM) ...... 46 3.4.2 Microscopy Sample List and Mounting Procedures ...... 47 3.4.3 Grinding and Polishing Procedures ...... 48 CHAPTER 4 - RESULTS AND DISCUSSION ...... 49 4.1 Electrochemical Analysis ...... 49 4.1.1 Embeddable Reference Electrode Measurements ...... 49 4.1.2 Open Circuit Potential Analysis...... 50 4.1.3 Electrochemical Impedance Spectroscopy Analysis ...... 53

4.1.4 Cyclic Voltammetry Analysis ...... 55 4.1.5 Coarse Aggregate Resistance Measurement ...... 56 4.2 Ion Chromatography Analysis ...... 56 4.2.1 GGBFS in Water Results ...... 57 4.2.2 GGBFS in Basic Solutions Results ...... 58 4.2.3 Aggregate and GGBFS in Basic Solutions Results ...... 60 4.3 Microscopy and Analysis ...... 61 4.3.1 Dry GGBFS ...... 62 4.3.2 GGBFS in Water ...... 63 4.3.3 GGBFS in Basic Solutions...... 64 4.3.4 OPC and GGBFS Paste ...... 65 4.3.5 High-Density Concrete ...... 66 4.3.6 High-Density Mortar ...... 67 CHAPTER 5 - SUMMARY AND CONCLUSIONS ...... 69 CHAPTER 6 – FUTURE WORK ...... 71 REFERENCES ...... 72

LIST OF TABLES

Table 1: Chemical composition of Type GU OPC in Canada ...... 7 Table 2: Chemical composition of GGBFS in Canada ...... 9 Table 3: Chemical composition and physical analysis of OPC used in experimental work...... 32 Table 4: Chemical composition and physical analysis of GGBFS used in experimental work ....32 Table 5: Properties of coarse and fine aggregates ...... 34 Table 6: Mix design results for high-density GGBFS concrete and mortar samples ...... 37 Table 7: Casting results for high-density GGBFS concrete and mortar samples ...... 37 Table 8: High-density concrete and mortar sample list summary ...... 38 Table 9: Ion chromatography sample list summary ...... 45 Table 10: Microscopy sample list summary ...... 48 Table A1: Specific gravity and absorption calculations ...... 78 Table A2: Surface moisture content calculations ...... 79 Table A3: Mix design for 50% OPC-50% GGBFS mortar with fine hematite sand ...... 82 Table A4: Mix design for 100% OPC mortar with fine hematite sand ...... 83 Table A5: Mix design for 50% OPC-50% GGBFS mortar with fine silica sand ...... 84 Table A6: Mix design for 100% OPC mortar with fine silica sand ...... 85 Table A7: Mix design for 50% OPC-50% GGBFS concrete with iron oxide aggregates ...... 86 Table A8: Mix design for 100% OPC concrete with iron oxide aggregates ...... 87 Table A9: Concrete to mortar data and calculations ...... 91 Table A10: Sulfur mass balance calculations for ion chromatography analysis ...... 97

vii

LIST OF FIGURES

Figure 1: Row of DSCs at the Pickering waste management facility ...... 3 Figure 2: Graphic representation of OPC grain cross-section ...... 8 Figure 3: Moisture conditions of aggregates ...... 12 Figure 4: OPC hydration products ...... 16 Figure 5: Early age, rosette-shaped AFm phases in GGBFS HCP ...... 16 Figure 6: Interfacial transition zone between GGBFS HCP and crushed basalt rock aggregate ...17 Figure 7: Three-stage corrosion damage model for reinforced concrete ...... 21 Figure 8: Schematic flowchart of the corrosion of steel in concrete ...... 23 Figure 9: Corrosion of steel in the presence of chloride ions ...... 26 Figure 10: Randles circuit model ...... 29 Figure 11: Warburg impedance circuit model ...... 29 Figure 12: Coarse aggregate grading chart ...... 33 Figure 13: Fine aggregate grading chart ...... 33 Figure 14: Schematic of high-density GGBFS concrete and mortar samples ...... 36 Figure 15: Cross-sections of concrete samples ((A) 50% OPC-50% GGBFS (B) 100% OPC) ....36

Figure 16: Experimental MnO2 reference electrode potentials versus time...... 49 Figure 17: Corrosion potentials of embedded carbon and stainless steels in high-density GGBFS concrete samples with iron oxide aggregates ...... 51 Figure 18: Corrosion potentials of embedded carbon and stainless steels in high-density GGBFS mortar samples with iron oxide aggregate ...... 51 Figure 19: High frequency electrolyte resistance of embedded carbon and stainless steels in high- density GGBFS concrete samples with iron oxide aggregates ...... 54 Figure 20: High frequency electrolyte resistance of embedded carbon and stainless steels in high- density GGBFS mortar samples with iron oxide aggregate...... 54 Figure 21: Cyclic voltammogram for silver in 100% OPC high-density mortar ...... 55 Figure 22: Cyclic voltammogram for platinum in 100% OPC high-density mortar ...... 55 Figure 23: Thiosulfate concentration versus hydration time for GGBFS in water ...... 57 Figure 24: Sulfate concentration versus hydration time for GGBFS in water ...... 57 Figure 25: Thiosulfate concentration versus hydration time for GGBFS in basic solutions ...... 58

viii

Figure 26: Sulfate concentration versus hydration time for GGBFS in basic solutions ...... 58 Figure 27: Thiosulfate concentration versus hydration time for aggregates and GGBFS in basic solutions ...... 61 Figure 28: Sulfate concentration versus hydration time for aggregates and GGBFS in basic solutions ...... 61 Figure 29: Typical EDX spectrum of GGBFS grains ...... 62 Figure 30: ESEM image of dry GGBFS grains mixed in epoxy ...... 63 Figure 31: ESEM image of dry GGBFS grains mixed in epoxy ...... 63 Figure 32: ESEM image of single dry GGBFS grain ...... 63 Figure 33: EDX quantification table for dry GGBFS ...... 63 Figure 34: ESEM image of GGBFS grains in aerated water ...... 64 Figure 35: ESEM image of GGBFS grain in aerated water ...... 64 Figure 36: ESEM image of GGBFS grains in deaerated water ...... 64 Figure 37: ESEM image of GGBFS grain in deaerated water ...... 64 Figure 38: EDX quantification table for GGBFS in aerated water ...... 64 Figure 39: EDX quantification table for GGBFS in deaerated water ...... 64 Figure 40: ESEM image of GGBFS in NaOH ...... 65 Figure 41: EDX sulfur mapping of GGBFS in NaOH ...... 65

Figure 42: ESEM image of GGBFS in Ca(OH)2 + NaOH ...... 65

Figure 43: EDX sulfur mapping of GGBFS in Ca(OH)2 + NaOH ...... 65 Figure 44: EDX quantification table for GGBFS in NaOH ...... 65

Figure 45: EDX quantification table for GGBFS in Ca(OH)2 + NaOH ...... 65 Figure 46: ESEM image of OPC and GGBFS paste...... 66 Figure 47: ESEM image of OPC and GGBFS paste...... 66 Figure 48: EDX sulfur mapping of GGBFS grains ...... 66 Figure 49: ESEM image of GGBFS grain ...... 66 Figure 50: EDX quantification table for GGBFS grain in OPC and GGBFS paste ...... 66 Figure 51: ESEM image of high-density GGBFS concrete ...... 67 Figure 52: ESEM image of high-density GGBFS concrete ...... 67 Figure 53: EDX iron mapping of coarse aggregates ...... 67 Figure 54: ESEM image of high-density 100% OPC concrete ...... 67

Figure 55: EDX quantification table for GGBFS grains in high-density GGBFS concrete ...... 67 Figure 56: EDX quantification table for coarse aggregates ...... 67 Figure 57: ESEM image of high-density GGBFS mortar ...... 68 Figure 58: ESEM image of high-density GGBFS mortar ...... 68 Figure 59: ESEM image of silica GGBFS mortar ...... 68 Figure 60: ESEM image of silica GGBFS mortar ...... 68 Figure 61: EDX quantification table for GGBFS grain in high-density GGBFS mortar ...... 68 Figure 62: EDX quantification table for GGBFS grain in silica GGBFS mortar ...... 68 Figure A1: XRD Spectrum of fine hematite sand ...... 79 Figure A2: Corrosion potential of embedded carbon and stainless steels in mortar sample type 2 ...... 93 Figure A3: Corrosion potential of embedded carbon and stainless steels in mortar sample type 3 ...... 93 Figure A4: Corrosion potential of embedded carbon and stainless steels in mortar sample type 4 ...... 93 Figure A5: Corrosion potential of embedded carbon and stainless steels in high-density concrete sample type 6 ...... 93 Figure A6: Pourbaix diagram for iron-water system at 298 K ...... 95 Figure A7: Pourbaix diagram for chromium-water system at 298 K ...... 95 Figure A8: Pourbaix diagram for silver ...... 95 Figure A9: Pourbaix diagram for platinum ...... 95 Figure A10: High frequency electrolyte resistance of embedded carbon and stainless steels in mortar sample type 2 ...... 96 Figure A11: High frequency electrolyte resistance of embedded carbon and stainless steels in mortar sample type 3 ...... 96 Figure A12: High frequency electrolyte resistance of embedded carbon and stainless steels in mortar sample type 4 ...... 96 Figure A13: High frequency electrolyte resistance of embedded carbon and stainless steels in high-density concrete sample type 6 ...... 96

LIST OF APPENDICES

A-1: Coarse and Fine Aggregate Property Calculations ...... 78 A-2: X-ray Diffraction (XRD) Spectrum of Fine Hematite Sand...... 79 A-3: Mix Design Calculations ...... 80 A-4: High-Density Concrete and Mortar Mixing Procedures ...... 88 A-5: Mortar Air Content Calculation ...... 90 A-6: Concrete to Mortar Calculations...... 91 A-7: OCP Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6 ...... 93 A-8: Sulfide to Oxygen Molar Ratio Calculation ...... 94 A-9: Pourbaix Diagrams for Metals ...... 95 A-10: EIS Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6 ...... 96 A-11: Sulfur Mass Balance Calculations for Ion Chromatography Analysis ...... 97

CHAPTER 1 - INTRODUCTION

1.1 Objectives and Motivations

The resurgence of nuclear power in Ontario as a safe, affordable and reliable energy source for electricity production will involve an increased production of used nuclear fuel from the nuclear fission process. The safe and responsible management of this high-level nuclear waste concerns several organizations, such as the Ontario Power Generation and the Nuclear Waste Management

Organization, who must effectively plan the adequate storage and disposal of the waste and its potential future interactions within the environment. Currently, the decommissioning process of the used fuel involves water storage to remove heat and radioactivity for approximately 10 years and then interim storage within dry storage containers (DSCs). The 70 tonne, double-walled steel DSCs made of high-density reinforced concrete are of interest and motivation for this study, due to the requirement of scientific evidence for the DSCs to withstand corrosion within their lifetime. Since it is likely that the used fuel will spend more than 50 years within the DSCs, it is of much significance to study the science and technology of the DSC materials to address any potential concerns that may affect the storage of the used fuel.

The DSC materials include reinforced high-density concrete, which is a proportioned mixture of ordinary Portland cement (OPC) and ground granulated blast-furnace slag (GGBFS), as cementitious material, as well as iron oxide aggregates and water. The varying proportions of coarse and fine iron oxide aggregates in the form of magnetite (Fe3O4) and hematite (α-Fe2O3) are fundamental for creating the high-density of the concrete and provide an effective barrier against any gamma radiation effects from the used fuel. The GGBFS is of particular interest since it

2 contains up to 2% by mass of calcium sulfide (CaS), which represents a stoichiometric excess over the oxygen present in the concrete, and should in principle create a reducing environment; however there is a theoretical possibility of steel corrosion due to the creation of oxidation products, such as thiosulfate. Therefore, the objectives of the experimental work involve:

1. Developing an understanding of the chemistry of the cementitious materials, aggregates

and chemical admixtures in concrete, mortar and simulated pore water environments.

2. Investigating corrosion mechanisms and kinetic stability of reduced sulfur species, such

as thiosulfate.

3. Monitoring the electrochemical behaviour of embedded steels in high-density concrete

and mortar environments.

This project was a part of a collaborative research interest with the Ontario Power Generation and

AMEC Earth and Environmental, with the major objectives to be addressed by experimental research in the areas of electrochemistry, ion chromatography and microscopy. The significance of this study is to develop knowledge on the underlying science of the DSC materials at a laboratory scale, in order to obtain a broader understanding of their application and usage in industry.

In the forthcoming sections, relevant literature on nuclear waste management, high-density concrete, mortar materials and steel corrosion will be presented in Chapter 2. The sample preparation of high-density concrete and mortar and the experimental procedures related to the key research areas mentioned above are presented in Chapter 3. The analysis of the results and discussion in the experimental areas are presented in Chapter 4. The main conclusions and summary of the study are drawn in Chapter 5. Finally, implications for future work are presented in

Chapter 6, followed by References and Appendices sections.

CHAPTER 2 - BACKGROUND AND LITERATURE REVIEW

2.1 Nuclear Waste Management

The current, interim management of high-level nuclear waste in Canada involves the dry storage process, as a safe and regulated technology for containing and shielding the used fuel in a dry state.

Currently seven licensed facilities across Canada, with four in Ontario, manage the used fuel with a

DSC processing building, indoor DSC storage warehouse (as depicted in Figure 1) and an amenities area (1).

Figure 1 - Row of DSCs at the Pickering waste management facility (1)

2.1.1 Dry Storage Container Processes and Challenges

In order to investigate background information on the construction of the DSCs, a site visit was conducted to Niagara Energy Products (NEP), one of the manufacturers of the DSCs. In essence, the DSC manufacturing process involves detailed and controlled welding operations to create the rectangular carbon steel container and lid sub-assemblies, loading of 12 batches of high-density concrete, drying stage and a helium leak detection test. Since the DSC is designed to contain up to

384 used fuel bundles it needs to be ensured that the requirements of a total radiation shielding of

21 inches be met, with the reinforced high-density concrete accounting for 20 inches and both the inner and outer carbon steel lining accounting for approximately 0.5 inches each. Upon placement of the used fuel bundles in the DSCs within the water storage bays, the DSCs need to be drained, decontaminated, vacuum dried and back-filled with helium gas to create an inert atmosphere for the used fuel (1). Currently, the DSC drain is the only stainless steel component of the DSC. The meticulous DSC fabrication and used fuel loading processes are carefully regulated at all important stages of the operation by the Canadian Nuclear Safety Commission (CNSC).

The relevant challenge identified in the DSC fabrication process involves maintaining uniformity in the high-density concrete loads. Industrial concrete batching is greatly affected by seasonal weather conditions, since the aggregate stockpiles either increase or decrease in temperature which affects their moisture content and absorption properties. The air content and slump can also be affected if the high-density concrete is not mixed properly from load to load. Since the main function is to serve as a radiation shielding material, the interaction of radiation with improperly batched high-density concrete can be detrimental to the safe and secure containment of the used fuel. However, if the consistency of the loads is maintained, experience has shown that high- density concrete is an effective, versatile and economical material for usage in radiation shielding applications (2). High-density concrete is already widely used as a containment material for large stationary installations such as nuclear power plants and particle accelerators, which makes it an applicable option for shielding high-level nuclear waste in the stationary DSCs.

2.1.2 Radiation and Shielding

During the interim storage of the high level nuclear waste, the attenuation of photon (gamma and

X-rays) radiation within the high-density concrete can occur due to the concrete’s varying proportions of light and heavy elements. The main interaction processes of photons with matter are the photoelectrical effect, pair production and Compton scattering, but actual interaction patterns within concrete are dependent on the density and proportion of elements within the concrete (3).

Since photon radiation is considered to be of great concern, natural, high-density mineral aggregates such as magnetite and hematite are useful for creating high-density concrete that can attenuate photons. An increase in density must result with the usage of these aggregates; however the thickness of concrete radiation shields can be effectively reduced to compensate (4).

Furthermore, since used fuel can contain up to 3% fission products with variable radioactive decay time and energy levels, high-density concrete needs to be able to withstand heat generation effects from gamma radiation over the period of storage (3, 5). Since energy captured from the photon radiation is deposited directly into the high-density concrete and liberated as heat, the thermal stresses can be signifigant to cause physical release of chemically bound water from the hydrated cement paste (HCP) (6). This ultimately leads to release of hydrogen and creates additional safety issues, such as the deteroriation of the high-density concrete by cracking, due to the decrease in compression and tensile strength of the concrete from long-term exposure to high-temperature (6).

Furthermore, the poor thermal properties, such as a low thermal conductivity, of most concretes are generally viewed as a disadvantage since high temperature gradients and thermal stresses may be created within the concrete and are important design considerations for maintaining the structural integrity (3).

2.2 High Density Concrete and Mortar

High-density concrete is defined as a proportioned mixture of hydraulic cement and water paste with embedded coarse and fine heavyweight aggregates, combined to weigh generally more than

3200 kg/m3 (7). High-density concrete is usually required for a dual compliance of strength and density and is generally more expensive than normal concrete. Since high-density concrete is more dependent on the aggregate properties than the cementitious material properties, it can be more prone to segregation or bleeding phenomena during hydration (2). Other important concrete properties such as the compressive and tensile strength, elasticity behaviour, shrinkage and workablility may be affected as well, but any expected changes can be accounted for in proper mix designing. High-density mortar is a proportioned mixture of hydraulic cement and water paste with embedded fine heavyweight aggregates, combined to weigh anywhere between 2900-4300 kg/m3

(2). It is usually produced for specialized applications and tends to have higher workability than concrete, due to its rheology properties. Flow testing is an important way to assess the consistenty and workability of a mortar, while slump testing is essential for freshly batched concrete. However, despite testing procedures, it is the chemistry of the materials that have the most significant effect on the final properties of high-density concrete and mortar.

2.2.1 Ordinary Portland Cement (OPC)

OPC is the most widely used cement in the world and there are various types manufactured to meet different physical and chemical requirements. Type General Use (GU, Type I or CSA Type 10)

OPC is of particular interest and upon manufacturing becomes a porous clinker that is ground to fine, dark-gray granular powder that has a typical composition as shown in Table 1. The standard requirements for OPC are detailed in ASTM 150 (Standard Specification for Portland Cement),

7 long with reference to testing properties such as strength, setting time and fineness. The fineness averages about 370 m2/kg (Blaine) and is an important parameter, which affects the water demand and rate of hydration of cement (9). Similarly, the particle size distribution is equally as important and can be estimated from the Rosin-Rammler function, used for analyzing particle size data, with typically 7-9% of OPC being finer than 2 µm and 0-4% being coarser than 90 µm (8, 10).

