Investigating the potential of CO2 sequestration in concrete through natural and accelerated carbonation An analysis of natural and accelerated CO2 sequestration in million program areas in the municipality of Gothenburg - a case study Master’s thesis in Industrial Ecology MATHIAS BERGQVIST & GUSTAV FRYKLUND DEPARTMENT OF ARCHITECTURE AND CIVIL ENGINEERING CHALMERS UNIVERSITY OF TECHNOLOGY Gothenburg, Sweden 2024 www.chalmers.se www.chalmers.se Master’s thesis 2024 Investigating the potential CO2 sequestration in concrete through natural and accelerated carbonation An analysis of natural and accelerated CO2 sequestration in million program areas in the municipality of Gothenburg - a case study MATHIAS BERGQVIST & GUSTAV FRYKLUND Department of Architecture and Civil Engineering Division of Energy and Environmental Systems Chalmers University of Technology Gothenburg, Sweden 2024 Investigating the potential of CO2 sequestration in concrete through natural and accelerated carbonation: an analysis of natural and accelerated CO2 sequestration in million program areas in the municipality of Gothenburg - a case study © MATHIAS BERGQVIST & GUSTAV FRYKLUND, 2024. Supervisor: Leonardo Rosado, Divison of Water Environment Technology Examiner: Sebastien Rauch, Department of Architecture and Civil Engineering Master’s Thesis 2024 Department of Architecture and Civil Engineering Division of Energy and Environmental Systems Chalmers University of Technology SE-412 96 Gothenburg Telephone +46 31 772 1000 Cover: a green plant growing in a concrete environment. Created with DALL-E, an image-generating deep-learning artificial intelligence built by OpenAI. Typeset in LATEX, template by Kyriaki Antoniadou-Plytaria Printed by Chalmers Reproservice Gothenburg, Sweden 2024 iv Investigating the potential of CO2 sequestration in concrete through natural and accelerated carbonation: an analysis of natural and accelerated CO2 sequestration in million program areas in the municipality of Gothenburg - a case study MATHIAS BERGQVIST & GUSTAV FRYKLUND Department of Architecture and Civil Engineering Chalmers University of Technology Abstract This thesis explores the potential End-of-Life (EoL) management of demolished concrete from Million Program buildings (MP) to act as a carbon sink through the process of natural and accelerated carbonation. The study presents speculative scenarios, natural carbonation in stockpiles and the use of accelerated carbonation through a fluidized bed unit, which involves the utilization of flue gases to enhance the carbon sequestration of concrete aggregates (CA). A significant amount of CO2 is sequestrated in the service life of a building but the largest potential is at the EoL, where the study indicates that concrete can reabsorb between 11.1-55.5 % of CO2 emissions from cement production. This is when accounting for process emissions related to transporting, crushing and using loaders to move the concrete into stock- piles. The thesis suggests the need for more processing sites and optimized stockpile dimensions to maximize carbon uptake. The thesis also highlights the complexity of the carbonation process and calls for further studies to accurately determine the carbonation degree and operational planning for CO2 sequestration in concrete ag- gregates. The research underscores the importance of natural carbonation and its role in reducing the carbon footprint of concrete. Keywords: CO2-sequestration, carbon sink, concrete, miljonprogrammet v Acknowledgements We would first of all like to thank our supervisor Leonardo Rosado for his guidance and positivity throughout the course of this project. We want to express our grati- tude to our examiner Sebastien Rauch for his feedback and support. We’d also like to thank our co-supervisors Barbro Brattström Grujovic and Ulf Kjellén at Skanska who have given us valuable input and support and we’d also like to express our thanks to Erik Liljeby at Skanska for his correspondence and knowledge. Lastly we’d like to extend our gratitudes to our families for their continuous support during this journey. Mathias Bergqvist & Gustav Fryklund, Gothenburg, June 2024 vii List of Acronyms Below is the list of acronyms that have been used throughout this thesis listed in alphabetical order: CA Concrete Aggregates DoC Degree of Carbonation EoL End-of-life IS Industrial symbiosis MP Million Program ix Contents List of Acronyms ix Nomenclature xi List of Figures xiii List of Tables xv 1 Introduction 1 1.1 Aim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.2 Research questions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2 Background 5 2.1 Production, emissions and waste management of concrete . . . . . . . 5 2.1.1 Calcination- and fuel emissions . . . . . . . . . . . . . . . . . 5 2.1.2 Contemporary post-demolition management of concrete . . . . 6 2.2 Carbonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 2.2.1 Estimating carbonation . . . . . . . . . . . . . . . . . . . . . . 8 2.2.2 Conditions for carbon uptake . . . . . . . . . . . . . . . . . . 9 2.3 Demolition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.3.1 Particle sizes . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 2.4 Methods for CO2 sequestration . . . . . . . . . . . . . . . . . . . . . 12 2.4.1 Optimizing natural carbonation of RCA . . . . . . . . . . . . 12 2.4.2 Accelerated carbonation . . . . . . . . . . . . . . . . . . . . . 13 2.4.2.1 Fastcarb: a case study in France . . . . . . . . . . . 14 3 Method 17 3.1 The case study: the million program 1965-1974 . . . . . . . . . . . . 17 3.1.1 The future of the million program . . . . . . . . . . . . . . . . 19 3.2 Effluence of concrete from MP building stock . . . . . . . . . . . . . . 19 3.3 Calculating the concrete stock and use-phase CO2 sequestration in the million program in the municpality of Gothenburg . . . . . . . . . 20 3.3.1 CO2 sequestration use-phase . . . . . . . . . . . . . . . . . . . 21 3.3.2 Transport distances . . . . . . . . . . . . . . . . . . . . . . . . 22 3.3.3 Resource parks . . . . . . . . . . . . . . . . . . . . . . . . . . 22 3.3.4 Natural carbonation . . . . . . . . . . . . . . . . . . . . . . . 22 3.3.5 Cone-geometry and resource park processing capacity . . . . . 24 xi Contents 3.3.6 Accelerated carbonation . . . . . . . . . . . . . . . . . . . . . 25 3.3.7 Emission factors . . . . . . . . . . . . . . . . . . . . . . . . . . 26 3.4 Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.4.1 Scope and scale . . . . . . . . . . . . . . . . . . . . . . . . . . 28 3.4.2 Theoretical limitations . . . . . . . . . . . . . . . . . . . . . . 28 3.4.3 Technical limitations . . . . . . . . . . . . . . . . . . . . . . . 28 4 Results & discussion 29 4.1 Location and concrete stock in the case study . . . . . . . . . . . . . 29 4.2 Lifespan and CO2 sequestration during use-phase . . . . . . . . . . . 29 4.3 Resource parks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 4.4 Natural carbonation . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.4.1 Pile geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . 33 4.4.2 Vikan resource park . . . . . . . . . . . . . . . . . . . . . . . 34 4.4.3 Net-emissions: Vikan . . . . . . . . . . . . . . . . . . . . . . . 36 4.4.4 K̊allered resource park . . . . . . . . . . . . . . . . . . . . . . 38 4.4.5 Net-emissions: K̊allered . . . . . . . . . . . . . . . . . . . . . 41 4.4.6 Gunnilse resource park . . . . . . . . . . . . . . . . . . . . . . 42 4.4.7 Net-emissions: Gunnilse . . . . . . . . . . . . . . . . . . . . . 45 4.4.8 Utilizing surplus volumes . . . . . . . . . . . . . . . . . . . . . 46 4.5 CO2 donors and Fastcarb technology . . . . . . . . . . . . . . . . . . 47 4.5.1 Fastcarb: Vikan . . . . . . . . . . . . . . . . . . . . . . . . . . 48 4.5.2 Fastcarb: K̊allered . . . . . . . . . . . . . . . . . . . . . . . . 49 4.5.3 Fastcarb: Gunnilse . . . . . . . . . . . . . . . . . . . . . . . . 50 4.6 Net-emissions: Fastcarb technology . . . . . . . . . . . . . . . . . . . 51 4.7 Aggregated results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 5 Conclusion 53 Bibliography 55 A Appendix I A.1 Process emissions, Vikan 0.4 m . . . . . . . . . . . . . . . . . . . . . I A.2 Process emissions, Vikan 2 m . . . . . . . . . . . . . . . . . . . . . . II A.3 Process emissions, K̊allered 0.4 m . . . . . . . . . . . . . . . . . . . . III A.4 Process emissions, K̊allered 2 m . . . . . . . . . . . . . . . . . . . . . IV A.5 Process emissions, Gunnilse 0.4 m . . . . . . . . . . . . . . . . . . . . V A.6 Process emissions, Gunnilse 2 m . . . . . . . . . . . . . . . . . . . . . VI A.7 Process emissions, Fastcarb Vikan . . . . . . . . . . . . . . . . . . . . VII A.8 Process emissions, Fastcarb K̊allered . . . . . . . . . . . . . . . . . . VIII A.9 Process emissions, Fastcarb Gunnilse . . . . . . . . . . . . . . . . . . IX xii List of Figures 1 World production in kg per capita of cement, steel, and wood between 1950-2015 (Monteiro et al., 2017). . . . . . . . . . . . . . . . . . . . . 1 2 Visual description the carbonation reaction on a concrete aggregate . 8 3 X-axis displays the service life of the concrete and Y-axis the relative carbonation potential. The largest potential is showcased in the waste handling stage (SIS, 2019). . . . . . . . . . . . . . . . . . . . . . . . . 12 5 Number of million program apartments in need of renovation per region in Sweden (NCC, 2021). . . . . . . . . . . . . . . . . . . . . . 18 6 Amount of concrete used in Sweden annually 1893-2011 . . . . . . . . 19 7 Effluence of concrete from the MP building stock in kilotonnes, per assumed year of demolition. . . . . . . . . . . . . . . . . . . . . . . . 21 8 Overview of required steps for utilizing concrete in natural & accel- erated carbonation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24 9 Sources of industrial CO2 emissions in the municipality of Gothenburg (European Environment Agency, 2024) . . . . . . . . . . . . . . . . . 26 10 Location of MP buildings in Gothenburg municipality, colour sorted by construction year. Picture captured from QGIS. . . . . . . . . . . 30 11 Circles with a radius of 8.7 km have been drawn around each pro- cessing site to visualize the shortest transportation route. Picture captured from QGIS. . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 12 Amount of concrete that is transported to each of the three resource parks annually. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 13 The total amount of concrete transported to Vikan resource park annually, in relation to the processing capacity for the two stockpiling scenarios. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35 14 Net emissions for Vikan for 0.4 m stockpiling scenario. . . . . . . . . 36 15 Net emissions for Vikan for 2 m stockpiling scenario. . . . . . . . . . 37 16 The total amount of concrete transported to K̊allered resource park annually, in relation to the processing capacity for each natural car- bonation scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40 17 Net emissions for K̊allered for 0.4 m stockpiling scenario. . . . . . . . 41 18 Net emissions for K̊allered for 2 m stockpiling scenario. . . . . . . . . 42 xiii List of Figures 19 The total amount of concrete transported to Gunnilse resource park annually, in relation to the processing capacity for each natural car- bonation scenario. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45 20 Net emissions for Gunnilse for 0.4 m stockpiling scenario. . . . . . . . 46 21 Net emissions for Gunnilse for 2 m stockpiling scenario. . . . . . . . . 46 22 Map over Gothenburg with plotted buildings, resource parks and CO2-donors. Picture captured from QGIS . . . . . . . . . . . . . . . 48 23 A Fastcarb units processing capacity in relation to inflow of concrete at Vikan resource park. . . . . . . . . . . . . . . . . . . . . . . . . . . 49 24 A Fastcarb units processing capacity in relation to inflow of concrete at K̊allered resource park . . . . . . . . . . . . . . . . . . . . . . . . . 50 25 A Fastcarb units processing capacity in relation to inflow of concrete at Gunnilse resource park . . . . . . . . . . . . . . . . . . . . . . . . 51 xiv List of Tables 1 Carbonation parameters from literature. . . . . . . . . . . . . . . . . 