Mechanical Properties, Micro-Mechanisms and Crack Evolution of Plant-Based Bio-Cement-Improved Loess Under Extreme Freeze–Thaw Environment
The extreme environment characterized by repeated freeze–thaw cycles poses a severe challenge to the stability and durability of loess in engineering applications. This study systematically investigates the improvement of Weinan loess using a plant-based bio-cement (BC) combined with fly ash (FA) under extreme freeze–thaw environments. Through unconfined compressive strength tests, permeability tests, calcium carbonate content measurements, and microscopic analyses (SEM and XRD), the mechanical properties, microstructural evolution, and crack development characteristics of the improved loess were comprehensively evaluated. The results demonstrate that BC-FA modification significantly enhances the mechanical strength and impermeability of loess. The unconfined compressive strength of the 7% FA-amended specimen increased by 201.6% compared to untreated loess, while the permeability coefficient decreased by 61.58%. Freeze–thaw-induced deterioration predominantly occurred within the first five cycles, with a maximum peak strength reduction of 33.29%, after which the soil structure gradually stabilized beyond ten cycles. Microscopic observations revealed that biomineralized calcium carbonate crystals (calcite, aragonite, and vaterite) filled pores and bridged soil particles, forming a continuous cementation network. Furthermore, a novel Crack Identification Method Based on Multi-Feature Mechanical Responses (CIMBMFMR) was proposed, which establishes a quantitative mapping between mechanical degradation, micro-damage, and crack evolution, offering superior accuracy and physical interpretability over traditional image-based techniques. The BC-FA system exhibits notable low-carbon and eco-friendly advantages, providing a promising green solution for loess reinforcement in seasonally frozen regions.