Investigating the mechanism of pH effect on saturated permeability of remolded loess

The pollution of remolded loess caused by acids and bases greatly affects its engineering performance, and thereby seriously threaten the service life of loess buildings or structures. To explore the changes in saturated permeability of the remolded loess polluted by hydrochloric acid or sodium hydroxide solutions, a series of the saturated seepage tests were designed in presence of different pH values. In addition, the hydrochemical components in the leachate along with the zeta potentials in water-soil solutions under different pH were measured. The changes in mineralogical, chemical, and microstructural properties of the loess samples before and after the seepage were also investigated. The results demonstrate gradual increase in the permeability of the loess samples with the decrease of pH of the solution. When acid enters the loess, the dissolution of carbonate cement and chemical cement (Al2O3 and Fe2O3) lays the foundation for the enlargement of pores. The decrease in thickness of the diffuse double layer results in aggregation of clay particles and enlarges the space of gravity water moving in the free layer. Hence, the significant increase in the permeability takes place over time during the seepage at pH = 3. The change in permeability at pH = 7 shows similar behavior to the acidic condition, and this is mainly responsible for the leaching of carbonate. However, the alkaline (pH = 11) condition inhibits the dissolution of carbonate minerals, and calcite is precipitated due to the incongruous dissolution of dolomite and dedolomitization. Al(OH)3, Fe(OH)3, and H2SiO3 colloids are formed and attached to the particles. These processes plug the pores during the seepage. Moreover, the development of the diffuse double layer results in wide distribution of the clay particles with highly cementation, which greatly reduces the effective connectivity of the pores. Therefore, its permeability decreases with the seepage time. These findings will offer reference for the practice of geological engineering in the loess areas.