23 2 Parameters affecting the carbon uptake for Fastcarb. Ranging from an uptake of 25-40 kg CO2 / t concrete. . . . . . . . . . . . . . . . . 25 3 Emissions from EoL processing of concrete . . . . . . . . . . . . . . . 27 4 Amount of concrete in the MP buildings per year of construction, the amount of sequestrated CO2 during their assumed lifetime of 100 years and the degree of carbonation. . . . . . . . . . . . . . . . . . . 30 5 Cone geometry data for a pile of 0.4 m in height . . . . . . . . . . . . 33 6 Cone geometry data for a pile of 2 m in height . . . . . . . . . . . . . 33 7 Processing capacity of the resource parks. . . . . . . . . . . . . . . . 33 8 Total inflow of concrete to Vikan annually and the share that can effectively be processed due to area restrictions for the 0.4 m scenario. 34 9 Total inflow of concrete to Vikan annually and the share that can effectively be processed due to area restrictions for the 2 m scenario. . 35 10 Total inflow of concrete to K̊allered annually and the share that can effectively be processed due to area restrictions for the 0.4 m scenario. 39 11 Total inflow of concrete to K̊allered annually and the share that can effectively be processed due to capacity restrictions for the 2 m scenario. 40 12 Total inflow of concrete to Gunnilse and the share that can effectively be processed due to area restrictions for the 0.4 m scenario. . . . . . . 43 13 Total inflow of concrete to Gunnilse annually and the share that can effectively be processed due to area restrictions for the 2 m scenario. . 44 14 Aggregated results for each scenario. . . . . . . . . . . . . . . . . . . 52 A1 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at Vikan. 0.4 m scenario. . . . I A2 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at Vikan. 2 m scenario. . . . . II A3 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at K̊allered. 0.4 m scenario. . . III A4 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at K̊allered. 2 m scenario. . . . IV A5 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at Gunnilse. 0.4 m scenario. . . V A6 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at Gunnilse. 2 m scenario. . . . VI xv List of Tables A7 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at Vikan. Fastcarb scenario (4 pcs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VII A8 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at K̊allered. Fastcarb scenario (1 pc). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . VIII A9 Emissions from demolishing MP buildings and from crushing concrete into smaller fractions for carbonation at Gunnilse. Fastcarb scenario (2 pcs). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IX xvi 1 Introduction Human activities, marked by the widespread use of fossil fuels, deforestation, and land use, have resulted in a substantial increase in the levels of greenhouse gases in the earth’s atmosphere, which is a driving force behind climate change. The pressing global challenges of climate change have underscored the need for innovative and sustainable solutions to mitigate greenhouse gas emissions, and to focus on sectors that contribute significantly to these can have a large positive impact if successfully implemented. The built environment, shaped by construction and demolition activi- ties, contributes substantially to CO2 emissions and is one of the leading contributors to climate change in Europe (Antunes et al., 2024). The construction industry is resource-intensive with extensive use of energy-demanding materials, such as con- crete. Concrete is the most common building material in the world, with 30 billion tons used annually (Huang et al., 2020; Nature, 2021). Chaudhury et al. (2023) ex- plains that concrete production alone contributes approximately 7% of global CO2 emissions yearly, emphasizing the urgency of research on how to reduce its environ- mental impact, and more so since its demand increases more than for other building materials such as steel and wood; see figure 1. To limit global warming to 1.5 ℃, Figure 1: World production in kg per capita of cement, steel, and wood between 1950-2015 (Monteiro et al., 2017). which is the target set forth in the Paris Agreement, there is a need to reduce anthro- pogenic greenhouse gas emissions. However, many climate scientists don’t believe that emission reduction alone will be enough to limit global warming to 1.5 ℃, there 1 1. Introduction is a need for complementary technologies to allow for a needed drastic reduction of atmospheric greenhouse gases and emissions (Plumer and Popovich, 2021; UNECE, 2015). A potential strategy is to expand the focus beyond emission reduction and explore carbon capture through carbonation, a natural process that uses concrete as a carbon sink. Carbonation occurs when CO2 from the atmosphere reacts with calcium hydroxide (Ca(OH)2) in the concrete to form calcium carbonate (CaCO3), a process that occurs naturally over time (Andersson et al., 2013). However, the natural carbonation of concrete can be a slow process, that takes place over months to years, but with large stocks of concrete in society the total amount of sequestrated CO2 can be significant over time. Urbanization has led to large amounts of concrete in cities, making cities a natural focal point for the role it plays in (Holcim, 2023; SCB, 2015; Sika, n.d.). During the 1960s and 1970s, more than 1.6 million homes were built in Sweden to combat a housing crisis during a time of great economic development (SCB, 2023). At its peak from 1965-1974, a period that came to be known as the million pro- gram, more than 100’000 homes were completed annually. With cities expanding and utilizing concrete for buildings, tunnels and bridges the stock of concrete only increases. An initial literature review was conducted in an attempt to find gaps in the studied area from which research questions could grow. The chemical reaction of carbonisation has long been a known phenomenon due to its adverse effects on re- inforcement in concrete. However, traction regarding utilizing concrete as a carbon sink has grown over the past decade which will be the focus of this study. From the literature review, it was observed that there was a lack of research on the subject of accelerated carbonation methods applied on industrial scales. Most studies con- ducted tests in laboratory settings and on low volumes of aggregates. One project stood out as it had undergone laboratory tests on accelerated carbonation before installing a pilot-project on a cement plant in France that handled volumes on an industrial scale (Torrenti et al., 2022). Other accelerated carbonation methods in- cluded carbon curing, which is out of the scope of this study as it delves into the carbonisation in the production phase and not in the end of life (EoL) (Šavija and Luković, 2016; Shang et al., 2023. Moreover, the geographical scopes on the initial literature review were mainly on northern European countries while the rest weren’t specific in their spatial locations (Ammenberg et al., 2014; Lagerblad, 2005; Patricio et al., 2017; Yu et al., 2021). The concept industrial symbiosis (IS) was of interest too investigate, mainly the utilization of flue gases from industries as an output and carbonisation of crushed concrete as an input. The concept of IS was observed in two studies, Patricio et al. (2017) and Van Oss and Padovani (2003), in the initial literature review but weren’t exploring the option of flue gas utilization. Overall, it could be observed that there was a gap in exploring IS alongside accelerated carbonation which is of interest to be explored within the context of Gothenburg municipality. 2 1. Introduction 1.1 Aim This report aims to quantify the natural carbon capture potential of demolished concrete, and to compare this to an accelerated carbonation technology. This is done through a case study in which the source of concrete is million program buildings built between 1965-1974 in the municipality of Gothenburg. 1.2 Research questions • Where are the buildings within the scope of this study located, when will they be demolished and how much CO2 can naturally be bound in demolished concrete from buildings constructed between 1965-1974 in the municipality of Gothenburg? • What could a system for the carbonation of concrete in Gothenburg look like? • What are the net emissions from processing concrete to be leveraged as a carbon sink via natural and accelerated carbonation? 3 1. Introduction 4 2 Background 2.1 Production, emissions and waste management of concrete Concrete is a mixture of three main components: aggregates, cement and water. Aggregates form the bulk and provide the structural backbone of concrete. The aggregates vary in size, ranging from fine sand particles to coarse gravel or crushed stone. The selection and size variation of aggregates are essential to ensure proper packing and interlocking within the concrete matrix. This, in turn, influences the density, workability, and strength of the concrete. Cement is a binding agent that holds the aggregates together. When mixed with water, cement undergoes a chemi- cal reaction called hydration, forming a paste that coats and surrounds the aggregate particles. As this paste hardens and cures, it provides the concrete with its compres- sive strength. Over time, the strength of concrete increases as the hydration process continues. The production of cement involves extracting limestone rock from the lithosphere. Limestone, primarily composed of calcium carbonate, undergoes a series of steps to become cement. After extraction, the limestone is crushed and trans- ported to a cement kiln. Inside the kiln, the limestone is heated to temperatures reaching 1450 °C, initiating a process known as calcination. During calcination, the limestone undergoes chemical decomposition, releasing carbon dioxide and leaving behind calcium oxide (lime, CaO). CaO reacts with other minerals present in the raw materials to form clinker, the main component of cement. The clinker is then finely ground to produce the powdered cement used in concrete production (PCA, n.d.). 2.1.1 Calcination- and fuel emissions The main constituents of clinker are alumina (Al2O3), iron oxide (Fe2O3), lime (CaO) and silica (SiO2). According to Tokheim (1999) a typical clinker would con- stitute 66% CaO, 21% SiO2, 5% Al2O3, 3% Fe2O3 and 5% of other materials. When heated, a number of complex chemical reactions occur which will not be explored in detail in this study. In short, four compounds are formed that in cement chemist notation are Dicalcium silicate (C2S), Tricalcium silicate (C3S), Tricalcium alumi- nate (C3A) and Tetracalcium alumino ferrite (C4AF). These four compounds are all important for the properties of the cement and affect aestethics, durability, strength development, and heat evolution of the cement. Without the addition of iron and aluminum compounds the calcination would take place at temperatures of more 5 2. Background than 3000 °C (Munthe, 2013). Depending on the chemical composition and internal proportion of these four compounds in the quarried raw material, varying additives are added to compensate for variations and to create the desired properties of the cement (Britannica, n.d.). The calcination of limestone is an endothermic reaction, requiring approximately 1.7 MJ per kg of CaCO3 (Kahawalage et al., 2017). The calcination of CaCO3 can be expressed by the following chemical reaction: CaCO3 → CaO + CO2 CO2 is an inherent by-product when producing clinker, and the emission-to-production ratio is in the range of 600-900 kg of CO2 / tons of cement produced, depending on the ratio between the clinker and the cement and the type of fuel used for heating. Approximately 2/3 of production emissions derive from the calcination process and 1/3 from burning fuel to heat the kiln. 2.1.2 Contemporary post-demolition management of con- crete Understanding today’s system of waste handling is imperative to shape and im- prove a more sustainable practice of managing the vast amounts of concrete waste generated from construction activities. By utilizing demolished concrete as or in a secondary product, the demand for virgin materials can be reduced, aligning with national and global climate goals of reducing the extraction of natural resources (Boverket, n.d.). When tearing down a concrete structure, the concrete is broken down into trans- portable pieces for intermediate stockpiling before the removal of steel reinforcement. Once further processed into smaller fractions, fresh uncarbonated surfaces of the concrete react with CO2 to enable further carbonation. Once at a processing site, the steel reinforcement is removed from the concrete and awaits further processing, these masses of crushed concrete are placed into stockpiles. The mixed fractions generated from the retrieval of reinforcement are placed in a stockpile which creates subpar conditions for carbon uptake, especially when exposed to rain which further compacts the stockpile, resulting in poor airflow and thus the carbonation rate (SIS, 2019). by stockpiling in similar fractions, more void space is created and thus in- creased airflow for a more efficient carbonation. See figure 5. With the current waste handling system in place, the conditions for carbonation is relatively low which means there is a potential for improvement in the CO2 uptake for waste concrete (Munthe, 2013). The current waste handling system in Sweden is described by Munthe (2013) in 5 steps: 6 2. Background 1. Demolition of used concrete products 2. Intermediate stockpiling of demolished concrete (storage 0.5 - 4 years) 3. Rebars are extracted for recycling and the crushed concrete is piled with mixed fractions. 