Effect of Freeze–Thaw Cycles on Black Cotton Soil Reinforced with Coir and Hemp Fibres in Alkali-Activated Binder

Expansive black cotton soil (BCS) is predominantly impervious and undergoes swelling and shrinkage when exposed to moisture fluctuation. This results in heaving of soil, causing it to lose its mechanical strength. Use of traditional cementitious binders like lime and cement has a significant impact on the environment and contributes almost 7% of the global CO₂ emissions. In the present study, an attempt was made to improve the geomechanical properties of BCS using envirosafe alkali-activated binder (AAB) with naturally available coir (CF) and hemp fibres (HF). The coir and hemp fibres were chemically treated to improve their durability. AAB was prepared by blending an alkali activator solution of sodium silicate and sodium hydroxide with class F fly ash at 0.4 water to solid ratio. This study also investigated the effectiveness of coir and hemp fibre reinforced AAB–BCS at different freeze–thaw cycles. The influence of varying dosages of fibres and freeze–thaw cycles in AAB-treated BCS showed a significant improvement in soil tensile strength and durability. The microstructural and geomechanical results of treated fibres showed higher serviceability and tensile resistance compared to the untreated ones. Furthermore, nonlinear regression equations were also proposed to relate experimental test results with model-predicted results in terms of unconfined compressive strength and indirect tensile strength.