• Energy Research

Abstract Mineral carbonation is a carbon utilisation technology in which an alkaline material reacts with carbon dioxide forming stable carbonates that can have different further uses, for instance as construction material. The alkaline material can be a residue from industrial activities (e.g. metallurgic slags) while CO2 can be recovered from industrial flue gasses. Mineral carbonation presents several potential environmental advantages: (i) industrial residues valorisation, (ii) CO2 sequestration and (iii) substitution of conventional concrete based on Portland cement (PC). However, both the carbonation and the CO2 recovery processes require energy. To understand the trade-off between the environmental benefits and drawbacks of CO2 recovery and mineral carbonation, this study presents a life cycle assessment (LCA) of carbonated construction blocks from mineral carbonation of stainless steel slags. The carbonated blocks are compared to traditional PC-based concrete blocks with similar properties. The results of the LCA analysis show that the carbonated blocks present lower environmental impacts in most of the analysed impact categories. The key finding is that the carbonated blocks present a negative carbon footprint. Nonetheless, the energy required represents the main environmental hotspot. An increase in the energy efficiency of the mineral carbonation process and a CO2 valorisation network are among the suggestions to further lower the environmental impacts of carbonated blocks production. Finally, the LCA results can promote the development of policy recommendations to support the implementation of mineral carbonation technology. Further research should enable the use of mineral carbonation on a broader range and large volume of alkaline residues.

Bio-based plastics are increasingly appearing in a range of consumption products, and after use they often end up in technical recycling chains. Bio-based plastics are different from fossil-based ones and could disturb the current recycling of plastics and hence inhibit the closure of plastic cycles, which is undesirable given the current focus on a transition towards a circular economy. In this paper, this risk has been assessed via three elaborated case studies using data and information retrieved through an extended literature search. No overall risks were revealed for bio-based plastics as a group; rather, every bio-based plastic is to be considered as a potential separate source of contamination in current recycling practices. For PLA (polylactic acid), a severe incompatibility with PET (polyethylene terephthalate) recycling is known; hence, future risks are assessed by measuring amounts of PLA ending up in PET waste streams. For PHA (polyhydroxy alkanoate) there is no risk currently, but it will be crucial to monitor future application development. For PEF (polyethylene furanoate), a particular approach for contamination-related issues has been included in the upcoming market introduction. With respect to developing policy, it is important that any introduction of novel plastics is well guided from a system perspective and with a particular eye on incompatibilities with current and upcoming practices in the recycling of plastics.

In recent years, cities have revealed themselves as being prominent actors in the circular economy transition. Besides supporting and initiating urban projects catalyzing circularity, cities are looking for monitoring tools that can make their progress towards circularity visible. Adopting Leuven’s pilot project for a building materials bank as a case study, this paper notes the particular challenges and opportunities in the pilot project to assess its progress and impact, in combination with gathering data for overall circular city monitoring purposes. Firstly, the paper names tensions between the “messy” transition process from policy ambitions to implementation and the question of data and monitoring. Secondly, the paper identifies relevant dimensions and scales to evaluate progress and impacts of a building materials bank, drawing from its development process. Thirdly, it proposes guidelines to monitor and evaluate circular city projects from the bottom up, combining quantitative indicators with guiding questions in a developmental evaluation. The analysis serves a critical reflection, distills lessons learned for projects contributing to circular cities and feeds a few concluding policy recommendations. The case study serves as an example that, in order to move beyond the tensions between circularity monitoring and actual circular city project development, monitoring instruments should simultaneously interact with and feed the circularity transition process. Therefore, dedicated data governance driven by enhanced stakeholder interactions should be inscribed in transition process guidance. Bottom-up projects such as a building materials bank provide opportunities to do this.