4. Intermediate stockpiling of mixed concrete fractions (storage 1-4 months) 5. The mixed concrete is used in construction applications such as landfillings, road base coarse, or building foundations. According to Stripple et al. (2018) a significant amount of the cement paste (20%) turns into fine dust during the processes of crushing and fractioning. With proper handling, these fine particles of hydrated cement can carbonate quickly in a couple of days or up to a few weeks. However, the amount of fine dust depends on equip- ment and crushing practices which may vary. 2.2 Carbonation Carbonation refers to the chemical process where atmospheric CO2 diffuses into the concrete and reacts with calcium hydroxide and calcium silicate hydrates (CSH) present in the pore surface of the hardened cement paste, resulting in the formation of limestone i.e. calcium carbonate (CaCO3) (Andersson et al., 2013). This chemical reaction binds the CO2, effectively acting as a permanent carbon sink. The chemical reaction is represented as follows: Ca(OH)2 + CO2 → CaCO3 + H2O CO2 initially penetrates the concrete at the surface by a number of chemical reac- tions, driven mainly by gas diffusion in the concrete pores and proceeds inwards the concrete element (Silva et al., 2015). There is an increase in density as CaCO3 is formed in the pores resulting in a reduction of water adsorption Zhang et al. (2020). The carbonation process needs moisture to be initiated as it’s in water where CO2 is dissolved into carbonate ions which react with calcium ions to form calcite. To summarise, the sequestration of CO2 involves three phases Li and Wu, 2022. The theoretical binding capacity is dictated by the amount of CaO in the portland clinker that can bind back CO2, i.e, the amount of CO2 that was released during calcina- tion (SIS, 2019). Portland cement is 95 % clinker which consists of 65% CaO that may potentially react with CO2. For Portland cement type CEM-I this is typically set to 0.49 kg CO2 / kg portland cement due to the molecular compositions. As showcased in SIS (2019), the molecular weights for the given elements are as follows: • Carbon (C): 12 • Oxygen (O): 16 • Carbon dioxide (CO2): 44 • Calcium (Ca): 40 • Calcium oxide (CaO): 56 7 2. Background Figure 2: Visual description the carbonation reaction on a concrete aggregate(Li and Wu, 2022). • Calcium carbonate (CaCO3): 40 + 12 + (3 × 16) = 100 1.00 kg Portland cement thus has the potential to bind: 1.00 X 0.95 X 0.65 = 0.62 CaO 0.62 X [44/56] = 0.49 kg CO2 This value is at the same time a way to define the degree of carbonation (DoC) (SIS, 2019) as it puts CO2 released from calcination in relation to what it potentially can bind back. In theory, the amount of released CO2 from calcination may bind back into the cement paste(Andersson et al., 2013) but is according to the literature often given a value of 75 %. In practicality, the carbonation is dictated by several factors (Stripple et al., 2018). To summarize, carbonation is a chemical process where carbon dioxide from the air reacts with calcium hydroxide in the cement paste to for calcium carbonate, which reduces the alkalinity of the concrete. This occurs via diffusion in the cement paste and not the aggregates ( sand and gravel). Accelerated carbonation may be achieved by increasing the surface area of concrete by crushing it and exposing more of the Ca(OH)2 in the cement paste to catalyze the sequestration. Thus, the management of crushed concrete plays a significant role in leveraging the crushed concrete as a carbon sink. 2.2.1 Estimating carbonation A way to measure the DoC is by applying phenolphthalein, a substance that changes colour corresponding to the pH on the applied surface SIS, 2019. For non-carbonated 8 2. Background areas it appears pink and for carbonated it is transparent Bui et al., 2023. The differ- ent levels of carbonation can be measured via image analysis where the carbonated areas (appears colorless) are divided by the fresh uncarbonated (appears pink) area to get the DoC. The advantage of this method is that its uncomplicated, relatively time efficient and economically beneficial compared to other methods. mentioned drawbacks regarding this method are that partially carbonated areas are hard to observe and that the phenolphtalein only measures the change in pH which indicates carbonation but doesn’t measure the actual uptake of CO2. Overall, the task of cal- culating CO2-sequestration is complicated as there is a lack of a definitive method. 2.2.2 Conditions for carbon uptake As explained in SIS (2019) there is no single general formula calculating CO2 uptake that is applicable for all concrete structures in all environments as there are many parameters taken into account. A good understanding of preferable conditions for carbon sequestration is necessary to evaluate post-demolition management of con- crete. Time is one important aspect as carbonation is a slow process, a process which Lagerblad (2005) means occurs primarily during the first 50 years of a structure’s lifetime. Carbonation is a surface phenomenon which slows down as it becomes progressively harder for CO2 to penetrate and reach uncarbonated surfaces in the interior of the concrete. The carbonation rate describes the correlation between the time and depth of the transportation of carbon dioxide in concrete (SIS, 2019). Transportation effects and reactions are described by the carbonation rate “k” and permits the degree of carbonation (DOC) to be calculated at any given time. No- tably, the carbonation rate is affected by several factors such as dimensions and interconnectivity of the pores, cement type and RH. Through laboratory tests, it has been established that the DOC is around 75% but present-day management of demolished concrete prohibits it from reaching those values and are instead much lower which further adds weight to explore EoL options to optimize the carbon se- questration (Andersson et al., 2019). Water is necessary as it enables the formation of ions which react with carbon hy- droxide or CSH to bind CO2. This is why very dry concrete doesn’t carbonate and reversely, in very wet concrete carbonation is instead slowed down as gas diffusion isn’t enabled. For a normal dry concrete with a RH of 50 – 80 % gas diffusion is the driving mechanism where the partial pressure between the surface of the concrete and the interior dictates the carbonation level. In SIS (2019) it was evident that a RH in the air of 60% was deemed optimal for carbonation . In Gothenburg, the RH varies during the year and reaches its maximum in December at 85%, its lowest in May at 62% and with an average annual RH of 75 %. There is a correlation between compressive strength, water/cement ratio, and car- bonation rate. A higher compressive strength indicates a lower w/c ratio which 9 2. Background lowers the permeability for CO2 diffusion. The amount of cement doesn’t dictate the carbonation rate. The carbonation rate is higher in urban areas and indoor envi- ronments. This is due to the higher-than-average partial pressure of CO2. Equation 1 is generally accepted (Pade and Guimaraes, 2007) as a good approximation to calculate the carbonation depth where t is time and k the carbonation rate: (1) dc = k · √ t Pade and Guimaraes (2007) summarized several studies on carbonation rate and exposure conditions and concluded that the carbonation rate typically is accelerated in indoor conditions, in comparison to outdoors, as the carbonation is affected by humidity levels. In this study, carbonation was found to be most effective at a relative humidity (RH) of 40-80%. Low RH prohibits CO2 from dissolving with the water in the pores and in contrast, in environments with too much water, e.g. submerged concrete, the permeation of CO2 will be slowed down. Equation 2 as described in SIS (2019), is used to calculate the carbon uptake in kg CO2 / m2: (2) Cu = (k · DOC) ( √ t 1000 ) · Utcc · C k is the carbonation rate in mm/ √ t, DOC the degree of carbonation about the sur- face and t is service life. Utcc is the theoretical uptake of CO2 / kg in cement and the value for portland cement (CEM-I) is 0.49. C is cement content in kg / m3 concrete. The result is then divided by the volume of the studied concrete to obtain a value on sequestered concrete during a buildings service life inkg CO2 / m3. In addition to this, a decrease in carbonation rate by 30-50 % can be expected if the concrete surfaces are covered. Exact rate depends on the thickness and properties of the cover. ”CEM” is the notation that refers to the European standard EN 197-1 that describes the proportions of different components in various types of cement in percentages (SIS, 2019). Historically, CEM-I has been the most common type of cement used in Sweden up until the 2000s (Andersson et al., 2013). For this type of cement, the portland clinker ratio is typically 95-100% of the mass ratio, whereas other cement types substitute some of the clinker with, e.g., furnace slag and fly ash, resulting in a lower clinker-to-mass ratio (Bäckman, 2022). This has been the trend since the turn of the century, where CEM-II has been more widely adopted in Europe for buildings which constitute of a lower rate of clinker and more substitutes (SIS, 2019). 10 2. Background 2.3 Demolition The European Union recognizes the inherent potential environmental improvements by recycling concrete & demolition waste (CDW) and established in 2020 a waste di- rective framework with a target of recovering 70% non-hazardous CDW for recycling or re-use. Generally, the most common use of recovered demolition waste is as filler in road construction or as backfilling material. These utilisations often subceeds the original quality of the materials, subsequently lowering the market value of them as a secondary material compared to the original one. Due to this, it is not deemed optimal in a circular perspective nor is it as efficient in substituting materials as it could be and therefore the overall environmental performance could be improved Caro et al., 2024. The studied buildings within the municipality of Gothenburg all have data on concrete quantity, but on average this constitutes 24% of CDW on av- erage in the EU. Demolition and collective practices plays a vital role in how much concrete is recovered and although the technology exists, there are other barriers that prevent use of recovered materials as a secondary product. Identified obstacles are: • Perceived expense of recovering and recycling in contrast to landfilling • Low demand amongst buyers • Regulatory challenges e.g. safety requirements • Chemical compositions • Competition with low-cost products stemming from virgin resources which may be cheaper Essentially, products from primary materials don’t necessarily factor in external costs such as pollution or resource depletion which are often borne by society or the environment rather than the producer. The amount of concrete that will be available for processing will thus depend on how much is recovered from a demolition and the purity of the waste as only the concrete is of interest in a carbonation perspective. Some concrete elements may be salvage- able and re-used which should be a priority according to the Swedish planning and construction law (“Riva, återvinna och återbruka – Boverket”, n.d.). Thus, not all concrete may be crushed and utilized for carbonation. However, this study aims to analyze the carbonation potential if all concrete where to be utilized for carbonation. 