Chemical Mass Composition Mineral Mass Composition Compounds (%) Compounds (%) CaO (Lime) 67 Tricalcium silicate (alite) 55 CaO3SiO5 (C3S) SiO2 (Silica) 22 Dicalcium silicate (belite) 19 CaO2SiO4 (C2S) Al2O3 5 Tricalcium aluminate 10 (Alumina) (aluminate) Ca3Al2O6 (C3A) Fe2O3 3 Tetracalcium 7 (Iron Oxide) aluminoferrite (ferrite) Ca2AlFeO5 (C4AF) Other (CaSO4, 3 Other (alkali sulfates, 9 CaO or MgO) calcium oxide or magnesium oxide) Table 1 – Chemical composition of Type GU OPC in Canada (8,9)

OPC grains are homogenous in composition and their microstructure is usually angular alite and rounded belite crystals embedded in an interstitial matrix of dendritic ferrite and aluminate, as depicted in Figure 2. The alite crystals range from 15-20 µm, the belite crystals range from 25-40

µm, while the ferrite and aluminate are variable due to their presence as a solid solution (8). Since the mineral compounds are all present as oxides, their cementing ability is highly dependent on oxygen’s ability to acquire electrons during chemical reaction with water (11). The actual phase equilibrium composition within the crystalline structure of the compounds also determines reactivity. A study performed on the synthesis of the pure OPC mineral compounds by Wesselsky, suggests that it is practically impossible to separate them into individual components due to the complexity of their crystal systems (12). Therefore, it is likely that the overall reactivity of OPC is

8 determined by a stable phase assemblage of the mineral compounds, rather than their individual hydraulicity.

Figure 2 - Graphic representation of OPC grain cross-section (13)

2- The sulfur chemistry in OPC is present as sulfate (SO4 ) in the form of calcium sulfate as gypsum, hemihydrate and/or anhydrite, respectively, to help control early setting properties. The total level is normally reported as a SO3 equivalent and is limited in Type GU cement at 3% by mass, with the majority of the sulfate being soluble within the crystal structure of the mineral compounds (14).

Elemental sulfur is almost never found in OPC, except in trace amounts, since cement clinker is produced in an oxidizing rotary kiln environment (15).

2.2.2 Ground Granulated Blast-Furnace Slag (GGBFS)

GGBFS is a whitish, glassy and granular cementitious material obtained from quenching and grinding molten slag, which is a by-product from iron production. Due to its amorphous structure,

GGBFS is not considered to be strongly self-cementing and is usually blended as a partial replacement to OPC at dosages between 5-70% by mass, depending on the required chemical and physical characteristics (9). Known as a green cementitious material, GGBFS can effectively lower the cost of cement when used as a partial replacement, by saving energy in the production process

and reducing CO2 emissions from cement manufacturing. The most important GGBFS properties that influence reactivity are the fineness, glass content, activity index and chemical constituents.

The standard requirements for GGBFS and GGBFS-blended cements are detailed in ASTM C989

(Standard Specification for Slag Cement for Use in Concrete and Mortars) and ASTM C595

(Standard Specification for Blended Hydraulic Cements), respectively. The fineness averages approximately 450 m2/kg (Blaine) for GGBFS produced in Canada and the fineness of GGBFS must usually be greater than that of OPC for acceptable performance upon hydration (9). Current research has shown that there is no correlation between glass content and hydraulicity, but it has been generally reported that the glass content should be in excess of 90% to show satisfactory properties (16). The chemical composition of Canadian GGBFS is shown in Table 2.

Chemical Composition CaO SiO2 Al2O3 MgO Fe2O3 MnO S (Lime) (Silica) (Alumina) (Magnesia) (Iron (Manganese (Sulfur) Oxide) Oxide) Mass 32-45 32-42 7-16 5-15 0.1-1.5 0.2-1 0.7-2.2 Composition (%) Table 2 - Chemical composition of GGBFS in Canada (9)

The glass microstructure of individual GGBFS grains consists of a continuous anionic network of oxygen and silicon in charge balance with calcium, magnesium and other electropositive elements, within the cavities of the network (8). The grains tend to be quite angular and studies have shown evidence of phase separation within the grains, due to compositional variation between glass and crystalline interfaces (17). Any crystalline phases present are usually in the form of unevenly distributed dendritic merwinite, melilite, calcite and quartz (11).

The sulfur chemistry in GGBFS exists primarily as CaS, with trace amounts of iron and manganese sulfides, and is speculated to exist homogeneously as a colloidal state throughout the glass

10 network. X-ray absorption spectroscopy in a study by Roy, indicated that most of the sulfur in

GGBFS is frozen in amorphous form, with only a minor amount appearing as oldhamite

(mineralogical CaS) (18). Research by Scott et al., reported that oldhamite occurred both as independent dendritic crystals and inclusions within melilite (19). Radwan also indicates that sulfide (S2-) is the main species of sulfur in GGBFS and if GGBFS is not properly stored, the sulfide can oxidize into sulfate, making it difficult to monitor the amount of initial sulfide (20).

2.2.3 Coarse and Fine Aggregates

Aggregates are natural or artificial particulate matter that are chemically inert and used in the production of composite materials such as concrete or mortar. Aggregates account for approximately 60-75% of the volume of concrete mixtures, and their properties are able to influence the workability of plastic concrete and the durability, strength, thermal properties and density of hardened concrete (21). The fine and coarse aggregates of interest are heavyweight, natural hematite (α-Fe2O3) and magnetite (Fe3O4) respectively, as mentioned for their radiation shielding properties and high specific gravity in the range of 4.6-5.2 (22). Magnetite is a ferromagnetic material with a mixed-valence Fe2+-Fe3+ structure and at room temperature very slowly oxidizes to ferromagnetic maghemite (γ-Fe2O3) and at higher temperatures to anti- ferromagnetic hematite (23). Research has shown that the oxidation of magnetite depends on the outward diffusion of iron cations from its center to the surface, but the kinetics are expected to be slow in alkaline conditions such as concrete, where iron is insoluble (24). The oxidation process is also complicated by the ratio of Fe2+ to Fe3+, oxygen fugacity and topotactic reorganization of the iron oxide crystal structures (25). Aggregate properties, such as hardness, may be affected by the compositional variation of oxidized magnetite, which also affects the abrasion resistance of the

11 aggregates when mixed with cementitious material and water. The hardness of both aggregates in their pure mineral state is approximately 5.5-6.5 on the Moh’s hardness scale, which means they are relatively scratch resistive (3). The electrical resistivity of magnetite at room temperature has been reported to be as low as 5 Ω·cm, which is much lower than hematite’s reported resistivity of

2000 kΩ·cm (8,25).

Heavyweight iron oxide aggregates are often found in weathered soils, clays and sedimentary rock with iron band formations (11). Due to their source variability, the heavyweight aggregates are processed differently depending on where they are quarried. However, the general procedure for aggregate processing is to extract, crush, screen and sort, in order to eliminate any undesirable constituents. Upon processing completion, the coarse aggregate tends to be angular in particle shape with a rough surface texture, while the fine aggregate is relatively granular. Test methods can also be performed after processing completion to determine necessary aggregate properties for concrete and mortar production.

2.2.3.1 Aggregate Properties and Test Methods

It is important to classify aggregates by performing test methods to determine their grading specific gravity, absorption and surface moisture content properties. Grading refers to the distribution of particle sizes present in an aggregate (21). Outlined in ASTM C136 (Sieve

Analysis of Fine and Coarse Aggregates) grading is performed by passing a sample of aggregates through a series of square-wired sieves that decrease in opening size, to obtain the mass percent passing each successive sieve. Coarse and fine aggregates are usually sieved separately with the coarse aggregates being sieved through sieve sizes decreasing from 28 to 5

12 mm and the fine aggregates being sieved through sieve sizes decreasing from 2.5 to 0.08 mm.

The individual percent of aggregates retained and the total percent passing between successive sieves are the key parameters of interest in evaluating the acceptability of a grading test.

Implications of a grading test can determine if there is well-distributed size range of aggregates, which is important for cement paste coverage on aggregates during mixing. The specific gravity of an aggregate is needed to properly account for the aggregate’s yield and mix density in a well- proportioned mix of concrete or mortar. It is fundamentally defined as the mass of the aggregate in air divided by the mass of equal volume of water and refers to the space occupied by the aggregate particles alone (i.e.-the volume of solid aggregate and internal aggregate pores), excluding the voids between particles (21). The specific gravity is determined at fixed moisture content and the four possible moisture conditions are shown in Figure 3. The damp or wet condition occurs when the aggregates pores are filled with water and free water exists on the surface. Saturated surface-dry (SSD) aggregates contain no free water on their surface and aggregates are usually brought to this condition for specific gravity determination. Air-dry refers to aggregates that contain some water in the pores, but have a dry surface. Oven or bone dry is the extreme condition, where the aggregates have no water in their pores or surface. Specific gravity determination involves measurement of an aggregate’s apparent mass in water, as well as saturated surface-dry and oven-dry masses in air.

Figure 3 - Moisture conditions of aggregates (21)

Absorption is a measure of the total pore volume accessible to water and can determined from saturated surface-dry and oven-dry masses, similarly to the specific gravity, as described in

ASTM C127 and ASTM C128 (Specific Gravity and Absorption of Coarse Aggregate) and

(Specific Gravity and Absorption of Fine Aggregate), respectively. Moisture content determination is summarized in ASTM C566 (Total Moisture Content of Aggregate by Drying) and is a calculation of the water evaporated from the aggregates’ natural to oven-dry condition.

Since natural aggregates are rarely at saturated surface-dry condition, their existing specific gravity, surface moisture content and absorption are necessary to determine, to account for the amount of aggregates and mixing water in concrete or mortar mix design.

2.2.4 Concrete and Mortar Microstructure

The hydration of cementitious material in the presence of water develops hydration products in a cement paste microstructure that changes from a plastic to hardened state. It is accomplished through stages of progressive strength development, such as stiffening and hardening. Stiffening refers to the initial setting of the HCP, as it changes from a fluid to rigid stage and usually occurs within 1 to 4 hours after mixing the paste (26). As the paste is able to withstand a prescribed pressure, usually after 3 to 6 hours, the final set is achieved and hardening is taking place due to the continuous strength development of the HCP (27). Furthermore, in order to achieve desired paste properties, the drying and hardening can be slowed down by curing processes to ensure optimal strength development. Abnormal setting behaviour can also occur, depending on the liberation of heat, in the form of false or flash set and is detrimental to the HCP if unobserved (9). As hydration progresses, the relative humidity decreases within the HCP, due to the volume of water changing as it is consumed by the cementitious material. The porosity of the HCP is also affected and

14 depends on the water to cementitious material ratio (w/cm), degree of hydration and ultimately affects strength, permeability and drying shrinkage (27). A high w/cm ratio indicates a more permeable HCP, while a low w/cm ratio signifies a much denser HCP. The main types of pores present are large interconnected capillary pores and ultra-fine gel pores that exist within the hydrated products. The capillary pores tend to hold free water which is lost upon drying, while the hydrated products contain chemically bound water that is not lost (26). The gel pores contain water in both states, and as hydration progresses, the gel pore volume generally increases while the capillary pore volume decreases (26). Spherical air voids, which are larger than the capillary voids, can also be present in the HCP at 2-6% by volume, due to mixing entrainment. The voids tend to be usually uniformly distributed throughout the HCP and do not affect the HCP permeability (26).

2.2.4.1 Hydration Products

The main hydration products that are formed when OPC reacts with water are calcium silicate hydrate (3CaO∙2SiO2∙2H2O or C-S-H), calcium hydroxide (Ca(OH)2), AFm phases such as monosulfoaluminate (C3A∙CaSO4∙12H2O), AFt phases such as ettringite (C3A∙3CaSO4∙32H2O) and hydrogarnet (C3A∙6H2O) (8). The C-S-H gel makes up 50-60% of the HCP and varies from poorly formed crystalline fibers to a reticular network of small particles (28). The calcium hydroxide exists as large hexagonal crystals and occupies up to 20-25% of the HCP (28). Calcium sulfoaluminates (AFm and AFt) consist of 15-20% of the HCP and initially favour formation of rod-like ettringite crystals, while rosette-shaped monosulfoaluminate forms later during hardening

(8). Eventually continued hydration releases aluminate and the ettringite is slowly converted to monosulfate hydrate. Aluminate and ferrite contribute to setting and early strength gain, as aluminate reacts early to form ettringite, while alite and belite contribute to hardening and long-

2- - term strength gain, as they react to form C-S-H and Ca(OH)2. The release of SO4 and OH into the HCP is important for activating the reaction of alite and belite. Equations [1] to [3] describe the formation of OPC hydration products from the OPC mineral compounds (3):

Ca3Al2O6 + 6H2O → 3CaO∙Ca3Al2O6∙6H2O + Ca(OH)2 [1] 2CaO3SiO5 + 6H2O → 3CaO∙2SiO2∙3H2O + 3Ca(OH)2 [2] 2CaO2SiO4 + 4H2O → 3CaO∙2SiO2∙3H2O + Ca(OH)2 [3]

GGBFS does not hydrate or harden in pure water in the same way as OPC, due to a different compositional and morphological structure. Hydration of GGBFS is very slow in water, since it is likely that hydration is delayed by an impermeable aluminosilicate coating that is deficient in Ca2+ and forms on the surface of GGBFS grains (29). The hydraulic reactivity of glassy GGBFS grains is dependent on the activation of the glass and water is not at a high enough pH to attack the glass

(8). However, upon contact with water, it has been observed that the initial slag hydration appears to be an incongruent dissolution and limited precipitation of C-S-H occurs at a pH below 9.5 (11).

The amorphous, foil-like C-S-H gel has high aluminum content and a lower CaO/SiO2 ratio, which defers from the fibrillar C-S-H gel in OPC (29). GGBFS is usually blended with OPC to increase its hydraulic reactivity and the hydration products formed are similar to that of OPC. The Ca(OH)2 produced from OPC hydration serves as an activator for GGBFS hydration and is consumed by

GGBFS grains to increase the amount of C-S-H gel in hydrated OPC-GGBFS pastes (11). Other differences from the OPC hydration products, include the formation of different AFm phases

(C4A∙13H2O and C2A∙8H2O) and a magnesium and aluminum rich hydrotalcite phase (30).

GGBFS hydration is relatively faster as a blended cement with OPC and during hydration the crystalline structure of GGBFS remains intact and is inert (31). The progressive strength gain of the blended cement is slower due to postponed setting, but higher strengths are achieved at later ages. GGBFS allows more capillary pores to be filled with C-S-H gel and has a greater gel pore

16 volume, thus creating a denser microstructure of lower porosity. The workability and slump of the hydrated blended paste are also improved with the addition of GGBFS, however bleeding may occur due to the greater fineness of GGBFS compared with that of OPC (32). Figures 4 and 5 show

SEM imagery of the hydration products and AFm phases in GGBFS HCP.

Figure 4 (left) – OPC hydration products (33) Figure 5 (right) – Early age, rosette-shaped AFm phases in GGBFS HCP (29)

Hydrated cements can contain mixtures of AFm phases and the fate of sulfur compounds is

2- 2- speculated to be linked with them. The SO4 and SO3 from OPC are accommodated as interlayer anions associated with AFm phases, but as hydration progresses the AFm phases become poorer in

2- 2- SO4 . The thermodynamic stability of the AFm phases is a key issue in retaining distinctive SO4 matter, as research has shown that monosulfoaluminate is calculated to be stable only above 40°C, since decomposition to AFt and hydrogarnet occurs at lower temperatures (34). Odler has shown

3+ 3+ 2+ 2- that OPC, in the absence of Al or Fe , had Ca and SO4 ions adsorbed by C-S-H (35). It was also shown that after hydration, ettringite and monosulfoaluminate combined to contain 32% of the sulfate content, while gypsum retained 46.5% of the sulfate content (34). GGBFS upon hydration develops a transient green colour that becomes darker over time, which suggests that polysulfides are formed from the S2- present in glass GGBFS grains. The S2- released from GGBFS has been assumed to enter AFm phases by many researchers and the possibility of interactions between

2- SO4 and sulfur in lower oxidation states cannot be overlooked (30, 36). Vernet found that in hydrated GGBFS cements the AFm phase, disulfur aluminate (C3A∙2CaS∙10H2O) was formed and

2- 2- consumed S , while decreasing sulfur ions in the pore solution (11). A decrease in SO4

2- concentration can also transform ettringite into monosulfoaluminate, which suggests that SO4 is key anion in stabilizing the formation of AFm (34). Furthermore, GGBFS that is activated by high

2- 2- pH solutions, have shown decrease in S and increase in crystalline SO4 , which suggests that the sulfur is exsolving from the glass to form distinct crystalline phases (18).

2.2.4.2 Aggregates and Hydrated Cement Paste

The aggregate phase within a concrete or mortar microstructure is predominantly responsible for the unit weight, elastic modulus and dimensional stability. These properties are dependent on an interfacial transition zone, which represents an interfacial region between aggregates and the HCP and is typically 10-50 µm thick around large aggregates (28). The aggregates and the HCP interact by Van der Waals forces of attraction and an example of the interfacial transition zone is shown in

Figure 6. The hydration product within the transition zone that is mostly in contact with the aggregates is C-S-H as found by Javelas, whereas Ca(OH)2 possesses less adhesion (37).

Figure 6 – Interfacial transition zone between GGBFS HCP and crushed basalt rock aggregate, (right side is the HCP and left side is the aggregate) (31)

As the surface area increases, more HCP is needed to coat the additional surface of aggregates or else the resulting concrete or mortar would be too stiff. Similarly, the reverse problem also exists, when a decrease in surface area occurs due to a more fluid HCP, which contributes to a decrease in strength and durability. Cement is more expensive than aggregates and it is recommended to have a uniform gradation of aggregates to ensure that the right amount of cement is used for aggregate coverage, workability and low void content (21). Phenomena such as microcracking or internal bleeding can also occur when there is poor adhesion, stress concentrations and water film accumulation next to the surface of aggregates, thus weakening the transition zone (26).

2.2.4.3 Pore Solution

The amount of water required for complete hydration of cementitious material is generally less than the calculated w/cm ratio and any excess water that is not consumed by the aggregates exists as an aqueous phase. The excess aqueous phase, termed the pore fluid, deposits itself within the pore structure of the HCP and has a varying composition depending on the pH and cementitious material used. Analysis of OPC pastes and mortars at a 0.5 w/c ratio for 6 months had shown that the pore solution is essentially an alkali hydroxide solution with dissolved ions of sodium, potassium and hydroxide and a solution pH ranging from 13.4-14.0 (11). Research has shown that typically 40-60 per cent of Na+ ions and 50-70 of K+ ions in OPC ended up in the pore solution

(38). The solubilities of silica, calcium, aluminum and magnesium in pore solutions from GGBFS pastes have been observed to be strong functions of the solution pH (8). GGBFS hydration is activated at high pH and studies by Song have shown that a high pH increases the concentration of silica and aluminum in the pore fluid, but reduces the amount of calcium and magnesium (39). The decrease in calcium is due to the low aqueous solubility of Ca(OH)2 in the pore fluid and

favourable thermodynamic stability of solid Ca(OH)2 at high pH (40). Similar results were observed by Longuet, with a decrease in the alkalinity of GGBFS pore solutions and increase in concentration of chemically reduced sulfur species over time (11). The pore solution of hydrated

GGBFS activated with gypsum also showed a high total reduced sulfur concentration for up to 56 days (41). Furthermore, reduced sulfur species, such as thiosulfate, were found to have stably formed in lixiviation water prepared from GGBFS pastes over time (42). Studies on OPC-GGBFS pore solutions have shown intermediate results, with an insignificant decrease in the alkalinity of the pore fluid and maintenance of high pH over time (11).

