Biochar and bamboo ash in black cotton soil improvement: A review of engineering performance

Black cotton soil is a problematic geotechnical material due to its pronounced swelling and shrinkage behavior. Recently, biochar and bamboo‐ash have emerged as sustainable alternatives for soil stabilization. This review evaluates their roles, properties, stabilization mechanisms, and effects on soil improvement. Biochar is produced through pyrolysis under limited oxygen conditions and is characterized by high porosity, low bulk‐density, and carbon sequestration capacity, which enhance soil structure and engineering properties. Bamboo‐ash, on the other hand, is obtained through complete combustion and exhibits pozzolanic characteristics that contribute to soil stabilization. Both materials improve compaction behavior, moisture retention, permeability, maximum dry density, and optimum moisture content. Studies showed that 4% bamboo‐ash combined with 3% lime and produced at 600°C increased the unconfined compressive strength (UCS) to 754 kN/m 2 while reducing the plasticity index by 72.2%. Similarly, 10% oyster shell ash with 8% cement or lime increased UCS to 2238 and 152 kN/m 2 , respectively, and improved the California Bearing Ratio (CBR) to 157% and 98%. Biochar derived from various feedstocks, including coconut coir, bamboo biomass, coconut shell, and mesquite hardwood, demonstrated optimum dosages of 2–6%. Coconut coir biochar at 2% increased UCS to 840 kPa, improved CBR by 60%, and reduced plasticity by 43.2%. Microstructural analyses revealed that curing enhances mineralogical transformations and promotes the formation of new minerals. Despite their benefits, variability in biochar properties, lack of standardization, and limited long‐term performance data remain challenges. Furthermore, no study has investigated the combined use of biochar and bamboo‐ash, highlighting an important area for future research.