2.3.1 Particle sizes Aggregates are a necessity for working infrastructure in society as it serves in many vital structures such as roads, railways, docks, airports, hospitals, residential build- ings and many more areas of usage (Bergmaterialindustrin, 2019). The size of the aggregates varies depending on the application, of which a few are presented below as described by (“Grus, makadam, stenmjöl och bergkross — NCC” (n.d.)): 11 2. Background Stone meal: 0/2 or 0/4 mm are very fine fractions and can be used for adjustments and hardening of walking paths, driveways and football fields. Ungraded crushed rock: Coarser material with finer fractions included. Fractions such as 0/16, 0/32 and 0/90. These can serve as sub-base road layers and foundation layers. 2.4 Methods for CO2 sequestration 2.4.1 Optimizing natural carbonation of RCA One alternative for increasing the carbon sequestration in concrete aggregates (CA) without applying any new technologies is by changing the way the aggregates are stockpiled at the end-of-life. Knowledge of conditions for optimizing storage of CA is essential to leverage it as a carbon sink. As shown in figure 3, the largest carbonation potential is evidently to be leveraged once the concrete is demolished into smaller fractions. Figure 3: X-axis displays the service life of the concrete and Y-axis the relative carbonation potential. The largest potential is showcased in the waste handling stage (SIS, 2019). These conditions were researched in a recycling center in Topinoja, Finland Skanska Finland and Toni Kekkonen Betoni (2023)to identify improvements in how the CA are stockpiled. Four piles with CA in the fractions 0-90 mm and 20-90 were mea- sured for 14 months by their carbon dioxide concentrations, and humidity, within 12 2. Background the pile at 4 different depths: 0.25, 0.5, 1.0 and 2.0 m from the upper surface. Two piles of 0-90 and 20-90 mm were sheltered from rain during the research as to find out if it would affect the carbonation. In a non-sifted conventional pile of 0-90 mm CO2 concentrations varied from 20-130 ppm. The lowest concentrations were seen at the bottom of the pile at 20 ppm and 130 ppm at the surface layer. It was ob- served that carbonation occurred at the bottom of the pile, although at a slower rate and after after 6 months 85% of concrete 0-90 was carbonated which corresponds with the same DoC as displayed in SIS, 2019 for concrete surfaces exposed to rain. Additionally, the tests conducted in Finland, no substantial difference in the piles that were sheltered from rain could be observed as they dried out quickly. Another study of CO2 sequestration in various fractions were presented in (SIS (2019)) and it was observed that smaller fractions carbonated at a faster pace, more specifically 0-4 mm. One cubic metre of concrete with a cement paste of 350 kg/m3, with an assumed water-cement ratio of 0.43, was crushed into three fractions with a depth of 0.4 m and placed in pallets sheltered from rain: 0-4, 4-8 and 4-16 mm. The three fractions generated a total uptake of 37 kg CO2/m3, about 8.3 kg / ton con- crete after 18 months of storage. These results were similar when compared to two other studies in SIS (2019). In comparison with mixed stockpiling, this approach generated a 4 time higher carbon sequestration. It is essential to sort the piles into similar fractions to create void space to allow airflow for a more efficient carbona- tion. According to the study by Leemann et al. (2023), it was concluded that finer particle sizes allow for a more efficient carbonation, with 0-4 mm achieving the best uptake of CO2. Interestingly, fractions of 4 mm of high-quality concrete didn’t car- bonate at all. Furthermore, in their findings, 80 % of the carbonated cement paste had carbonated after 1.5 months of their total 6-8 months of storage. Similar to results from Skanska Finland and Toni Kekkonen Betoni, 2023, it was observed that the DoC was only marginally higher at the outer layer of the studied stockpiles, and still carbonated at a similar degree at a depth of 1-2.5 m. Initially, a DoC is needed in order to estimate storage time, where after that time period the CA can be tested by thermogravimetric analysis (TGA) which measures weight loss between temperatures 500 - 1000 degrees celsius to calculate captured CO2. Carbonation occurs rapidly after demolition concrete has been crushed when a new fresh surface comes into contact with the air. It is necessary to determine a degree of carbonation in order to calculate the amount of captured CO2. Since this process slows down over time, there is thus an optimal storage period. 2.4.2 Accelerated carbonation This chapter will introduce two methodologies possible for accelerating the seques- tration, both of which are mainly theoretical and not as of yet applied in a larger scale. This will however serve as a foundation for exploring potential future man- agement plans for CA by examining the literature on carbonation and combining it 13 2. Background with the case study of the MPPs. Carbonation of concrete is desirable for the construction sector as it reduces the carbon footprint, which consequently have increased studies on the subject along with accelerated carbonation technologies. The amount of accelerated carbonation technologies that have been tested on an industrial-scale are rather limited, and so most are under development in laboratory settings and falls roughly into two methods; Applying pressurized CO2 in a chamber or adding a solvent in which the sequestration takes place (Li and Wu, 2022). By applying an accelerated carbona- tion technology on concrete aggregates (CA) one can achieve the following: 1. Speeding up the CO2 sequestration process. 2. An increase in uptake efficiency compared to letting the concrete carbonate naturally, effectively increasing the DOC. 2.4.2.1 Fastcarb: a case study in France In France, an initiative aimed at studying the feasibility in accelerated carbon cap- ture via RCA. Two pilot installations within cement plants utilized untreated in- dustrial gases emitted from cement kilns and processed quantities on an industrial scale. The crushed aggregates (CA) used were in the fractions 0-4 mm and 4-16 mm Torrenti et al. (2022). The first installation was a rotating drum of 11 m in length and with a radius of 2m, where the RCA was placed for carbonation with a connected pipe with flue gases emitted from a cement kiln. The temperature of the gas was 60-80 degrees celsius with a CO2 concentration of 11-16 % and a relative humidity (RH) in the drum of 90 %. With a capacity of 3 tonnes of RCA it showcased an optimal residence time of 1h, which in the end resulted in 2h when including filling and emptying of the drum. The rotating drum resulted in an average carbon capture of 31 kg CO2/ton for the fractions 0-4 mm and 5kg / t for 4-16 mm. Another installation was made which utilized a fluidized bed which resulted in slightly higher carbon uptake at 39 kg CO2 / t for the fraction 0-4 mm and 12 kg CO2 / t for 4-16. The flue gas in this case had a CO2 concentration of 20%. Overall, the Fastcarb project showcased that the fraction 0-4 mm was of most inter- est in a carbonation perspective and could capture 25-40 kg CO2 per tonne, results that were similar to tests under in laboratory settings. Industrial flue gases consti- tute of several substances apart from CO2 such as NOx and SO2 and was observed to adversely affect carbonation Torrenti et al. (2022) when compared to a stream of air without NOx and SO2. Furthermore, In the summary of the Fastcarb project, it was evident that emissions and the gains of carbon sequestration were largely affected by the transportation of aggregates. Bergmaterialindustrin (2019) argues that Exceeding a transportation distance of 20 km with aggregates is not deemed reasonable in an economical nor environmental perspective. 14 2. Background As previous research has concluded, particle size matters, and finer fractions in- creases the surface area of concrete that can be carbonated SIS (2019), Stripple et al. (2018), Torrenti et al. (2022). In addition to this, a RH of 40-80 % was deemed optimal for carbonation as too little moisture wouldn’t enable the carbonation pro- cess and too much would slow down the diffusion of CO2 into the cement pores. In Li and Wu, 2022 it was however argued that a higher water content with elevated temperatures may boost carbonation as it promotes two important aspects for the process: Diffusion of CO2 and leaching of calcium ions. The solubility of those two is however reduced, which is why an addition of water is needed. Li and Wu (2022) further explains that a significant acceleration of the carbonation occurred when increasing the CO2 concentrations from atmospheric levels (0.03 %) to 20-40 %. A continuous increase of concentration from 40-100 % showed marginal improvements on the carbonation. The study concluded that surface area (particle size) of the aggregates and temperature were deciding factors in carbonation effi- ciency. The process could be sped up by increasing pressure and CO2 concentrations, these factors did not however prove to increase the overall degree of carbonation. Increasing temperatures accelerate the diffusion of CO2 and speed up carbonation re- actions, which boosts the capacity for CO2 absorption and reaction rates. However, elevated temperatures also promote the formation of calcite layers and particles, as depicted in Figure 5. These calcite layers adhering to mineral surfaces may hinder the release of calcium ions, while calcite particles can densify the porous structures of cement paste, thereby slowing down the diffusion and dissolution of CO2 [102]. Consequently, the influence of rising temperatures on the carbonation process of materials entails two conflicting factors. 15 2. Background 16 3 Method Figure 4: Overview of steps in methodology The methodology comprises data acquisition related to the physiology of cement, concrete, and carbonation through a literature review, the 3.1 The case study: the million program 1965- 1974 The case study aims to quantify the carbon capture potential in the concrete stock in million program (MP) buildings built between 1965-1974, in the municipality of Gothenburg. 