Optimization and Mechanical Performance of Alkali-Activated Bottom Ash and Polypropylene Fiber in Deep Mixing Columns

Deep mixing columns (DMC) are one of the widely used soil improvement methods in cohesive and cohesionless soils. Cement and lime are predominantly used as conventional binders in DMC grout. Due to high CO2 emissions from conventional binders, the feasibility of using alkali-activated binders as DMC grout is being investigated. This study was conducted in three main stages. In the first stage, the proportions of bottom ash and activator (mixture of 10M NaOH and Na2SiO3) were optimized to achieve the highest UCS value using response surface methodology (RSM). In the second stage, the influence of polypropylene fiber content was examined in detail by incorporating varying amounts of fibers into the optimized binder-activator mixture to determine the optimal fiber content. Since DMC are fully buried, they are cured in the soil media. In this case, the soil temperature, which varies along the depth of the soil, also becomes the curing temperature of the DCM. Due to climate, soil condition and thermodynamic based parameters, curing temperature of the DMC varies throughout the soil depth. Therefore, in the third stage, the effect of curing temperature on the mechanical properties of the stabilized samples was evaluated under different temperatures. Results of the study revealed optimal UCS values with AAB content of 0.3-0.5 and activator content of 0.45-0.5. Polypropylene fiber reinforcement further improved strength, reaching an optimum at 1.50-1.75% content. Curing temperature played a significant role, with UCS and modulus of elasticity increasing up to 30 °C, indicating the importance of considering environmental factors in DMC design. These findings suggest that the optimized mixture has the potential to replace conventional cement-based binders in DMCs, offering a more sustainable solution with lower carbon emissions. Furthermore, the observed influence of curing temperature highlights the need to account for subsurface thermal variations when designing DMCs, as these factors significantly affect strength gain and long-term performance.