Shear strength and durability enhancement of fly ash and bottom ash mixed soil using microbial-induced calcite precipitation: A comparative study

Abstract

Coal ash, a byproduct of coal combustion, is produced in large volumes globally, posing environmental risks due to landfill disposal. Reusing or stabilizing coal ash is essential for mitigating these risks. Fly ash (FA) and bottom ash (BA) exhibit pozzolanic properties that improve soil properties, making them suitable for geotechnical applications, especially as backfill materials. However, slow curing and insufficient strength limit their use. To overcome these limitations, this study explores microbial-induced calcite precipitation (MICP), a sustainable and eco-friendly ground improvement technique, to enhance the strength and durability of FA and BA-mixed soils. Biotreatment was performed using Sporosarcina pasteurii and a cementation solution, applied at one pore volume per cycle over a 9-d period. The effects of FA and BA on biocementation were evaluated at varying concentrations relative to soil weight. Unconsolidated undrained (UU) triaxial tests were conducted to assess shear strength parameters. Calcite content and microstructural analyses, including scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), were used to evaluate biocementation. The results showed that FA and BA improved soil strength, with optimal performance at 3 % ash content. Excessive FA and BA reduced bacterial mobility, limiting calcite precipitation. Durability tests under wetting-drying (WD) and freeze-thaw (FT) cycles revealed that FA-mixed samples exhibited better resistance to degradation than BA-mixed samples, with lower mass loss and minimal strength reductions. Overall, both FA and BA-mixed samples exhibited significant strength and durability, making them suitable for high-performance backfill materials. However, FA-mixed samples outperformed BA-mixed samples, offering superior strength and durability. © 2026 Institute of Rock and Soil Mechanics, Chinese Academy of Sciences