Recycling of phosphogypsum and red mud in low carbon and green cementitious materials for vertical barrier

MgO activated slag and bentonite (MASB) slurry is a new and promising vertical barrier material along with excellent performances. Some solid wastes, such as phosphogypsum (PG), red mud (RM), fly ash and so on, show a positive effect on the performances of alkali activated slag. However, few studies focus on the recycling of these solid wastes in the system of MgO activated slag. The purpose of this paper is to study the incorporation of phosphogypsum and red mud on the mechanical property, permeability and hydration process of MASB slurry. The results showed that the addition of PG could significantly improve the mechanical strength and anti-permeability of the MASB slurry at early age (7 days), where the unconfined compressive strength (UCS) increased from 793.1 kPa to 1395.7 kPa and the permeability coefficient declined from 16.1 × 10⁻⁷ cm/s to 1.7 × 10⁻⁷ cm/s. In contrast, the introduction of RM had some negative effects on its macroscopic properties, resulting the UCS decreased to 580.4 kPa and the permeability coefficient rose to 25.9 × 10⁻⁷ cm/s at 7 days. The ettringite formed in the PG blended MASB slurry led to a notable increase in the absolute solid volume, which could satisfactorily fill the pores and block the pore channels. The combined addition of RM and PG had a synergistic effect on the promotion of hydration process and optimization of the pore structure, contributing to establish a low permeability and high mechanical strength matrix. The overall findings indicate that the use of solid wastes in the MASB slurry can not only improve its engineering properties, but also promotes its sustainability and economical efficiency, holding a great potential for popularization and application.