Stabilization of dredged clay slurry with carbide sludge (CS)-ground granulated blastfurnace slag (GGBS) for land reclamation

Land reclamation projects have been extensively conducted in Singapore to accommodate its economic development and to tackle the challenge of sea level rise. Huge amounts of filling materials are then demanded, and sand is the most favorable filling material in land reclamation. However, Singapore is now facing a shortage of sand because the surrounding countries banned their sand exports to Singapore. Dredging activities in Singapore generates million tonnes of high water content clay slurry every year, and these materials has the potential to be used as fills in land reclamation to partially mitigate the demand for sand. Conventional vertical drain method has been widely used in soil improvements due to its relatively low cost, but this method is time-consuming and would result in surface settlement. Cement stabilization can achieve rapid stabilization by mixing clay slurry with binders. Nevertheless, a relatively large amount of ordinary Portland cement (OPC) is often required in slurry stabilization, and the production of OPC is associated with significant environmental impacts, including high CO2 emission and high energy consumption. Therefore, this study focuses on the use of two industrial by-products, carbide sludge (CS) and ground granulated blastfurnace slag (GGBS), as OPC alternatives in the stabilization of dredged clay slurry, aiming to reduce construction costs, enhance the stabilization efficacy, and mitigate the environmental impacts related to OPC production. The first part of this study (Chapters 3) investigates the use of CS to replace hydrated lime (HL) for GGBS activation. The strength and hydration products of CS-GGBS and HL-GGBS pastes at different curing ages are compared and studied to evaluate the activation mechanisms of CS-GGBS. Results show that CS-GGBS pastes can achieve close compressive strength as that of HL-GGBS pastes, and the optimum activator content of CS/HL-GGBS pastes are the same at all the curing ages. Besides, similar types of hydration products are identified in CS/HL-GGBS pastes, and the microstructure of these specimens are presented. Hence, CS can be used in GGBS activation, and these findings lay the foundation for follow-up research. The second part of this study (Chapters 4-5) investigates the efficacy of using CS-GGBS in clay slurry stabilization with OPC used as control. The unconfined compressive strength (UCS), water content, void ratio, and hydration products of the clay slurry stabilized by these two types of binders are compared and analyzed. Moreover, the influence of CS content on the UCS and hydration products of stabilized clay slurry are investigated, and a method is proposed to rapidly estimate the optimum CS content. Results show that the CS-GGBS-stabilized clay slurry can achieve 28-day UCS up to 4 times greater than that of the corresponding OPC-stabilized clay slurry. Therefore, compared with OPC, less CS-GGBS is required to achieve a same target UCS in clay slurry stabilization. Since the unit price of GGBS is close to that of OPC and CS is free, using CS-GGBS can significantly reduce material costs. It is also indicated that the remarkably different strength discrepancy between CS-GGBS- and OPC-stabilized clay slurry is not attributed to the slightly different void ratio but to the significantly different microstructures in these specimens. For the CS-GGBS-stabilized clay slurry with a fixed GGBS content, the optimum CS content can be estimated based on the initial pH plot. This method shows considerable accuracy in the stabilization of three types of clay slurry, which helps the design of CS-GGBS-stabilized clay slurry. The third part of this study (Chapters 6 and 7) evaluates the triaxial undrained shear strength, compressibility, and permeability of CS-GGBS-stabilized clay slurry. The strength behaviors and compression behaviors of CS-GGBS-stabilized clay slurry are studied under different levels of isotropic consolidation pressure and vertical stress, respectively. Bilinear Mohr-Coulomb failure envelopes and bilinear compression curves were observed for stabilized clay slurry in triaxial shear tests and oedometer tests, respectively. These observed engineering behaviors could be explained by the evolution of soil-cementation structure. Compared with unstabilized clay, CS-GGBS-stabilized clay slurry presents significant enhancement in shear strength and deformation resistance. Thus, it is feasible to use CS-GGBS-stabilized clay slurry in land reclamation, and a series of benefits, including reducing construction costs and mitigating the environmental impacts caused by OPC production, can be achieved. This research advances the understanding of CS-GGBS reactions as well as the geotechnical properties and microstructure of CS-GGBS-stabilized clay slurry. Moreover, the findings also pave the way for subsequent efforts to reveal the intricate reactions and microscopic evolutions that occur in stabilized clay slurry.Doctor of Philosoph