1.4 million homes were built in Sweden between 1961-1975, of which a million were built between 1965-1974 in what became known as the million program. The million program was a response to housing shortages and poor living conditions in Sweden during a period of rapid economic growth and labor migration after the Second World War. Employment opportunities and vacancies in urban areas led to an influx of citizens from the hinterlands during the 1950s and 1960s (Molinder, 2018). Sweden’s three largest cities; Gothenburg, Malmö, and Stockholm; together had 330’000 new multi-family housing built during the million program (Johansson, 2012), an indicator of the influx of citizens to urban areas that Sweden experi- enced during this time. Today, buildings constructed during these years make up approximately 30% of all homes in Sweden according to data from Statistics Swe- den (2023). These buildings are approaching their technical lifespan, and are in need of reconditioning, as shown in figure 5. Research by Thuvander et al. (2015) indicates that only a third of the buildings from this era had been refurbished by 2015. A report by the Swedish construction firm NCC (2021) estimates the total cost of renovating these buildings at 500 billion SEK, the equivalent of about 10% 17 3. Method Figure 5: Number of million program apartments in need of renovation per region in Sweden (NCC, 2021). of Sweden’s GDP in 2021 (Our World In Data, 2022). Of all homes con- structed during the million program, ca. 75’000 were built in the mu- nicipality of Gothenburg, which to- day amount to ca. a third of its housing stock (Göteborgs Statistik- databas, 2022). The plan of erecting one million homes in one decade ne- cessitated industrialized construction techniques with many prefabricated components. The implementation of these innovative building practices in- tended to ensure affordability, with the goal that a two-room apartment should cost no more than a fifth of an industrial worker’s salary, which be- came possible with these new build- ing practices, as well as through ben- eficial government loans (SCB, 2024; Sveriges Allmännytta, n.d.). Re- inforced concrete was the preferred material to construct MP buildings (Björk et al., 2016). It was cost-efficient, robust, and low-maintenance, all attractive characteristics for constructing functional, multi-story apartment buildings. During the height of the MP the domestic use of concrete reached levels that have not been seen since, see figure 6. This extensive use comes with environmental drawbacks related to its production, but there are also issues with end-of-life waste manage- ment of concrete, which will be important to address in the future when this large building stock reaches its technical lifespan. The MP was successful in that it mitigated the housing crisis and provided cheap apartments, but it was already criticized from the beginning for its architectural uniformity, lack of green areas, and concerns about social issues that arise from lo- cating large groups of low-income citizens within specific geographical areas (Bover- ket, 2020; Lundberg, 2018). With these buildings reaching the end of their technical lifespan in the coming 40-60 years, there is an active public debate on the future of the million program, with stakeholders voicing a number of concerns. Segregation, energy efficiency of buildings, the growing number of dwellings in need of renova- tion, as well as the costs related to this, are all points raised in the last 15 years (Johansson, 2012; Mossfeldt, 2011; NCC, 2021; Nytt & Viktigt, 2014; Vicktor, 2009). 18 3. Method Figure 6: Amount of cement and concrete used in Sweden 1893-2011 (Andersson et al., 2013)1. 3.1.1 The future of the million program The million program is, as explained by Johansson (2012), a polarizing subject in Sweden. The subject of an urgent need of refurbishment and the importance of addressing social issues linked to MP areas are brought up in public debates. For the sake of the problem formulation of this thesis it will be assumed that all buildings are demolished after a service life of 100 years, arguments for this assumption are presented in 3.2. 3.2 Effluence of concrete from MP building stock The temporal material flow rate in the end-of-life phase of buildings is poorly under- stood and is the focus of ongoing research. The useful life of a building is difficult to assess with certainty due to the many involved variables that lead to the demolition of a building. Previous research on this topic has used various types of probability distributions to predict future demolition rates, but Miatto et al. (2017) acknowl- edges that there is no agreed upon model that estimates the amount of accumulated construction material at urban or national levels. The choice of probability distri- bution depends on demolition patterns, the nature of the built environment, and on the availability of data, none of which currently is available at any reliable rate for Sweden Tiberg, 2024. Predicting the lifetime of buildings in Sweden using previous research is further complicated by the fact that they have mainly been carried out in Asia, where the average lifespan of buildings can be as short as 30 years, which does not reflect the situation in Europe, where more than 40% of the building stock in 2011 was constructed before the 1960s (Andersen and Negendahl, 2023; Daigo et al., 2017; Economidou, 2011; Ji et al., 2021; Liu et al., 2014; Miatto et al., 2017). 1Reprinted with permission. ©2024 American Chemical Society 19 3. Method Wiedenhofer et al. (2015) have modeled the flow of material stock in the EU25 and found that Sweden has one of the lowest annual mean demolition rates with 0.03% for the years 2003-2009. This demolition rate is based on a collection of the most common residential buildings in the EU in 2003, which the authors acknowledge only represent 80% of the building stock leaving a considerable uncertainty. The low demolition rate points to the fact that buildings have a longer life in Sweden, compared to other nations. Data from Göteborgs Statistikdatabas (2022) shows that the total building stock from 1961-1980 in Gothenburg grew ca. 1% between 2014-2022. According to email correspondence with Göteborgs Stad this is due to variations in the building stock due to reconstructions and corrections of error of reporting from property owners. Johansson (2012) argues that the million program, in comparison to other large-scale housing programs in Europe during the same era, was well built and meant for a growing middle class and that it will surely last 100 years. In 1994 new regulations for the lifetime classification of concrete structures were introduced, with the L2 class assuring a lifetime of 100 years, and L1 class 50 years. With the MP being constructed before these regulations were implemented, it can still be assumed that they were built with such robustness that a lifetime of 100 years can be assumed. Multiple studies investigating carbonation uses 100 years as the assumed time hori- zon for the service life of concrete buildings for calculation purposes (Andersson et al. (2013) and SIS (2019). This will mean that a 100 year lifetime for the MP will be fulfilled in 2065 for the buildings constructed in 1965 and so on, see figure 7 for concrete output per demolition year. 3.3 Calculating the concrete stock and use-phase CO2 sequestration in the million program in the municpality of Gothenburg To properly assess the carbonation potential in the concrete stock from the case study, a first step is to analyze how large the concrete stock is, and how much car- bon has been bound in the buildings during their assumed lifetime of 100 years. This will showcase the degree of carbonation and how much of the concrete that remains available for CO2 sequestration. The data of the concrete stock was obtained from a provided excel file (Adamu and Bhattarai, 2022), which had data on the quantity of concrete in buildings constructed in the municipality of Gothenburg. These values were divided into groups per construction year and summarised, to get a picture of how much concrete waste could be generated under the hypothetical scenario that the technical lifetime of a building is 100 years and that it is immediately demol- ished once it reaches its assumed lifetime. The data in the Excel document was based upon the analysis of 46 typical residential buildings in Sweden constructed over the time frame 1880-2010, comprising 12 single-family and 36 multi-family type buildings. In the context of this thesis, only the multi-family buildings are to be studied. The report from which the Excel file is derived used material intensity data to calculate the concrete stock in Gothenburg. 20 3. Method The Excel file also provided coordinates for each building which was inserted into the geographical information system software QGIS to visualize the geographical location of the concrete stock to investigate transportation emissions, see figure 10. To understand the spatial spread of buildings per construction year each building was assigned a specific colour, as seen in figure 10. Figure 7: Effluence of concrete from the MP building stock in kilotonnes, per assumed year of demolition. 3.3.1 CO2 sequestration use-phase To properly assess the carbon uptake of a building during its service life, it is im- portant to obtain as much data as possible on the buildings. Surface areas, wall coatings, indoor and outdoor environment are all factors determining how much carbon is captured during a buildings service life (Munthe, 2013; SIS, 2019; Stripple et al., 2018). Many parameters beyond the exposure levels also contribute to the actual carbonation depth of a concrete element during its technical lifespan, such as the compressive strength of the concrete and the clinker ratio. SIS (2019) calculated a mean use-phase carbonation value of a concrete multi-family residential building to be 20 kg CO2/m3. This value is for an assumed technical lifetime of 100 years, a cement with clinker ratio of 95%, a cement ratio of 330 kg cement/m3 of concrete, and a compressive strength of 25 MPa. These values are representative for a MP building, as explained by Andersson et al. (2013), and as well as by construction documents of an MP building from 1967 obtained from the Gothenburg city plan- ning office. To calculate the amount of sequestrated CO2 during the technical lifespan the equa- tion 3 is used: 21 3. Method (3) Ca = βV where C is the total amount of sequestrated CO2 in kg, β is the mean amount of CO2 in kilograms that is sequestrated per m3 of concrete during 100 years, and V is the total amount of concrete in m3. To calculate the carbonation degree, equation 4 is used: (4) Cd = β eCc where Cd is the carbonation degree in %, e is the ratio of emitted CO2 from calci- nation in kg to the production of 1 kg of cement, and Cc is the ratio of cement to m3 of concrete. 3.3.2 Transport distances Using the software QGIS enables measurement of the mean average linear distance between emitter and resource parks. In QGIS the coordinates for the resource parks were identified through the use of pythagoras equation the shortest linear distance between all MP buildings and the resource parks was calculated. The same method was used to find the shortest distance between the resource parks and CO2-donors to highlight the shortest routes of transportation from processing sites to emitters to apply an industrial symbiosis and to keep the transportation distances as short as possible. 3.3.3 Resource parks An initial step was to identify where the eventual masses of demolished concrete will be transported and processed. Information about the location of potential re- source parks were acquired by research and discussions with Skanska. These were then plotted on a map in QGIS and mean transportation distances were determined by adding an 8.7 km radius around each processing site in QGIS as shown in 12. The radius was chosen to avoid an overlapping. This appliance will illustrate the areas of buildings that are closest in a linear distance to the processing sites. Build- ings outside of the circles or in-between are measured in a linear distance in QGIS to sort them to the closest processing sites. Once distances are established, the transportation emissions can be calculated accordingly by multiplying an emission- transportation factor presented in table 3. 