Sodium carbonate and sodium silicate promote the Ca-montmorillonite: the nucleation, stabilization and hydrophilicity mechanisms

Montmorillonite is widely utilized in catalysis, environmental science, and civil engineering. Previous studies have demonstrated that Na₂CO₃ and Na₂SiO₃ enhance the stability of Ca-montmorillonite-rich clayey soils in chemical soil stabilization. However, the microscopic mechanisms underlying their effects on nucleation, stabilization, and hydrophilicity remain unclear. This study investigates these mechanisms using Scanning Electron Microscopy (SEM) and Density Functional Theory (DFT) calculations. SEM results show that Na₂CO₃ and Na₂SiO₃ enhance the strength of the stabilized soils by promoting the formation of cementitious and crystalline substances. DFT calculations reveal that SiO₃ ^2- and CO₃ ^2- exhibit the most negative adsorption energies of -6.2 eV and -5.1 eV, respectively, in the exchangeable layers of montmorillonite, significantly higher than those of Na⁺ and Ca²⁺. On the montmorillonite surface, SiO₃ ^2- and CO₃ ^2- display even lower adsorption energies of -8.7 eV and -6.8 eV, respectively. Water molecules preferentially adsorb dissociatively on the montmorillonite surface with an energy of -3.1 eV; however, their adsorption is suppressed following the adsorption of Ca²⁺, Na⁺, CO₃ ^2-, and SiO₃ ^2-, with energies decreasing to between -1.1 eV and -2.5 eV. Differential charge density plots indicate that ion adsorption leads to charge redistribution and the formation of chemical bonds. Specifically, Ca²⁺ and Na⁺ donate cationic charge, while CO₃ ^2- and SiO₃ ^2- accept electrons. The study further explains why Na₂CO₃ and Na₂SiO₃, in combination with lime, are more effective than lime alone in soil stabilization. A mechanism model for nucleation, stabilization, and hydrophilicity is proposed to explain the role of Na₂CO₃ and Na₂SiO₃ in promoting Ca-montmorillonite stabilization. This work provides valuable insights into the chemical properties of montmorillonite and the synergistic effects of calcium-based stabilizers combined with Na₂CO₃ and Na₂SiO₃ for soil stabilization.