Effects of Ion-Regulated Mechanisms on Calcite Precipitation in the Enzyme-Induced Carbonate Precipitation Treatment of Loess

This study examines the effects and mechanisms of different Enzyme-Induced Carbonate Precipitation (EICP) treatments on loess structure improvement. The study focuses on ordinary EICP and three modified methods using MgCl2, NH4Cl, and CaCl2. A series of unconfined compressive strength (UCS) tests, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and elemental mapping were used to assess both macroscopic performance and microscopic characteristics. The results indicate that ordinary EICP significantly enhances loess particle bonding by promoting calcite precipitation. MgCl2-modified EICP achieves the highest UCS (820 kPa) due to delayed urea hydrolysis and the formation of aragonite alongside calcite, which results in stronger and more continuous cementation. In contrast, NH4Cl reduces urease activity and reverses the reaction, which limits carbonate precipitation and weakens structural cohesion. Excessive CaCl2 leads to a “hijacking mechanism” where hydroxide ions form Ca(OH)2, restricting carbonate formation and diminishing the overall enhancement. This study highlights the mechanisms behind enhancement, degradation, and diversion in the EICP process. It also provides theoretical support for optimizing loess subgrade reinforcement. However, challenges such as uneven permeability, environmental variability, and long-term durability must be addressed before field-scale applications can be realized, necessitating further research.