Life cycle assessment of bottom ash valorization into foam glass-ceramic

The increasing generation of Municipal Solid Waste (MSW) and the need for sustainable management strategies present a critical environmental challenge. Among the residues from MSW incineration, bottom ash (BA) is the largest solid output by mass and poses long-term risks due to heavy metals and other pollutants. Conventional disposal methods like landfilling or low-grade reuse fail to address the environmental burden or resource potential of BA. In this context, thermal valorisation into high-value construction materials such as foam glass-ceramics has emerged as a promising solution. This thesis aims to assess and compare the environmental performance of different BA conversion pathways through a Life Cycle Assessment (LCA) approach, following ISO 14040–44:2006 standards. The functional unit of the study was 1 kg of BA, and the boundary conditions were from the grave to the gate, which included MSWI incineration, energy, and ashes (BA and fly ashes), and product productions. Inventory data were derived from peer-reviewed literature and adapted to Italian energy and waste composition contexts. The study focuses on four product scenarios derived from two main technological routes. Product 1A represents the foam glass-ceramic route, in where BA is vitrified and foamed using minimal additives. In contrast, Products 2A, 2B, and 2C represent three variants of the foam glass-ceramic route, using 60%, 50%, and 40% bottom ash respectively, supplemented with various additives. The environmental assessment was carried out using the ReCiPe 2016 Midpoint (H) method within SimaPro 9.6.0.1 and database Ecoinvent 3.5.0.1. The impact categories were: Global Warming Potential (GWP), Terrestrial Ecotoxicity, and Human Toxicity, Non-Carcinogenic. The results revealed that Product 1A, which benefits from energy recovery through incineration, outperformed the other scenarios across the GWP impact category, while it showed the highest human toxicity and ecotoxicity levels, highlighting the environmental benefits and challenges of integrating energy recovery with material valorization. Among the 2-series products, Product 2A (with 60% BA content) showed the best performance, while Product 2C (40% BA) exhibited the highest environmental impacts, mainly due to the increased use of external materials and energy inputs. In conclusion, the thesis showed that the thermal conversion of bottom ash into foam glass-ceramic (as in Product 1A) can significantly reduce climate change impacts when energy recovery from incineration is integrated into the process. However, this environmental benefit comes with trade-offs, as Product 1A also exhibits the highest impacts in terms of terrestrial ecotoxicity and human non-carcinogenic toxicity. These findings highlight the need to balance carbon efficiency with toxicity control in bottom ash valorisation strategies. The results further emphasize the critical role of careful material formulation and LCA modelling. The analysis of the 2-series products suggests a clear trend: higher bottom ash content and reduced reliance on external additives are associated with lower overall environmental impacts, reinforcing the value of maximizing waste utilization in sustainable material design. Future perspectives include extending the LCA to endpoint impact categories and assessing economic feasibility and scale-up potential. On the basis of LCA results and the identified hotspots, further experimental work should be done to reduce the final environmental impacts.