Treating fine incineration bottom ash (IBA) of municipal solid waste using marine clay (MC) and ground granulated blastfurnace slag (GGBS) for land reclamation

• Main Author: Sun, Xinlei
• Other Authors: Yi Yaolin
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2022
• Subjects: Engineering::Civil engineering
• Online Access: https://hdl.handle.net/10356/163895
• Institution: Nanyang Technological University

The growing economy and rapid urbanization have accelerated the generation of municipal solid wastes (MSW) in Singapore. Incineration is an effective method to deal with the increasing amount of MSW because it can largely reduce the mass and volume of MSW by 70-90%. However, MSW incineration also produces incineration bottom ash (IBA), which is currently landfilled in Singapore. Since IBA, especially fine fraction of IBA, contains a certain amount of heavy metals, it may pose a threat to the environment and human beings without proper treatment. The lack of space for waste disposal is another arising problem in Singapore, which makes the land-shortage problem even worse. Land reclamation is one of the solutions for enlarging the territories, which requires a huge amount of filling materials. However, Singapore is now short of conventional filling materials such as sand and gravel. Marine clay (MC) is a common geological material in Singapore, but it is not preferred as a filling material due to its high water content and compressibility. Nevertheless, the combination of IBA and MC (IBA-MC) may complement each other’s limitations, e.g., leaching of heavy metals from IBA and poor engineering properties of MC. To further improve the strength and reduce the leaching of heavy metals of IBA-MC, the stabilization/solidification (S/S) method can be used. Ordinary Portland cement (OPC) was the main binder to stabilize/solidify the mixture of IBA and soil. However, the production of OPC is associated with high-energy consumption and enormous CO2 emissions. To substitute OPC, this research uses an industrial byproduct, ground granulated blast furnace slag (GGBS), to stabilize/solidify IBA-MC. Moreover, in practical applications, IBA-MC and stabilized IBA-MC may be immersed in groundwater or even seawater. However, despite its importance, the stability of IBA-MC and stabilized IBA-MC in water environments has not been studied. Therefore, this research aims to investigate the feasibility of using fine IBA and excavated MC treated by GGBS as filling materials for land reclamation, focusing on its stability in water environments. The first part of this study (Chapters 3 to 5) investigates the utilization of air-cured mixture of fine IBA, MC, and GGBS as filling materials in three steps. The properties of the mixture of IBA and MC are firstly investigated ( Chapter 3) to evaluate the effect of the IBA/MC ratio on the compactability, strength, and leaching of heavy metals of IBA-MC. The results show that IBA reduces the water content of MC, improving the compactability and strength of MC, while MC reduces the leaching of heavy metals from IBA. Afterward, GGBS is used to treat fine IBA for the first time and compared with OPC (Chapter 4). The results show that GGBS-IBA achieves better strength and lower leaching of heavy metals than OPC-IBA. IBA and GGBS complement each other, i.e., GGBS binds IBA and reduces the leaching of heavy metals, and IBA activates the GGBS hydration and increases the strength. Based on the findings from Chapters 3 and 4, Chapter 5 investigates the properties of IBA-MC stabilized by GGBS (GGBS-IBA-MC) and OPC (OPC-IBA-MC) cured in the air for the first time. The results show that for a relatively high IBA/MC ratio, GGBS-IBA-MC has better strength and lower leaching of heavy metals than OPC-IBA-MC. The second part of this study (Chapters 6 to 7) investigates the stability of IBA-MC and stabilized IBA-MC in water environments for the first time. As a further work of Chapter 5, Chapter 6 investigates the stability of compacted IBA-MC, GGBS-IBA-MC, and OPC-IBA-MC soaked in distilled water and seawater. The results show that after soaked in water environments, especially seawater, IBA-MC and OPC-IBA-MC tend to generate cracks on the surfaces and have much lower strength compared with GGBS-IBA-MC. The decreasing suction force inside specimens and formation and growth of ettringite were two possible reasons for the instability of IBA-MC and OPC-IBA-MC in water environments. In Chapter 7, IBA-MC, GGBS-IBA-MC, and OPC-IBA-MC are made as pumpable filling materials to further examine their stability mechanism in water environments. IBA-MC and OPC-MC-IBA soaked in water environments, especially seawater, exhibit instability and higher strength reduction compared with GGBS-IBA-MC. The stability of GGBS-IBA-MC was significantly higher than OPC-IBA-MC with the same binder content. These findings were consistent with those for the compacted specimens. Since the pumpable specimens had a high water content and saturation degree, the suction inside was negligible. Moreover, no weak interfaces were generated during specimen preparation. Therefore, formation and growth of ettringite was deemed as the primary cause for the differences in the stability of OPC-IBA-MC and GGBS-IBA-MC.