A comparative study on stabilization efficiency of kaolinite and montmorillonite clays with fly ash (FA) and rice husk ash (RHA)-based geopolymers

This research compares the stabilization efficiency of kaolinite and montmorillonite clayey soils using two industrial and agricultural by-products, namely fly ash (FA) and rice husk ash (RHA), activated by sodium hydroxide (NaOH). To this end, various proportions of FA and RHA (i.e., 0%, 5%, 10%, 15%, and 20%), along with NaOH solutions at 2 M and 4 M concentrations, are utilized to treat both low- and high-plasticity clayey soils. The resulting geopolymers are then subjected to a wide range of mechanical and micro-structural tests, including standard compaction, unconfined compressive strength (UCS), ultrasonic pulse velocity (UPV), swelling potential, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR). Results show that incorporating both FA and RHA into kaolinite and montmorillonite clays up to their respective optimal contents significantly enhances all their mechanical properties. However, FA-based geopolymers exhibit superior mechanical properties compared to RHA-based ones under similar additive contents and curing conditions. Accordingly, the optimal FA content is found to be 15%, while for the RHA-based geopolymers, the peak UCS is observed at 15% and 10% RHA for kaolinite and 10% and 5% RHA for montmorillonite when treated with 2 M and 4 M NaOH solutions, respectively. The results also suggest that FA is more effective than RHA in controlling the swelling potential of both kaolinite and montmorillonite soils. Microstructural analyses further corroborate the findings of macro-scale experiments by showcasing the comparative occurrence of geopolymerization, as well as the formation of cementitious gels, and synthesis of new chemical products.