Alkali-Activated Geopolymer Using Municipal Solid Waste Incineration (MSWI) Ash for Soil Stabilisation: Beyond Conventional Life Cycle Assessment (LCA)

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Abstract
The use of municipal solid waste incineration (MSWI) ash as an alternative to conventional soil stabilisers offers an opportunity to promote circular economy practices. However, the environmental benefits of using waste-derived materials may be affected by additional processing and chemical inputs. This study calculates the carbon footprint of MSWI ash-based geopolymer and relevant uncertainties of design components using a comparative life cycle assessment (LCA). The functional unit is defined as the stabilisation of a 1 km-long road subgrade, with a width of 7 m and a stabilised layer thickness of 0.3 m, assuming a soil density of 1.8 tonnes/m3. Conducted LCA expands beyond the conventional system boundary by considering four scenarios: (1) direct environmental impacts, (2) benefits avoiding landfill diversion, (3) resource conservation avoiding virgin materials extraction and (4) combined credits from avoiding landfills and materials extractions. The assessment considers the production of alkali activators, MSWI ash processing, and the use of supplementary precursors, including ground granulated blast furnace slag (GGBFS) and kaolin. Life cycle impact assessment (LCIA) shows that the replacement of conventional stabiliser with MSWI ash contributes to a significant greenhouse gas emission. On the contrary, a higher emission has been observed for geopolymer production due to the additional chemical inputs but gained satisfactory strength. Landfill diversion of waste alone is insufficient to offset the emissions associated with hydrated lime. However, the environmental benefits become more evident when the avoidance of limestone extraction is included in the assessment. Correlation analysis further shows that sodium silicate (Na2SiO3) has a strong positive relationship with the total carbon footprint (r ≥ 0.80). Sodium hydroxide (NaOH) also shows a moderate relationship, with Pearson and rank correlation coefficients of 0.569 and 0.551, respectively. These results highlight the importance of alkali activators in determining the overall carbon footprint of geopolymer production. Critically, the environmental credits associated with landfill diversion and resource conservation are necessary to reflect the actual emission reduction strategies. Overall, the novelty of this research lies in diverting waste from landfills not necessarily translated into substantial carbon reductions. Instead, the environmental performance of waste-derived construction materials needs to be assessed by considering both their production impacts and the resources they help conserve.

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