2.2.4.4 Kinetics of Oxygen Diffusion

The kinetics of oxygen diffusion in concrete or mortar microstructure affect the electrochemical behaviour of embedded steels. Oxygen diffusion has been shown to be a strong function of the moisture content of HCPs (43). A moisture content of 100% indicates a low diffusion coefficient, since oxygen slowly diffuses through capillary pores filled with water. A moisture content of 0% indicates a high diffusion coefficient, since oxygen quickly diffuses through capillary pores filled with air. Page and Lambert measured the oxygen diffusivity for hydrated OPC pastes at different w/cm ratios and temperatures, with the primary finding indicating that they were of similar magnitude to chloride ion diffusivities in HCPs (44). At 25°C and at w/cm ratio of 0.5, the oxygen diffusivity was measured to be 1.52 x 10-8 cm2/sec (44). This differs greatly from Kobayashi’s reported value of 10-3 cm2/sec for 0.6 w/cm OPC curing in air, but is within the range of 10-3-10-9 cm2/sec found in Hunkeler’s work (45, 46). OPC-GGBFS blended mortars were found to have a lower diffusivity than OPC, as the GGBFS content increased, most likely due to the slower transport of oxygen through silicate glasses (45, 47). Furthermore, the oxygen diffusivity was

20 found to also increase when the w/cm ratio and temperature of the HCP both increased (44). At lower w/cm ratios, the capillary pores would be filled with C-S-H which acts as a barrier for diffusion. The kinetics of oxygen diffusion are also affected by the path tortuosity and activation energy of the cementitious material-water adsorption isotherm, which are favourable at high w/cm ratios (43).

2.2.5 Admixtures

Chemical admixtures are defined as materials other than hydraulic cements, water or aggregates that are added before or during mixing to improve the properties of concrete or mortar in its plastic or hardened state (48). The two types of admixtures that are of primary interest are air-entraining agents (AEAs) and superplasticizers. AEAs are liquid admixtures that have the ability to trap or entrain tiny air bubbles during mixing, while improving workability and plasticity. AEAs tend to consist of long chain organic molecules with polar groups at one end, allowing them to lower surface tension and stabilize hydrophobic air molecule interactions with water (8). Along with

AEAs, other important factors that affect the air content are the fineness of cement, an increase in slump and efficiency of the mixer. Superplasticizer, also known as a high-range water-reducing

(HRWR) admixture, is used to reduce the water requirement of cementitious material, by more than 30% without retardation side effects and essentially produce normal workability concrete or mortar at a lower w/cm ratio (48). By controlling the rheology of the HCP, superplasticizers can improve strength development, finishability and surface appearance. The polycarboxylate chemistry of superplasticizers can also disperse the cementitious material, which helps reduce permeability and shrinkage (11).

2.3 Steel Corrosion in Concrete

The corrosion of steel in concrete is one of the main causes of damage and failure of reinforced concrete structures. The high alkalinity of concrete pore water allows embedded steel to passivate and remain protected, unless the passive film is destroyed by a corrosion mechanism. Under aerobic conditions, the passive film is a hard, non-reactive surface film that is self-maintaining and consists primarily of metal oxides. The development of the corrosion mechanism and its ensuing level of damage can be described in three stages as shown in Figure 7 (49). The first stage is an initiation period before corrosion activation, where contaminants, such as carbon dioxide and chloride, ingress into the concrete matrix. This is followed by propagation period where corrosion products and propagation of micro-cracking develops. The third stage, which is most detrimental, is the acceleration stage where the corrosion rate of the steel increases, due to the low permeability of concrete to corrosion agents, such as oxygen and water. Well-known corrosion mechanisms that are applicable to this model include carbonation-induced corrosion and chloride attack, both which have been widely studied; in both cases oxygen reduction being the cathodic reaction.

Figure 7 - Three-stage corrosion damage model for reinforced concrete (49)

2.3.1 Corrosion Mechanisms

The breakdown of the steel’s passive layer will cause rust formation as lepidocricite (γ-FeOOH).

The chemical reactions for the formation of rust, shown in equations [4] to [7], and most notably involve the coupled reactions of anodic oxidation of the steel’s metallic iron and cathodic reduction of oxygen, in equations [4] and [5]:

Fe → Fe2+ + 2e- [4] - - 2H2O + O2 + 4e → 4OH [5] 2+ - Fe + 2OH → Fe(OH)2 [6]

2Fe(OH)2 + O2 → 2 γ-FeOOH + H2O [7]

If a reduction in concrete alkalinity occurs, as the iron is being removed by anodic oxidation, the ferrous ions will dissolve into the surrounding pore water solution of the concrete and the steel loses mass, due to its cross-sectional area becoming smaller (50). The ferrous ions then react with hydroxide ions to form ferrous hydroxide, which reacts with oxygen to ultimately produce lepidocricite. The corrosion process is dependent on the flow of electrons between the anodic and cathodic sites on the steel, as well as the flow of ions through the capillary pores of the concrete

(50). If the pores are dried out, the flow becomes difficult and the corrosion process slows down. It is also dependent on the oxygen rate of diffusion to the steel surface and availability within the concrete. The stability of the steel’s passivation film is also a key issue and different metals can help improve the stability. Stainless steels can form passivation layers that consist of iron, chromium and rarely molybdenum oxides, while carbon steels are stable at high pH, due to the formation of iron (III) oxide (51). The general properties of a stable passivation layer include having a low ionic conductivity, low chemical solubility, good adhesion to the steel, considerable electron conductivity and a large range of potential thermodynamic stability (52). Both carbonation and chloride attack undergo fundamentally different corrosion mechanisms, as shown in Figure 8, and to be discussed in the following sections.

Figure 8 – Schematic flowchart of the corrosion of steel in concrete (53) The usage of GGBFS as a cementitious material in concrete creates another corrosion mechanism of interest, related to the reducing characteristics of sulfide. Sulfides can affect steels in the following two ways:

(1) Oxidation of sulfides to sulfate by the available oxygen present in concrete, which

depletes the oxygen concentration near the steel, to create a beneficial, reducing

environment. Oxidation to thiosulfate can result in pitting corrosion on stainless steel, if

there is a decrease in alkalinity.

(2) Precipitation as iron (II) sulfide on the steel surface or sulfide incorporation into the

oxide layer, which limits the formation of the passivation layer on steel (54,55).

The cathodic reduction of oxygen is affected by both mechanisms and if passivation is insufficient, the embedded steel may become more susceptible to carbonation and chloride attack.

2.3.1.1 Carbonation

Carbonation is a neutralizing chemical reaction, where carbon dioxide gas in the atmosphere can react with solid calcium hydroxide, C-S-H gel, alkali and calcium ions in the pore water solution

(50). This can result in a reduction in the pH of the HCP from above 12 to below 9 (56). Carbon dioxide dissolves in the pore water solution of concrete to form carbonic acid, which neutralizes the alkali hydroxides and reacts to form calcium carbonate, as shown in equations [8] and [9].

CO2 + H2O → H2CO3 [8] H2CO3 + Ca(OH)2 → CaCO3 + 2H2O [9]

The decrease in pH by carbonation causes depassivation of the steel and subsequently the corrosion mechanism responsible for rust formation. Carbonation is primarily dependent on the diffusion of carbon dioxide and the length of carbonation can be measured from the surface of the concrete to the depth to which it occurs at in the concrete matrix (52). This is known as the carbonation depth and its relationship is approximately parabolic with exposure time (57). Other factors that affect carbonation are the moisture content of the concrete, temperature, w/cm ratio and concrete composition. Carbonation does not occur if the relative humidity is very low, since moisture is required to form carbonic acid, or very high, since carbon dioxide diffusivity becomes low. Numerous studies have shown that OPC-GGBFS concrete mixtures that contained an increasing amount of GGBFS, generally showed an increase in carbonation depth with time (58).

Song and Saraswathy suggested that in environmental situations, where there is a risk of excessive carbonation over a long period of time, the GGBFS levels should be restricted to 50%, since higher

GGBFS levels increase carbonation significantly (59). Similarly, in a study on the long term behaviour of concrete in nuclear waste repositories, it was found that the depth of carbonation was greater for specimens containing GGBFS rather than OPC only, after 28 years of storage at approximately 60% relative humidity (60). GGBFS increases the carbonation depth, since the

25 addition of GGBFS with OPC leads to less calcium hydroxide content upon hydration. This limits the extent of reaction [9] and as a result less calcium carbonate is formed in the pore space.

Although there is no standard method to measure carbonation, many studies reference Parrott’s approach to determining carbonation rates from empirical equations involving air permeability data in concrete (51). ASTM C856 (Standard Practice for Petrographic Examination of Hardened

Concrete) presents a qualitative approach to determine carbonation, based on colour indication of differential pH areas using 1-2% phenolphthalein solution.

2.3.1.2 Chloride Attack

Chloride attack is a locally-induced pitting corrosion phenomenon, which requires hydrolysis of

Fe2+ and surrounding of chloride ions at anodic sites of the steel surface at a significant concentration to form hydrochloric acid (50). This subsequently decreases the pH of the HCP and breaks down the steel passivation layer. The depassivation of steel has been suggested to be a competing process between hydroxide, as it stabilizes the passivation layer, and chloride ions, as it disrupts the passivation layer (50). The locally activated anodes are small areas, while the large steel surface area is the cathode for reactions between Fe2+ and Cl- to form chloride or oxychloride compounds (52). The process is self-sustaining due to the hydrolysis of these compounds which leads to recycling of chlorine ions, increased acidity of the anodic areas and continuous oxidation of iron (52). Chloride attack can occur internally within the concrete if the source of chlorides is from using seawater, calcium chloride as an admixture or aggregates that contain chlorides (51). It can also occur externally from the environment by sources such as seawater spray or the deicing salt (51). However, regardless of the method of attack, chloride movement is dependent on diffusion inside the capillary pores of the concrete, similarly to oxygen and carbon dioxide. In

Figure 9, the corrosion of steel in the presence of chloride ions is depicted.

Figure 9 – Corrosion of steel in the presence of chloride ions (52) Concrete that consists primarily of OPC is considered at high risk to corrosion when the chloride contents (by weight of the cement) are greater than 1% and therefore GGBFS is generally added to the concrete to reduce this occurrence (57). The corrosion resistance of reinforced steel in OPC-

GGBFS concrete has been widely researched with an indication that the chloride diffusion coefficients and permeability decrease, as an increasing amount of GGBFS is used. Yeau and Song were both able to independently prove that the free chloride content was much lower in OPC-

GGBFS concrete, due to a finer pore structure that reduced the mobility of chloride ions (59, 61).

Furthermore, the high alumina content in the pore fluid of OPC-GGBFS concrete can contribute to binding chloride ions, which also leads to a decrease in mobility (62). Therefore the corrosion risk of chloride attack is primarily dependent on free chloride ions, rather than bound ones, but if bound ions are released, due to the reversible nature of binding reactions, then they may pose a similar risk (52). The total chloride ion concentration necessary to depassivate the steel, which is known as the chloride threshold level and is usually described as a Cl-/OH- ratio, is useful for predicting the extent of corrosion caused by chloride attack (51).

2.3.2 Measurement Techniques

The corrosion behaviour of steel in concrete can be measured using electrochemical techniques that are suitable for the alkaline, solidifying nature of reinforced concrete. Techniques are useful if they do not involve the removal of the steel in concrete for separate study and account for the high ionic resistivity of concrete compared with normal aqueous environments (50). Since the steel is permanently embedded in concrete, over time as it passivates it will come to an steady state condition where the anodic and cathodic reaction rates are equal. The potential at which this occurs is known as the corrosion potential, Ecorr, and is determined by measurement of the potential difference between the steel and a reference electrode that is within the same environment (50).

The measurement of the corrosion potential is identical to the open-circuit potential (OCP), under the conditions of no external current flow between the two electrodes, and is useful for assessing whether an oxidizing or reducing environment exists within the concrete. Several studies have shown from corrosion potential measurements that a reducing environment exists with OPC-

GGBFS mortars, where it is speculated that oxygen consumption is occurring by the sulfide. The study performed by Benjamin had revealed negative drifting potentials as low as -750 mV vs. saturated calomel electrode (SCE) after 30 days, when the silica sand to cementitious material ratio was 1:1 (62). However, when the ratio was 3:1 (which is more representative of a mortar), higher potentials were observed at -200 mV vs. SCE with no negative drift observed (62). The negative drift in potentials was also observed by Yeau, after only 20 days, in OPC-GGBFS concrete with limestone as coarse aggregate (61). In OPC-GGBFS blends with greater than 70% percent

GGBFS, Angus observed potentials of -400 mV (63). Furthermore, Pal was able to determine similar behaviour in the influence of reducing characteristics of GGBFS on OPC-GGBFS pore solutions (64). Despite GGBFS’s capability to create a reducing environment, due to the reducing

28 effects of sulfur species, there is no established standard for the potential at which it can become a severe corrosion risk. ASTM C876-91 (Test Method for Half Cell Potentials of Uncoated

Reinforcing Steel in Concrete) indicates that an OCP measurement less than -0.5 V vs. CSE means there is a risk of severe corrosion in air-exposed concrete.

The overall concrete resistivity is one factor that can also be measured to determine the risk of corrosion. A concrete resistivity less than 5000 ohm·cm means there is risk of severe corrosion, but the risk is variable since the resistivity is dependent on the w/cm ratio, porosity, relative humidity, as well as the chloride content and carbonation depth (65). Electrochemical impedance spectroscopy (EIS) is a useful technique in determining the resistance of concrete by applying an alternating potential, with respect to Ecorr of the embedded steel, at varying frequencies and measuring the resulting alternating current (AC) (66). The output impedance (Z) is usually plotted as a function of frequency and is a measure of the ability of a circuit to resist the flow of electrical current, at relative amplitudes and phase angles of the current and voltage (50). The impedance is a sum of both real and imaginary parts and can also be represented by Nyquist (real Z versus imaginary Z) and Bode plots (|Z| and phase angle versus frequency) (67). Regardless of the data representation, the data can be fit accordingly to electrical equivalent circuits, with elements such as resistors, capacitors and inductors, to determine useful electrochemical properties about the reinforced concrete. The series ionic resistance is a property that can be obtained in this manner and is a measure of the permeability of its pore structure. It is dependent on the conductivity of the

HCP, geometry of the reinforced steel and is indicated by the high-frequency intercept on the

Nyquist plot (65). One of the most common cell models that is used as a starting point to fit impedance data in reinforced concrete is the Randles circuit, which is shown in Figure 10 (67). The

Randles circuit has a series ionic resistance, double layer capacitor or constant phase element and a polarization resistance, that is charge transfer resistance in the diagrams shown below (67).

Figure 10 (left) – Randles circuit model (67) Figure 11 (right) - Warburg impedance circuit model (69)

OPC-GGBFS concretes have a denser microstructure with lower porosity than OPC concrete, which leads to a greater resistance and reduction in corrosion rate of the embedded steels. Song and Saraswathy reported that the resistivity of concrete is increased, as an increasing amount of

GGBFS up to 60% is used (59). Furthermore, Macphee and Cao indicated that GGBFS has a drastic alteration of the pore structure and reported increases in concrete resistivity and reduction in the corrosion rate of the steel, due to good protection by the GGBFS microstructure (68). In the study performed by Benjamin, the polarization resistance of steel in OPC-GGBFS mortars had a tendency to decrease after 10 days, which suggests that the corrosion rate is affected most likely by the reducing characteristics of sulfide in GGBFS (62). Although the polarization resistance is affected by fitting the impedance data, Sanchez has shown that certain equivalent circuits are more accurate at modeling the formation of a passive layer on steel in concrete. The investigated circuits had constant phase elements and a Warburg impedance component, which represents diffusion affected impedance at low frequencies, as shown in Figure 11 (69).

2.3.3 Reference Electrodes

Reference electrodes are used in the monitoring of the corrosion potential of reinforced steel by either being permanently embedded in the concrete or externally contacting the concrete surface.

They are generally fabricated from material with behaviour that is independent of the environment and are non-polarizable with a stable electrode potential (50). Embeddable reference electrodes are useful for remote monitoring and are advantageous due to their ability to conduct long-term measurements without risk of oxygen exposure to the concrete (71). However, unlike non- embeddable reference electrodes, they cannot be periodically tested for accuracy and can be affected by carbonation and moisture content within concrete. The embeddable manganese dioxide

(MnO2) pseudo-reference electrode is of interest, due to the MnO2 and Mn2O3 reaction in alkaline environments. It is designed as a double junction electrode with an interface between the metal and inner electrolyte, as well as an interface between the inner electrolyte and concrete (70). The inner electrolyte is typically a NaOH solution, at pH 13.5 (71). The MnO2 electrode also contains a concrete porous plug, shaped to give contact with the concrete specimen (71). At 25°C, the MnO2 electrode in a saturated Ca(OH)2 solution is +0.150 V vs. SCE or +0.396 V vs. SHE (70). In theory however, the MnO2 electrode is not a true reversible reference electrode and the possibility of a liquid junction potential error across its porous concrete plug is an issue, due to difference in mobilities of Na+ and OH- in the inner electrolyte solution (70). In experiments performed by

Muralidharan, MnO2 sensors showed long-term stability and little variations in potential, in concrete samples under laboratory conditions (72). The non-embeddable mercury-mercury oxide

(Hg/HgO) reference electrode can also be used in alkaline environments and at 25°C, the standard electrode potential is +0.098 V vs. SHE in 20% potassium hydroxide (KOH) (73). Its inner electrolyte consists of 20% KOH and it is also designed as a double junction electrode (73).

CHAPTER 3 – EXPERIMENTAL DETAILS

3.1 High-Density Concrete and Mortar Sample Preparation

The basis for the experimental work was to design and replicate the reinforced high-density concrete, used in the DSCs, in a laboratory environment. In order to effectively accomplish this, the concrete engineers at AMEC Earth and Environmental were consulted for the usage of their facilities and materials to establish the mix design and prepare samples. The mix designs were created for both high-density concrete and mortar, with the purpose of calculating the amount of components needed to successfully batch correctly proportioned concrete or mortar mixtures. The mix design incorporates yield and batch calculations and accounts for the air content, w/cm ratio and specific gravity of all components. Mix designs were prepared for producing the high-density concrete and mortar samples with a 50% OPC and 50% GGBFS mix, as well as 100% OPC only.

The fine aggregate of the high-density mortar samples was also varied between fine hematite (iron oxide) sand and silica sand. Therefore in total, two types of high-density concrete samples and four types of high-density mortar samples were prepared, as indicated in Table 8.

