The freeze–thaw resistance of bio-cement used for enhancing the stability of fractured rock masses: insight from ultradepth-of-field 3D microscopic observation and nano-indentation testing

In recent years, microbially induced calcium carbonate precipitation (MICP) technology has shown considerable promise in enhancing the bonding strength and reducing the permeability of rock fractures. However, research on the freeze–thaw resistance of bio-cemented rock fractures in cold regions remains limited. This study applies a “three-stage” MICP injection strategy to rock fracture cell models and subjects them to 0, 5, 10 freeze–thaw cycles (F-Ts) to evaluate the extent of deterioration. The degree of deterioration was quantified using P-wave velocity tests, direct shear tests, and solution analysis of the precipitated minerals within the cemented fractures. Microscopic deterioration processes were investigated using ultradepth-of-field 3D microscopic observation, nano-indentation testing, and X-ray diffraction techniques. Results indicate that F-Ts reduce the shear strength of bio-cemented rock fractures, with more pronounced deterioration observed in models with larger fracture apertures and higher bacterial concentrations. Specifically, the mechanical strength of the fracture cell models decreased by 46.5% and 49.5% after 5 and 10 F-Ts. Models cemented by a 2 × bacterial solution concentration exhibited 17.3 and 12.8% less degradation than those by 3 × and 5 × BS concentrations, respectively. Furthermore, low-concentration one-phase injection yielded the highest physical properties, whereas two-phase injection achieved superior spatial uniformity of mineral deposition. Microscopic analyses revealed that ice crystal growth and expansion during F-Ts disrupted the interfacial bonding between calcium carbonate crystals, leading to crack propagation and crystal shedding. This study elucidates the deterioration mechanisms of bio-cemented fractures under freeze–thaw conditions, providing a scientific foundation for the practical application of rock engineering in cold regions.