Enhancing Mechanical Strength and Slake Durability of Remolded Loess via Microbial-Induced Carbonate Precipitation (MICP): A Microstructural Study

Loess has a metastable microstructure and high water sensitivity. When exposed to water, it undergoes rapid structural damage and disintegration, posing significant risks to the stability and durability of geotechnical structures such as foundations and slopes. Unconfined compressive strength (UCS) tests, direct shear tests, uniaxial tensile strength tests, and slake durability tests were conducted to evaluate the treatment performance. Optical microscopy and SEM were used to characterize the changes in microstructure to explain the potential reinforcement mechanism. The results show that microbial-induced carbonate precipitation (MICP) treatment leads to substantial improvement. Compared with untreated loess, the UCS, cohesion, internal friction angle, and uniaxial tensile strength increased by 370%, 663%, 43.7%, and 480%, respectively. Empirical refinements to the Mohr-Coulomb criterion were established to relate the measured UCS and uniaxial tensile strength to their theoretical values predicted from cohesion and friction angle. Both correlation models achieved R2 > 0.82, quantifying the additional structural strength contributed by bio-cementation. At the same time, the treatment significantly improved water stability, and the slaking index was reduced from 100% to less than 20%. Microstructural analysis shows that precipitated calcium carbonate crystals bond soil particles at contact points and fill inter-particle pores, constructing a bonding framework, which enhances the mechanical strength and water stability of the soil mass. These research results further illustrate the potential of MICP in enhancing the performance of loess in engineering projects.