3.1.1 Cementitious Materials Specifications

OPC and GGBFS were used as the cementitious materials in producing the high-density concrete and mortar. Prior to usage, the cementitious materials were verified with the OPC certification

CSA A3001-08 (Type 10 OPC) and the GGBFS certification CSA A3001 (Type S GGBFS). The chemical compositions and physical analysis of the OPC and GGBFS used are specified in Tables

3 and 4, respectively. It should be noted that only the sulfur content was determined for the

GGBFS and the remaining composition is expected to be similar to Table 2.

Chemical Mass Mineral Mass Physical Compounds Composition Compounds Composition Analysis (%) (%) CaO (Lime) 62.50 Tricalcium 57.33 Blaine 385 silicate (alite) Fineness 2 CaO3SiO5 (C3S) (m /kg) SiO2 (Silica) 19.20 Dicalcium 11.79 Air Content 6.22 silicate (belite) (%) CaO2SiO4 (C2S) Al2O3 5.43 Tricalcium 10.88 Initial 115 (Alumina) aluminate Setting Time (aluminate) (minutes) Ca3Al2O6 (C3A) Fe2O3 2.08 Tetracalcium 6.32 Compressive 33.09 (Iron Oxide) aluminoferrite Strength, (ferrite) 7 days Ca2AlFeO5 (MPa) (C4AF) MgO 2.32 Compressive 40.70 (Magnesia) Strength, SO3 (Sulfur 4.12 28 days trioxide) (MPa) Total Alkali 0.99 Loss on 2.49 Free Lime 1.18 Ignition (%) Table 3 – Chemical composition and physical analysis of OPC used in experimental work

Chemical Physical Composition Analysis S (Sulfide Blaine Air Compressive Strength Slag Activity Sulfur) Fineness Content (50% OPC-50% GGBFS) Index (%) (m2/kg) (%) 7 days 28 days 7 28 (MPa) (MPa) days days Mass 1.14 717 6.80 22.81 38.37 80.1 110.8 Composition (%) Table 4 - Chemical composition and physical analysis of GGBFS used in experimental work

3.1.2 Coarse and Fine Aggregate Specifications and Testing

Magnetite (Fe3O4) and hematite (α-Fe2O3) were used as the coarse and fine aggregates, respectively and their grading, specific gravity, absorption and surface moisture content properties were determined by the ASTM specification testing procedures mentioned in section 2.2.3.1.

Samples of the aggregates from the raw stockpile were taken, washed, dried and immersed in water for 24 hours, prior to testing. They were then removed and dried to a saturated-surface dry state before being evenly split into two equal batch sizes for duplicate testing. For the grading analysis, the aggregates were shaken through successive CSA sieves in an 8 inch shaker chamber with a shaking time of 10 minutes. The particle sizes present in the coarse and fine aggregates were well distributed and for the most part within the allowable limits, as shown in Figures 12 and 13, the grading charts which are used to graphically represent the sieve analysis results.

Figure 12 (left) – Coarse aggregate grading chart (Red lines indicate maximum and minimum limits) Figure 13 (right) – Fine aggregate grading chart (Red lines indicate maximum and minimum limits)

The specific gravity and absorption properties were determined together with the usage of calibrated equipment, such a wire basket suspended in water for the coarse aggregate and volumetric flask filled to capacity with water for the fine aggregate, to determine the apparent mass in water. Along with the determination of the saturated surface-dry and oven-dry masses, the properties were then experimentally calculated as shown in Appendix A-1. The surface moisture content of the aggregates was also determined similarly, however the aggregate samples that were tested did not need to be washed or dried, as the actual wet mass of sampled raw stockpile aggregates was needed to find their existing moisture content. Table 5 summarizes the average properties of the coarse and fine aggregates from the duplicate testing results.

Specific gravity Absorption Moisture content (SSD) (kg/m3) (%) (%) Coarse aggregate 4310 0.183 0.563 Fine aggregate 4754 0.140 0.120 Table 5- Properties of coarse and fine aggregates

Compositional testing was performed using X-ray diffraction (XRD) analysis on the crushed hematite aggregate to determine its purity existing as hematite. The XRD analysis was performed at the University of Toronto, Department of Chemistry, X-ray Powder Diffraction Lab using a

Siemens D5000 conventional θ/2θ diffractometer. Although the fine aggregate sand is considered to consist entirely of hematite, the XRD analysis showed that it consisted of 89.6% hematite, 7.9% magnetite and 2.5% impurities. The XRD spectrum is shown in Appendix A-2.

3.1.3 Admixtures

The AEA and superplasticizer materials that were used were the BASF brand Micro Air and PS

1466. Micro Air offers excellent stability of air-entrainment and the recommended dosage was 2 mL/kg of cementitious material (74). PS 1466 is useful for concrete mixtures containing additional cementitious materials other than OPC and the recommended dosage was 3.06 mL/kg of cementitious material (75). Although these dosages were suggested by the manufacturer, in practice, it was found that the recommended dosages were high and the actual dosages used had to be experimentally determined depending on the mixing and batching conditions. Furthermore, since the air content is affected by the AEA and superplasticizer dosage rate, experimentally verifying the correct dosage rates to obtain the required air content is important.

3.1.4 Mix Designs

The first part of the mix design involved a yield calculation that ensured that the components were

35 properly proportioned at theoretical masses to create a concrete volume of 1 m3 at the desired density, if they were mixed together. Concrete is normally produced with the components being specified and measured on a mass basis; however it is sold on a volume basis, since the mass required will vary depending on the specific gravity of the concrete’s components. The calculation involved adding the volumes (m3) of the cementitious material, aggregates and water, determined from their masses and specific gravities, to total 1 m3 of concrete. The air content was subtracted from the 1 m3 basis to give the total theoretical yield, while the volumes of the AEA and superplasticizer were not accounted for since they were an insignificant amount. The second part of the mix design involved a batch calculation, which was a scale-down of the yield calculation, to calculate the actual component masses that would be mixed together to create a desired amount of concrete or mortar. The batch calculation took into the account the non-SSD state of the raw stockpile aggregates, by incorporating their absorption and existing surface moisture content properties, which corrected the aggregates’ water demand and thus the amount of mix water to be added. The mixing water needs to be accounted for correctly, since excess water will cause sedimentation of the aggregates or deficient water will affect the HCP formation. Mix design calculations for both the high-density concrete and mortar samples are shown in Appendix A-3.

3.1.5 Mixing and Casting Procedures

Upon creation of the mix design, the cementitious material, aggregates and water were then weighed accurately to three decimal places, in kg, on an electronic balance and kept in dry, separate containers. An industrial sized, flat-pan mixer was used to prepare 80 kg concrete batches, while a Hobart N-50 Quart mixer was used to prepare 4.5 kg mortar batches. Concrete and mortar batches have different mixing times, but the essential procedure is to mix, rest and then mix again.

The detailed mixing procedures were documented and are presented in Appendix A-4. The fluidity, wetness and workability of the batches were monitored to ensure adequate mixing and uniformity of the batch in the mixer. Upon completion of mixing, the slump (for the concrete batches only) and air test were performed on the freshly mixed concrete or mortar to ensure they were within specifications. The slump specification was determined, from the NEP visit, to be 110-135 mm, while the air content was 5.5% +/- 1.5%. A Humboldt concrete air meter was used as per ASTM

C231 (Standard Test Method for Air Content of Freshly Mixed Concrete by the Pressure Method) to test the air content of the concrete. For the mortar, the ASTM C185 (Standard Test Method for

Air Content of Hydraulic Cement Mortar) mathematical derivation and formula was modified to calculate the air content as shown in Appendix A-5. Samples were cast in cylindrical, propylene/ethylene copolymer blend plastic molds. The concrete samples were cast in 4” diameter

(D) by 8” length (L) molds and the mortar samples were cast in smaller 2” D by 4” L molds.

Figure 14 presents a schematic of the prepared samples and the compositional difference between high-density concrete and mortar, while Figure 15 shows the two types of high-density concrete samples after 7 days of setting.

Figure 14 (left) - Schematic of high-density GGBFS concrete and mortar samples Figure 15 (right) - Cross-sections of concrete samples ((A) 50% OPC-50% GGBFS (B) 100% OPC)

The concrete and mortars were tapped and rodded to ensure that the HCP and aggregates were consolidated within the molds and not segregated. Carbon and stainless steel (CS and SS) electrode

embedment into the samples was performed using a plastic ring support apparatus to hold the

electrodes in place during the setting stages. The carbon steel was specified to be of a 99% pure

iron basis and the stainless steel was grade 308L welding rod (76). The steels were approximately

2 mm D by 200 mm L in the concrete and 2 mm D by 100 mm L in the mortar, with 3 cm length

shrink tubing around their middle. After embedment, the molds were capped, sealed with water

resistant silicone, electrically taped and transported to a deaerated glove box for storage and

electrochemistry experimentation, as indicated in section 3.2. The glove box was purged via

vacuum pump to eliminate oxygen and filled with nitrogen at a high flow rate, as per the

manufacturer’s recommended procedure (77). Tables 6 and 7 summarize the mix design and

casting results for the high-density GGBFS concrete and mortar samples with iron oxide

aggregates. Appendix A-3 presents the detailed results for all the samples.

Mass Percentages (%) AEA Superplasticizer W/CM Density OPC GGBFS Coarse Fine Mix (mL) (mL) Ratio (kg/m3) Sample Aggregate Aggregate Water

Concrete 5.09 5.09 49.09 36.59 4.12 7.50 7.50 0.42 3535.20 Mortar 8.90 8.90 - 75.14 7.06 1.60 2.45 0.42 3471.76 Table 6 – Mix design results for high-density GGBFS concrete and mortar samples (with iron oxide aggregates)

Sample Air Slump Compressive Content (%) (mm) Strength 7 days 28 days (MPa) (MPa) Concrete 7 110 28 42 Mortar 6.1-23.7 - - - Table 7 – Casting results for high-density GGBFS concrete and mortar samples (with iron oxide aggregates)

3.1.6 Concrete to Mortar Procedure and Theory

The high-density mortar samples were made to replicate the high-density concrete, with the major

assumption that the coarse aggregate in the concrete does not react with the HCP. In order to have

replication of the concrete’s mortar reaction chemistry in mortars themselves, the concrete batches

that were not cast, were taken after mixing and the coarse aggregate was sieved out using a 4.75

inch sieve. The fine aggregate that was coated on the coarse aggregate was determined by washing

the coarse aggregate and collecting the remains in a 200 mm sieve. The cementitious material and

water that were coated on the stone were then determined to be the remaining amount. A

theoretical mortar batch calculation was then performed from these lost amounts to obtain mass

percentages of the cementitious material, fine aggregate and water in a correctly proportioned

mixture with no coarse aggregate. Appendix A-6 indicates in detail the concrete to mortar

calculations. By using these calculated mass percentages, the mortar mix design was then

developed similar to the concrete mix design. Other assumptions involved in this calculation were

that the HCP contains all the water and the sieved coarse aggregate was at SSD condition.

3.1.7 High-Density Concrete and Mortar Sample List Summary

The following table presents a summary of the high-density concrete and mortar samples that were

produced, with an indication of the different reference and working electrodes used.

Sample Type Mortar Concrete Sample 1 2 3 4 5 6 Number and a b c d e f g a b c a b c a b c a b c a b Letter Cementitious 50% OPC 100% 50% OPC 100% 50% OPC 100% Material 50% GGBFS OPC 50% GGBFS OPC 50% GGBFS OPC Coarse - Magnetite Stone Aggregate Fine Hematite Sand Silica Sand Hematite Sand Aggregate Reference Hg/HgO MnO Hg/HgO MnO Hg/HgO MnO Hg/HgO Electrodes 2 2 2 Working CS and SS Electrodes Table 8 - High-density concrete and mortar sample list summary

3.2 Electrochemistry Experiments

The corrosion mechanism that is of interest was mentioned in section 2.3.1 and involves a relatively beneficial, reducing environment being created in the high-density GGBFS, reinforced concrete or mortar environment, due to oxygen consumption by CaS. It was hypothesized that this reducing environment could be created in the prepared samples, outlined in section 3.1, where the only oxygen availability is within the HCP and not from the external environment. In order to understand the effect of this sulfur corrosion mechanism on the embedded steels, electrochemistry experiments involving corrosion potential and EIS measurements were performed. Additionally, voltammetry scans on embedded noble metals were performed to try to detect dissolved species including oxygen, chloride, sulphur compounds, etc.

3.2.1 Electrochemical Cell

The electrochemical cell was set-up using a PARSTAT® 2263 potentiostat as the electronic hardware to control the three-electrode system. The cell consisted of the reinforced high-density concrete or mortar molded samples with designated carbon and stainless steel working electrodes to be tested, a designated carbon steel counter electrode, similar to Figure 14, and a 1 cm D drilled hole for the reference electrode/sample surface contact. The available surface area of the embedded electrodes, neglecting the shrink tubing surface area, was approximately 2.23 cm2 in the mortars and 5.37 cm2 in the concrete. The reference electrodes used were both embeddable

(MnO2 with 0.1 M NaOH as the inner electrolyte) and non-embeddable (Hg/HgO with 20%

KOH as the inner electrolyte) types, with electrolytic contact between the sample surface and the non-embeddable reference electrodes being created by a sponge soaked in 20% KOH. During non-measurement periods, the samples were stored in the deaerated, nitrogen-filled glove box.

3.2.2 Reference Electrode Preparation

The embeddable (MnO2) pseudo-reference electrodes were prepared using stainless steel 304 tubing with silver-soldered copper wire to provide the external electrical connection. The internal design of the reference electrode consisted of packed manganese (IV) oxide powder, an inner 0.l

M NaOH gel at pH 13.5, an OPC plug at 0.5 w/c ratio at one end and epoxy resin plug at the other. The NaOH gel was prepared by adding 2% granulated agar by mass in water and heating the mixture to a dissolving point of 90°C. The mixture was then allowed to cool to 55°C and 0.1

M NaOH was added to achieve a pH of 13.5. Prior to embedment, the four electrodes were stored in a saturated Ca(OH)2 solution at 25°C, with their potential being periodically monitored for about 6 weeks with a commercial Force Technology MnO2 reference electrode, using a voltmeter. The non-embeddable (Hg/HgO) reference electrode did not require any preparation, however it was periodically calibrated against other reference electrodes and during non- measurement periods it was stored in a 20% KOH solution at 25°C.

3.2.3 Corrosion Potential Measurements

Corrosion potential measurements were performed on the embedded steels for approximately 10 weeks. The PARSTAT® 2263 potentiostat measured the open circuit potential, between the working electrode and reference electrode, in volts (V) versus time, relative to the type of reference electrode used. The total measurement time was approximately 5 minutes and the collected data points for the last 50 seconds were averaged and taken as the potential value.

3.2.4 Electrochemical Impedance Spectroscopy Measurements

EIS measurements were performed on the embedded steels for approximately 10 weeks. The

41 measurements consisted of the PARSTAT® 2263 potentiostat applying an alternating sinusoidal potential of 10 mV with amplitude (rms) respect to Ecorr of the embedded steel and measuring the resulting alternating current, at varying frequencies between 100,000 to 0.1 Hz. The output impedance measurements were represented as Bode and Nyquist plots and the total measurement time was approximately 5 minutes.

3.2.5 Cyclic Voltammetry Measurements

Cyclic voltammetry scans were performed on metals embedded in mortar samples made from mix design 2, as indicated in Table 8. The measurements consisted of the PARSTAT® 2263 applying a scan rate of 10 mv between potentials of -0.15-1.1 V for silver and -0.75-0.2 V for platinum. The available surface area of the silver and platinum electrodes was approximately 0.434 and 0.347 cm2, respectively. It was theorized that silver can be used to detect free chloride and be used as a practical reference electrode, due to its double oxidation state chemistry of Ag2O and AgO, while platinum voltammetry can imply evidence about the effect of redox reactions involving sulfide and oxygen in the OPC-GGBFS samples.

3.2.6 Coarse Aggregate Resistance

The resistance of the coarse aggregate was determined by a wet resistivity measurement. One magnetite stone was taken and two holes were drilled into the stone. The stone was then ground to rectangular geometry with 4 cm length and total surface area of 66 cm2. Granulated agar salt gel, prepared from dissolving 2% agar by mass in salt water, was set into the holes and was the contact medium for copper wire to magnetite. The magnetite was abraded to ensure surface removal of maghemite, which is more resistive. The resistance was measured by a voltmeter.

3.3 Ion Chromatography Experiments

The possible reducing chemistry of the CaS in GGBFS, used in the high-density concrete and mortar samples, was investigated by ion chromatography experiments to provide complementary information relevant to the sulfur corrosion mechanism, as discussed in section 3.2. CaS is capable of consuming oxygen in the concrete and mortar samples, which may cause conversion into other sulfide forms such polysulfide(s), soluble sulfide and most notably thiosulfate.

Thiosulfate is of particular interest since it is kinetically stable in alkaline environments such as concrete, and capable (in principle) of corrosively attacking the embedded carbon and stainless steels, were the pH to drop locally. Ion chromatography was used to determine the concentration of these anions produced in simulated pore water solutions. Simulated pore water solution environments were preferred, rather than concrete or mortar environments, since it was easier to extract solution from liquefied cementitious material at a high solution to GGBFS ratio, rather than the pore fluid of concrete or mortar. The advantages of ion chromatography as an analytical technique over other wet-chemical methods are its speed, sensitivity, selectivity and stability of the separation column (79). It is a favourable option for solutions with ppm concentrations of thiosulfate (retention time ~11 minutes) and sulfate (retention time ~5 minutes).

3.3.1 Ion Chromatography Column

The ion chromatography column at the University of Toronto, Department of Chemistry,

ANALEST lab was used for performing the experimental work. The system consists of a

PerkinElmer Series 200 binary pump, Alltech ERIS 1000HP Autosupressor and Alltech 550

Conductivity Detector. Essentially the system utilizes temperature compensated conductivity detection, which can eliminate thermally induced background noise, as well as autosupression of

43 the background conductivity of the eluent passing through the column (79). This allows an increased signal detection of the analyte. The basic operation of the column is to inject the sample solutions through the pressurized chromatograph column, which contains a resin with covalently bonded charged functional groups (79). The ions then become adsorbed onto the column, while an ionic extraction eluent, 7.2 mol/L of sodium carbonate at a flowrate of 1.5 mL/min, runs through the column causing the ions to separate based on size and type. Prior to operation, the pump feed line is purged at a flowrate of 10 mL/min. The sample run time is approximately 16 minutes. The output data is a chromatograph with distinctive peaks that represent ionic species at different retention times. By testing standard solutions of known thiosulfate and sulfate concentrations, the corresponding peak areas that are produced and displayed on the chromatograph can then be used to create a correlation between area and concentration. This correlation then can be used to approximate unknown solution concentrations from outputted chromatograph peak areas.

