Durability of bio-carbonated soft soil under extreme freeze-thaw climates: Mechanical-physical-chemical insights

Bio-carbonation of reactive magnesia (BCRM) has emerged as a low-carbon and efficient technology for soft soil stabilization, yet its long-term weathering durability remains underexplored. This study systematically evaluates the freeze-thaw resistance of bio-carbonated soft soil under extreme climate conditions through macro-micro analyses, focusing on the evolution of mechanical (splitting tensile strength, STS), physical (water absorption, WA; ultrasonic pulse velocity, UPV), and chemical (pH; electrical conductivity, EC; ammonia nitrogen concentration, ANC) properties. The influence of fiber reinforcement is also investigated. Results show that STS decreases progressively with increasing freeze-thaw cycles, with the sharpest drop after the first cycle. Increases in WA and decreases in UPV indicate that strength loss is attributed to cementation failure and structural damage from frost heave, confirmed by the observed microcracks. Higher MgO content promotes denser structure and stronger cementation, improving the initial STS and mitigating the deterioration of physical-mechanical properties. Additionally, repeated leaching lowers pH, EC, and ANC in both soil matrix and leachate. However, during the early freeze-thaw cycles, the release of encapsulated reactive MgO and the disintegration of soil aggregates, induced by frost heave, temporarily elevate the pH and ANC of soil. Fiber reinforcement significantly improves freeze-thaw resistance but does not enhance initial strength due to interfacial voids and fiber clustering. This study fills the research gap in the durability and improvement strategies of bio-carbonated soft soil, supporting the advancement of BCRM technology in soft soil stabilization.