Mechanical properties and micro-mechanism of loess roadbed filling using by-product red mud as a partial alternative

Abstract

Abstract The disposal of red mud waste has induced critical environmental issue worldwide. While adding red mud waste as a partial alternative in loess subgrade projects can effectively reduce the red mud waste accumulation and engineering cost. Therefore, the feasibility study of red mud waste as loess subgrade filling is in urgent demand. In this study, the mechanical characteristics and micro-mechanism of red mud-treated loess with a small amount of cement additive (RMCL) were analyzed systematically. In order to study the effect of red mud addition on RMCL, tests on the compaction characteristics, strength properties, dynamic behaviors of RMCL were carried. The action mechanism of red mud was also revealed through a series of X-ray diffraction (XRD), light microscope (LM) and scanning electron microscope (SEM) with energy dispersive spectrometer (EDS) test. The results show that the red mud content CR has a significant effect on the unconfined compressive strength (UCS) of RMCL. An appropriate amount of red mud waste can effectively improve the mechanical properties of loess subgrade filling. The optimal CR is approximately 15%–20% for loess roadbed filling in this study. In addition, the UCS of RMCL increases with the increasing curing time, but first increases and then decreases with the increase of CR. Considering the effect of CR and curing time, an empirical Gauss 2D model is strongly proposed to predict the UCS value. A good linear relationship is also observed between the UCS value and the resistivity for RMCL. Moreover, the dynamic elastic modulus of RMCL is four times higher than that of untreated loess. Both dynamic stress and confining pressure have significant effect on the dynamic characteristics of RMCL. Based on the microanalysis of XRD, LM, SEM, and EDS, red mud can promote the generation of more cementitious hydrate such as C-S-H/C-A-S-H and Aft due to its strong alkalinity and hydrolysis. These hydrates, red mud fine particles, and aggregates can adhere to the surface of loess particles or fill inter-granular pores to obtain a more compact and stable soil structure, leading to a significant improvement of mechanical properties. The study provides an effective way to recycle red mud waste as a substitutable filler in loess roadbed, exhibiting environmental and economic benefits.