3.3.2 GGBFS in Water

The hydration of GGBFS in deionized water was examined at 10:1 water to GGBFS ratio. It was hypothesized that the reaction would be slow, as indicated in Juenger’s work, and the amounts of thiosulfate and sulfate produced would be relatively low (29). An experimental comparison between a controlled, limited oxygen environment and unlimited oxygen environment was made by creating two duplicate GGBFS in water samples within cylindrical, plastic molds. The controlled case was sealed from the external atmosphere, similarly to how the high-density concrete and mortar samples were sealed as explained in section 3.1.5. The controlled case was also made carbon dioxide free, by being sealed in a CO2 free glove box containing limewater as a

CO2 absorbing medium. The unlimited oxygen samples were designated as non-CO2 free samples. The purpose of creating CO2 free and non-CO2 free samples was to investigate and compare the effect of carbonation on the solution pH. The purpose of the limited oxygen environment solution was to have it replicate a concrete or mortar environment, where the oxygen content is low. The solutions were allowed to hydrate within the moulds and filtered samples were periodically extracted from the moulds on a weekly basis for ion chromatography testing.

3.3.3 GGBFS in Basic Solutions

The hydration of GGBFS in basic solutions of 0.1 M NaOH and saturated Ca(OH)2 in NaOH

(produced to create a total 0.1 M OH- concentration) at a 10:1 solution to GGBFS ratio were also examined. It was anticipated that the presence of calcium would slow down slag dissolution and sulfide release for oxidation. The latter experiment was also performed with the hypothesis that an aqueous calcium equilibrium would effectively be established in the solution, due to saturation with Ca2+, which would affect the dissolution of sulfide from GGBFS into the solution. If no calcium equilibrium is established then the high pH of the solution should be able to attack the glass GGBFS particles to release any frozen sulfide for oxidation, as theorized in the NaOH solution experiment. Duplicate samples of both types of the basic solutions were prepared, with

CO2/non-CO2 free and limited oxygen/unlimited oxygen environment comparison.

3.3.4 Aggregate and GGBFS in Basic Solutions

- The hydration of GGBFS in a saturated Ca(OH)2 + NaOH (total 0.1 M OH concentration) at a

10:1 solution to GGBFS ratio, with the addition of 1 g of cleansed, fine aggregate, was investigated

45 to determine if hematite is capable of reacting with reduced sulfur compounds. Duplicate samples were prepared, with CO2/non-CO2 free and limited oxygen/unlimited oxygen comparison, similarly to the previous experiments. Another sample set was prepared, with 300 ppm of thiosulfate and 1 g of cleansed, hematite added to the basic solutions to determine if dissolved thiosulfate in solution reacts differently with hematite, in the absence of GGBFS. Furthermore, a third sample set of the basic solution was prepared, with 300 ppm of thiosulfate and 1 g of synthetic magnetite, to investigate if magnetite reacts with thiosulfate. It was expected that the iron oxide aggregates and thiosulfate would react in some manner, however the reaction chemistry is theorized to be complex and the results may not be representative of the reaction chemistry in the high-density concrete and mortar samples. A comparison between the first two sets of experiments, involving aggregate in the absence of GGBFS, would also help understand the limitations a

GGBFS environment may introduce on the iron oxide aggregates’ capability to react with CaS.

3.3.5 Ion Chromatography Sample List Summary

The following table presents a summary of the ion chromatography samples that were produced, with the amount and type of sample medium and components indicated.

Sample Sample Amount Sample Amount of Number Medium of Components Components Medium (g, unless stated) (mL) 1 Deionized Water GGBFS 10 2 0.1 M NaOH GGBFS 10 3 Saturated Ca(OH)2 + NaOH GGBFS 10 4 Saturated Ca(OH) + NaOH GGBFS, Hematite 10, 1 2 100 5 Saturated Ca(OH)2 + NaOH Thiosulfate, 300 ppm, 1 Hematite 6 Saturated Ca(OH)2 + NaOH Thiosulfate, 300 ppm, 1 Magnetite Table 9 - Ion chromatography sample list summary

3.4 Microscopy Experiments

The microstructures of the cementitious materials, high-density concrete and mortar samples were investigated by microscopy and analysis. Environmental scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) techniques, such as high-resolution imaging, quantification and mapping, were used to view and analyze the materials and samples. The primary objective was to understand the composition and elemental distribution of GGBFS environments upon hydration, as well as any visual evidence of the origin and transport of sulfide from GGBFS.

The general composition of the microstructure was desired to be practically studied rather than a time-dependent one, since it is difficult to track the hydration products in their unsteady state of growth and the fate of sulfide during GGBFS hydration has been widely speculated, as discussed in section 2.2.4. Furthermore, due to the fact that the GGBFS contained only 1.14% sulfide by mass, it was expected that the sulfur quantification may be unrealistic due to the low mass % and

EDX signal detection limitation. Other challenges that were considered during the microscopy experiments were relief and morphology effects due to the porous, uneven hydrated microstructure, as well as the superimposition of the sample geometry during EDX mapping.

3.4.1 Environmental Scanning Electron Microscope (ESEM)

The Hitachi SU6600 analytical variable pressure (environmental) scanning electron microscope

(ESEM) at the University of Toronto, Department of Materials Science and Engineering was used for microscopy. The ESEM instrument parameters used with the backscatter electron (BSE) detector and Bruker Quantax EDX detector, were an electron accelerating voltage of 20 kV, variable chamber pressure of 20 Pa, working analysis distance of 10 mm, medium probe current intensity and both mechanical apertures set at a focus level of 3. At these parameters, the Schottky-

47 field emission electron source of the microscope provided good beam current stability with a small probe diameter. The beam stability is affected by the voltage of 20 kV, as charging tends to become significant as the voltage increases, however the variable pressure in the chamber tends to dissipate this. Prior to analysis of each sample, aperture, beam and stigma alignment were performed to stabilize the beam current and any oscillations in the x and y directions. The coarse and fine adjustment of the resolution was also modified at the appropriate brightness, contrast and scan refresh rate. The majority of experimentation was performed using the Bruker EDX detector, however occasionally an Oxford Instruments EDX detector was used in its place. The EDX quantification for the ESEM was also capable of quantifying lighter elements than sodium’s atomic weight, such as oxygen. It should also be noted that the samples did not have to be conductively carbon coated prior to inserting them into the ESEM chamber with a 2 inch diameter stage, as the accumulation of electric charge on the specimen surface is avoided with an ESEM (80).

3.4.2 Microscopy Sample List and Mounting Procedures

Microscopy experiments were performed on a variety of different samples, as summarized in Table

10. The mounting procedure performed for the wet GGBFS samples in different environments was identical. The procedure involved initially preparing the sample at a 0.5 solution to GGBFS ratio in a disposable mounting cup and allowing the sample to hydrate for 1 week. Following hydration, the sample would then be placed in a larger plastic mounting cup and a mixture of epoxy resin and hardener would be cast into the cup to permanently embed the sample. The epoxy resin and hardener were mixed by mass at a 100 to 14 g ratio for approximately 7 minutes. After epoxy placement into the cup, the embedded sample was then placed in a desiccator and ran under a vacuum, for approximately 10 minutes, to ensure removal of all air bubbles from the epoxy

mixture. The sample was then allowed to set for approximately 1 day, so the epoxy mixture could

permanently harden around the embedded specimen. The mounting procedure performed for the

dry GGBFS sample was similar, however the GGBFS was directly mixed with the epoxy mixture

at an equal volume amount. The mounting procedure performed for the high-density concrete and

mortar samples was also similar, however there was no initial sample preparation, as small, cubic

sections were cut from the high-density concrete and mortar sample types, indicated in table 8.

Sample Dry Wet GGBFS in High- High- Types GGBFS Different Environments Density Density Mortar Concrete

Sample GGBFS GGBFS GGBFS Components + OPC

Sample - Aerated Deaerated NaOH Saturated Aerated 1 2 3 4 5 6 Medium Water Water Ca(OH)2 Water + NaOH

Table 10 - Microscopy sample list summary (The high-density concrete and mortar samples are the six types of samples listed in Table 8)

3.4.3 Grinding and Polishing Procedures

The grinding and polishing procedures were identical for all the different samples. The samples

were first cut with a diamond saw to expose the surface of the embedded specimen. They were

then ground from a 78 to 15.3 µm finish with successively finer silicon carbide grinding paper and

polished from a 9 to 1 µm finish on polishing discs, both using a rotating grinding/polishing wheel.

Water was used as the lubricant during the grinding stages, while diamond paste suspensions from

9 to 1 µm were used as the polishing media. During successive grinding and polishing stages, the

samples were ultra-sonically cleaned in deionized water to ensure removal of any debris and

examined under an optical microscope to ensure a satisfactory surface appearance.

CHAPTER 4 – RESULTS AND DISCUSSION

4.1 Electrochemical Analysis

Electrochemical analysis of the embedded steels has important implications for understanding their corrosion behaviour in high-density concrete and mortar samples. Due to the high alkalinity of the samples, the embedded steels are likely to passivate and remain protected, unless the total available oxygen in the samples is significantly depleted by CaS oxidation (only in the high-density GGBFS concrete and mortar samples) leaving little or no oxygen available for cathodic reduction on the steels. The electrochemistry experiments, involving OCP and EIS measurements on the embedded steels, were subsequently analyzed to confirm whether this type of reducing environment exists within the samples. The voltammetry scans on the noble metals also provide complementary information relevant to this issue.

4.1.1 Embeddable Reference Electrode Measurements

To ensure accurate OCP and EIS electrochemical measurements with the embeddable MnO2 reference electrodes, their potentials were periodically monitored against a Force MnO2 reference electrode to ensure a known, stable potential value prior to embedment, as shown in Figure 16.

Figure 16 – Experimental MnO2 reference electrode potentials versus time

The results indicate that the electrodes reached a relatively steady state potential after 6 weeks of measurement. A deviation in the saturated Ca(OH)2 solution pH by +/- 0.5 pH units is likely responsible for the potential variation by approximately +/- 10 mV in the electrodes for samples

1d, 1e and 3c, since changes in alkalinity can affect the MnO2-Mn2O3 reaction (71) . Furthermore, the electrode for sample 5a was constructed first, using the procedure described in section 3.2.2, and the potential variation from the other electrodes, may be due to the fact that it was constructed individually and not concurrently with the other electrodes. An average steady state potential value of -0.192 V versus the Force MnO2 reference electrode was taken from the results and calculated to be approximately +0.108 V versus the Hg/HgO reference electrode, based on the literature potential values presented in section 2.3.3. The large difference in potential from the Force electrode can be attributed to the experimental electrodes’ stainless steel tubing, which can adopt its own potential, and is a different type of housing than used in the Force electrode.

4.1.2 Open Circuit Potential Analysis

The OCP measurements of the embedded steels were analyzed to determine the existence of a reducing environment within the high-density GGBFS concrete and mortar samples, by observing a negative drift in the OCP with time. A negative drift indicates that oxygen is being consumed by CaS oxidation, whereas a positive drift would mean that an oxidizing environment exists, despite the fact that CaS is present in stoichiometric excess to the oxygen. The OCP measurements for the high-density GGBFS concrete and mortar with iron oxide aggregates, sample types 1 and 5 indicated in Table 8, are presented in Figures 17 and 18. The OCP measurements for the remaining concrete and mortar samples are shown in Appendix A-7. After approximately 10 weeks, in the high-density concrete samples, there are some low potentials

51 suggesting oxygen consumption by the sulfide, however there is also positive to negative potential variation, which may be influenced by the hematite-magnetite equilibrium potential.

Since magnetite is present at approximately 49% by mass, the contact it has with the embedded steels is at an unpredictable extent, due to its relatively larger surface area. The lowest potential observed is approximately -0.4 V versus SHE, which is slightly higher than the potential of the hematite-magnetite equilibrium at pH 13, shown in the iron Pourbaix diagram, attached in

Appendix A-9. It is reasonable to suggest that if the sulfide can consume the oxygen to very low levels then the embedded steels will not passivate, and if there is sufficient contact with the insulating hematite, the potential can rise from near the hematite-magnetite equilibrium potential to a higher oxidizing potential, as observed on the carbon steel in sample 5c. For the stainless steel in sample 5c, the potential appears to remain reducing and hematite may not be in sufficient contact to cause positive potential variation. However, the large amount of hematite in the samples, approximately 37%, ensures that the potential will not go lower than the magnetite- hematite equilibrium potential and that this potential dominates the direction that the steel’s potential takes, rather than the sulfide-oxygen redox potential.

Figures 17 (left) and 18 (right) – Corrosion potentials of embedded carbon and stainless steels in high-density GGBFS concrete and mortar samples with iron oxide aggregates (samples type 5 and 1)

The high-density mortar samples, 1a-1e, show no negative drift towards reducing potentials, but rather a stable negative potential that is slowly oxidizing, most likely due to the diffusion of external oxygen into the samples over the 10 weeks of measurement. The high air content

(23.7%), within these samples and lower CaS content, due to the smaller sample size, are likely the reasons that a negative drift was not achieved. Since these samples were created with the recommended dosages of AEA and superplasticizer, as shown in mix design 1 in Appendix A-3, their high air content was not initially expected and accounted for only after the electrochemical measurements, by a digital point-count method on their hardened state. This led to the development of the mortar air content formula, as described in Appendix A-6, to ensure that the air content, of any successive samples produced, could be measured in the mortar’s plastic state.

Successive samples were produced with a lower air content, 1f and 1g at 6.1%, similar to the concrete air content results. A slightly negative drift in the OCP for approximately 4 weeks was observed, before eventual oxidizing potentials occurred similarly to samples 1a-1e. The lowest potentials observed in both the high-density concrete and mortar samples were around -0.5 V versus Hg/HgO, which is similar to the results of Benjamin and Angus (62, 63).

The variation in the mortar’s OCP results prompted stoichiometric analysis to determine criteria for the negative drift observance from CaS oxidation. It was found that the molar ratio between sulfide and oxygen is approximately 4 times greater in samples 1f and 1g than 1a to 1e. The ratio for samples 1f and 1g was twice as great as for the concrete samples, due to the higher mass percentage of GGBFS, rather than their similar oxygen content. Since the air content is desired to be fixed at 5.5% +/- 1.5%, it is reasonable to establish a minimum molar ratio, where a reducing environment has been shown not to occur. Based on this analysis and the worst-case scenario

53 assumption that all the total available oxygen will react with sulfide and not be consumed in other HCP reactions, the molar ratio of sulfide to oxygen is approximately 55. The detailed stoichiometric calculations are shown in Appendix A-8.

The OCP results for the high-density 100% OPC concrete and mortar were similar, as generally higher oxidizing potentials were observed due to the absence of GGBFS. The OCP results for the

GGBFS mortar with silica sand show no signs of reducing potentials, most likely due to the high air content of 17.9% and an approximate molar ratio of sulfide to oxygen of 50. However, the potentials observed were more negative than the 100% OPC mortar with silica sand. The difference between silica and hematite sand on the OCP results is not evident, however silica sand seems to have an effect on creating lower stainless steel potentials than carbon steel potentials. In general, the OCP results of the carbon steels were higher than the stainless steels, as expected, since the presence of 19-21% chromium lowers the oxygen reduction kinetics responsible for the formation of the chromium (III) oxide, passive film. Furthermore, Pourbaix diagrams for both iron and chromium at pH 13, attached in Appendix A-9, indicate that both types of metals form stable surface phases and corrosion is relatively unlikely.

4.1.3 Electrochemical Impedance Spectroscopy Analysis

The EIS measurements of the embedded steels were analyzed to determine the value of the series electrolyte resistance between the working and counter electrodes in the high-density concrete and mortar samples. Although the electrolyte resistance does not directly reveal information about the existence of a reducing environment, it gives information on the continuity of conductive pore water throughout the HCP, and also (for the concrete samples) on the possible shorting-out effect

54 of the large magnetite aggregate. The electrolyte resistance for the high-density GGBFS concrete and mortar with iron oxide aggregates, sample types 1 and 5 indicated in Table 8, are presented in

Figures 19 and 20. The electrolyte resistance measurements for the remaining concrete and mortar samples are shown in Appendix A-10. From the results, it can be seen that for both types of samples, the electrolyte resistance has a tendency to increase with time, due to the increase in strength and decrease in permeability of the HCP. As less free water becomes available, due to the consumption by the HCP, the total porosity will decrease which creates a denser paste structure.

Figures 19 (left) and 20 (right) – High frequency electrolyte resistance of embedded carbon and stainless steels in high-density GGBFS concrete and mortar samples with iron oxide aggregates (samples type 5 and 1)

The carbon steel in sample 5c initially shows an increasing resistance with time, however there is a decrease after approximately 50 days. The decrease is due to the short circuit effect of the conductive magnetite, which also causes an increase in the double-layer capacitance and correlates with the potential rise observed in the OCP results. The high resistance results of the stainless steel in sample 5c also correlate to its reducing OCP results, as there is no potential variation or decrease in resistance over time, due to the lack of conductive magnetite contact with the stainless steel. In general, considering the results of all the samples, the stainless steels did not show a trend of having higher resistances than the carbon steel, however this may be a geometrical effect since the geometrical environment for current flow may vary for the electrodes. Furthermore, the mortar

55 samples containing silica sand had a much higher resistance than the mortar samples with hematite sand, most likely since the silica sand is finer and has a higher water demand. Both the concrete and mortar samples with GGBFS were observed to have a higher resistance than the samples with

100% OPC, due to the effect that the GGBFS has on the HCP. Nevertheless, the concrete had a lower resistance than the mortar samples, due to the presence of conductive magnetite.

4.1.4 Cyclic Voltammetry Analysis

Cyclic voltammetry measurements of the silver electrode, as shown in Figure 21, confirm the presence of free chloride within the HCP, which makes it difficult to evaluate the effectiveness of pure silver as a reference electrode. The broad peak originating around 0.22 V is near the Ag/AgCl equilibrium potential and the OPC measurement prior to voltammetry measurement was approximately 0.25 V, suggesting that the present free chloride concentration is sufficient enough to create a stable potential consistent within the Ag/AgCl potential range, rather than the silver oxide potentials. The Ag/Ag2O and Ag2O/AgO oxidation peaks were observed at higher potentials, as expected from their potential-pH curves on the silver Pourbaix diagram in Appendix A-9, however the second oxidation peak was sufficiently close to the oxygen equilibrium potential.

Figures 21 (left) and 22 (right) – Cyclic voltammograms for silver and platinum in 100% OPC high-density mortar

Analysis of the platinum voltammogram indicates a charging effect related to the double layer capacitance, as observed by the loop at positive potentials above 0 V, in Figure 22. The observed cathodic current densities are likely too high for oxygen reduction to be occurring and the cycled potential range is too positive for hydrogen evolution. Furthermore, the large IR drop observed suggests that ohmic control may be a factor in contributing to the total impedance of the cell, as the overall resistance of the cell is observed to be greater than the solution resistance.

4.1.5 Coarse Aggregate Resistance Measurement

The wet resistance (R) measurement of the magnetite, was approximately 0.65 Ω and based on the

aggregate’s geometry (A), length (l) and from the resistivity equation, , the aggregate’s

resistivity is 10.7 Ω·cm. The low resistivity is comparable to the literature values discussed in section 2.2.3 and suggests that magnetite can be fairly conductive in the high-density concrete samples, if sufficient abrasion of surface maghemite occurs by the HCP.

