Life cycle assessment of waste materials in deep cement mixing for land reclamation in Hong Kong

In recent years, researchers have shown increasing concern on the environmental impact attributed to traditional binders like cement, especially in mega-scale reclamation projects utilizing Deep Cement Mixing (DCM). The substitution of cement with alternative binders derived from construction and demolition waste as well as industrial slag waste is viewed as a potential strategy for mitigating the carbon footprint of this construction sector. In this study, the potential of using ground granulated blast furnace slag (GGBS), steel slag (SS), and waste concrete powder (WCP) to partially replace cement in construction projects involving DCM is assessed based on a case study in Hong Kong. A detailed comparison through Life Cycle Assessment (LCA) is conducted to examine the carbon footprint of alternative binders. Various scenarios that involve substituting cement to different extents with alternative binders generated from waste materials have been considered. The results reveal that the grinding process is the main contributor to the Global Warming Potential (GWP) for GGBS and SS, accounting for over 57% in each case. For WCP, the GWP is distributed across separation, sieving, and loading. Utilizing alternative waste leads to a GWP reduction in all scenarios, primarily due to lower emissions from these secondary raw materials. Overall, substituting 60% of Portland cement with GGBS, SS, or WCP in DCM construction resulted in GWP reductions of 34.7%, 34.5%, and 35.8% of greenhouse gas emissions, respectively. Various transportation scenarios are explored when moving the supply of construction materials from a local to a regional level by considering various material suppliers and the associated transportation distance. The sensitivity analysis highlighted the significant impact of transportation distance on GWP. This underscores the importance of logistics analysis in land reclamation projects and the benefits of integrating alternative binders into DCM projects.