The Challenge of Circular Economy in the Concrete Sector: Life Cycle Thinking Approaches to Assess the Environmental and Economic Impact of Alternatives to Conventional Concrete

Circular economy (CE) is receiving increasing attention worldwide as a way to replace the current linear production/consumption model. Within the "European Circular Economy package", a set of actions supported by the European Commission in 2015, the construction sector has a prominent role in the transition towards a CE. Among the traditional construction materials, concrete is the most commonly used in modern construction. Concrete is a proportionated mix of hydraulic cement, acting as a binder, and fine coarse aggregates. Due to the massive use of natural aggregates and the emissions released during cement production, concrete raises considerable environmental concerns. Concrete uses up to 42% of natural aggregates produced globally in 1 year, and the ordinary Portland cement (OPC), the most commonly used binder in concrete, is responsible for the 8-10% of the global anthropogenic CO2 emissions. Therefore, the interest for CE in the concrete sector has rapidly increased, to find alternatives to OPC and natural aggregates, but also to create new opportunities for businesses and employment. To assess the environmental performances of the proposed alternatives, attributional life cycle assessment (ALCA) is the most commonly used methodology. However, this assessment method presents some methodological issues when applied to the evaluation of the CE for the construction sector. The goal of the thesis is, therefore, to analyse the environmental and economic costs/benefits of alternative construction blocks produced using metallurgic slag and recycled construction and demolition waste. The thesis also explores different methodological approaches in LCA, to investigate how different approaches may lead to different results. The results presented in the thesis can provide useful information for policymakers to promote the aspects contributing to sustainability and to limit the ones creating a barrier for CE in the construction sector. To achieve the goal of the thesis, chapter 2 and chapter 3 present an ALCA evaluation of two newly developed construction materials that use metallurgic slags (stainless steelslag and industrial goethite) as an alternative to OPC. The thesis analysed threedifferent valorisation routes: alkali activation, carbonation, combined "plasma fuming + inorganic polymerisation". The analysis highlighted the environmental benefits of the slag-based materials, such as the avoided landfilling of the slag and the reduction of carbon emissions for alkali activation and carbonation. At the same time, the analysis highlighted some environmental hotspots that can still represent a barrier to the development of the proposed technologies. In particular the production of chemicals in the alkali activation, the electricity consumption and the CO2 production for carbonation, and the electricity consumption and direct emission for plasma fuming. In chapter 4 an ALCA is combined with life cycle costing (LCC) to analyse the use of construction and demolition waste as an alternative to natural aggregates in concrete production. The study highlighted the main environmental benefits/hotspots, and the economic barriers of high-quality recycling of construction and demolition waste (CDW). In particular, transport distances play an essential role in the environmental sustainability of the system. The selling price of the recycled aggregates and the investment costs for the construction of the recycling plant are among the leading economic factors affecting the economic viability of CDW high-quality recycling schemes. In chapter 5 a consequential life cycle assessment (CLCA) is used to include market dynamics into the environmental LCA analysis.The study identifies the marginal suppliers that are potentially affected by the production of slag-based construction blocks. The study shows the importance of including market dynamics when assessing the large-scale changes. Chapter 6 presents a comparison between ALCA and dynamic life cycle assessment (DLCA), using a time-dependent life cycle impact assessment (LCIA) on climate change, to analyse the long-term effect on global warming of slag-based construction blocks . DLCA has shown more consistency when stating the actual time-frame of the study, allowing a more informed analysis of emission flows and global warming potential (GWP) effect over time. In general, the thesis demonstrated the potential for the implementation of a CE in the concrete sector, and highlights some of the main environmental and economic bottlenecks of the technologies. This provides useful information to help in developing the technologies. The results presented in the thesis can provide more informed decision support on the implementation of CE in the concrete sector, by helping decisionmakers to better understand the environmental and economic consequences of their decision With the presented case studies, the thesis also contributes to the development of methodologies to combine and integrate environmental and economic analysis when assessing the effects of the CE.

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