4.2 Ion Chromatography Analysis

The oxidation of CaS from GGBFS into thiosulfate could potentially affect the corrosion tendency of the embedded steels. The kinetic stability of thiosulfate is an important factor in determining its ability to cause corrosion – equation 10 – rather than oxidizing to relatively non- corrosive sulfate (equation 11). Both anodic reactions occur alongside oxygen reduction.

2- 2+ - 2CaS + 2O2 + H2O → S2O3 + 2Ca + 2OH [10] 2- 2- + S2O3 + H2O + 2O2 → 2SO4 + 2H [11]

The concentrations of thiosulfate and sulfate formed are important for determining the total mass percentage of sulfur that was oxidized. Since a standard of 10 g of GGBFS was used in all the experiments, the maximum concentration of thiosulfate that could potentially form from 100%

57 conversion of the CaS (equation 10), regardless of the GGBFS environment, would be approximately 2250 ppm. If this amount of thiosulfate was to be completely converted into sulfate or CaS oxidizes directly to sulfate without partially forming any thiosulfate, then the maximum concentration of sulfate formed would be approximately 3850 ppm. Appendix A-11 presents these calculations in detail, which are an important basis, for the experimental concentrations of the sulfur anions formed in different GGBFS environments, since they are likely to be lower than these theoretical concentrations, due to partial conversion.

4.2.1 GGBFS in Water Results

The hydration of GGBFS in deionized water yielded relatively low amounts of thiosulfate and sulfate, compared to the maximum theoretical concentrations calculated. As shown in Figures 23 and 24, the amounts of thiosulfate and sulfate produced from CaS oxidation in water were monitored as a function of hydration time, for both the CO2 free and non-CO2 free samples. The pH of both types of the samples was found to be slightly basic, ranging from 9-11.

Figures 23 (left) and 24 (right) –Thiosulfate and sulfate concentrations versus hydration time for GGBFS in water

The hydration was expected to be very slow in water, since the low pH is insufficient to attack the glass GGBFS grains and release sulfide. The thiosulfate and sulfate concentrations were higher in

the CO2 free sample than in the carbonated sample, suggesting that there was more release of sulfide from the GGBFS grains, due to the higher pH level in the CO2 free sample. The sulfate concentrations were generally higher than the thiosulfate concentrations; however it was not much of a difference to suggest that thiosulfate is not kinetically stable in water. Furthermore, based on the maximum amounts of thiosulfate and sulfate formed (comparing both samples), the mass percentage of sulfur oxidized to form thiosulfate was 10.6%, while the mass percentage of sulfide oxidized to form sulfate was 19.7%. The calculations are detailed in Appendix A-11 and it can be understood that a fair amount of sulfide has been left unoxidized, possibly remaining in the grains.

4.2.2 GGBFS in Basic Solutions Results

The formation of thiosulfate and sulfate from GGBFS in basic solutions varied, as CaS oxidation is primarily dependent on the solution pH to attack the glass GGBFS grains to release sulfide. As shown in Figures 25 and 26, the amounts of thiosulfate and sulfate produced in basic solutions were monitored as a function of hydration time, for both the CO2 free and non-CO2 free samples.

The pH of the Ca(OH)2 + NaOH samples was monitored to be basic over time, as it ranged from

13-13.5, while the pH of the NaOH samples was slightly less basic in the range of 11-13.

Figures 25 (left) and 26 (right) – Thiosulfate and sulfate concentrations versus hydration time for GGBFS in basic solutions

The results indicate that the thiosulfate and sulfate concentrations are lower in the Ca(OH)2 +

NaOH experiment than the NaOH experiment. Since the simulated GGBFS and Ca(OH)2 + NaOH pore solution is saturated with Ca2+, there is more formation of C-S-H gel within the HCP, which may become a barrier in preventing sulfide from diffusing out into the pore solution for oxidation.

Furthermore, the forward rate of thiosulfate formation may also be affected by an aqueous equilibrium being reached by the reaction, in equation 10, due to the reservoir of OH- and Ca2+ ions.

In general, for the GGBFS and Ca(OH)2 + NaOH pore solution that was CO2 free, the thiosulfate concentration was higher and the sulfate concentration was lower, when compared to the carbonated sample. The CO2 free sample was monitored to be at a higher pH than the carbonated sample, which suggests that more sulfide was released and oxidized into thiosulfate, however further oxidation into sulfate may have been inhibited due to the C-S-H barrier. The carbonated sample was at slightly lower pH, however most likely resistant to carbonation, due to the reservoir of OH- ions. Therefore, the thiosulfate concentration was observed to be lower, which also implies that GGBFS grains may have a certain oxidation depth that is sensitive to pH. Therefore, a lower pH may only be sufficient to attack the outer glass structure of a GGBFS grain, where the sulfide may already be present in a higher oxidation state, hence the slightly higher sulfate levels observed in the carbonated sample.

The high concentrations of thiosulfate and sulfate observed in the NaOH experiment, signify the absence of a saturated Ca2+ equilibrium, since the sulfur oxidation products are easily able to diffuse out into the simulated pore solution. The sulfate concentration is approximately 2.5 times

60 greater than the thiosulfate concentration, suggesting that rapid thiosulfate oxidation is occurring with no maximum thiosulfate concentration observed, as theoretically anticipated. However, due to the slow nature of the HCP reactions, a decline in thiosulfate concentration and complete oxidation to sulfate may take longer to occur. Similarly to the GGBFS in water analysis, the maximum mass percentage of sulfur oxidized to form thiosulfate was 15.9% and 21.3% in the Ca(OH)2 + NaOH and NaOH experiments, respectively. The maximum mass percentage of sulfur oxidized to form sulfate was 4.1% and 29.8% in the Ca(OH)2 + NaOH and NaOH experiments, respectively. The calculations are shown in detail in Appendix A-11. From these results, it can be understood that thiosulfate is quite kinetically stable in basic solutions and oxidation into non-corrosive sulfate is pH and time dependent.

4.2.3 Aggregate and GGBFS in Basic Solutions Results

The addition of hematite to GGBFS in basic solutions provides a substrate for surface reactions to take place between sulfur compounds and oxygen, since the aggregate becomes coated with HCP.

As shown in Figures 27 and 28, the concentrations of thiosulfate and sulfate were observed to be higher in the carbonated, unlimited oxygen sample than both the CO2 free, limited oxygen sample and GGBFS in basic solutions samples. This was anticipated, since the unlimited oxygen may allow hematite to oxidize thiosulfate into higher oxidation state sulfur compounds, as theorized in equations 13 and 14. Although, similarly to the GGBFS in basic solutions samples, the sulfate concentrations were lower than the thiosulfate concentrations, due to the aqueous Ca2+ equilibrium and C-S-H barrier.

2- + 2- 2+ 2S2O3 + Fe2O3 + 6H → S4O6 + 2Fe + 3H2O [13] 2- 2- + 3Fe2O3 + S2O3 + O2 + H2O → 2Fe3O4 + 2SO4 + 2H [14]

Figures 27 (left) and 28 (right) - Thiosulfate and sulfate concentrations versus hydration time for aggregates and GGBFS in basic solutions

There was little sulfate oxidation observed when thiosulfate was dissolved in solution, as the thiosulfate concentrations remained fairly constant in both sample types, suggesting that hematite cannot oxidize thiosulfate when there is no surface reaction mechanism. Furthermore, synthetic magnetite was observed to react with thiosulfate and oxidize it into sulfate in the CO2 free sample, however this was not the case in the carbonated sample, which signifies that magnetite may possibly be reacting with sulfide in the high-density GGBFS concrete.

4.3 Microscopy and Analysis

Microscopy and analysis was performed on the samples listed in Table 8, with the majority of the high-resolution imaging, quantification and mapping focusing on the microstructure of GGBFS environments upon hydration. The visual analysis is consistent with literature theory and the ion chromatography results, which suggests that GGBFS grains are hydrated from their outer glass structure to their internal core. If this is the case, then it is likely that the core of the grains contains most of the sulfide. However contrary to this theory, it may appear that the sulfur could be evenly distributed throughout the particle making it difficult to assess if it exists as core inclusions or particulate phases. The elements of interest at their characteristic peak positions in GGBFS grains

62 are shown in Figure 29, an EDX spectrum. It is evident from the spectrum that calcium, silicon, magnesium, aluminum and oxygen are the dominating elements, as expected from the chemical composition of GGBFS, shown in Table 2 in section 2.2.2. EDX quantification tables indicating the weight and atomic mass percentages of elements in areas of interest will be presented in subsequent analysis of the individual microscopy samples, rather than EDX spectra.

cps/eV 20

O Fe K Mn Na Mg Al Si S K Ca Mn Fe 10 Ca

2 Ca

0 0 1 2 3 4 5 6 7 keV Figure 29 – Typical EDX spectrum of GGBFS grains 4.3.1 Dry GGBFS

Dry GGBFS was analyzed to determine the elemental composition and average particle size of

GGBFS grains, as a reference, prior to any particle hydration. As shown in Figures 30 and 31, the size of the GGBFS particles varies anywhere between 5-40 µm, but the average particle size is quite small, as expected due to the GGBFS’s high Blaine fineness of 717 m2/kg. Furthermore, the grains tend to appear quite angular and jagged in nature, which is understandable due to their amorphous structure. The EDX quantification is detailed in Figure 33, on a selected area in Figure

32, where it is apparent that calcium and silicon are present in abundance, evidently in the form of calcium and silicon oxides. An approximate 1% sulfur weight percentage was observed in the

GGBFS grains and was expected due to their unreactive nature when dry and unhydrated. The remaining elements are within the composition ranges of their chemical constituents, listed in

Table 2.

Figures 30 (left) and 31 (right) – ESEM images of dry GGBFS grains mixed in epoxy

Figures 32 (left) and 33 (right) – ESEM image of single GGBFS grain and EDX quantification table for dry GGBFS

4.3.2 GGBFS in Water

GGBFS mixtures with aerated and deaerated, deionized water were analyzed, respectively. The imaging analysis showed that GGBFS hydration in water appears to be relatively slow, which is consistent with the ion chromatography results for GGBFS in water. The comparison between both the aerated and deaerated sample, shows that a lack of oxygen in water has an effect on GGBFS hydration. It is apparent from Figures 34 and 36, that the average GGBFS grain size appears to be larger in the deaerated sample than in the aerated sample. This suggests that due to the lack of oxygen, the formation of hydration products such as C-S-H gel, AFm and hydrotalcite phases, which are essential for the supporting microstructure, may be inhibited. The rate of hydration in

64 both types of samples also appears to be delayed, which is possible due to the aluminosilicate coating that has been observed to form on the surface of GGBFS grains (29).

Figures 34 (left) and 35 (right) – Figures 36 (left) and 37 (right) - ESEM images of GGBFS grain(s) in aerated water ESEM images of GGBFS grain(s) in deaerated water The EDX quantification results were similar, shown in Figures 38 and 39, for the GGBFS grains in

Figures 35 and 37 respectively, indicating a decrease in calcium and silicon content, compared to the dry GGBFS grains, which implies that hydration is indeed occurring, regardless how slow.

Figures 38 (left) and 39 (right) – EDX quantification tables for GGBFS in aerated and deaerated water

4.3.3 GGBFS in Basic Solutions

The hydration of GGBFS in basic solutions was analyzed to yield similar trends to the ion chromatography experiments involving basic solutions. In the GGBFS and NaOH sample, it was visually understood that high pH is effective at attacking the glass GGBFS grains, as there appears to be more dispersion and hydration of the grains, as shown in Figure 40. In Figure 41, the sulfur

EDX mapping, there were similar dispersion effects suggesting that the sulfide may be reacting in the basic environment. Furthermore, a lower sulfur weight percentage of 0.59%, was observed in

GGBFS grains as indicated in Figure 44, the EDX quantification table corresponding to the grain

area in Figure 40. In the GGBFS and Ca(OH)2 + NaOH sample, the sulfur weight percentage is higher at approximately 1%, shown in Figure 45, which may be due to the Ca2+ equilibrium and additional C-S-H gel barrier that is preventing oxidized sulfur compounds from being released into the surrounding microstructure. There also appears to be calcite formation on the surface of the grains, which may be precipitation favoured, due to the high dissolved Ca2+ concentration.

Figures 40 (left) and 41 (right) – ESEM image and Figures 42 (left) and 43 (right) – ESEM image and EDX sulfur mapping of GGBFS in NaOH EDX sulfur mapping of GGBFS in Ca(OH)2 + NaOH

Figures 44 (left) and 45 (right) - EDX quantification tables for GGBFS in NaOH and Ca(OH)2 + NaOH

4.3.4 OPC and GGBFS Paste

The hydration of OPC and GGBFS mixed with aerated water was fairly active, as the GGBFS grain sizes were relatively smaller compared to the dry GGBFS grain sizes, as observed in Figures

46 and 47. OPC is an excellent activator for GGBFS and the amount of C-S-H gel appears to have increased as the cementitious materials hydrate together. In Figure 47, the formation of calcium hydroxide crystals is somewhat evident, which is also important for activating GGBFS hydration.

Figures 46 (left) and 47 (right) – Figures 48 (left) and 49 (right) – ESEM images of OPC and GGBFS paste EDX sulfur mapping and ESEM image of GGBFS grain

The EDX quantification results, shown in Figure 50 for the GGBFS grain in Figure 49, were similar to the previous samples. It was observed that the topography of the samples, tended to yield comparable results, despite the possibility of nonuniform absorption of X-rays, due to the porous, uneven microstructure. The sulfur weight percentage of 0.7% was higher than the GGBFS in

NaOH result, although lower than the dry GGBFS result, suggesting that there may be release of sulfide from the GGBFS grains. However, despite the high pH of the OPC-GGBFS paste, the increase in C-S-H gel production can inhibit sulfide transport, as observed in the GGBFS in

Ca(OH)2 + NaOH sample. The majority of the sulfide appears to remain within the GGBFS grains as indicated by Figure 48, the EDX sulfur mapping of several GGBFS grains within the paste.

Figure 50 - EDX quantification table for GGBFS grain in OPC and GGBFS paste

4.3.5 High-Density Concrete

The microstructure of the high-density concrete was investigated, with the high-resolution imaging and EDX quantification indicating that the internal concrete structure is quite complex, due to the presence of coarse and fine aggregate of varying proportions and sizes. In Figure 51, it is apparent

67 that there are partially hydrated or unhydrated GGBFS grains in the high-density GGBFS concrete, similar to the OPC and GGBFS paste, which suggests that hydration may be slow or delayed. The complicated aggregate structure is shown in Figure 52, where the bright, large particles represent magnetite and some of the dark, small particles signify hematite. The internal composition of magnetite also has some variation, due to the different iron oxidation states present.

Figures 51 (left) and 52 (right) - Figures 53 (left) and 54 (right) – EDX iron mapping ESEM images of high-density GGBFS concrete and ESEM image of high-density 100% OPC concrete

The EDX quantification results, shown in Figure 55 for the GGBFS grains in Figure 51, were similar to the previous samples, although there was a higher weight percentage of iron and lower weight percentage of calcium observed. The coarse aggregates in Figure 53 were found to be composed of approximately 64% iron and 34% oxygen by weight percentage.

Figures 55 (left) and 56 (right) – EDX quantification tables for GGBFS grains in high-density GGBFS concrete and coarse aggregates

4.3.6 High-Density Mortar

The hydration of high-density GGBFS mortar was observed to be comparable to the high-density

GGBFS concrete, as most of the GGBFS grains appeared to be partially hydrated or unhydrated, as

68 shown in Figure 57. In the silica GGBFS mortar, the GGBFS grain size varies considerably, as observed in Figure 59, suggesting that the larger grains may be undergoing very slow hydration.

The significance of this observation is that the finer silica sand has a greater effect on interacting with the HCP and hydrating most of the GGBFS grains. In Figure 60, the silica GGBFS mortar appears to be denser around the GGBFS grains, as compared to the hematite microstructure observed in Figure 58. The hematite aggregate generally appears to be segregated from the

GGBFS grains in a mortar, as compared to concrete, possibly due to variations in the mixing procedures, which affects the surface area coverage of the aggregates by the HCP.

Figures 57 (left) and 58 (right) - Figure 59 (left) and 60 (right) – ESEM images of high-density GGBFS mortar ESEM images of silica GGBFS mortar

EDX quantification results, shown in Figure 59 and 60 for the GGBFS grains in Figures 55 and 57, yielded similar results. The sulfur content was also similar to the high-density GGBFS concrete.

Figures 61 (left) and 62 (right) – EDX quantification tables for GGBFS grains in high-density GGBFS mortar and silica GGBFS mortar

CHAPTER 5 – SUMMARY AND CONCLUSIONS

Electrochemistry, ion chromatography and microscopy have been used to investigate the chemistry and potential corrosion mechanisms associated with reduced sulfur compounds, such as calcium sulfide, in high density GGBFS concrete, mortar and simulated pore-water solution environments.

Calcium sulfide is capable of consuming oxygen in high-density GGBFS concrete and mortar environments, which may cause conversion into reduced sulfur states that can affect the potential of embedded steels.

High-density concrete is important for shielding photon radiation from used nuclear fuel in dry storage containers. Through extensive testing and understanding of the physical and chemical properties of cementitious materials, aggregates and chemical admixtures, replication of the high- density concrete was performed from mix design calculations. The exact chemistry of the high- density concrete was also proven to be reproducible in high-density mortars, by sieving out the coarse aggregate and accounting for the lost cementitious material, fine aggregate and water.

Corrosion potential and electrochemical impedance spectroscopy analyses indicate that a reducing environment exists within some of the high-density GGBFS concrete samples, where some of the oxygen is being consumed by the calcium sulfide from GGBFS. However, if hematite is in sufficient contact with the embedded steel, there can be potential variation and a decrease in the high-frequency electrolyte resistance, due to magnetite’s short-circuiting ability. Some of the high- density GGBFS mortars showed a reducing environment as well and the molar ratio of sulfide to oxygen was determined to be one of the key factors in the creation of this corrosion mechanism.

Furthermore, the high-frequency electrolyte resistance was observed to increase with hydration time, due to the increase in strength, decrease in permeability and loss of moisture within the hydrated cementitious paste. The addition of GGBFS, type of embedded steel and fine aggregate used, also had effects on the corrosion potential and high-frequency electrolyte resistance results.

Ion chromatography analysis on simulated pore water solutions has shown than thiosulfate is quite kinetically stable in neutral and basic GGBFS solutions with or without aggregate. The theoretical decrease in thiosulfate concentration was not observed over several weeks of monitoring, however at any given time there was no more than 25% of the initial sulfur oxidizing to thiosulfate. Other conclusions suggest that high pH is effective at attacking the glass GGBFS grains to release sulfide for oxidation, saturated Ca2+ pore solutions are able to inhibit the mass transport of sulfide and that magnetite is capable of oxidizing sulfide into higher oxidation state sulfur compounds.

Microscopy has provided visual evidence of the particle size distribution in dry GGBFS and

GGBFS hydration in a variety of different environments such as water, basic solutions, OPC paste, high-density concrete and mortar. The majority of the analysis suggests that GGBFS grains decrease in particle size, as they are hydrated from their outer glass structure to their internal core.

