Calcium ions and calcium carbonate: key regulators of the enzymatic mineralization for soil dispersivity control

Dispersive soil is a widely distributed problematic soil in arid or semiarid areas of the world and can cause pipe erosion, gully damage and other seepage failures. This study analyzed the effect of environmentally friendly enzyme-induced carbonate precipitation (EICP) on the dispersivity of dispersive soils. This methodology was tested for the stabilization of three dispersive soil types (two high-sodium soils, two low-clay-content soils, and two soils with both high sodium and low clay contents) to examine the impact on dispersivity based on the results of pinhole tests and mud ball tests. Physical, chemical, mechanical, and microscopic tests were also conducted to investigate the effects of the components in the EICP reaction solution on dispersive soil modification. The experiments showed that the concentration of the reaction solution and the curing time required to limit the dispersivity decreased with increasing clay content in the soil. Ca²⁺ limited the dispersivities of dispersive soils via four distinct mechanisms. The first mechanism was ion exchange; Ca²⁺ decreased the percentage of exchangeable sodium ions to less than 7% while reducing the thickness of the diffuse double layer such that the spacings between soil particles were reduced and the chemical dispersivity was limited. Second, Ca²⁺ increased the viscosity of the solution by salting out the organic matter present in the soybean urease. Subsequently, the D1-class physically dispersive soil was converted into an ND2-class nondispersive soil. Third, Ca²⁺ decreased the soil pH by reducing the CO₃²⁻ content, which could hydrolyze to increase the soil alkalinity. Finally, the presence of Ca²⁺ led to the generation of cementitious minerals through the precipitation of CaCO₃ crystals that continuously generated CO₃²⁻, filling and cementing soil particles and thereby limiting their physical dispersivity. These results indicated that a low-concentration EICP reaction solution efficiently controlled the dispersivities of the three dispersive soils.