Enabling circular economy: An evaluation framework based on alkali-activated materials – From material environmental impacts to regional integration through supply chain optimization and stakeholder perspectives
Embargoed until 2025-03-08
Author
Date
2023Type
- Doctoral Thesis
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Abstract
Global objectives to mitigate climate change, reduce dependency on raw materials, and minimize waste require transformations in multiple sectors across the global economy, including the construction and building sector. Multiple alternative binders to carbon-intensive cement are being researched, among them alkali-activated materials (AAMs). The technology of AAM production involves activating aluminosilicate precursors with soluble solutions of alkaline sources. The most widely used precursors are industrial by-products, such as ground granulated blast furnace slag (GBFS) and fly ash (FA) from the steel industry and coal power plants, respectively, as well as the primary product metakaolin (MK). Recent research shows that alternative urban, industrial, and agricultural waste streams can also be used in AAMs. Thus, AAM technology can potentially contribute to open-loop waste recycling within the circular economy framework. While AAMs are regarded as promising materials for niche applications, capable of achieving similar mechanical properties to conventional cement and thereby having lower CO2 eq. impacts, the adaptation of AAMs at a real scale is rather limited.
Therefore, this thesis aims to assess the sustainability of AAMs at different levels, starting from the evaluation of environmental impacts at the material level, then optimizing supply chain networks for waste streams at the regional context, and finally identifying the benefits and barriers along emerging value chains based on pilot-scale mineral wool waste recycling in AAMs.
The first part of this thesis examines the environmental impacts of alkali-activated concrete mixes with proven international replicability, using conventional precursors GBFS, FA, and MK in their design. The objective is to identify the parameters that contribute the most to the uncertainty in LCA, thereby providing indications on the areas for further improvement. Results show that AAMs have lower CO2 eq. emissions compared to Portland cement concretes, even considering the variability in production processes of mix constituents for both types of binders, as well as uncertainty in transportation. However, activators contribute up to 33%, and precursors contribute up to 54% to the total global warming potential of the mixes, while variability in the production processes of sodium silicates and precursors are the main contributors to the uncertainty in results. This highlights the need to pay attention to the sources of activators and precursors, including waste-derived constituents, where site-specific data from producers would facilitate decision-making processes.
With a vast range of AAM mix constituents, the lack of long-term durability data often prevents researchers from examining impacts beyond the 'cradle-to-gate' framework. In this thesis, the results of the probabilistic service life time model developed within the DuRSAAM project were incorporated into LCA to determine environmental impacts of AAMs throughout the target service life time for reinforced concrete structures. Results indicate that uncertainty in service life time has the highest contribution to the total variance in LCA results and can affect the choice of the optimal mix for the targeted application, highlighting the need for further advancement of reliable and robust service life time models to cover the diversity of AAM mixes.
Within the second part of this thesis, LCA and geospatial analysis are combined with a mixed-integer linear optimization model to examine optimal supply chain network design for waste recycling in AAMs. The focus is on minimizing environmental impacts and costs associated with transportation between waste production sites, potential waste pretreatment plants, and alkali-activated concrete producers. Results show that for a distributed waste source, such as bottom ashes stemming from municipal solid waste incinerators, the optimal supply chain networks involve direct symbiosis between waste producers and concrete plants. While the total production of AAMs is associated with higher costs than cement concrete, for waste with scattered source locations, such as mineral wool insulation stemming from the renovation and demolition of building stock, centralized supply chains involving a single pre-treatment plant with subsequent production of AAMs are more cost-effective. Results indicate that by nudging open-loop recycling of available wastes, nearly 35% of the annual cement demand can be substituted in Switzerland, associated with 0.4 Mt avoided CO2 eq. emissions.
The third part of this thesis evaluates the benefits and barriers along emerging value chains based on interviews with stakeholders involved in pilot-scale mineral wool AAM production in five European countries. The assessment, based on selected CE indicators and the analysis of strengths, weaknesses, opportunities, and threats, indicates that technological and economic barriers faced by producers of AAMs at the pilot scale can potentially be overcome at industrial up-scale, where cities and policymakers shall take actions in the proposed urban-industrial symbiosis framework to help overcome barriers associated with waste sourcing and stimulate demand for alternative construction materials. Show more
Permanent link
https://doi.org/10.3929/ethz-b-000663485Publication status
publishedExternal links
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Contributors
Examiner: Habert, Guillaume
Examiner: Andersen, Birgitte Holt
Examiner: Matthys, Stijn
Examiner: Amor, Ben
Publisher
ETH ZurichSubject
Alkali-activated materials (AAM); Circular economy; Life cycle assessment; Uncertainty quantification; Supply chain; Alternative Cement; Multi objective optimization; Municipal solid waste incineration; Spatial analysis; Mineral wool waste; Value chain analysis; Stakeholder analysis; SWOT Analysis; Waste management; Recycling; Construction sector; Sustainability; Urban-industrial metabolismOrganisational unit
03972 - Habert, Guillaume / Habert, Guillaume
02655 - Netzwerk Stadt u. Landschaft ARCH u BAUG / Network City and Landscape ARCH and BAUG
Funding
813596 - Durable, Reliable and Sustainable Structures with Alkali-Activated Materials (EC)
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