The high pH of the basic solutions was found to hydrate GGBFS the most effectively, while hydration appeared to be the slowest in deoxygenated water. Additionally, the EDX quantification results showed that the sulfur composition within GGBFS grains decreased after hydration.

CHAPTER 6 – FUTURE WORK

Future work in this field of study should take into account the following considerations to improve the experimental details, as well as further investigating relevant areas of interest:

1. It may be beneficial to extract pore solutions from GGBFS mortar samples during the

initial hydration stages and perform ion chromatography analysis. The concentration of

reduced sulfur species in the actual pore solutions could then be compared to their

concentrations in the simulated pore solutions. Computer simulation of the ion

chromatography experiments using aqueous equilibrium modeling software would also

provide relevant details about the expected concentration of reduced sulfur species.

2. Time dependent or wet microscopy experiments with a staining or marking chemical

should be performed to better determine the growth of the hydration products. This type of

experimentation would be especially valuable during the first few days of hydration.

3. An experimental rate law that empirically fits the oxygen concentration over time in the

high-density GGBFS concrete and mortar samples would be beneficial to develop to better

understand the kinetics of oxygen consumption by calcium sulfide.

REFERENCES

1. Ontario Power Generation, Pickering waste management facility fact sheet, (2011), Retrieved from http://www.opg.com/power/nuclear/waste/facilities.asp.

2. E. Miller and J. Newman, Advanced concrete technologies: processes, Oxford: Butterworth-Heinemann, 5/1-5/14 (2003).

3. M.F. Kaplan, Concrete radiation shielding: nuclear physics, concrete properties, design and construction, England: Longman Scientific & Technical (1989).

4. M.G. Alexander and S. Mindness, Aggregates in Concrete, New York: Taylor and Francis, 365 (2005).

5. C. Lee, Y.H. Lee and K.J. Lee. Cracking effect of on gamma-ray shielding performance in concrete structure, Progess in Nuclear Energy, 49, 303 (2007).

6. A.S. Mollah, G.U. Ahmad and S.R. Husain. Measurements of neutron shielding properties of heavy concretes using a Cf-252 source, Nuclear Engineering and Design, 135, 325 (1992).

7. E.G. Nawy, Fundamentals of high-performance concrete, New York: John Wiley and Sons, 93 (2001).

8. H.F.W. Taylor, Cement chemistry (2nd edition), London: Thomas Telford Publishing Ltd (1997).

9. American Concrete Institute Committee, ACI education bulletin E3-01-Cementitious materials for concrete, Michigan: American Concrete Institute Committee (2007).

10. S.H. Lee, Effect of particle size distribution of fly ash–cement system on the fluidity of cement pastes, Cement and Concrete Research, 33, 768 (2003).

11. P.C Hewlett, Lea's chemistry of cement and concrete, Oxford: Butterworth- Heinemann (2004).

12. A. Wesselsky and O.M. Jensen, Synthesis of pure Portland cement phases, Cement and Concrete Research, 39, 973 (2009).

13. Halliburton, Cement microscopy, Okhaloma: Halliburton Services (2007).

14. ASTM International, ASTM C150-Standard specifications for Portland cement, United States: ASTM Committee (2009).

15. Portland Cement Association, http://www.cement.org/tech/faq_sulfate.asp, in Concrete technology.

16. S.C. Pal, A. Mukherjee and S.R. Pathak, Investigation of hydraulic activity of ground granulated furnace slag in concrete, Cement and Concrete Research, 33, 1482 (2003).

17. M. Regourd, 8th International Congress on the Chemistry of Cement, 1, 199 (1986).

18. A. Roy, Sulfur speciation in granulated blast furnace slag: an X-ray absorption spectroscopic investigation, Cement and Concrete Research, 39, 659 (2009).

19. P.W. Scott, S.R. Critchley and F.C.F Wilkinson, The chemistry and mineralogy of some granulated and pelletized blastfurnace slags, Mineralogical Magazine, 50, 142 (1986).

20. M.M. Radwan, Influence of the type of BFS on some characteristics of high slag cement, Silicates Industries, 67, 90 (2002).

21. American Concrete Institute Committee, ACI education bulletin E1-07-Aggregates for concrete, Michigan: American Concrete Institute Committee (2007).

22. ASTM International, ASTM C637-Standard specifications for aggregates for radiation- shielding concrete, United States: ASTM Committee (2010).

23. J. Tang, M. Myers, K.A. Bosnick and L.E. Brus, Magnetite Fe3O4 nanocrystals: spectroscopic observation of aqeuous oxidation kinetics, Journal of Physical Chemistry, 107, 7501 (2003).

24. R.C. Newman, Assessment of the use of BFS in concrete for DSCs, University of Toronto (2005).

25. L.E. Lagoeiro, Transformation of magnetite to hematite and its influence on the dissolution of iron oxide minerals, Journal of Metamorphic Geology, 16, 418 (1998).

26. A.M. Neville, Properties of concrete, New York: John Wiley and Sons (1996).

27. Portland Cement Association, http://www.cement.org/basics/concretebasics_concretebasics.asp, in Concrete basics.

28. P.K. Mehta, Concrete: microtructure, properties and materials, California: McGraw-Hill (2006).

29. M.C.G. Jeunger, P.J.M. Monteiro and E.M. Gartner, In situ imaging of ground granulated blast furnace slag hydration, Journal of Materials Science, 41, 7074-7081 (2006).

30. W. Chen and H.J.H. Brouwers, The hydration of slag, part 1: reaction models for alkali- activated slag, Journal of Materials Science, 42, 429 (2007).

31. J.M. Gao, C.X. Qian, H.F. Liu, B. Wang and L. Li, ITZ microstructure of concrete containing GGBS, Cement and Concrete Research, 35, 1303 (2005).

32. American Concrete Institute Committee, ACI education bulletin 233R-95-Ground Granulated Blast-Furnace Slag as a Cementitious Constituent in Concrete, Michigan: American Concrete Institute Committee (2000).

33. American Concrete Institute, Presentation 236 D-role of interlayer water, Los Angeles: American Concrete Institute Committee (2008).

34. T. Matschei, B. Lothenbach and F.P. Glasser, The AFm phase in Portland cement, Cement and Concrete Research, 37, 120 (2007).

35. I. Regourd, 7th International Congress on the Chemistry of Cement, 4, 493 (1981).

36. J.M. Richardson and J.J. Biernacki, Stoichiometry of slag hydration with calcium hydroxide, Journal of American Ceramic Society, 85, 947 (2002).

37. R. Javelas, J.C. Maso, J.P. Ollivier, and B. Thenoz, Direct observation by transmission electron microscopy of the binding cement paste aggregate in mortars of calcite and quartz, Cement and Concrete Research, 4, 285 (1975).

38. H.F.W. Taylor, A method for predicting alkali ion concentrations in cement pore solutions, Advances in Cement Research, 1, 5-17 (1987).

39. S. Song and H.M. Jennings, Pore solution chemistry of alkali-activated ground granulated blast-furnace slag, Cement and Concrete Research, 29, 168 (1999).

40. S.Song, D. Sohn, H.M. Jennings and T.O. Mason, Hydration of alkali-activated ground granulated blast-furnace slag, Journal of Materials Science, 35, 254 (2000).

41. T.Matschei, F. Bellmann and J. Stark, Hydration behaviour of sulphate-activated slag cements, Advances in Cement Research, 17, 170 (2005).

42. H. Satake, T. Hisano and S. Ikeda, The rapid determination of sulfide, thiosulfate and polysulfide in the lixiviation water of blast-furnace slag by means of argentimetric potentiometric titration, Bulletin of the Chemical Society of Japan, 54, 1970 (1981).

43. Böhni, H, Corrosion in reinforced concrete structures, Boca Ranton Florida: CRC Press, 7-9 (2005).

44. C.L. Page and P.Lambert, Kinetics of oxygen diffusion in hardened cement pastes, Journal of Materials Science, 22, 942-946 (1987).

45. K. Kobayashi, Oxygen diffusivity of various cementitious materials, Cement and Concrete Research, 21, 280 (1991).

46. F. Hunkeler, The resistivity of pore water solution - a decisive parameter of rebar corrosion and repair methods, Construction and Builing Materials, 10, 382 (1996).

47. R.H. Doremus, Transport of oxygen in silicate glasses, Journal of Non-Crystalline Solids, 349, 243 (2004).

48. American Concrete Institute Committee, ACI education bulletin E4-03-Chemical admixtures for concrete, Michigan: American Concrete Institute Committee (2003).

49. A.Bentur, S. Diamond and N.S Berke, Steel corrosion in concrete: fundamentals and civil engineering practice, London: Taylor and Francis (1998).

50. M.A. El-Reedy, Steel-reinforced concrete structures: assessment and repair of corrosion, United States: CRC Press (2007).

51. L. Bertolini, B. Elsener, P. Pedeferri and R.B. Polder, Corrosion of steel in concrete: prevention, diagnosis and repair, Germany: Wiley-VCH (2004).

52. Concrete Materials and Structural Integrity Research Group, Transport mechanisms, corrosion of steel and corrosion assessment in concrete, University of Cape Town (2010).

53. V. L'Hostis, F. Foct and P. Dillmann, Corrosion behaviour of reinforced concrete: laboratory experiments and archaeological analogues for long-term predictive modelling, Journal of Nuclear Materials, 379, 126 (2008).

54. K. West, Basic Corrosion and Oxidation, London: Ellis Horwood Ltd (1980).

55. D. Tromans, Anodic polarization behavior of mild steel in hot alkaline sulfide solutions, Journal of Electrochemical Society, 127, 1253-1256 (1980).

56. L.J. Parrott, The influence of cement type and curing on the drying and air permeability of cover concrete, Magazine of Concrete Research, 171, 103 (1995).

57. C.L. Page and K.W.J. Treadaway, Aspects of the electrochemistry of steel in concrete, Nature, 297, 109-114 (1982).

58. K. Horiguchi, T. Chosokabe, T. Ikabata, and Y. Suzuki, The rate of carbonation in concrete made with blended cement. Michigan: American Concrete Institute Committee, 917-31 (1994).

59. H. Song and V. Saraswathy, Studies on the corrosion resistance of reinforced steel in concrete with ground granulated blast-furnace slag - an overview, Journal of Hazardous Materials, B138¸ 226-233 (2006).

60. F. Jacobs and F.H. Wittmann, Long term behaviour of concrete in nuclear waste repositories, Nuclear Engineering and Design, 138, 163 (1992).

61. K.Y. Yeau and E.K. Kim, An experimental study on corrosion resistance of concrete with ground granulate blast-furnace slag, Cement and Concrete Research, 35, 1391-1399 (2005).

62. S. Benjamin, F.A. Khalid and R.A. Khan, Performance of steel in ordinary Portland, fly ash and slag cement mortars during the hydration period, Journal of Materials Processing Technology, 103, 385 (2000).

63. M.J. Angus and F.P. Glasse, Scientific basis for nuclear waste managment IX. Materials Research Society Symposium Proceedings, 547-86 (1986).

64. S.C. Pal, A. Mukhurjee and S.R. Pathak, Corrosion behaviour of reinforcement of slag concrete, ACI Materials Journal, 99, 1-7 (2002).

65. A.P. Crane, Corrosion of reinforcement in concrete construction, London: Society of Chemical Industry (1983).

66. M.E. Orazem and B. Tribollet, Electrochemical impedance spectroscopy, New Jersey: John Wiley and Sons (2008).

67. Gamry Instruments, Basics of electrochemical impedance spectroscopy, Pennsylvania: Gamry Instruments (2010).

68. D.E. Macphee and H.T. Cao, Theoretical description of impact of BFS on steel passivation in concrete, Magazine of Concrete Research, 162, 63-69 (1993.

69. M. Sanchez, J. Gregori, M.C. Alonso, J.J. Garcia-Jareno and F. Vicente, Anodic growth of passive layers on steel rebars in an alkaline medium simulating the concrete pores, Electrochimica Acta, 52, 50 (2006).

70. Mrydal, R. The electochemistry and characteristics of embeddable reference electrodes for concrete, Cambridge: Woodhead 14-15 (2007).

71. S. Muralidharan, A promising embeddable sensor for corrosion monitoring application in concrete structures, Measurement, 40, 601 (2007).

72. S. Muralidharan, T. Ha, J. Bae, Y. Ha, H. Lee, K. Park and D. Kim, Electrochemical studies on the solid embeddable reference sensors, Materials Letters, 60, 652 (2006).

73. Koslow Scientific Testing Instruments, http://koslow.com/reference_electrode.html, in Koslow Scientific Testing Instruments reference electrodes.

74. BASF The Chemical Company, Micro Air product data, United States: BASF Construction Chemicals, LLC (2007).

75. BASF The Chemical Company, PS1466 product data, United States: BASF Construction Chemicals, LLC (2007).

76. Alfa Aesar, Iron wire product data, United States: Johnson Matthey Company, (2007).

77. Plas-Labs Inc., http://www.plas-labs.com/content.cfm?id=101, in Glove Box Applications: How do I purge a Glove Box.

78. J. Weiss, Ion chromatography, New York: John Wiley and Sons, 5 (1995).

79. D. Mathers, http://www.chem.utoronto.ca/facilities/analest/equipment/large/55.htm, in Ion chromatography.

80. P.J. Goodhew, F.J. Humphreys and R. Beanland, The scanning electron microscope, London: Taylor and Francis, 166 (2001).

81. E. Mccafferty, Introduction to corrosion science, New York: Springer, 104 (2010).

82. D.A. Jones, Principles and prevention of corrosion (2nd edition), Upper Saddle River: Prentice Hall, 116 (1996).

83. D.S. Cameron, The chemistry of silver, Reading: Elsevier, 878 (2009).

84. A.J. Betts, D.P. Dowling, M.L. McConnell and C. Pope, The influence of platinum on the performance of silver-platinum anti-bacterial coatings, Materials and Design, 26, 219 (2005).

APPENDICES

A-1 Coarse and Fine Aggregate Property Calculations

The specific gravity and absorption properties discussed in section 3.1.2 were experimentally

calculated from the measurements and formulae listed in Table A-1. The surface moisture content

calculation is shown in Table A-2. The formulae were obtained from ASTM C127 and C128, as

indicated in section 2.2.3.1.

Measurements Symbols and Fine Measurements Symbols and Coarse and Properties Formulae Aggregate and Properties Formulae Aggregate 1 2 1 2 Saturated 504.3 500.3 Saturated 4458 5243.2 Surface-dry Surface-dry Mass in Air (g) Mass in Air (g)

Oven-dry Mass 503.6 499.6 Oven-dry Mass 4445.6 5238.6 in Air (g) in Air (g)

Mass of Flask 1071.8 1062 Saturated Mass 3429 4024.6 with Specimen in Water (g) and Water to Fill Mark (After 1 hr) (g)

Mass of Flask 673.3 666.9 with Water to Fill Mark (g) Bulk Specific 4.760 4.749 Bulk Specific 4.320 4.299 3 3 Gravity (g/cm ) Gravity (g/cm ) AVERAGE = AVERAGE = 4.754 4.310 Bulk Specific 4.767 4.756 Bulk Specific 4.332 4.303

Gravity SSD Gravity SSD (g/cm3) AVERAGE = (g/cm3) AVERAGE = 4.762 4.318 Absorption % 0.139 0.140 Absorption % 0.279 0.088

AVERAGE = AVERAGE = 0.140 0.183 Table A1 – Specific gravity and absorption calculations

Measurements Symbols and Fine Measurements Symbols and Coarse and Properties Formulae Aggregate and Properties Formulae Aggregate

Wet Mass (g) 500.1 Wet Mass (g) 500.4

Oven-dry 497.3 Oven-dry 499.8 Mass in Air (g) Mass in Air (g)

Moisture 0.563 Moisture 0.120

Content (%) Content (%)

Table A2 – Surface moisture content calculations

A-2 X-ray Diffraction (XRD) Spectrum of Fine Hematite Sand

Figure A1 – XRD spectrum of fine hematite sand

quote from the document or the summary of an 80

A-3 Mix Design Calculations

The mix design calculations discussed in section 3.1.4 were experimentally determined for the six different types of samples (two high-density concrete and four mortar, as listed in Table 8 in section 3.1.7) and the results are presented here in Tables A3 to A8. The following points should be noted about the calculations:

 The yield and batch calculations for the mortars (mix designs 1-4) were combined into one

calculation, since the mortar mass percentages, obtained from the concrete to mortar

calculations, discussed in section 3.1.6 and shown in Appendix A-6, were used to calculate

the actual batch masses (defined below).

 Yield (m3) of each component is calculated by the following formula:

 SSD mass (kg) of each component is calculated by the following formula:

 The actual batch mass refers to the mass of the components that were used in the mix,

since the aggregates were not at SSD condition. Depending on whether the surface

moisture content is greater or less than the absorption, for the aggregates, the overall

amount of water in the mix is affected and mix water needs to be removed or added. Based

on the coarse and fine aggregate property calculations discussed and calculated in sections

3.1.2 and A-1, only the fine aggregate’s surface moisture content is significantly greater

than its absorption and needs to be accounted for. The resulting formulae calculate how

much additional fine aggregate need to be added and how much mix water needs to be

removed.

 In mix designs 3 and 4, for the mortars containing silica sand, the total yield was kept at the

same value as the total yield for the mortars containing hematite sand. This was done in

order to reverse calculate the actual batch mass of silica sand required, from its specific

gravity and the same yield (volume) of hematite sand.

 In mix design 5, the total yield of 0.945 m3 accounts for 5.5% air content, hence a total of

1 m3. In mix design 6, the total yield is similarly calculated; however there is a slight

discrepancy due to the OPC and GGBFS specific gravity variation.

 In mix designs 5 and 6, the AEA and superplasticizer amounts indicated in the yield

calculation are calculated from the recommended dosage amounts specified in section

3.1.3. AEA and superplasticizer are not accounted for in the total yield, because of their

insignificant amount. The actual amounts that were used are indicated in the batch

calculation and have been experimentally determined on a trial and error basis to give the

desired air content. For mix designs 1 to 4, the recommended dosage amounts were used.

Mix design 1 was specifically of interest and the dosages were also experimentally varied

to reduce the air content in successive samples after the initial samples, 1a-1e, were made.