3.3.4 Natural carbonation The following parameters as presented in 3 are used to calculate potential CO2 sequestration and time frames for the respective scenarios. As the concrete elements are crushed into various fractions, the volume subsequently increases and is assumed 22 3. Method Table 1: Carbonation parameters from literature. Carbonation method Fractions [mm] CEM-type Degree of carbonation [%] Residency time Study Natural carbonation 0/90 n.m 85 6 months [2023] Natural carbonation 0/4,4/8 & 8/16 I 34.4 18 months [2019] Fastcarb 0/4 I,II,III 37-59 60 min [2022] Fastcarb 4/16 I,II,III 18 60 min [2022] to be similar to that of virgin aggregates of similar fractions i.e. 1.6 t/ m3 which results in 220 kg cement / m3 concrete. The method of storing 0-90 mm in piles of 2 meters in height which, according to Co2ncrete Solution (n.d.), would result in 85% carbonation after a period of 6 months was one of the few studies which analyzed larger stockpiles of concrete closer to the operational realities of aggregate production. Each resource park will have a varying maximum capacity of storing concrete depending on which pile dimension Results provided in a study conducted in SIS (2019) presented a total of 37 kg of CO2 / m3 after 18 months by piling in 3 separate fractions of: 0-4, 4-8 and 8-16 mm. For this thesis, the piles were stored in 0.4 m and on pallets. Utilizing the parameters from both studies, the outcomes in this study will provide a range of po- tential CO2 sequestration estimates, spanning from a minimum to a maximum value. As described by Munthe (2013), the current waste handling system in Sweden re- sults in an intermediate stockpiling of demolished concrete for 0.5-4 years and 1-4 months for the storage of mixed fractions. If approximately 6 months are needed for intermediate stockpiling and crushing of demolished concrete, and if the interme- diate stockpiling of mixed fractions (0-90 mm) also takes 6 months to achieve 85% carbonation, the total processing time for CA would be 12 months. It is assumed that the storage space for an inflow of concrete prior to processing is not an issue. This time frame of 12 months will be used in the calculation examples when inves- tigating the 2m scenario. Time frame: 2 m scenario • 6 months initial processing & intermediate storage • 6 months carbonation to achieve 85% DoC • Tot. processing time: 12 months Time frame: 0.4 m scenario • 6 months initial processing & intermediate storage • 18 months carbonation to achieve 34.4% DoC • Tot. processing time: 24 months An overview of the processes is presented in figure 11 and includes the demolition process from which the concrete is crushed into transportable pieces, use of load- 23 3. Method Figure 8: Overview of required steps for utilizing concrete in natural & accelerated carbonation ers for loading and unloading crushed concrete onto trucks and into stockpiles, and transportation for further processing at the resource parks where its crushed into the desired fractions of either 0/90 or 0/4,4/8 and 8/16. 3.3.5 Cone-geometry and resource park processing capacity By using the provided height from the studies of Stripple et al. (2018) (0.4 m height) and Skanska Finland and Toni Kekkonen Betoni (2023) (2 m height) along with stan- dard practices of storing cone-shaped or rectangular-shaped piles, a first observation of required space for the concrete aggregates can be made. For each of the two cases the volume of a conical stockpile with an assumed 37° angle of repose, which is a common value for stockpiling concrete as per Concrete Plants Inc. (n.d.), can be calculated by equation 5: (5) Vc = 1.836h3 where Vc is the volume of a cone and h is the height of a cone in meters. To calculate the maximum amount of concrete that can be stored at the three identified resource parks Vikan, K̊allered, and Gunnilse the approximate size of each site, hereafter notated Arp, was measured with the area measuring tool in QGIS. To 24 3. Method calculate the maximum amount of concrete that can be processed with respect to the size of the resource parks the radius of a cone is calculated with equation 6: (6) r = √ 3Vc πh The floor area, A, of a cone is then calculated with equation 7: (7) Ac = πr2 To calculate the maximum volume of concrete that can be processed at each resource park, Vrp, equation 8 is used: (8) Vrp = Arp Ac Vc 3.3.6 Accelerated carbonation As a Fastcarb unit requires a steady inflow of CO2, the initial step involves identify- ing industrial sites within reasonable proximity that emit CO2 in order to investigate a potential industrial symbiosis. As previously mentioned in 2.4.2 a transportation distance exceeding 20 km for aggregates surpasses the threshold where costs and emissions become too high (Bergmaterialindustrin (2019)). It is assumed that one Fastcarb unit is attached to the chosen industries outflow of flue gases where the performance is equal to that of the pilot installation in the case study of Torrenti et al. (2022). The steps involved for accelerated carbonation are represented in 8 Observing the distances and analyzing maps provided by The European Pollutant Release and Transfer Register (E-PRTR) facilitates the identification of potential sites to implement an industrial symbiosis with the industries as CO2 donors for the Fastcarb unit, see figure 9 for selected CO2 donors. The seven sites that have been included as CO2 donors in this thesis are selected upon their emissions of CO2, the other sites within the municipality of Gothenburg did not have data for CO2 emissions. Table 2: Parameters affecting the carbon uptake for Fastcarb. Ranging from an uptake of 25-40 kg CO2 / t concrete. Processing time [h] Concrete processed [t/yr] Min. sequestration potential [t] Max. sequestration potential [t] 2 024 3 036 75.9 121.44 Similar to the stockpiling scenarios, the time frame for processing CA in the Fast- carb scenario is presented as follows: 25 3. Method Figure 9: Sources of industrial CO2 emissions in the municipality of Gothenburg (European Environment Agency, 2024) • 6 months initial processing & intermediate storage • 12 months carbonation to achieve 37-59% DoC 3.3.7 Emission factors In an attempt to quantify the net-gains of carbon sequestration in the different sce- narios, various emission posts connected to leveraging concrete as a carbon sink have been identified. Among the processes are demolition, transport, loaders, crushing and emissions connected to operating a Fastcarb unit, referred to as carbonation infrastructure in table 3. The emission factors are presented in table 3. Emissions from the procedure of producing CA in different fractions varies depending on how many crushing steps are necessary. 0/90 requires 2 crushing steps and for 0/4, 4/8 and 8/16 3 crushing steps are required. The crushing emissions is a mean value calculated from 4 different resource parks in the region of Västra Götaland deriving from information supplied by Skanska. The emissions will be allocated to the amount of processed concrete in each scenario. This is to make the potential CO2 sequestration for the actual processed concrete comparable to the emissions for this same amount of concrete. In other words: emissions from excess concrete that can’t be processed due to capacity limitations wont be accounted for in the net emissions. The total emissions for 26 3. Method each scenario are subtracted from the sequestrated CO2 to present the net carbon capture. Table 3: Emissions from EoL processing of concrete Process Emissions Study Demolition 0.007 [kg CO2 / kg concrete] 2021 Transportation 0.106 [kg CO2 / tkm] 2019 Crushing 1.63-1.92 [kg CO2 / t concrete] 2024 Carbonation 1.3 [kg CO2 / t concrete] 2023 Loader 1.18 [kg CO2 / t concrete] 2024 27 3. Method 3.4 Limitations 3.4.1 Scope and scale The thesis is confined to evaluating the CO2 sequestration and emissions connected to the cement bound in the MP buildings constructed 1965-1974 in the municipality of Gothenburg. The time frame of the thesis is firstly based on the assumption of a 100 year service life for each of the MP buildings. After demolition, there are three different scenarios of carbonation occurring which will process the CA in three separate time frames. As it is difficult to know what the future holds in terms of energy mixes, fuel use, processing equipments, and demolition practices the emission coefficients and scenarios are based on a business as usual scenario. 3.4.2 Theoretical limitations As there is no standardised method of measuring the uptake of CO2 in CA it results in studies presenting sometimes contradictory data that the scenarios in the thesis are based upon. Moreover, there is a lack of studies investigating the industrial scale of carbonation in stockpiles. Flue gases have various compositions depending on the industry. The calculations in this thesis are based on flue gases being similar to that of the cement kiln in the pilot installation in France, and that the fastcarb machine can capture 25-40 kg when paired with flue gases from the chosen industries in VGR. This thesis does not account for economic aspects of constructing this carbonation system. 3.4.3 Technical limitations A mean average linear distance is applied to get an overview of distances and sub- sequently emissions connected to transportation of demolished concrete. Future research directions are recommended to delve deeper into demolition of buildings, as there is very limited data on predicting the demolition rates which is the founda- tion for the material flow rate and thus the on potential sequestration in concrete processed at the EoL. The calculations presented in 2.2.2 are explained as a back- ground for carbonation calculations, but this method is not viable for use in this thesis due to time constraints and the sheer volume of buildings examined. Instead in-situ carbonation values are used. 28 4 Results & discussion The following section presents the results and discussion for the location, the amount of concrete bound in, and the assumed year of demolition for the MP buildings of interest to this thesis: multi-family concrete buildings constructed in Gothenburg municipality between 1965-1974. The CO2 sequestrated during the MP buildings lifetime is also presented as well as the potential for CO2 sequestration after the demolition of the buildings in three different scenarios: stockpiling in 0.4 m or 2 m and using Fastcarb technology. 4.1 Location and concrete stock in the case study The data in the provided excel-file resulted in 1 142 buildings being identified after removing buildings which lacked coordinate data or were located in the archipelago, which is presented in figure 10. Table 4 presents the amount of concrete bound in the MP buildings aggregated per year of construction. For Gothenburg municipality it can be seen that the largest amount of concrete is found in buildings built year 1965 and 1967-1971, with a steep drop-off for 1972-1974 and with 1966 having quite low volume as well. This can be interpreted as the mid 60s to early 70s being the peak years for the completion of MP buildings in Gothenburg municipality. As for the location of the buildings, figure 10 shows that the location of the buildings are in large south-west to north-east in relation to Gothenburg city-center. Each dot represent a building, and they are color coded by construction year. It can be seen that for the early years, 1965-1970, the buildings are located in the south-west and north-west, while buildings constructed later have more central or northly locations. 4.2 Lifespan and CO2 sequestration during use- phase Table 4 presents the results for the amount of sequestrated CO2 during the lifetime of MP buildings constructed in the years 1965-1974. A total of 19 857 tons of carbon dioxide can be bound in the concrete of the 1 142 buildings during an assumed lifetime of 100 years, see 3.2 for arguments for assumed lifespan. This amounts to a lifetime DoC of 12.4 %, which agrees well with established use-phase carbonation degrees of 10-20 % in the aggregated data from SIS, 2019. It should be emphasized that the used carbonation value to retrieve this result of 20 kg CO2/m3 derives from 29 4. Results & discussion Figure 10: Location of MP buildings in Gothenburg municipality, colour sorted by construction year. Picture captured from QGIS. Table 4: Amount of concrete in the MP buildings per year of construction, the amount of sequestrated CO2 during their assumed lifetime of 100 years and the degree of carbonation. Construction year Amount of concrete [t] CO2 sequestration use-phase [t] DoC [%] 1965 297 708 2 481 12.4 1966 148 127 1 234 12.4 1967 352 344 2 936 12.4 1968 369 298 3 077 12.4 1969 208 169 1 735 12.4 1970 353 272 2 945 12.4 1971 313 253 2 610 12.4 1972 147 607 1 231 12.4 1973 100 717 839 12.4 1974 92 315 769 12.4 TOT 2 382 810 19 857 12.4 a case study on a singular building to capture data on exposure conditions of all concrete elements in great detail. This method would however be excessively time- 30 4. Results & discussion consuming to conduct for the number of buildings examined in this study. This result implies that 12.4% of the reactive CaO in the cement has reacted with atmospheric CO2 during the use-phase to form chemically stable CaCO3, subsequently scaling down the remaining amount of CaO available for carbonation. This highlights the potential of carbonation in the EoL, as there is a remaining 87.6% CaO available for carbonation, which provides valuable insight into the environmental performance of concrete buildings during their EoL-stages. 