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.

The increasing volume of surplus soil generated from excavation works in infrastructure projects such as roads, railroads, and subway facilities poses significant environmental and logistical challenges, particularly in terms of its disposal. Therefore, the development of engineering technologies that promote the effective use of surplus soil has intensified. Among surplus soils, clay is soft and must be treated when used as a backfill or fill material. Cement-based stabilizers are commonly used for soil treatment; however, the production of cement involves high carbon-dioxide emissions, which conflicts with Japan's carbon-neutrality goals. This study investigates the use of alternative stabilizers derived from biomass waste, specifically palm kernel shell ash (PKSA) and rice husk ash (RHA), to treat clayey soils intended for use as a backfill material in road construction. Experiments are conducted to evaluate the compaction and consolidation properties of clayey soils treated with PKSA and RHA. The results indicate that both stabilizers reduced the maximum dry density and increased the optimum water content of the treated soils. PKSA and RHA treatments enhanced compaction control, particularly on the wet side above the optimum water content, thus facilitating the achievement of a high compaction degree of 95 % under high initial water contents. Consolidation test results indicate that treatment with PKSA and RHA increases the consolidation yielding stress and reduces the volume compressibility, and that these effects are more pronounced at higher compaction levels. These results suggest that adding PKSA or RHA can expand the load range that exhibits elastic settlement and may reduce the consolidation settlement. At the same addition rate, PKSA treatment increases the consolidation yielding stress more significantly than RHA treatment. Additionally, PKSA treatment improves the stiffness and reduces the hydraulic conductivity at lower consolidation pressures compared with RHA treatment, thus indicating the greater effect of PKSA. Based on results of scanning electron microscopy, the enhancement in the stiffness and permeability afforded by PKSA is attributed to the formation of needle-like ettringite crystals, which strengthened the soil structure. By contrast, RHA treatment results in densely packed particles, which is attributable to limited hydration reactions caused by low CaO and high SiO2 contents. Thus, different mechanisms can result in different consolidation parameters of PKSA- and RHA-treated clays. However, both the PKSA- and RHA-treated clays indicate reduced coefficients of volume compressibility and permeability at a compaction degree of 95 %, with a stabilizer-to-clay ratio of up to 15 % by dry mass. Because neither PKSA nor RHA require high addition rates to improve the properties of surplus soft clays, these results suggest that PKSA and RHA can effectively enhance the compaction and consolidation properties of clayey soils. PKSA and RHA treatments are sustainable alternatives to the conventional cement-based treatments and support the environmental goals of construction projects.