Mix Design 1 Mass Actual Specific Yield Required Bach 4.5 Percentages Batch Gravity (m3) Mass (kg) (%) Mass (kg) (kg/m3) W/CM ratio 0.42 (On a per Mortar 3471.76 1 m3 Density basis) (Total mix kg/ per m3 of mortar)

OPC 8.90 0.4 3150 0.00013 Air Content 1a-1e 23.7 Measured (%) 1f and 6.1 1g GGBFS 8.90 0.4 2860 0.00014

Fine 75.14 3.381 4754 0.00071 Aggregate (Hematite Sand) Mix Water 7.06 0.318 1000 0.00032 Total ~100 4.5 - 0.00130

AEA 1a-1e 1.60 mL 1f and 1g 0.3 mL Super 1a-1e 2.45 mL Plasticizer 1f and 1g 0.5 mL Table A3 – Mix design for 50% OPC-50% GGBFS mortar with fine hematite sand

Mix Design 2 Mass Actual Specific Yield Required Bach 4.5 Percentages Batch Gravity (m3) Mass (kg) (%) Mass (kg) (kg/m3) W/CM ratio 0.42 (On a per Mortar 3487.87 1 m3 Density basis) (Total mix kg/ per m3 of mortar)

OPC 18.05 0.812 3150 0.00026 Air Content 2a-2e 18.9 Measured (%)

Fine 74.73 3.363 4754 0.00071 Aggregate (Hematite Sand) Mix Water 7.22 0.325 1000 0.00032 Total ~100 4.5 - 0.00129

AEA 1.62 mL

Super 2.48 mL Plasticizer Table A4 – Mix design for 100% OPC mortar with fine hematite sand

Mix Design 3 Mass Actual Specific Yield Required Bach 3.014 Percentages Batch Gravity (m3) Mass (kg) (%) Mass (kg) (kg/m3) W/CM ratio 0.42 (On a per Mortar 2332.60 1 m3 Density basis) (Total mix kg/ per m3 of mortar)

OPC 8.90 0.4 3150 0.00013 Air Content 3a-3c 17.9 Measured (%)

GGBFS 8.90 0.4 2860 0.00014

Fine 75.14 1.896 2680 0.00071 Aggregate (Silica Sand) Mix Water 7.06 0.318 1000 0.00032 Total ~100 3.014 - 0.00130

AEA 1.60 mL

Super 2.45 mL Plasticizer Table A5 – Mix design for 50% OPC-50% GGBFS mortar with fine silica sand

Mix Design 4 Mass Actual Specific Yield Required Bach 3.024 Percentages Batch Gravity (m3) Mass (kg) (%) Mass (kg) (kg/m3) W/CM ratio 0.42 (On a per Mortar 2349.92 1 m3 Density basis) (Total mix kg/ per m3 of mortar)

OPC 18.05 0.812 3150 0.00026 Air Content 4a-4c 21.9 Measured (%)

Fine 74.73 1.887 2680 0.00070 Aggregate (Silica Sand) Mix Water 7.22 0.325 1000 0.00032 Total ~100 3.024 - 0.00129

AEA 1.62 mL

Super 2.48 mL Plasticizer Table A6 – Mix design for 100% OPC mortar with fine silica sand

Mix Design 5 Component Yield Calculation Batch Calculation Casting Specifications and Results Component Specific Yield SSD Actual Mass Required Bach 80 Mass (kg) Gravity (m3) Mass Batch Percentages Mass (kg) (On a per 1 (kg/m3) (kg) Mass (%) Required Bach 0.023 m3 basis) (kg) Volume (m3) W/CM ratio 0.42 OPC 180 3150 0.057 4.073 4.073 5.09 Concrete 3535.2 Density (Total mix kg/ per m3 of concrete)

GGBFS 180 2860 0.063 4.073 4.073 5.09 Slump 110-135 Range (mm) Coarse 1735.776 4310 0.403 39.280 39.280 49.09 Slump 120 Aggregate Measured (Magnetite (mm) Stone) Fine 1288.224 4754 0.271 29.152 29.275 36.59 Air Content 5.5 +/- Aggregate Range (%) 1.5 (Hematite Sand) Mix Water 151.2 1000 0.151 3.422 3.298 4.12 Air Content 7 Measured (%) Total 3535.2 - 0.945 80.041 80.015 ~100 Compressive 7 28 Strength (MPa) days 28 42 days AEA 0.72 L 7.5 mL Temperature 21.3 of Concrete (°C) Super 1.01 L 7.5 mL Temperature 17.3 Plasticizer of Air (°C) Table A7 – Mix design for 50% OPC-50% GGBFS concrete with iron oxide aggregates

Mix Design 6 Component Yield Calculation Batch Calculation Casting Specifications and Results Component Specific Yield SSD Actual Mass Required 80 Mass (kg) Gravity (m3) Mass Batch Percentages Bach Mass (On a per 1 (kg/m3) (kg) Mass (%) (kg) m3 basis) (kg) Required 0.023 Bach Volume (m3) W/CM ratio 0.42 OPC 360 3150 0.114 8.147 4.073 10.18 Concrete 3535.2 Density (Total mix kg/ per m3 of concrete)

Coarse 1735.776 4310 0.403 39.280 39.280 49.09 Slump 150 Aggregate Measured (Magnetite (mm) Stone) Fine 1288.224 4754 0.271 29.152 29.275 36.59 Air Content 7.2 Aggregate Measured (Hematite (%) Sand) Mix Water 151.2 1000 0.151 3.422 3.298 4.12 Compressive 7 30.7 Strength days (MPa) 28 37.4 days Total 3535.2 - 0.939 80.041 80.015 ~100 Temperature 21.5 of Concrete (°C) Temperature 21.3 of Air (°C) AEA 0.72 L 7.5 mL

Super 1.01 L 7.5 mL Plasticizer Table A8 – Mix design for 100% OPC concrete with iron oxide aggregates

A-4 High-Density Concrete and Mortar Mixing Procedures

The mixing procedures briefly discussed in section 3.1.5 for producing the high-density concrete and mortar samples are sequentially detailed here.

Procedure for Mixing Concrete

1. Ensure mixer is completely clean and dry.

2. Add coarse and fine aggregate into the mixer together.

3. Turn mixer on.

4. Add cementitious material into the mixer. Pour approximately 75% of the mix water at the

same time into the mixer, while holding back approximately 25% of the mix water. This is

performed depending on the mix appearance. If it appears too dry, add the remaining mix

water. If it appears too wet, hold back the mix water. Add the AEA by syringe injection.

5. Start the timer and mix for 3 minutes. Add the superplasticizer by syringe injection, holding

back if necessary depending on the appearance of the mix fluidity, wetness and workability.

6. Off the mixer and rest for 2 minutes. Cover the mixer with a plastic sheet to ensure water

does not evaporate during the rest period.

7. Remove plastic sheet, turn the mixer back on and mix for 3 minutes.

8. Off the mixer. Measure the temperature of the concrete. Perform slump and air test.

Procedure for Mixing Mortar

1. Pour mix water into the mixer. Add AEA by syringe injection.

2. Set mixer on low speed (speed 1) and turn mixer on, start the timer and add cementitious

material into the mixer. Mix for 30 seconds.

3. At 30 seconds, add fine aggregate while mixing.

4. At 60 seconds (1 minute), add superplasticizer by syringe injection. Turn off mixer and set

mixer at medium speed (speed 2). Turn mixer on. Mix to 90 seconds (1.5 minutes).

5. At 90 seconds (1.5 minutes), turn off mixer and rest for 90 additional seconds (1.5 minutes).

Scrape down sides of mixer bowl with spatula.

6. At 180 seconds (3 minutes), turn mixer on. Mixer should be at medium speed (speed 2). Mix

for additional 60 seconds (1 min). At 240 seconds (4 minutes) total, turn mixer off. Perform air

test.

A-5 Mortar Air Content Calculation

The air content measurement of a mortar, as discussed in section 3.1.5, can only be performed when the mortar remains in a plastic state, thus upon the completion of mixing and before stiffening occurs. ASTM C185 outlines the test procedure and mathematical formula used to calculate the air content of a mortar. However, modification of the formula is necessary to ensure that the calculated air content is consistent with the actual batch masses of the mortar mix design.

The following derivation uses the data in Mix Design 1, Table A3, to determine an explicit formula for the air content. Similar derivations were performed to calculate the air content for the other types of mortars (Mix Designs 2-4, Tables A4-A6).

Therefore based on the experimentally measured mass of the mortar in a specified mould volume, the air content can be calculated.

A-6 Concrete to Mortar Calculations

The mortar mix designs in Appendix A-3 were created after determining the mass percentages of the components required to replicate the high-density mortar chemistry from the concrete, in mortars themselves. Section 3.1.6 outlines the procedure used to obtain the relevant data needed to create a mortar that replicates the mortar chemistry of a concrete. The theoretical calculation performed to obtain the mortar mass percentages from high-density OPC-GGBFS concrete, mix design 5, is shown here. A similar calculation was performed for the high-density OPC concrete, mix design 6.

Sieving Data

Sieved Coarse Aggregate Mixture Mass 6409 from Concrete Batch (Before Wash) (g) (contains Coarse Aggregate, Fine Aggregate, Cementitious Material and Water) Sieved Coarse Aggregate Mass from Sieved 5046.20 Coarse Aggregate Mixture (After Wash) (g) (Any Coarse Aggregate retained on the 4.75 inch sieve) Fine Aggregate, Water and Cementitious = 6409-5046.20 1362.80 Material Mass coated on the Sieved Coarse Aggregate (g) (Remaining mixture after Coarse Aggregate was sieved out) Sieved Fine Aggregate Mass from Sieved 853.90 Coarse Aggregate Mixture (After Wash) (g) (Any Fine Aggregate passing 4.75 inch sieve and retained on a 200 mm sieve) Water and Cementitious Material Mass coated = 1362.80-853.90 508.90 on Sieved Coarse Aggregate (g) (remaining mixture after Fine Aggregate sieved out)

Water and Cementitious Material Calculation W/CM ratio 0.42

Cementitious Material Mass (x) 358.38 coated on Sieved Coarse Aggregate (g) Water (1-x) Mass coated on Sieved Coarse 150.52 Aggregate (g)

Concrete Mass Percentages (%) (from Mix Design 5) OPC 5.09 GGBFS 5.09 Coarse Aggregate 49.09 Fine Aggregate 36.59 Mix Water 4.12 Total ~100

Theoretical Concrete Batch Calculation from the Sieved Coarse Aggregate in the Sieved Coarse Mixture (Using Concrete Mass Percentages) Sieved Coarse Aggregate Mass from Sieved 5046.20 Coarse Aggregate Mixture (After Wash) (g) Total Batch Mass (g) = 5046.20/49.09% 10279.36 Fine Aggregate Mass (g) = 10279.36*36.59% 3760.93 OPC Mass (g) = 10279.36*5.09% 523.39 GGBFS Mass (g) = 10279.36*5.09% 523.39 Water Mass (g) = 10279.36*4.12% 423.72

Theoretical Mortar Batch Calculation (Accounting for lost Fine Aggregate, Cementitious Material and Water that was coated on the Sieved Coarse Aggregate from the Sieved Coarse Aggregate Mixture) Fine Aggregate Mass (g) = 3760.93 – 853.90 2907.03 OPC Mass (g) = 523.39 – 0.5*358.38 344.2 GGBFS Mass (g) = 523.39 – 0.5*358.38 344.2 Water Mass (g) = 423.72 – 150.52 273.2 Total Mass (g) = 3868.63

Mortar Mass Percentages (%) Fine Aggregate = (2907.03/3868.63)*100 75.14% OPC = (344.2/3868.63)*100 8.90% GGBFS = (344.2/3868.63)*100 8.90% Water = (273.2/3868.63)*100 7.06% Total - ~100% Table A9 – Concrete to mortar data and calculations

A-7 OCP Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6

The OCP measurements for the remaining high-density concrete and mortar samples, not presented in section 4.1.2, are attached in this appendix.

Figures A2 (left) and A3 (right) – Corrosion potentials of embedded carbon and stainless steels in mortar samples types 2 and 3

Figures A4 (left) and A5 (right) – Corrosion potentials of embedded carbon and stainless steels in mortar sample type 4 and high-density concrete sample type 6

A-8 Sulfide to Oxygen Molar Ratio Calculation

The molar ratio of sulfide, existing as CaS, to oxygen in the high-density GGBFS concrete and mortar samples, as discussed in section 4.1.2, is determined by the following calculation.

Stoichiometrically, under ideal reaction conditions where the reactants have direct access to each other to chemically combine and react, one mole of oxygen is needed to oxidize one mole of calcium sulfide, as shown in equation 10, in section 4.2. However, the sulfide is present in excess to the oxygen in the high-density concrete and mortar samples and that excess amount is important in determining whether the sulfide is consuming oxygen, under non-ideal reaction conditions in the samples. The ratio calculation for the high-density GGBFS mortar samples, 1a-1e, is shown below, with the ratio for the other samples being similarly calculated:

A-9 Pourbaix Diagrams for Metals

Pourbaix diagrams used in the analysis of equilibrium phases for the embedded steels and noble metals are presented here, with the dashed line signifying the approximate alkaline pH of the high- density concrete and mortar samples. The potential-pH equilibrium diagrams are for iron, chromium, silver and platinum as metal-water systems, at 25°C.

Figures A6 (left) and A7 (right) – Pourbaix diagrams for iron-water and chromium-water systems at 298 K (81, 82)

Figures A8 (left) and A9 (right) – Pourbaix diagrams for silver and platinum (83, 84)

A-10 EIS Measurements for Mortar Sample Types 2, 3, 4 and Concrete Sample Type 6

The EIS measurements for the remaining high-density concrete and mortar samples, not presented in section 4.1.3, are attached in this appendix.

Figures A10 (left) and A11 (right) – High frequency electrolyte resistance of embedded carbon and stainless steels in mortar samples types 2 and 3

Figures A12 (left) and A13 (right) – High frequency electrolyte resistance of embedded carbon and stainless steels in mortar sample type 4 and high-density concrete sample type 6

A-11 Sulfur Mass Balance Calculations for Ion Chromatography Analysis

In section 4.2, the maximum amounts of thiosulfate and sulfate that can be formed from CaS oxidation and thiosulfate oxidation are discussed, respectively. The stoichiometric calculations based on the reactions described in equations A1 to A3, are detailed as follows:

2- 2+ - 2CaS + 2O2 + H2O → S2O3 + 2Ca + 2OH [A1] 2- 2+ CaS + 2O2 → SO4 + Ca [A2] 2- 2- + S2O3 + H2O + 2O2 → 2SO4 + 2H [A3]

The mass of sulfur in thiosulfate and sulfate are calculated from the following relationships:

The mass of sulfur from CaS oxidized to form thiosulfate and sulfate are calculated from the following relationship (with the exception of the experiments involving no GGBFS):

Sulfur Mass Balance Calculations Theoretical Maximum Amount of Sulfur in GGBFS CaS in GGBFS (Weight %) 1.14 Mass of GGBFS used (g) 10 Mass of CaS in GGBFS used (mg) 114 Mass of S (sulfide) in GGBFS used (mg) 114 Volume of Water used (L) 0.1 Max Concentration of S (mg/L, ppm) 1140 Molecular Weight of S (g/mol) 32.07 Moles of S in GGBFS 0.004

Theoretical Maximum Thiosulfate Concentration Produced (from [A1]) Moles of Thiosulfate Stoichiometrically Produced 0.002 Molecular Weight of Thiosulfate (g/mol) 112.13 Mass of Thiosulfate Produced (mg) 224.26 Max Concentration of Thiosulfate Produced (mg/L, ppm) 2- (if 100% conversion of S to S2O3 ) 2242.6

Theoretical Maximum Sulfate Concentration Produced (from [A2] or [A3]) Moles of Sulfate Stoichiometrically Produced 0.004 Molecular Weight of Sulfate (g/mol) 96.06 Mass of Sulfate Produced (mg) 384.24 Max Concentration of Sulfate Produced (mg/L, ppm) 2- 2- (if 100% conversion of CaS or S2O3 to SO4 ) 3842.4

GGBFS in Water Calculations Max Concentration of Thiosulfate (After 28 days) (mg/L, ppm) 211.22 Mass of Thiosulfate (After 28 days) (mg) 21.12 Mass of Sulfur in Thiosulfate (After 28 days) (mg) 12.08 Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 10.60

Max Concentration of Sulfate (After 28 days) (mg/L, ppm) 671.11 Mass of Sulfate (After 28 days) (mg) 67.11 Mass of Sulfur in Sulfate (After 28 days) (mg) 22.40 Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 19.65

Total Mass % of Sulfur Oxidized 30.25

GGBFS in Basic Solutions Calculations GGBFS in Ca(OH)2 + NaOH Calculation Max Concentration of Thiosulfate (After 35 days) (mg/L, ppm) 316.44 Mass of Thiosulfate (After 35 days) (mg) 31.64 Mass of Sulfur in Thiosulfate (After 35 days) (mg) 18.10 Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 15.87

Max Concentration of Sulfate (After 35 days) (mg/L, ppm) 138.77 Mass of Sulfate (After 35 days) (mg) 13.88 Mass of Sulfur in Sulfate (After 35 days) (mg) 4.63 Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 4.06

Total Mass % of Sulfur Oxidized 19.93

GGBFS in NaOH Calculation Max Concentration of Thiosulfate (After 28 days) (mg/L, ppm) 424.71 Mass of Thiosulfate (After 28 days) (mg) 42.47 Mass of Sulfur in Thiosulfate (After 28 days) (mg) 24.29 Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 21.31

Max Concentration of Sulfate (After 28 days) (mg/L, ppm) 1016.62 Mass of Sulfate (After 28 days) (mg) 101.66 Mass of Sulfur in Sulfate (After 28 days) (mg) 33.93 Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 29.76

Total Mass % of Sulfur Oxidized 51.07

Aggregate and GGBFS in Basic Solutions Calculations 300 ppm Thiosulfate and Hematite in Ca(OH)2 + NaOH Calculation Maximum Concentration of Thiosulfate (mg/L, ppm) 300 Mass of Thiosulfate (mg) 30 Mass of Sulfur in Thiosulfate (mg) 17.16 Max Concentration of Sulfate (After 35 days) (mg/L, ppm) 15.58 Mass of Sulfate (After 35 days) (mg) 1.56 Mass of Sulfur in Sulfate (After 35 days) (mg) 1.34 Mass % of Sulfur Oxidized to Form Sulfate 7.81

GGBFS and Hematite in Ca(OH)2 + NaOH Calculation Max Concentration of Thiosulfate (After 35 days) (mg/L, ppm) 352.6 Mass of Thiosulfate (After 35 days) (mg) 35.26 Mass of Sulfur in Thiosulfate (After 35 days) (mg) 20.17 Mass % of Sulfur (from CaS) Oxidized to Form Thiosulfate 17.69

Max Concentration of Sulfate (After 35 days) (mg/L, ppm) 154.93 Mass of Sulfate (After 35 days) (mg) 15.49 Mass of Sulfur in Sulfate (After 35 days) (mg) 13.27 Mass % of Sulfur (from CaS) Oxidized to Form Sulfate 11.64

Total Mass % of Sulfur Oxidized 29.33

300 ppm Thiosulfate and Magnetite in Ca(OH)2 + NaOH Calculation Maximum Concentration of Thiosulfate (mg/L, ppm) 300 Mass of Thiosulfate (mg) 30 Mass of Sulfur in Thiosulfate (mg) 17.16 Max Concentration of Sulfate (After 42 days) (mg/L, ppm) 142.11 Mass of Sulfate (After 42 days) (mg) 14.21 Mass of Sulfur in Sulfate (After 42 days) (mg) 4.74 Mass % of Sulfur Oxidized to Form Sulfate 27.62 Table A10 – Sulfur mass balance calculations for ion chromatography analysis