4.3 Resource parks Three resource parks were identified in the scope of this report: Vikan, K̊allered and Gunnilse. Their respective geographical contexts are presented in 11. Vikan resource park is an approximately 40-hectare large area and is today used for quar- rying stone. It is located on Hisingen island approximately 7 km to the west of Gothenburg city center. K̊allered is an approximately 42-hectare large quarry that is located approximately 13 km to the south of Gothenburg city center, and Gunnilse is, as of this reports completion, a resource park that is not yet in use. Gunnilse is a 5-hectare area which is located 11 km to the northeast of Gothenburg city center. For calculation and logistical purposes, within the scope of this report, these three resource parks will solely be used for retrieving and storing crushed concrete in three different scenarios: • Crushing and storing concrete in cone-shaped stockpiles with a height of 2 meters for a DoC of 85% • Crushing and storing concrete in cone-shaped stockpiles with a height 0.4 me- ters with a DoC of 34.3 % • Crushing concrete for utilization in Fastcarb units at identified CO2 donors Figure 11 illustrates the geographical context of the resource parks with a radius of 8.7 km drawn around each site. Figure 12 shows the annual inflow of concrete from MP buildings prone to demolition, based on the shortest mean average linear distance to each resource park. Figure 12 together with figure 11 explains the dif- ference in the amount of concrete transported to each site. The method used in this report aimed at keeping the transport distance of the concrete from each demol- ished building to each resource park as short as possible to avoid excess transport emissions. With Vikan and Gunnilse being more centrally located in Gothenburg, and therefore in a closer proximity to the MP buildings as visualized in figure 11, they will receive more concrete than K̊allered resource park which is located in the far south. It is noteworthy to mention that due to this choice of method, K̊allered resource park will not recieve any concrete in 2067 and 2071-2074 as Vikan and Gunnilse are closer to the buildings that are demolished these years. 31 4. Results & discussion Figure 11: Circles with a radius of 8.7 km have been drawn around each processing site to visualize the shortest transportation route. Picture captured from QGIS. Figure 12: Amount of concrete that is transported to each of the three resource parks annually. 32 4. Results & discussion 4.4 Natural carbonation Natural carbonation will occur in two separate scenarios: • 1. Stockpiled in 2 meter height with 85% DoC (Skanska Finland and Toni Kekkonen Betoni, 2023) • 2. Stockpiled in 0.4 meter height with 34.3 % DoC (Stripple et al., 2018) The following sections will present the results for how much of the inflow of concrete to each site that is able to be processed in relation to the capacity for each of the two scenarios. In some cases, there will be enough space for stockpiling all concrete that is sent to the resource park, resulting in a surplus volume available for processing and in some cases, reversely, there won’t be enough space to process the inflow and will thus result in concrete not being able to be processed at the site. 4.4.1 Pile geometry Table 5 and table 6 presents the parameters for determining the total volume of concrete that can be stockpiled at each resource park, for the 0.4 m case and the 2 m case respectively. Table 7 shows the results for the processing capacity of each resource site in m3. Table 5: Cone geometry data for a pile of 0.4 m in height Radius [m] Volume [m3] Height [m] Floor area [m2] 0.53 0.118 0.4 0.881 Table 6: Cone geometry data for a pile of 2 m in height Radius [m] Volume [m3] Height [m] Floor area [m2] 2.648 14.688 2 22.033 Table 7: Processing capacity of the resource parks. Resource park Processing capacity 0.4 m scenario [m3] Processing capacity 2 m scenario [m3] Vikan 53 333 266 667 K̊allered 56 000 280 000 Gunnilse 6 667 33 333 33 4. Results & discussion 4.4.2 Vikan resource park The results for the amount of concrete that can be processed by Vikan in the 0.4m stockpiling scenario is presented in table 8. While the maximum processing capac- ity of crushed concrete for this scenario at Vikan resource park is 53 333 m3, the amount of concrete that is transported to the site exceeds this limit 2065-2071. In table 8, column four, the amount of concrete that is not processed these years are shown. The amount of concrete that can not be processed varies between 5 472 tons (2069) up to 118 504 tons (2067), a consequence of the varying amounts of concrete that is transported to the site each year. For all ten years Vikan can process 58.2 % of all concrete that is transported to this site. In table 9, the scenario of 2 m stockpiling provides a capacity of processing 266 667 m3 of concrete. As the inflow doesn’t exceed the capacity during any of the years 2065-2074 meaning 100% of the inflow of concrete can be processed. Subsequently, there will be a surplus volume available these years that can be used to process concrete from other resource parks. For the period spanning 2072-2074 the storage capacity is projected to be adequate. This is attributed to the diminished inflow of concrete, which is a consequence of the lower number of buildings constructed during the years 1972-1974. Figure 13 illustrates the total inflow of concrete to Vikan resource park annually with the horizontal lines displaying the max capacity for each stockpiling scenario, visualizing the large available volume for the 2 m scenario in comparison to the 0.4 m scenario. Table 8: Total inflow of concrete to Vikan annually and the share that can effec- tively be processed due to area restrictions for the 0.4 m scenario. Resource park: Vikan Approx. size: 40 ha Processing capacity, 0,4 m scenario: 53 333 [m3] YoD Inflow of concrete [m3] Capacity [m3] Concrete not processed [m3] Concrete processed of total inflow [%] 2065 158 450 53 333 105 117 34 % 2066 75 572 53 333 22 239 71 % 2067 171 837 53 333 118 504 31 % 2068 105 770 53 333 52 437 50 % 2069 58 805 53 333 5 472 91 % 2070 63 946 53 333 10 613 83 % 2071 65 182 53 333 11 849 82 % 2072 18 931 18 931 0 100 % 2073 30 054 30 054 0 100% 2074 33 780 33 780 0 100% TOT 782 387 456 095 - 58.2 % 34 4. Results & discussion Table 9: Total inflow of concrete to Vikan annually and the share that can effec- tively be processed due to area restrictions for the 2 m scenario. Resource park: Vikan Approx. size: 40 ha Processing capacity, 2 m scenario: 267 667 [m3] YoD Inflow of concrete [m3] Capacity [m3] Surplus volume available for processing [m3] Concrete processed of total inflow [%] 2065 158 450 266 667 108 217 100 % 2066 75 572 266 667 191 095 100 % 2067 171 837 266 667 94 830 100 % 2068 105 770 266 667 160 897 100 % 2069 58 805 266 667 207 862 100 % 2070 63 946 266 667 202 721 100 % 2071 65 182 266 667 201 485 100 % 2072 18 931 266 667 247 736 100 % 2073 30 054 266 667 236 613 100% 2074 33 780 266 667 232 887 100% TOT 782 387 2 666 670 - 100 % Figure 13: The total amount of concrete transported to Vikan resource park an- nually, in relation to the processing capacity for the two stockpiling scenarios. 35 4. Results & discussion 4.4.3 Net-emissions: Vikan The following section presents net-emissions for the processed concrete at Vikan. In figure 14 the net-emissions for the 0.4 m stockpiling are presented. In this case the emissions from processing the concrete are larger than the potential carbon sink, leading to CO2 emissions ranging from 32 tons for 2072 up to 103 tons for 2069. The variation is due to the amount of concrete varying over the years. The lower amount of concrete for the latter years 2072-2074 is attributable to a lower amount of buildings being completed these years. The low variation in net emissions for 2065- 2071 is due to the capacity limit of 53 333 m3 being reached these years, see figure 13. Excess concrete which can’t be processed due to capacity constraints wont be accounted for when calculating sequestration potential and process emissions. For the aggregated emissions, the demolition process is the largest contributor to CO2, see appendix A1. Demolition, along with the other emission posts, highlight where measures can be taken to mitigate emissions as to increase the net-sequestration and the environmental benefits. Demolition, along with the other emission posts, highlight where measures can be taken to mitigate emissions as to increase the net- sequestration and the environmental benefits. In figure 15 a net-gain in CO2 sequestration can be seen for all years. The larger CO2 values, compared to the emissions, are in part attributed to the larger DoC for the 2 m scenario, but also due to Vikan being able to process all concrete that is sent to it, meaning that more concrete can be utilized as a carbon sink. As for the 0.4 m scenario, the demolition emissions are the largest, see appendix table A2, followed by loader emissions, contributing 54.6% and 27.6 % of the total emissions for the 2 m scenario respectively. Figure 14: Net emissions for Vikan for 0.4 m stockpiling scenario. 36 4. Results & discussion Figure 15: Net emissions for Vikan for 2 m stockpiling scenario. 37 4. Results & discussion 4.4.4 K̊allered resource park Figure 16 visualize the results for both scenarios for K̊allered resource park. It can be seen that it is able to process 100% of the inflow of concrete for all years, for both scenarios. This result is a combination of the low inflow of concrete to K̊allered, see table 10 and table 11, and the large processing capacity of K̊allered due to its size of 42 ha. An important comment is that due to K̊allereds location in the far south of Gothenburg, see figure 10, it is further away from most of the buildings investigated in this thesis in comparison to Vikan and Gunnilse. While it recieves miniscule amounts of concrete compared to its processing capacity, this opens up an opportunity for processing excess concrete from other resource parks, see 4.4.8 for comments on this. 38 4. Results & discussion Table 10: Total inflow of concrete to K̊allered annually and the share that can effectively be processed due to area restrictions for the 0.4 m scenario. Resource park: K̊allered Approx. size: 42 ha Processing capacity, 0.4 m scenario: 56 000 [m3] YoD Inflow of concrete [m3] Capacity [m3] Surplus volume available for processing [m3] Concrete processed of total inflow [%] 2065 18 709 56 000 37 291 100 % 2066 8 801 56 000 47 199 100 % 2067 - 56 000 56 000 100 % 2068 5 136 56 000 50 864 100 % 2069 1 128 56 000 54 872 100 % 2070 18 557 56 000 37 443 100 % 2071 - 56 000 56 000 100 % 2072 - 56 000 56 000 100 % 2073 - 56 000 56 000 100% 2074 - 56 000 56 000 100% TOT 52 331 560 000 - 100 % 39 4. Results & discussion Table 11: Total inflow of concrete to K̊allered annually and the share that can effectively be processed due to capacity restrictions for the 2 m scenario. Resource park: K̊allered Approx. size: 42 ha Processing capacity, 2 m scenario: 280 000 [m3] YoD Inflow of concrete [m3] Capacity [m3] Surplus volume available for processing [m3] Concrete processed of total inflow [%] 2065 18 709 280 000 261 291 100 % 2066 8 801 280 000 271 199 100 % 2067 - 280 000 280 000 100 % 2068 5 136 280 000 274 864 100 % 2069 1 128 280 000 278 872 100 % 2070 18 557 280 000 261 443 100 % 2071 - 280 000 280 000 100 % 2072 - 280 000 280 000 100 % 2073 - 280 000 280 000 100% 2074 - 280 000 280 000 100% TOT 52 331 2 800 000 - 100 % Figure 16: The total amount of concrete transported to K̊allered resource park annually, in relation to the processing capacity for each natural carbonation scenario. 