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

Abstract

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.

Keywords: Ca-montmorillonite; Na2CO3 and Na2SiO3; calcium-based stabilizer; density functional theory (DFT); soil stabilization; stabilization and hydrophilicity mechanism.

FIGURE 1

The top and side view of montmorillonite. The red, blue, gray, and dark gray represent O, Si, Al and Ca, respectively.

FIGURE 2

SEM images of soil samples (a,b) are the B2 stabilizer-soil mixture without Na₂CO₃ and Na₂SiO₃; (c,d) are the B2 stabilizer-soil mixture with the addition of Na₂CO₃ and Na₂SiO₃.

FIGURE 3

The structures of (a) Ca²⁺, (b) Na⁺, (c) CO₃ ^2- and (d) SiO₃ ^2- on the exchangeable layer of montmorillonite.

FIGURE 4

The adsorption energies of Ca²⁺, Na⁺, CO₃ ^2- and SiO₃ ^2- on the exchangeable layer of montmorillonite.

FIGURE 5

The structures of (a) Ca²⁺, (b) Na⁺, (c) CO₃ ^2- and (d) SiO₃ ^2- on the surface of montmorillonite.

FIGURE 6

The adsorption energies of Ca²⁺, Na⁺, CO₃ ^2- and SiO₃ ^2- on the surface of montmorillonite.

FIGURE 7

The structures of H₂O and OH on the montmorillonite; (a) H₂O physical adsorption, (b) H₂O molecular adsorption, (c) H₂O dissociative adsorption.

FIGURE 8

The adsorption energies of H₂O and OH on the montmorillonite.

FIGURE 9

The structures of H₂O on the (a) Ca²⁺, (b) Na⁺, (c) CO₃ ^2- and (d) SiO₃ ^2- absorbed montmorillonite.

FIGURE 10

The adsorption energies of H₂O on the Ca²⁺, Na⁺, CO₃ ^2- and SiO₃ ^2- absorbed montmorillonite.

FIGURE 11

The differential charge density plots of (a) Ca²⁺, (b) Na⁺, (c) CO₃ ²⁻ and (d) SiO₃ ²⁻ on the montmorillonite surface. Yellow: charge accumulation; Cyan: charge depletion. The isosurface value is set to 0.008 e/Bohr3.

FIGURE 12

The structures of (a) OH on the montmorillonite, and the structures of (b) H₂O on the OH absorbed montmorillonite.

FIGURE 13

The adsorption energy of OH on the montmorillonite, and the adsorption energy of H₂O on the OH absorbed montmorillonite.

FIGURE 14

The structures of CO₂ on the (a) Ca-montmorillonite and (b) CaCO₃-montmorillonite surface.

FIGURE 15

The nucleation, stabilization and hydrophilicity mechanisms schematic diagram.