Red mud modified phosphogypsum based self-leveling mortar: Hydration mechanism, heavy metal migration law and spatial distribution characteristics

Abstract

In the production processes of phosphoric acid and alumina, chemical enterprises generate solid waste such as red mud (RM), phosphogypsum (PG), and fly ash (FA). The environmentally friendly utilization of these waste materials is of crucial significance. Therefore, this study focuses on investigating the impact of collaborative modification using diverse solid wastes on the macroscopic properties and internal microstructure of self-leveling mortar. Through a careful selection process, the optimal ratio for red mud and phosphogypsum self-leveling mortar(RPSM) has been identified during the research. This study not only provides substantial support for the effective utilization of solid waste but also contributes to the improvement of overall performance and structural characteristics of the mortar. Based on this, the hydration characteristics, leaching patterns, and spatial distribution characteristics of heavy metals in RPSM were evaluated. The research revealed that the optimal ratios of RM, PG, FA, PC, river sand, and heavy calcium carbonate powder in RPSM were 20.7%, 13.8%, 17.1%, 13.8%, 17.3%, and 17.3%, respectively. Under a water-binder ratio of 0.18, the flexural and compressive strength of the mortar specimens after 14 d of curing can reach 4.3 and 31.6 MPa. Through comprehensive analysis and discussion of the performance, microstructure, and dynamics of the hydration process of RPSM, the main processes of mortar hydration were elucidated. Environmental risk assessments indicated no environmental risks from heavy metals in RPSM. Human health risk assessment suggested carcinogenic risks from Cr and Cd in RPSM through oral exposure, leading to restrictions on the content of characteristic heavy metals in different application scenarios. Furthermore, fitting results of the release kinetics of heavy metals indicated that the leaching of heavy metals in RPSM can be well fitted with kinetic models, allowing the determination of the optimal safe age of RPSM containing harmful heavy metals. Finally, using interpolation and vertical distribution simulation by software such as ArcGIS, the spatial distribution characteristics of heavy metals in RPSM during use were analyzed. The research can supply fundamental academic support for the recycling of solid wastes like RM and the control of environmental risks associated with heavy metal leaching in building materials.