Coupling Effect of Fly Ash and Lime Sludge Geopolymer in Stabilizing Kaolin Clay: Mechanical and Microstructural Characteristics

This study investigates the combined impact of fly ash (FA) and lime sludge (LS) based geopolymer treatment of Edgar plastic kaolin (EPK) clay. Geopolymers are increasingly favored over traditional soil stabilization methods, with various alkali-activated materials (AAMs) such as fly ash (FA) and ground granulated blast furnace slag (GGBS) being used to treat weak and compressible soils. However, limited documentation exists on the use of fly ash (FA) and lime sludge (LS) as a combined geopolymer treatment. FA-LS geopolymer-stabilized soil specimens were characterized through unconfined compressive strength (UCS) testing, scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX), and an X-ray diffraction (XRD) test. Curing was done at room temperature (RT, 23°C) and elevated temperature (ET, 70°C) for 7 and 28 days, respectively. Experimental findings revealed that varying FA-LS proportions led to increased UCS, Young’s modulus (E), and failure strain (εf), with the highest UCS achieved with a 15% FA-LS combination at ET compared to at RT. Statistical analysis of the experimental results showed a strong positive correlation between the UCS and other properties, such as Young’s modulus (E) and failure strain (εf). Microstructural analysis from SEM images confirmed the formation of geopolymer gels in the stabilized FA-LS samples, showing a more homogeneous and compact microstructure after stabilization at ET. Additionally, X-ray diffraction (XRD) analysis showed the formation of C-S-H and new minerals that were responsible for the strength enhancement of the geopolymer-stabilized clay. Finally, this study shows that FA-LS-based geopolymers at ET are more effective and can be used as a binder for soil stabilization.