Atomic-Scale Insights into the Regulatory Mechanisms of Impurity Ions on the Stability and Growth Pathways of CaCO3 Pre-Nucleation Clusters in Tunnel Drainage Systems

Crystallization and blockage in tunnel drainage systems represent a major challenge in the operation and maintenance of tunnels in karst regions. This study focuses on a tunnel in Guilin, Guangxi, employing a combined approach of field investigation, laboratory characterization, and molecular dynamics (MD) simulations to explore the atomic-scale mechanism of CaCO3 crystallization within the drainage system. Field investigations reveal that the groundwater is dominated by Ca2+ and HCO3− ions, and the crystalline products consist primarily of high-crystallinity single-phase calcite, characterized by typical rhombohedral geometric structures and heterogeneous stacking. Molecular dynamics simulations indicate that the CaCO3 nucleation process is accompanied by the desolvation of Ca2+, while background electrolyte ions exert distinct regulatory effects on the nucleation kinetics. SO42− participates in cluster construction through strong coordination, inducing the formation of loose, chain-like aggregates; conversely, Cl− delays cluster coalescence primarily through charge shielding and steric hindrance effects. Additionally, Na+ influences the overall solution dynamics and the stability of pre-nucleation clusters by constructing stable hydration shells and providing charge neutralization. This research reveals the formation mechanism of tunnel crystallization from a microscopic perspective, providing theoretical support for the prevention and control of crystallization in tunnel drainage systems.