Effects of carbonate on the structure and properties of loess and the corresponding mechanism: an experimental study of the Malan loess, Xi’an area, China

Abstract

The structural, physical and morphological characteristics of loess will change as a result of the dissolution of mineral components, mainly carbonate, when it is exposed to acid rain, industrial acid leakage or other acidic environments. This study investigates the physical and structural characteristics of the Malan loess, Xi’an area, China, after carbonate dissolution. The presence of carbonate is determined by scanning electron microscopy (SEM)/energy-dispersive X-ray (EDX) and X-ray diffraction (XRD) analysis. The loess was soaked with 0.2 mol/L acetic acid. The degree of degradation of the loess, carbonate and other minerals was determined by the abundances of elements in the solution. Microstructural analysis combined with the loess characteristics of adsorption, water stability, compressive yield stress and shear strength are used to explain the changes in the soil structure after carbonate dissolution. The results show that bonds in the cementation are irreversibly lost during the remoulding process, and additional bonds are destroyed with the removal of large amounts of carbonate, causing the skeletal particle surface area, specific surface area, adsorption capacity and water stability to decrease. The compressive yield stress and cohesion of the soil are considerably more sensitive at higher water contents. Calcium carbonate is the main carbonate component in the loess, and the secondary calcium carbonate connects the skeletal particles by coating and bridging; the strong cementation stops the particles from dispersing in water. The change in strength and deformation of the soil change its microstructure, which indicates that the secondary calcium carbonate affects the soil structure and stability by reducing the aggregate wettability and changing the mechanical strength of the soil aggregate. This paper will contribute to elucidating the formation and destruction of loess.