40 4. Results & discussion 4.4.5 Net-emissions: K̊allered In the case for K̊allered 0,4 m scenario, there is a net gain in CO2 sequestration for all years that it recieves concrete. As the net-emissions are similar in characteristics to the 0.4 m scenario for Vikan, see figure 14 for comparison. Due to the low DoC in this scenario, the net emissions will subsequently be smaller. As K̊allered will have the capacity to process all concrete it receives, the difference in net-emissions between the two scenarios are very clear when comparing figure 17 with figure 18. As it is the same amount of concrete that is processed in both scenarios, the emissions are equally large, but due to the 2m stockpiling scenario having a DoC of 85% the net savings of CO2 are substantially higher. Here it clearly shows what impact the DoC has, not only on amounts CO2 sequestrated but also from a planning perspective by knowing the time it takes to reach a certain DoC, Figure 17: Net emissions for K̊allered for 0.4 m stockpiling scenario. 41 4. Results & discussion Figure 18: Net emissions for K̊allered for 2 m stockpiling scenario. 4.4.6 Gunnilse resource park Table 13 illustrates the scenario using the dimensions for 2 m stockpiles and it can be observed that there won’t be adequate space for processing the concrete inflow during the years 2067-2072 largely due to the fact that Gunnilse is operating on a smaller area of 5 ha compared to the other resource parks. 42 4. Results & discussion Table 12: Total inflow of concrete to Gunnilse and the share that can effectively be processed due to area restrictions for the 0.4 m scenario. Resource park: Gunnilse Approx. size: 5 ha Processing capacity, 0.4 m scenario: 6 667 [m3/yr] YoD Inflow of concrete [m3] Capacity [m3] Concrete not processed [m3] Concrete processed of total inflow [%] 2065 8 909 6 667 2 242 74.8 % 2066 8 207 6 667 1 540 81.2 % 2067 48 376 6 667 41 709 13.8 % 2068 119 904 6 667 113 237 5.6 % 2069 70 173 6 667 63 506 9.5 % 2070 138 291 6 667 131 604 4.8 % 2071 130 602 6 667 123 935 5.1 % 2072 73 323 6 667 66 656 9.1 % 2073 32 894 6 667 26 227 20.3 % 2074 23 918 6 667 17 251 27.9 % TOT 654 598 66 670 - 10.2 % 43 4. Results & discussion Table 13: Total inflow of concrete to Gunnilse annually and the share that can effectively be processed due to area restrictions for the 2 m scenario. Resource park: Gunnilse Approx. size: 5 ha Processing capacity, 2 m scenario: 33 333 [m3/yr] YoD Inflow of concrete [m3] Capacity [m3] Concrete not processed [m3] Concrete processed of total inflow [%] 2065 8 909 8 909 0 100 % 2066 8 207 8 207 0 100 % 2067 48 376 33 333 15 043 68.9 % 2068 119 904 33 333 86 571 27.8 % 2069 70 173 33 333 36 840 47.5 % 2070 138 291 33 333 104 958 24.1 % 2071 130 602 33 333 97 269 25.5 % 2072 73 323 33 333 39 990 45.5 % 2073 32 894 32 894 0 100 % 2074 23 918 23 918 0 100 % TOT 654 598 273 926 41.8 % 44 4. Results & discussion Figure 19: The total amount of concrete transported to Gunnilse resource park annually, in relation to the processing capacity for each natural carbonation scenario. 4.4.7 Net-emissions: Gunnilse The results for the net-emissions related to Gunnilse resource park are presented in figure 20 for the 0.4 m scenario and figure 21 for the 2 m scenario. Figure 20 show that there will be net-emissions of 11-14 tons of CO2 for the 0.4 m scenario. This is both a consequence of the DoC of 34.3 % compared to that of 85 % for the 2 m scenario in figure 21, and due to the processing capacity being exceeded for all years in the 0.4 m scenario. The 2 m stockpiling scenario results in 214-870 tons of CO2 being sequestrated, depending on how much concrete that is processed. For the years 2067-2073 the amount of concrete that is transported to Gunnilse exceeds its processing capacity of 33 333 m3. The difference in net-emissions for these years, ca 10 tons, is attributed to a difference in transport distance over the years, see A6. 45 4. Results & discussion Figure 20: Net emissions for Gunnilse for 0.4 m stockpiling scenario. Figure 21: Net emissions for Gunnilse for 2 m stockpiling scenario. 4.4.8 Utilizing surplus volumes The method of measuring mean average linear distances, most concrete from demol- ished buildings will be allocated to Vikan as it is more centrally located compared to K̊allered and Gunnilse. Due to this, there will be scenarios where resource parks get a larger inflow of concrete than they are able to process, and some scenarios where the capacity won’t be exceeded and thus have a surplus volume available. A re-evaluation of certain transports could result in sending excess concrete that cant 46 4. Results & discussion be processed to sites with a surplus volume available. This is illustrated in 16, where there are years of no concrete inflow, and years where there is potential to stockpile larger volumes. 4.5 CO2 donors and Fastcarb technology The identified industries that could provide an inflow of CO2 to the Fastcarb units are presented in figure 22. However, not all sites emit CO2 and not all sites provided sufficient data for further analysis. The heaviest emitters operates as combustion plants and oil refineries, with various compositions of emissions in the industries flue gases. This thesis will only regard the CO2 emitted from these sites. Furthermore, The sites should prefer- ably be chosen with distances in mind to optimize logistical efficiencies parallel with environmental sustainability objectives. Moreover, a single Fastcarb unit has a constant annual processing capacity of 1 897.5 m3 with the ability to capture between 25-40 kg of CO2, see table 2. The capacity of the units remain constant over the years 2065-2074 which is why the net emissions only exhibit a slight variation. This variation is attributed to the calculations being based on different mean average transportation distances. As the thesis progressed, it was clear that transportation emissions were only a small share in comparison to the other emission posts, especially demolition, see appendix A1 through A6. If the transportation emissions are offset by the carbon sequestration, then it can be worth exploring CO2-donors located further away in order to increase the processing capacity for the Fastcarb scenario. More noticeably is the small volume of concrete that a Fastcarb unit can process. However, if there were to be a smaller stream of demolished concrete to the resource parks, which may very well be the case since the assumption of a 100 year lifetime is yet to be proven, then a Fastcarb unit would possibly be a more viable technology, due to its rapid carbonation. Adding to this, if the carbonated CA is used as an input in when producing concrete, it can reduce the need for virgin material ex- traction, strongly connected to the goal set by EU to implement aa environmentally sound management of construction demolition waste Caro et al., 2024. When comparing these three scenarios, a Fastcarb unit that can sequestrate 25-40 kg CO2 per t concrete with an annual capacity of 3 036 tons falls short when com- pared to stockpiling 2 m with a DoC of 85% for 6 months. Attributed mainly to the high DoC and amounts of concrete that can be processed with natural carbonation, which is closer to the operational realities of the aggregate industry. However, if it would be a case with lower DoC, smaller stockpiles and longer time-scales for sequestrating CA, such as the 0.4 m scenario, then a Fastcarb unit could be deemed more attractive. 47 4. Results & discussion Figure 22: Map over Gothenburg with plotted buildings, resource parks and CO2- donors. Picture captured from QGIS 4.5.1 Fastcarb: Vikan In figure 23 the processing capacity of four Fastcarb units is illustrated. Vikan, located in an industrial area where four of the identified potential CO2 donors operate, see figure 22, presents an opportunity to increase the share of processed concrete for that site. With four Fastcarb units a total of 7 590 m3 of concrete can be processed annually, which however still is not enough compared to the amount of concrete that the resource park recieve every year. 48 4. Results & discussion Figure 23: A Fastcarb units processing capacity in relation to inflow of concrete at Vikan resource park. 4.5.2 Fastcarb: K̊allered In figure 24 the processing capacity of a single Fastcarb unit operating at K̊allered is illustrated. For K̊allered there is only one identified emittor near-by, see figure 22, Riskullaverket. As previously mentioned, due to the location, year 2067 and 2071-2074 will result in no inflow of concrete to the resource park. However, as the Fastcarb unit only processes a small amount of the previous years volumes, there is a potential to use the excess non-processed concrete the years where there is no inflow of concrete. 49 4. Results & discussion Figure 24: A Fastcarb units processing capacity in relation to inflow of concrete at K̊allered resource park 4.5.3 Fastcarb: Gunnilse In figure 24 the processing capacity of a two Fastcarb units operating at Gunnilse is illustrated as two nearby potential CO2 donors where identified. This increases the annual processing capacity of the concrete sent from Gunnilse resource park to 3 795 m3. 50 4. Results & discussion Figure 25: A Fastcarb units processing capacity in relation to inflow of concrete at Gunnilse resource park 4.6 Net-emissions: Fastcarb technology Due to the Fastcarb unit processing an equal amount annually, sequestrating 25-40 kg CO2 / t concrete, there will only be slight variations of net emissions, which are attributed to different mean average distances. Interestingly, a Fastcarb unit has a DoC ranging from 37-59 %, which at its lowest is still higher than the scenario of 0.4 m stockpiling which generates a DoC of 34.4%, indicating that it sequestrates more kg CO2 / t concrete, and in a much shorter time span as well. See table 14 for aggregated performance results on a Fastcarb unit. 51 4. Results & discussion 4.7 Aggregated results In table 14 the aggregated results are presented in kg CO2 emitted and seques- trated per ton of processed concrete. It is clear that the scenario of 2 m stockpiling sequestrate more than what is possible during the use-phase with a maximum se- questration of 37.4 kg CO2 / t concrete. Even if a Fastcarb unit is used and perform at 40 kg CO2 / t concrete it can at most sequestrate 26.2 kg when accounted for process emissions. The 0.4 m scenario is the poorest choice when accounting for process emissions with a net sequestration of 7.1 kg CO2 / t concrete. The use- phase sequestration is in the table for reference, but it may not be truly comparable as this thesis account for all emissions in relation to natural carbonation of crushed concrete and using Fastcarb technology, see figure 8 for the system boundaries. If removing the demolition emissions from the use-phase sequestration it would result in a net sequestration of 1.33 kg CO2 / t concrete highlighting the large emissions connected to demolition practices. When accounting for how time consuming these different practices are, the Fastcarb unit may be a viable choice as it sequestrate CO2 much faster than the process of natural carbonation. A barrier for effiency is its limited capacity and man hours needed for loading and unloading concrete. Natural carbonation occurs constantly and without the need for human interaction, beside loading the concrete into piles. If a suitable area of land is available it may be a viable practice for storing CO2, which then is perpetually locked in as CaCO3. More research is however needed in how the natural carbonation performance is affected on larger stockpiles as smaller stockpiles will consume large land areas, which may make it unattractive for use. The two natural carbonation scenarios used in this thesis are however contradictive, with the smaller fractions (0.4 m scenario) consuming less CO2 than larger stockpiles (2 m scenario), which should not generally be the case according to literature (SIS, 2019; Stripple et al., 2018). Table 14: Aggregated results for each scenario. Scenario Emissions [kg CO2 / t concrete] Sequestration [-kg CO2 / t concrete] Net emissions [kg CO2 / t concrete] Time requirement Use-phase 0 -8.33 -8.33 100 [yr] Vikan 0.4 m 13.1 -20.3 -7.1 1,5 [yr] Vikan 2 m 12.8 -50.2 -37.4 0,5 [yr] K̊allered 0.4 m 13.1 -20.3 -7.1 1,5 [yr] K̊allered 2 m 12.9 -50.2 -37.3 0,5 [yr] Gunnilse 0.4 m 12.7 -20.3 -7.5 1,5 [yr] Gunnilse 2 m 12.5 -50.2 -37.7 0,5 [yr] Fastcarb (1 unit) 13.2-13.5 -40 - (-25) -26.2 - (-11.2) 1,5 [h] 52 5 Conclusion The studied concrete buildings sequestrate 12.4% of